# The 2016 oxide electronic materials and oxide interfaces roadmap

Wilfrid Prellier, M Lorenz, M Ramachandra Rao, T. Venkatesan, E Fortunato, P Barquinha, R Branquinho, D Salgueiro, M. Corriea Martins, E Carlos, et al.

► To cite this version:

Wilfrid Prellier, M Lorenz, M Ramachandra Rao, T. Venkatesan, E Fortunato, et al. The 2016 oxide electronic materials and oxide interfaces roadmap. Journal of Physics D: Applied Physics, 2016, 49 (43), pp.433001. 10.1088/0022-3727/49/43/433001. hal-02118554

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TOPICAL REVIEW • OPEN ACCESS

# The 2016 oxide electronic materials and oxide interfaces roadmap

To cite this article: M Lorenz et al 2016 *J. Phys. D: Appl. Phys.* **49** 433001

View the [article online](#) for updates and enhancements.

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TOPICAL REVIEW • OPEN ACCESS

# The 2016 oxide electronic materials and oxide interfaces roadmap

To cite this article: M Lorenz et al 2016 *J. Phys. D: Appl. Phys.* **49** 433001

View the [article online](#) for updates and enhancements.

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  Wenzhi Zheng, Ping Tan, Zhenhua Zhang et al.

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Advancing solid state &
electrochemical science & technology

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DISCOVER
how sustainability
intersects with
electrochemistry & solid
state science research

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Topical Review

# The 2016 oxide electronic materials and oxide interfaces roadmap

M Lorenz<fnref n="1" />, M S Ramachandra Rao<fnref n="2" />, T Venkatesan<fnref n="3" />, E Fortunato<fnref n="4" />,
P Barquinha<fnref n="4" />, R Branquinho<fnref n="4" />, D Salgueiro<fnref n="4" />, R Martins<fnref n="4" />, E Carlos<fnref n="4" />, A Liu<fnref n="5" />,
F K Shan<fnref n="5" />, M Grundmann<fnref n="1" />, H Boschker<fnref n="6" />, J Mukherjee<fnref n="2" />, M Priyadarshini<fnref n="2" />,<fnref n="7" />,
N DasGupta<fnref n="7" />, D J Rogers<fnref n="8" />, F H Teherani<fnref n="8" />, E V Sandana<fnref n="8" />, P Bove<fnref n="8" />,
K Rietwyk<fnref n="9" />, A Zaban<fnref n="9" />, A Veziridis<fnref n="10" />, A Weidenkaff<fnref n="10" />, M Muralidhar<fnref n="11" />,
M Murakami<fnref n="11" />, S Abel<fnref n="12" />, J Fompeyrine<fnref n="12" />, J Zuniga-Perez<fnref n="13" />, R Ramesh<fnref n="14" />,
N A Spaldin<fnref n="15" />, S Ostanin<fnref n="16" />, V Borisov<fnref n="16" />, I Mertig<fnref n="16" />, V Lazenka<fnref n="17" />,
G Srinivasan<fnref n="18" />, W Prellier<fnref n="19" />, M Uchida<fnref n="20" />, M Kawasaki<fnref n="20" />, R Pentcheva<fnref n="21" />,
P Gegenwart<fnref n="22" />, F Miletto Granzio²³, J Fontcuberta²⁴ and N Pryds²⁵

<figure><img src="image_4.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 3.0 licence](https://creativecommons.org/licenses/by/3.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig, Germany</footnote>

<footnote n="2">² Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600036, India</footnote>

<footnote n="3">³ National University of Singapore, Singapore</footnote>

<footnote n="4">⁴ i3N/CENIMAT, Department of Materials Science from Faculty of Science and Technology, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal</footnote>

<footnote n="5">⁵ College of Physics and Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, People's Republic of China</footnote>

<footnote n="6">⁶ Max Planck Institute for Solid State Research, Stuttgart, Germany</footnote>

<footnote n="7">⁷ Microelectronics and MEMS laboratory, Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India</footnote>

<footnote n="8">⁸ Nanovation, 8 Route de Chevreuse, 78117 Châteaufort, France</footnote>

<footnote n="9">⁹ Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar Ilan University, Ramat Gan 52900, Israel</footnote>

<footnote n="10">¹⁰ University of Stuttgart, Institute for Materials Science, DE-70569 Stuttgart, Germany</footnote>

<footnote n="11">¹¹ Superconductivity Research Laboratory, Graduate School of Science and Engineering, Shibaura Institute of Technology, 3-7-8 Toyosu, Koto-ku, Tokyo 135-8548, Japan</footnote>

<footnote n="12">¹² IBM Research GmbH, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland</footnote>

<footnote n="13">¹³ CRHEA-CNRS, Rue Bernard Gregory, Sophia-Antipolis, 06560 Valbonne, France</footnote>

<footnote n="14">¹⁴ Departments of Materials Science & Engineering and Physics, University of California, Berkeley, CA 94720, USA</footnote>

<footnote n="15">¹⁵ Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland</footnote>

<footnote n="16">¹⁶ Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 1, 06099 Halle, Germany</footnote>

<footnote n="17">¹⁷ Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium</footnote>

<footnote n="18">¹⁸ Physics Department, Oakland University, Rochester, MI 48309, USA</footnote>

<footnote n="19">¹⁹ Laboratoire CRISMAT, ENSICAEN, CNRS UMR 6508, 6 Boulevard Maréchal Juin, 14050 Caen, France</footnote>

<footnote n="20">²⁰ Department of Applied Physics, Faculty of Engineering, University of Tokyo, Tokyo, Japan</footnote>

<footnote n="21">²¹ Theoretical Physics, Department of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47057 Duisburg, Germany</footnote>

<footnote n="22">²² Experimentalphysik VI, Center for Electronic Correlations and Magnetism, Augsburg University, 86159 Augsburg, Germany</footnote>
Topical Review

# The 2016 oxide electronic materials and oxide interfaces roadmap

M Lorenz<fnref n="1" />, M S Ramachandra Rao<fnref n="2" />, T Venkatesan<fnref n="3" />, E Fortunato<fnref n="4" />,
P Barquinha<fnref n="4" />, R Branquinho<fnref n="4" />, D Salgueiro<fnref n="4" />, R Martins<fnref n="4" />, E Carlos<fnref n="4" />, A Liu<fnref n="5" />,
F K Shan<fnref n="5" />, M Grundmann<fnref n="1" />, H Boschker<fnref n="6" />, J Mukherjee<fnref n="2" />, M Priyadarshini<fnref n="2" />,<fnref n="7" />,
N DasGupta<fnref n="7" />, D J Rogers<fnref n="8" />, F H Teherani<fnref n="8" />, E V Sandana<fnref n="8" />, P Bove<fnref n="8" />,
K Rietwyk<fnref n="9" />, A Zaban<fnref n="9" />, A Veziridis<fnref n="10" />, A Weidenkaff<fnref n="10" />, M Muralidhar<fnref n="11" />,
M Murakami<fnref n="11" />, S Abel<fnref n="12" />, J Fompeyrine<fnref n="12" />, J Zuniga-Perez<fnref n="13" />, R Ramesh<fnref n="14" />,
N A Spaldin<fnref n="15" />, S Ostanin<fnref n="16" />, V Borisov<fnref n="16" />, I Mertig<fnref n="16" />, V Lazenka<fnref n="17" />,
G Srinivasan<fnref n="18" />, W Prellier<fnref n="19" />, M Uchida<fnref n="20" />, M Kawasaki<fnref n="20" />, R Pentcheva<fnref n="21" />,
P Gegenwart<fnref n="22" />, F Miletto Granzio²³, J Fontcuberta²⁴ and N Pryds²⁵

<figure><img src="image_5.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 3.0 licence](https://creativecommons.org/licenses/by/3.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig, Germany</footnote>

<footnote n="2">² Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600036, India</footnote>

<footnote n="3">³ National University of Singapore, Singapore</footnote>

<footnote n="4">⁴ i3N/CENIMAT, Department of Materials Science from Faculty of Science and Technology, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal</footnote>

<footnote n="5">⁵ College of Physics and Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, People's Republic of China</footnote>

<footnote n="6">⁶ Max Planck Institute for Solid State Research, Stuttgart, Germany</footnote>

<footnote n="7">⁷ Microelectronics and MEMS laboratory, Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India</footnote>

<footnote n="8">⁸ Nanovation, 8 Route de Chevreuse, 78117 Châteaufort, France</footnote>

<footnote n="9">⁹ Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar Ilan University, Ramat Gan 52900, Israel</footnote>

<footnote n="10">¹⁰ University of Stuttgart, Institute for Materials Science, DE-70569 Stuttgart, Germany</footnote>

<footnote n="11">¹¹ Superconductivity Research Laboratory, Graduate School of Science and Engineering, Shibaura Institute of Technology, 3-7-8 Toyosu, Koto-ku, Tokyo 135-8548, Japan</footnote>

<footnote n="12">¹² IBM Research GmbH, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland</footnote>

<footnote n="13">¹³ CRHEA-CNRS, Rue Bernard Gregory, Sophia-Antipolis, 06560 Valbonne, France</footnote>

<footnote n="14">¹⁴ Departments of Materials Science & Engineering and Physics, University of California, Berkeley, CA 94720, USA</footnote>

<footnote n="15">¹⁵ Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland</footnote>

<footnote n="16">¹⁶ Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 1, 06099 Halle, Germany</footnote>

<footnote n="17">¹⁷ Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium</footnote>

<footnote n="18">¹⁸ Physics Department, Oakland University, Rochester, MI 48309, USA</footnote>

<footnote n="19">¹⁹ Laboratoire CRISMAT, ENSICAEN, CNRS UMR 6508, 6 Boulevard Maréchal Juin, 14050 Caen, France</footnote>

<footnote n="20">²⁰ Department of Applied Physics, Faculty of Engineering, University of Tokyo, Tokyo, Japan</footnote>

<footnote n="21">²¹ Theoretical Physics, Department of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47057 Duisburg, Germany</footnote>

<footnote n="22">²² Experimentalphysik VI, Center for Electronic Correlations and Magnetism, Augsburg University, 86159 Augsburg, Germany</footnote>
E-mail: mlorenz@physik.uni-leipzig.de and msrrao@iitm.ac.in

Received 5 April 2016, revised 18 July 2016

Accepted for publication 21 July 2016

Published 3 October 2016

<figure><img src="image_5.png" /></figure>

CrossMark

Abstract

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio.

Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on ‘oxide electronic materials and oxide interfaces’.

This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan.

The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action ‘towards oxide-based electronics’ which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies.

In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.

Guest editors: M S Ramachandra Rao and Michael Lorenz

Keywords: oxides, interfaces, materials, oxide electronics

(Some figures may appear in colour only in the online journal)

<footnote n="23">23 CNR-SPIN UOS Napoli, Complesso Universitario di Monte Sant'Angelo, Via Cinthia 80126, Napoli, Italy</footnote>

<footnote n="24">24 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193, Catalonia, Spain</footnote>

<footnote n="25">25 Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Roskilde 4000, Denmark</footnote>
23 CNR-SPIN UOS Napoli, Complesso Universitario di Monte Sant'Angelo, Via Cinthia 80126, Napoli, Italy

24 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193, Catalonia, Spain

25 Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Roskilde 4000, Denmark

E-mail: mlorenz@physik.uni-leipzig.de and msrrao@iitm.ac.in

Received 5 April 2016, revised 18 July 2016

Accepted for publication 21 July 2016

Published 3 October 2016

<figure><img src="image_6.png" /></figure>

CrossMark

Abstract

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio.

Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on ‘oxide electronic materials and oxide interfaces’.

This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan.

The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action ‘towards oxide-based electronics’ which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies.

In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.

Guest editors: M S Ramachandra Rao and Michael Lorenz

Keywords: oxides, interfaces, materials, oxide electronics

(Some figures may appear in colour only in the online journal)
Contents

<table><tr><td>1. Oxide films and heterostructures for electronics, memory, magnetics, photonics, energy and health</td><td>4</td></tr><tr><td>2. Is the new oxide electronics (r)evolution solution based?</td><td>6</td></tr><tr><td>3. Bipolar oxide devices</td><td>8</td></tr><tr><td>4. Complex-oxide field-effect transistors</td><td>10</td></tr><tr><td>5. Oxide electronic materials for resistive switching and TFT applications</td><td>12</td></tr><tr><td>6. Application aspects of ZnO</td><td>14</td></tr><tr><td>7. All-oxide photovoltaics</td><td>17</td></tr><tr><td>8. Perovskite-type materials for energy converters</td><td>20</td></tr><tr><td>9. Superconductivity in oxides: from processing to applications</td><td>23</td></tr><tr><td>10. Silicon photonics enhanced with functional oxides: new opportunities</td><td>25</td></tr><tr><td>11. New oxide microcavities applications: from polariton lasers to fundamental physics at room-temperature</td><td>28</td></tr><tr><td>12. Multiferroic bismuth ferrite</td><td>30</td></tr><tr><td>13. Novel functionalities at oxide (multi)ferroic domain walls</td><td>32</td></tr><tr><td>14. Switchable 2D electron gas at the multiferroic interfaces</td><td>34</td></tr><tr><td>15. Magnetoelectric coupling in multiferroic epitaxial thin film composites</td><td>36</td></tr><tr><td>16. Self-assembled multiferroic nanocomposites</td><td>38</td></tr><tr><td>17. Combinatorial substrate epitaxy: a novel approach of epitaxy for oxide films</td><td>40</td></tr><tr><td>18. Topological oxide electronics</td><td>42</td></tr><tr><td>19. Control of orbital reconstruction, metal-to-insulator transitions and topologically nontrivial phases at oxide interfaces</td><td>44</td></tr><tr><td>20. Kitaev exchange in hexagonal iridates and rhodates</td><td>46</td></tr><tr><td>21. A European action tracing the roadmap towards oxide electronics: perspective on nonvolatile oxide memories</td><td>48</td></tr></table>
Contents

<table><tr><td>1. Oxide films and heterostructures for electronics, memory, magnetics, photonics, energy and health</td><td>4</td></tr><tr><td>2. Is the new oxide electronics (r)evolution solution based?</td><td>6</td></tr><tr><td>3. Bipolar oxide devices</td><td>8</td></tr><tr><td>4. Complex-oxide field-effect transistors</td><td>10</td></tr><tr><td>5. Oxide electronic materials for resistive switching and TFT applications</td><td>12</td></tr><tr><td>6. Application aspects of ZnO</td><td>14</td></tr><tr><td>7. All-oxide photovoltaics</td><td>17</td></tr><tr><td>8. Perovskite-type materials for energy converters</td><td>20</td></tr><tr><td>9. Superconductivity in oxides: from processing to applications</td><td>23</td></tr><tr><td>10. Silicon photonics enhanced with functional oxides: new opportunities</td><td>25</td></tr><tr><td>11. New oxide microcavities applications: from polariton lasers to fundamental physics at room-temperature</td><td>28</td></tr><tr><td>12. Multiferroic bismuth ferrite</td><td>30</td></tr><tr><td>13. Novel functionalities at oxide (multi)ferroic domain walls</td><td>32</td></tr><tr><td>14. Switchable 2D electron gas at the multiferroic interfaces</td><td>34</td></tr><tr><td>15. Magnetoelectric coupling in multiferroic epitaxial thin film composites</td><td>36</td></tr><tr><td>16. Self-assembled multiferroic nanocomposites</td><td>38</td></tr><tr><td>17. Combinatorial substrate epitaxy: a novel approach of epitaxy for oxide films</td><td>40</td></tr><tr><td>18. Topological oxide electronics</td><td>42</td></tr><tr><td>19. Control of orbital reconstruction, metal-to-insulator transitions and topologically nontrivial phases at oxide interfaces</td><td>44</td></tr><tr><td>20. Kitaev exchange in hexagonal iridates and rhodates</td><td>46</td></tr><tr><td>21. A European action tracing the roadmap towards oxide electronics: perspective on nonvolatile oxide memories</td><td>48</td></tr></table>
# 1. Oxide films and heterostructures for electronics, memory, magnetics, photonics, energy and health

T Venkatesan

National University of Singapore, Singapore

**Status.** Since the invention of the pulsed laser deposition (PLD) process and the advent of high $T_c$ superconductivity [1] in the 1987 time frame, the field of oxides has accelerated at an extraordinary pace with potential impact on numerous fields. I will highlight some of the key driving contributions of oxides in each area of interest and also point out what is needed to make significant progress.

*Current and future challenges.*

**Electronics.** From the perspective of electronic devices one of the key issues is achieving higher mobility for carriers in oxides and the transport in most oxides is determined by the overlap of the cationic and anionic orbital. In the case of $TiO_2$ this is the Ti 3d and oxygen 2p overlap which is very sensitive to the O-Ti-O bond angle. The results were clearly demonstrated for 2D electron transport in the $TiO_2$ system where the maximum metallic behaviour is seen for the case of $TiO_2$ on $SrTiO_3$ (where the bond angles are 180 degrees), weak localization in the case of anatase $TiO_2$ (153 degrees) and strong localization in the case of rutile $TiO_2$ (81 and 93 degrees). Unlike conventional metals where molecular orbitals do not directly affect transport except indirectly via phonons, in the case of oxides they are very important and give us a way to engineer the carrier mobility by tuning this overlap via, strain, doping etc [2]. From a fundamental perspective this system is a good example where electron-polaron interactions can be well studied. By minimizing the strain at the surface of $SrTiO_3$ mobility in excess of $100\,000\,cm^2\,V^{-1}\,s^{-1}$ has been achieved at low temperatures [3].

**Memory.** In tunnel junction memories integrating ferroelectrics with conventional tunnel junctions may give us much lower switching energies with practical ON/OFF ratios. Recent work on ferroelectric tunnel junctions shows that the ON/OFF ratio can be enhanced significantly by increasing the number of interfaces in addition to the FE tunnel barrier [4]. The switch ON and OFF characteristics are determined by the ferroelectric only and while the ON/OFF ratio increases with number of barriers it also causes an increase in the ON state resistance which is not desirable. FE tunnel barriers as thin as two unit cells can switch while the ON state resistance can be made comparable to today's commercial MTJs. An order of magnitude reduction in the FE coercive fields are necessary to make this a competitive, low energy consuming memory technology.

**Photonics.** Ferroelectrics on silicon enable the making of wide band width optical modulators that can work at a variety of optical wavelength and speeds approaching $40\,MB\,s^{-1}$ [5]. These devices have the potential for $100\,MB\,s^{-1}$ modulation

speeds but the best devices today have only been demonstrated on MgO. Such all wavelength modulators would compete against established technologies such as ones based on $LiNbO_3$ (handicapped by large sizes) or the III-V semiconductors (wavelength specific due to electro absorption modulation mechanism). For commercial applications these oxide based devices need to be built on a silicon platform where high quality electro-optic (ferroelectric) layers need to be deposited with suitable buffer layers to preserve the high crystalline quality of the over layers. In order to achieve mode confinement the low index buffer layer needs to have a thickness of at least 200 nm. A scalable process for the fabrication of such oxide heterostructure layers for large areas does not exist today and there is a clear need for this.

**Magnetics.** Oxide magnetism received a significant boost with the advent of high temperature superconductors followed by a resurgence of interest in colossal magneto-resistive magnetites followed closely by diluted magnetic semiconducting oxides. While the origin of magnetism in these systems was not fully understood, the notion of defect mediated magnetism in oxides was advanced by Sawatzky et al [6]. The first such demonstration was in $TiO_2$ observed with Ta substitution at the Ti site. Spectroscopic analysis clearly established titanium vacancies as the primary source of magnetism with the donor electrons from Ta substitution being responsible for the magnetic exchange [7]. The observed magnetism was weak as the concentration of Ti vacancies was below $10^{21}\,cm^{-3}$ and stability consideration would dictate the difficulty of producing higher levels of sustainable defects leading a limit on the strength of magnetism possible via such an approach. However, extremely novel magnetic exchange is possible in oxide systems mediated by polar layers and the exact mechanism of the exchange which involves spin-orbit interactions is not fully understood but holds promise for novel magnetic devices utilizing magnetic exchange [8].

**Energy.** A number of oxides have been strong candidates in catalysis applications and owing to their stability in harsh chemical environments and reasonable band match with reaction pathways, materials such as $Fe_2O_3$ and $TiO_2$ have been popular. However, recently a water splitting metallic oxide was introduced which seems to have fascinating properties [9]. Besides being a conductor, this material is an efficient water splitter and the mechanism of the optical absorption, carrier decay have been now clearly understood. It turns out that this family of materials $MNbO_3$ ($M$ = Sr, Ca, Ba) is a degenerate wide bandgap semiconductor with a bandgap of over 4 eV but an electronic carrier density of over $10^{22}\,cm^{-3}$, an order of magnitude short of elemental metals [10]! The optical absorption in the material occurs through a plasmonic absorption at 1.9 eV arising from the large carrier density and the hot carrier lifetime after plasmonic excitation is very long (>250 ps) which explains the efficiency of these materials in water splitting. Such a strongly correlated electronic material promises a novel approach in the design of catalysts.
# 1. Oxide films and heterostructures for electronics, memory, magnetics, photonics, energy and health

T Venkatesan

National University of Singapore, Singapore

**Status.** Since the invention of the pulsed laser deposition (PLD) process and the advent of high $T_c$ superconductivity [1] in the 1987 time frame, the field of oxides has accelerated at an extraordinary pace with potential impact on numerous fields. I will highlight some of the key driving contributions of oxides in each area of interest and also point out what is needed to make significant progress.

*Current and future challenges.*

**Electronics.** From the perspective of electronic devices one of the key issues is achieving higher mobility for carriers in oxides and the transport in most oxides is determined by the overlap of the cationic and anionic orbital. In the case of $TiO_2$ this is the Ti 3d and oxygen 2p overlap which is very sensitive to the O-Ti-O bond angle. The results were clearly demonstrated for 2D electron transport in the $TiO_2$ system where the maximum metallic behaviour is seen for the case of $TiO_2$ on $SrTiO_3$ (where the bond angles are 180 degrees), weak localization in the case of anatase $TiO_2$ (153 degrees) and strong localization in the case of rutile $TiO_2$ (81 and 93 degrees). Unlike conventional metals where molecular orbitals do not directly affect transport except indirectly via phonons, in the case of oxides they are very important and give us a way to engineer the carrier mobility by tuning this overlap via, strain, doping etc [2]. From a fundamental perspective this system is a good example where electron-polaron interactions can be well studied. By minimizing the strain at the surface of $SrTiO_3$ mobility in excess of $100\,000\,cm^2\,V^{-1}\,s^{-1}$ has been achieved at low temperatures [3].

**Memory.** In tunnel junction memories integrating ferroelectrics with conventional tunnel junctions may give us much lower switching energies with practical ON/OFF ratios. Recent work on ferroelectric tunnel junctions shows that the ON/OFF ratio can be enhanced significantly by increasing the number of interfaces in addition to the FE tunnel barrier [4]. The switch ON and OFF characteristics are determined by the ferroelectric only and while the ON/OFF ratio increases with number of barriers it also causes an increase in the ON state resistance which is not desirable. FE tunnel barriers as thin as two unit cells can switch while the ON state resistance can be made comparable to today's commercial MTJs. An order of magnitude reduction in the FE coercive fields are necessary to make this a competitive, low energy consuming memory technology.

**Photonics.** Ferroelectrics on silicon enable the making of wide band width optical modulators that can work at a variety of optical wavelength and speeds approaching $40\,MB\,s^{-1}$ [5]. These devices have the potential for $100\,MB\,s^{-1}$ modulation

speeds but the best devices today have only been demonstrated on MgO. Such all wavelength modulators would compete against established technologies such as ones based on $LiNbO_3$ (handicapped by large sizes) or the III-V semiconductors (wavelength specific due to electro absorption modulation mechanism). For commercial applications these oxide based devices need to be built on a silicon platform where high quality electro-optic (ferroelectric) layers need to be deposited with suitable buffer layers to preserve the high crystalline quality of the over layers. In order to achieve mode confinement the low index buffer layer needs to have a thickness of at least 200 nm. A scalable process for the fabrication of such oxide heterostructure layers for large areas does not exist today and there is a clear need for this.

**Magnetics.** Oxide magnetism received a significant boost with the advent of high temperature superconductors followed by a resurgence of interest in colossal magneto-resistive magnetites followed closely by diluted magnetic semiconducting oxides. While the origin of magnetism in these systems was not fully understood, the notion of defect mediated magnetism in oxides was advanced by Sawatzky et al [6]. The first such demonstration was in $TiO_2$ observed with Ta substitution at the Ti site. Spectroscopic analysis clearly established titanium vacancies as the primary source of magnetism with the donor electrons from Ta substitution being responsible for the magnetic exchange [7]. The observed magnetism was weak as the concentration of Ti vacancies was below $10^{21}\,cm^{-3}$ and stability consideration would dictate the difficulty of producing higher levels of sustainable defects leading a limit on the strength of magnetism possible via such an approach. However, extremely novel magnetic exchange is possible in oxide systems mediated by polar layers and the exact mechanism of the exchange which involves spin-orbit interactions is not fully understood but holds promise for novel magnetic devices utilizing magnetic exchange [8].

**Energy.** A number of oxides have been strong candidates in catalysis applications and owing to their stability in harsh chemical environments and reasonable band match with reaction pathways, materials such as $Fe_2O_3$ and $TiO_2$ have been popular. However, recently a water splitting metallic oxide was introduced which seems to have fascinating properties [9]. Besides being a conductor, this material is an efficient water splitter and the mechanism of the optical absorption, carrier decay have been now clearly understood. It turns out that this family of materials $MNbO_3$ ($M$ = Sr, Ca, Ba) is a degenerate wide bandgap semiconductor with a bandgap of over 4 eV but an electronic carrier density of over $10^{22}\,cm^{-3}$, an order of magnitude short of elemental metals [10]! The optical absorption in the material occurs through a plasmonic absorption at 1.9 eV arising from the large carrier density and the hot carrier lifetime after plasmonic excitation is very long (>250 ps) which explains the efficiency of these materials in water splitting. Such a strongly correlated electronic material promises a novel approach in the design of catalysts.
**Health.** Oxide surfaces show a strong chemistry dependence on bio processes such as bio film formation, cell growth and cell differentiation. The origin of this behavior has to do most likely with the adhesion of macromolecules such as extracellular matrix proteins or lipids on specific surfaces where these molecules exhibit adherence. In addition, surface chemistry can also play an enhancing or inhibitory role in the cellular proliferation process. Oxides such as $Y_2O_3$ show significant lack of adhesion for cells such as fetal fibroblasts compared to $ZrO_2$ surfaces [11]. A comparable behavior was also seen for keratinocytes and neuronal stem cells. In the case of stem cells the proliferation after differentiation was doubled over $ZrO_2$ surfaces in comparison with standard glass (or plastic) surfaces. With the large number of oxide surfaces available this field is likely to expand rapidly in the near future. The applications are numerous including coatings for implants, cell localization on surfaces and also for antibacterial coatings and for harvesting drug resistant bacteria for development of antibiotics for so called super bugs.

**Advances in science and technology to meet challenges.** Large area deposition of high quality crystalline oxides to preserve their functionality is of extremely important for the commercial applications of the developments in this material. Another major challenge for the proliferation of oxide electronic devices is the compatibility with silicon technology. Growth of high quality functional oxides on silicon with a minimal native oxide barrier on the surface of silicon will be a key requirement. High aspect ratio patterning techniques (similar to reactive ion etching) need to be developed. For biological applications a low cost method that results in highly adherent, stoichiometric coatings as well as a highly repellent coatings would be of value.

**Concluding Remarks.** The field of oxides is reaching an interesting stage where potential large commercial applications are emerging and the challenges are in the deposition and processing of the materials and the success of this technology rests on how well these challenges are overcome.
**Health.** Oxide surfaces show a strong chemistry dependence on bio processes such as bio film formation, cell growth and cell differentiation. The origin of this behavior has to do most likely with the adhesion of macromolecules such as extracellular matrix proteins or lipids on specific surfaces where these molecules exhibit adherence. In addition, surface chemistry can also play an enhancing or inhibitory role in the cellular proliferation process. Oxides such as $Y_2O_3$ show significant lack of adhesion for cells such as fetal fibroblasts compared to $ZrO_2$ surfaces [11]. A comparable behavior was also seen for keratinocytes and neuronal stem cells. In the case of stem cells the proliferation after differentiation was doubled over $ZrO_2$ surfaces in comparison with standard glass (or plastic) surfaces. With the large number of oxide surfaces available this field is likely to expand rapidly in the near future. The applications are numerous including coatings for implants, cell localization on surfaces and also for antibacterial coatings and for harvesting drug resistant bacteria for development of antibiotics for so called super bugs.

**Advances in science and technology to meet challenges.** Large area deposition of high quality crystalline oxides to preserve their functionality is of extremely important for the commercial applications of the developments in this material. Another major challenge for the proliferation of oxide electronic devices is the compatibility with silicon technology. Growth of high quality functional oxides on silicon with a minimal native oxide barrier on the surface of silicon will be a key requirement. High aspect ratio patterning techniques (similar to reactive ion etching) need to be developed. For biological applications a low cost method that results in highly adherent, stoichiometric coatings as well as a highly repellent coatings would be of value.

**Concluding Remarks.** The field of oxides is reaching an interesting stage where potential large commercial applications are emerging and the challenges are in the deposition and processing of the materials and the success of this technology rests on how well these challenges are overcome.
## 2. Is the new oxide electronics (r)evolution solution based?

E Fortunato¹, P Barquinha¹, R Branquinho¹, D Salgueiro¹, E Carlos¹, A Liu², F K Shan², and R Martins¹

¹ i3N/CENIMAT, Department of Materials Science from Faculty of Science and Technology, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal

² College of Physics and Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China

**Status.** The evolution from rigid silicon-based electronics to flexible electronics requires the use of new materials with novel functionalities that allow non-conventional, low-cost and environmental friendly processing technologies. Among the alternatives, metal oxide semiconductors have brought to attention as backplane materials for the next generation of flat panel displays [12]. After the huge success and revolution of transparent electronics and with the worldwide interest in displays where metal oxide thin films (MOTF) have proved to be truly semiconductors, display backplanes have already gone commercial in a very short period of time, due to the huge investment of several high profile companies: Sharp, Samsung, LG, BOE [13]. These materials have demonstrated exceptional electronic performance as active semiconductor components and can be tuned for applications where high transparency/electrical conductivity is demanded and the global market for transparent electronics is expected to grow to nearly $7.1 billion by 2018, as it was stated by several forecast agencies [14].

In terms of the actual thin-film transistor (TFT) technology, In–Ga–Zn–O (IGZO) has been in mass-production since 2012. The main advantages of IGZO based TFTs are: high resolution, lower leakage current, less power consumption and less noise influence. Although IGZO technology is currently strongly targeted for active-matrix liquid crystal display (AMLCD) and active-matrix organic light emitting diode (AMOLED) applications, IGZO is proved to be a better semiconductor platform than a-Si:H for migrating towards other platforms like: photodetectors; memories; x-ray sensors and biosensors. In order to better visualize the main advantages of this technology in display applications, the merits of IGZO system are highlighted in figure 1.

**Current and future challenges.** The current fabrication techniques require high vacuum and photolithographic processes that are expensive and keep production at high costs or similar to those of a-Si:H. Solution based fabrication methods have been pursued as an alternative for economically viable large-scale electronics [15]. The cost can be lowered by 64% since solution-based processes do not require vacuum, gases and lithography [16]. Consequently the developments of the solution-based processes that can be used with low temperature techniques are highly desirable, such as self-combustion synthesis, water-inducement (WI) route, and deep-ultraviolet

(DUV) photochemical activation, etc. Among these, WI route makes significant contributions and its popularity is expected to increase with years. The WI system is only composed of metal nitrates and deionized water, which is considered to be healthier and environmental friendlier [17, 18]. In addition, the WI route has been proved to be compatible with various film fabrication techniques, such as spin coating, spray pyrolysis, or printing.

Most MOTF semiconductors reported are based on ZnO and In₂O₃ and combinations of these oxides such as In-Zn-O (IZO) or IGZO [19, 20]. Alternative semiconductor materials that rely on abundant and non-toxic elements such as zinc tin oxide (ZTO) are being explored as a In and Ga free alternative showing promising results [21]. The current trend is the use of non-critical (sustainable) oxide semiconductor materials combined with low cost and low temperature deposition systems. Recently, following these recommendations there has been remarkable development in solution processed inorganic MOTF semiconductor materials for high-performance TFTs without using critical raw materials. Comparing the evolution of the annealing temperature of MOTF by solution process we can observe a decrease of nearly 500 °C in the last eight years, reaching in some cases values comparable to the ones produced by physical techniques, as can be seen in figure 2.

**Advances in science and technology to meet challenges.** The rapid advances of n-type oxide semiconductors such as IGZO, ZTO, ZnO and In₂O₃ are pushing forward the realization of high-performance oxide TFTs. The missing key ingredient for the development of next-generation transparent electronics is their hole-transporting (p-type) counterparts with comparable performance, since the p-type is thought to be better suited for conventional OLED pixels and will enable the development of the strongly desired complementary metal-oxide-semiconductor (CMOS) circuits. To date, only a few p-type MOTF (i.e. CuₓO, SnO, and NiO) were reported and incorporated into transistors as p-type channel. Unfortunately, their electrical properties and fabrication techniques are not yet good enough for practical applications, necessitating further studies. For this reason, future attention should concentrate on the development of high-performance p-type TFTs, especially using low-cost solution process, and explore their applications in advanced displays and all-oxide CMOS electronics.

**Concluding remarks.** Multicomponent MOTF are leading the next generation of high performance materials for a wide range of applications. Transparent electronics is currently approaching its third generation, revealing to be a very promising technology. MOTF processing by physical vapour deposition (PVD) techniques like rf magnetron sputtering has been well established and has demonstrated high performance devices, however these require complex high vacuum equipment which is a major drawback, especially if we are targeting low cost applications. In contrast, the solution process has many advantages such as large-area deposition, roll-to-roll capability, easy control of composition, atmospheric processing, and low cost. The new paradigm of transparent electronics has attracted much interest as a novel technical
## 2. Is the new oxide electronics (r)evolution solution based?

E Fortunato¹, P Barquinha¹, R Branquinho¹, D Salgueiro¹, E Carlos¹, A Liu², F K Shan², and R Martins¹

¹ i3N/CENIMAT, Department of Materials Science from Faculty of Science and Technology, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal

² College of Physics and Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China

**Status.** The evolution from rigid silicon-based electronics to flexible electronics requires the use of new materials with novel functionalities that allow non-conventional, low-cost and environmental friendly processing technologies. Among the alternatives, metal oxide semiconductors have brought to attention as backplane materials for the next generation of flat panel displays [12]. After the huge success and revolution of transparent electronics and with the worldwide interest in displays where metal oxide thin films (MOTF) have proved to be truly semiconductors, display backplanes have already gone commercial in a very short period of time, due to the huge investment of several high profile companies: Sharp, Samsung, LG, BOE [13]. These materials have demonstrated exceptional electronic performance as active semiconductor components and can be tuned for applications where high transparency/electrical conductivity is demanded and the global market for transparent electronics is expected to grow to nearly $7.1 billion by 2018, as it was stated by several forecast agencies [14].

In terms of the actual thin-film transistor (TFT) technology, In–Ga–Zn–O (IGZO) has been in mass-production since 2012. The main advantages of IGZO based TFTs are: high resolution, lower leakage current, less power consumption and less noise influence. Although IGZO technology is currently strongly targeted for active-matrix liquid crystal display (AMLCD) and active-matrix organic light emitting diode (AMOLED) applications, IGZO is proved to be a better semiconductor platform than a-Si:H for migrating towards other platforms like: photodetectors; memories; x-ray sensors and biosensors. In order to better visualize the main advantages of this technology in display applications, the merits of IGZO system are highlighted in figure 1.

**Current and future challenges.** The current fabrication techniques require high vacuum and photolithographic processes that are expensive and keep production at high costs or similar to those of a-Si:H. Solution based fabrication methods have been pursued as an alternative for economically viable large-scale electronics [15]. The cost can be lowered by 64% since solution-based processes do not require vacuum, gases and lithography [16]. Consequently the developments of the solution-based processes that can be used with low temperature techniques are highly desirable, such as self-combustion synthesis, water-inducement (WI) route, and deep-ultraviolet

(DUV) photochemical activation, etc. Among these, WI route makes significant contributions and its popularity is expected to increase with years. The WI system is only composed of metal nitrates and deionized water, which is considered to be healthier and environmental friendlier [17, 18]. In addition, the WI route has been proved to be compatible with various film fabrication techniques, such as spin coating, spray pyrolysis, or printing.

Most MOTF semiconductors reported are based on ZnO and In₂O₃ and combinations of these oxides such as In-Zn-O (IZO) or IGZO [19, 20]. Alternative semiconductor materials that rely on abundant and non-toxic elements such as zinc tin oxide (ZTO) are being explored as a In and Ga free alternative showing promising results [21]. The current trend is the use of non-critical (sustainable) oxide semiconductor materials combined with low cost and low temperature deposition systems. Recently, following these recommendations there has been remarkable development in solution processed inorganic MOTF semiconductor materials for high-performance TFTs without using critical raw materials. Comparing the evolution of the annealing temperature of MOTF by solution process we can observe a decrease of nearly 500 °C in the last eight years, reaching in some cases values comparable to the ones produced by physical techniques, as can be seen in figure 2.

**Advances in science and technology to meet challenges.** The rapid advances of n-type oxide semiconductors such as IGZO, ZTO, ZnO and In₂O₃ are pushing forward the realization of high-performance oxide TFTs. The missing key ingredient for the development of next-generation transparent electronics is their hole-transporting (p-type) counterparts with comparable performance, since the p-type is thought to be better suited for conventional OLED pixels and will enable the development of the strongly desired complementary metal-oxide-semiconductor (CMOS) circuits. To date, only a few p-type MOTF (i.e. CuₓO, SnO, and NiO) were reported and incorporated into transistors as p-type channel. Unfortunately, their electrical properties and fabrication techniques are not yet good enough for practical applications, necessitating further studies. For this reason, future attention should concentrate on the development of high-performance p-type TFTs, especially using low-cost solution process, and explore their applications in advanced displays and all-oxide CMOS electronics.

**Concluding remarks.** Multicomponent MOTF are leading the next generation of high performance materials for a wide range of applications. Transparent electronics is currently approaching its third generation, revealing to be a very promising technology. MOTF processing by physical vapour deposition (PVD) techniques like rf magnetron sputtering has been well established and has demonstrated high performance devices, however these require complex high vacuum equipment which is a major drawback, especially if we are targeting low cost applications. In contrast, the solution process has many advantages such as large-area deposition, roll-to-roll capability, easy control of composition, atmospheric processing, and low cost. The new paradigm of transparent electronics has attracted much interest as a novel technical
<figure><img src="image_6.png" /><figcaption>**Figure 1.** The IGZO TFTs own more pixel per area and thinner wiring than current a-Si:H TFTs, resulting in a higher display resolution; The IGZO mobility is higher and the leakage current vastly lower. The on/off ratio is phenomenal >10⁹. Low leakage current, combined with smaller size, makes it a clear winner for smart phones and tablets.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Left graph: evolution of the annealing/processing temperature of solution based TFTs over the last 10 years. We have plotted only TFTs presenting device mobility higher than 1 cm² V⁻¹ s⁻¹. Right graph: evolution of device mobility of solution based TFTs over the last 10 years [22–56]. Due to the high number of references (>500), we have plotted only the highest values reported for device mobility. All the data will be available soon in a review paper.</figcaption></figure>

solution in the field of the next generation of consumer electronics. The ultimate goal of this ‘see-through’ device is to realize an integrated system equipped with ubiquitous

functions of information storage, image display and net-working, which strongly demands an embeddable transpar-ent array of non-volatile memory.
<figure><img src="image_7.png" /><figcaption>**Figure 1.** The IGZO TFTs own more pixel per area and thinner wiring than current a-Si:H TFTs, resulting in a higher display resolution; The IGZO mobility is higher and the leakage current vastly lower. The on/off ratio is phenomenal >10⁹. Low leakage current, combined with smaller size, makes it a clear winner for smart phones and tablets.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Left graph: evolution of the annealing/processing temperature of solution based TFTs over the last 10 years. We have plotted only TFTs presenting device mobility higher than 1 cm² V⁻¹ s⁻¹. Right graph: evolution of device mobility of solution based TFTs over the last 10 years [22–56]. Due to the high number of references (>500), we have plotted only the highest values reported for device mobility. All the data will be available soon in a review paper.</figcaption></figure>

solution in the field of the next generation of consumer electronics. The ultimate goal of this ‘see-through’ device is to realize an integrated system equipped with ubiquitous

functions of information storage, image display and net-working, which strongly demands an embeddable transpar-ent array of non-volatile memory.
# 3. Bipolar oxide devices

Marius Grundmann

Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig

**Status.** The majority of oxides are unipolar materials, i.e. they exist only either as n-type or as p-type material. Only a few oxides such as SnO are known to entertain n- and p-type conductivity. The difficulties of finding a reproducible and stable process for the p-doping of ZnO are well known. Therefore presently, devices based on oxides are mostly unipolar. The most prominent materials for application are TCOs (transparent conductive oxides) for transparent, highly conductive (ohmic) contacts [57]; these materials, such as ZnO:Ga and SnO₂:F, are n-type due to the much superior mobility compared to transparent p-type oxides. Using oxide semiconductors, high rectification diodes have been built with Schottky (metal–semiconductor) contacts to various oxides such as ZnO or Ga₂O₃, also allowing the fabrication of high efficiency photodetectors [58]. Amorphous InGaZnO is used as channel material in thin film transistors offering significant higher performance (mobility >10 cm² V⁻¹ s⁻¹) than a-Si (μ < 1 cm² V⁻¹ s⁻¹) [59], commercialized nowadays in TFT displays.

While the rectification of bipolar (p–n) diodes based on oxides has idled for the last twenty years at low values not surpassing 10³, recently high rectification (>10¹⁰) oxide p–n diodes were reported by us, namely based on ZnO/ZnCo₂O₄ (ZnO/ZCO) [60] and ZnO/NiO [61] (figure 3). Also highly rectifying (>10⁹) completely transparent ZnO/CuI diodes have been reported [62]. A review has been recently published [63]. Thus the path has been opened for high performance bipolar oxide devices such as photodetectors, solar cells, sensors, transistors (junction field effect transistor, JFET) [63, 64] and integrated circuits based on them. While crystalline materials generally are expected to provide the highest performance, amorphous materials allow advantages in fabrication (room temperature processing, low energy budget, roll-to-roll deposition, amorphous substrates) and applications (large area devices, flexible electronics). The said ZnO/ZnCo₂O₄ and ZnO/NiO diodes already contain a room temperature deposited p-type oxide and transistors have been built with these materials [64, 65]. Also fully amorphous bipolar oxide diodes based on ZTO/ZCO (ZTO: zinc-tin-oxide) with a rectification >10⁶ and ideality factor close to 1 have been reported [66] (figure 3). Bipolar devices in principle offer advantages such as low reverse current, low voltage operation and ease of fabrication.

**Current and future challenges.** Still there are some basic things that need to be understood regarding the physical mechanisms present in bipolar oxide heterostructure diodes. For type-II band lineup, the transport mechanism has been found to be interface recombination [61]; this limits the ideality factor to values around two. The interface recombination velocity, however, will depend on interface properties and fabrication details. Both fast and slow interface recombination can have their merits for devices. Type-I band lineups and also

type-III interfaces ('broken gap', e.g. for CdO/NiO [63]) need to be studied in more detail regarding the relative importance of thermionic, tunnel and recombination currents and their design and control.

Generally, p-type oxides suffer from low mobility, often due to large hole mass or hopping transport mechanisms. But also higher mobility n-type materials (towards and beyond 100 cm² V⁻¹ s⁻¹) are desirable for higher speed or energy efficient devices. Additional properties such as transparency (using wide gap oxides, limiting mid-gap absorption) and mechanical flexibility (using amorphous oxides) need to be fully explored. Also there is a drive to avoid rare, expensive and toxic metals, indium being one of them.

While so far mostly diodes and a few transistors have been demonstrated, higher integrated circuits are the ultimate goal, requiring reproducibility and stability. Initial results are very promising for certain materials [60], but surely much more work is needed in this direction. A big challenge is the transfer of the CMOS concept to oxide devices. The combination of n-type and p-type channels is a formidable task which so far has not led to devices superior to those achieved with unipolar concepts, mostly due to the low performance of the p-type oxides. Besides electronics, also photovoltaics has become interesting and wide gap oxides allow for invisible energy sources [67].

As a fabrication method, magnetron sputtering seems the method of choice since it is generally an established industrial process and allows large area fabrication and roll-to-roll processing. Wet chemical processes and printing so far had very little impact on the fabrication of bipolar oxide devices, however, further progress in this direction may make them interesting also for industrial scale production.

**Advances in science and technology to meet challenges.** A key to the device performance of bipolar (but also unipolar) devices is the improvement of mobility. While possibly a limit seems to have been reached for improving multi-cation oxides, multi-anion (n-type) oxides offer a possible solution. One of the promising materials is ZnON [68]; more such compounds should be searched for and investigated as channel materials and as part of p–n structures. The control of carrier density and its temperature and long-term stability in any of these materials remains a prime issue; often defects are the source of carriers while generally impurity doping is less important or impossible.

The fabrication and control of the p–n interfaces is a challenge and must be combined and transferred to large area and fast fabrication technologies such as sputtering. Many more hetero-structure combinations have to be tested and the promising ones systematically improved. A focus should be on the use of low toxicity, readily available cations (Zn, Sn, Cu, Ni, ...), replacing indium which is currently prominently used. The available materials and heterostructures need to be increasingly studied in device contexts and high performance bipolar devices need to be demonstrated in order to establish and realize the many possible advantages such as technological usefulness, superior performance and cost savings. A first step has been taken by us by reporting high gain (g > 250 at 3V supply voltage) inverters based on oxide JFETs with
# 3. Bipolar oxide devices

Marius Grundmann

Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig

**Status.** The majority of oxides are unipolar materials, i.e. they exist only either as n-type or as p-type material. Only a few oxides such as SnO are known to entertain n- and p-type conductivity. The difficulties of finding a reproducible and stable process for the p-doping of ZnO are well known. Therefore presently, devices based on oxides are mostly unipolar. The most prominent materials for application are TCOs (transparent conductive oxides) for transparent, highly conductive (ohmic) contacts [57]; these materials, such as ZnO:Ga and SnO₂:F, are n-type due to the much superior mobility compared to transparent p-type oxides. Using oxide semiconductors, high rectification diodes have been built with Schottky (metal–semiconductor) contacts to various oxides such as ZnO or Ga₂O₃, also allowing the fabrication of high efficiency photodetectors [58]. Amorphous InGaZnO is used as channel material in thin film transistors offering significant higher performance (mobility >10 cm² V⁻¹ s⁻¹) than a-Si (μ < 1 cm² V⁻¹ s⁻¹) [59], commercialized nowadays in TFT displays.

While the rectification of bipolar (p–n) diodes based on oxides has idled for the last twenty years at low values not surpassing 10³, recently high rectification (>10¹⁰) oxide p–n diodes were reported by us, namely based on ZnO/ZnCo₂O₄ (ZnO/ZCO) [60] and ZnO/NiO [61] (figure 3). Also highly rectifying (>10⁹) completely transparent ZnO/CuI diodes have been reported [62]. A review has been recently published [63]. Thus the path has been opened for high performance bipolar oxide devices such as photodetectors, solar cells, sensors, transistors (junction field effect transistor, JFET) [63, 64] and integrated circuits based on them. While crystalline materials generally are expected to provide the highest performance, amorphous materials allow advantages in fabrication (room temperature processing, low energy budget, roll-to-roll deposition, amorphous substrates) and applications (large area devices, flexible electronics). The said ZnO/ZnCo₂O₄ and ZnO/NiO diodes already contain a room temperature deposited p-type oxide and transistors have been built with these materials [64, 65]. Also fully amorphous bipolar oxide diodes based on ZTO/ZCO (ZTO: zinc-tin-oxide) with a rectification >10⁶ and ideality factor close to 1 have been reported [66] (figure 3). Bipolar devices in principle offer advantages such as low reverse current, low voltage operation and ease of fabrication.

**Current and future challenges.** Still there are some basic things that need to be understood regarding the physical mechanisms present in bipolar oxide heterostructure diodes. For type-II band lineup, the transport mechanism has been found to be interface recombination [61]; this limits the ideality factor to values around two. The interface recombination velocity, however, will depend on interface properties and fabrication details. Both fast and slow interface recombination can have their merits for devices. Type-I band lineups and also

type-III interfaces ('broken gap', e.g. for CdO/NiO [63]) need to be studied in more detail regarding the relative importance of thermionic, tunnel and recombination currents and their design and control.

Generally, p-type oxides suffer from low mobility, often due to large hole mass or hopping transport mechanisms. But also higher mobility n-type materials (towards and beyond 100 cm² V⁻¹ s⁻¹) are desirable for higher speed or energy efficient devices. Additional properties such as transparency (using wide gap oxides, limiting mid-gap absorption) and mechanical flexibility (using amorphous oxides) need to be fully explored. Also there is a drive to avoid rare, expensive and toxic metals, indium being one of them.

While so far mostly diodes and a few transistors have been demonstrated, higher integrated circuits are the ultimate goal, requiring reproducibility and stability. Initial results are very promising for certain materials [60], but surely much more work is needed in this direction. A big challenge is the transfer of the CMOS concept to oxide devices. The combination of n-type and p-type channels is a formidable task which so far has not led to devices superior to those achieved with unipolar concepts, mostly due to the low performance of the p-type oxides. Besides electronics, also photovoltaics has become interesting and wide gap oxides allow for invisible energy sources [67].

As a fabrication method, magnetron sputtering seems the method of choice since it is generally an established industrial process and allows large area fabrication and roll-to-roll processing. Wet chemical processes and printing so far had very little impact on the fabrication of bipolar oxide devices, however, further progress in this direction may make them interesting also for industrial scale production.

**Advances in science and technology to meet challenges.** A key to the device performance of bipolar (but also unipolar) devices is the improvement of mobility. While possibly a limit seems to have been reached for improving multi-cation oxides, multi-anion (n-type) oxides offer a possible solution. One of the promising materials is ZnON [68]; more such compounds should be searched for and investigated as channel materials and as part of p–n structures. The control of carrier density and its temperature and long-term stability in any of these materials remains a prime issue; often defects are the source of carriers while generally impurity doping is less important or impossible.

The fabrication and control of the p–n interfaces is a challenge and must be combined and transferred to large area and fast fabrication technologies such as sputtering. Many more hetero-structure combinations have to be tested and the promising ones systematically improved. A focus should be on the use of low toxicity, readily available cations (Zn, Sn, Cu, Ni, ...), replacing indium which is currently prominently used. The available materials and heterostructures need to be increasingly studied in device contexts and high performance bipolar devices need to be demonstrated in order to establish and realize the many possible advantages such as technological usefulness, superior performance and cost savings. A first step has been taken by us by reporting high gain (g > 250 at 3V supply voltage) inverters based on oxide JFETs with
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Rectification of various oxide homo- and heterojunction diodes versus the year of publication. Filled symbols represent results from our laboratory. Reprinted with permission from [63].</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** (a) Optical image of bipolar inverter based on ZnO/ZCO JFETs and diodes. The ZnO channels are seen as brighter blue green rectangles. The ZnCo₂O₄/Au gates and diodes are the yellow areas. (b) Inverter characteristics and gain values including the effect of the input voltage shift at the diodes for three values of the supply voltage VDD = 1, 2, and 3V. © 2015 IEEE, reprinted with permission from [69].</figcaption></figure>

bipolar oxide diode voltage shifters based on ZnO/ZCO (figure 4) [69] and oxide JFET-based ring oscillators [70]. CMOS approaches and devices, combining n-type and p-type devices either from the same or different channel materials, featuring high performance await realization.

*Concluding remarks.* Bipolar oxide devices offer many advantages such as ease of fabrication, stability, low

voltage operation and high performance over rival concepts. However, steps towards better understanding of the physical mechanisms of transport across the p-n interface, the exploration of advanced p- and n-type oxides featuring high mobility and made from readily available elemental constituents as well as the development of a related technological platform are necessary to harvest all the potential benefits.
<figure><img src="image_9.png" /><figcaption>**Figure 3.** Rectification of various oxide homo- and heterojunction diodes versus the year of publication. Filled symbols represent results from our laboratory. Reprinted with permission from [63].</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 4.** (a) Optical image of bipolar inverter based on ZnO/ZCO JFETs and diodes. The ZnO channels are seen as brighter blue green rectangles. The ZnCo₂O₄/Au gates and diodes are the yellow areas. (b) Inverter characteristics and gain values including the effect of the input voltage shift at the diodes for three values of the supply voltage VDD = 1, 2, and 3V. © 2015 IEEE, reprinted with permission from [69].</figcaption></figure>

bipolar oxide diode voltage shifters based on ZnO/ZCO (figure 4) [69] and oxide JFET-based ring oscillators [70]. CMOS approaches and devices, combining n-type and p-type devices either from the same or different channel materials, featuring high performance await realization.

*Concluding remarks.* Bipolar oxide devices offer many advantages such as ease of fabrication, stability, low

voltage operation and high performance over rival concepts. However, steps towards better understanding of the physical mechanisms of transport across the p-n interface, the exploration of advanced p- and n-type oxides featuring high mobility and made from readily available elemental constituents as well as the development of a related technological platform are necessary to harvest all the potential benefits.
# 4. Complex-oxide field-effect transistors

Hans Boschker

Max Planck Institute for Solid State Research, Stuttgart,
Germany

**Status.** This section of the roadmap focuses on complex-oxide field-effect transistors (FETs). Here, complex oxides are defined as those oxides where the conducting electron/hole systems comprise *d* or *f* electrons. For an overview of oxide transistors using materials with *s* and *p* electrons, such as ITO, ZnO and BaSnO₃, I refer to [16]. The electron systems of the complex oxides are strongly correlated and show rich emergent collective electronic behaviour. Therefore the main motivation to study complex-oxide transistors is to reveal how electronic correlations affect field-effect transistor operation and whether the correlated behaviour facilitates new types of logic devices [71].

Recently, FETs based on complex oxides that actually show current and voltage gain have been realized [72]. The key enabler is the electron system at the LaAlO₃–SrTiO₃ interface, discovered in 2004 by Ohtomo and Hwang [73]. This 2D electron system has a carrier density of $2 \times 10^{14} \text{ cm}^{-2}$ at room temperature and it is therefore possible to completely deplete the electron system with gate voltage smaller than a few V [72]. The electron system has a relatively low electron mobility of $10 \text{ cm}^2 \text{ V}^{-1} \text{ s}^{-1}$ at room temperature, mainly limited by strong electron-phonon scattering.

The state of the art LaAlO₃–SrTiO₃ transistors have a composite gate dielectric comprising a five unit-cell-thick LaAlO₃ layer and a nine unit-cell-thick BaTiO₃ layer (one unit cell is 0.4 nm) [74]. The dielectric constant of LaAlO₃ is 24 and that of BaTiO₃ is 300. This gate stack has a very high specific capacitance and is thick enough to prevent large gate leakage currents. The gate metal is gold and the source and drain contacts to the electron system are made from titanium. Figure 5(a) shows the transfer characteristic of a LaAlO₃–SrTiO₃ transistor. The transistors have been integrated into a ring oscillator circuit, whose output is shown in figure 5(b). Next to the previously discussed top-down lithographically patterned devices, also nanoscale devices have been written with AFM lithography [75].

Other materials that have been investigated for use as phase-transition transistors are VO₂ [76–78] and SmNiO₃ [79]. Recent field-effect studies using electrolyte gating of VO₂ thin films demonstrate a huge switching effect where the channel conductance increases much more rapidly than the gate-induced charge-carrier density [77]. By inducing a metallic state in the surface layer with the gate, metallicity is induced in the entire thin film. This provides a large gain in the operation of transistors. However, it is not yet clear whether this switching behaviour is purely electronic; the field-induced creation of oxygen vacancies has been argued to be intimately related to the large increase of the conductivity [78]. Furthermore, transfer characteristics have not yet been measured, so it is unclear how these transistors compare to semiconductor devices.

**Current and future challenges.** The crucial and as yet unanswered question is whether electronic correlations will facilitate electronics that can outperform standard FETs. With silicon technology reaching its fundamental performance limits, it is uncertain how electronics will advance beyond this stage. Graphene nanoribbon and carbon nanotube transistors receive much attention as a beyond-Si technology, because these materials have a much higher saturation velocity and thereby offer the prospect of faster devices. The physical basis of these devices, however, is unchanged with respect to silicon FETs. Correlated electron systems could facilitate logic devices based on fundamentally different switching mechanisms. In traditional field-effect devices, the applied voltages result in band bending but the band structure itself remains unchanged. In correlated electron systems, applied voltages can change the band structure, possibly leading to different physical limitations for transistor performance. For example, can an electron system that is driven through an intrinsic metal–insulator transition switch faster than 60 mV $\text{dec}^{-1}$, the limit imposed by filling a rigid band structure with a thermal distribution of charge carriers?

The ideal switching device would be a high-density electron system that is switched with a small electric field between an insulating state with a sufficiently large bandgap, stabilized by either direct Coulomb interactions (Mott physics) or by electron-lattice interactions, and a metallic state, ideally with ballistic transport. Therefore one of the challenges is to obtain a material system where the intrinsic metal–insulator transition occurs far above room temperature, but that is still switchable. Interesting candidates are NbO₂ and 2D Mott insulators such as LaTiO₃–LaAlO₃ and SrTiO₃–SrVO₃ heterostructures [80, 81]. Another challenge is to improve the high-field mobility of the complex-oxide materials. Currently mobilities are still too low for ballistic transport, but with a small improvement in materials quality and a drastic reduction of the device dimensions, high-enough mobilities are expected to be possible. Nanoscale devices are not only required to improve the device properties but also to study the underlying physics of the devices. Domain physics plays an important role in the physics of metal–insulator transitions [82]. To find out how fast these devices can switch, devices that incorporate only a single electronic domain are required. Thus, nanoscale patterning and processing of the complex-oxides materials is also a key challenge.

So far, LaAlO₃–SrTiO₃ transistors show characteristics similar to that of canonical semiconductor devices [74, 83], indicating that the effects of electronic correlations are small. The electron system at the LaAlO₃–SrTiO₃ interface has intermediate correlation strength, with effects due to electronic correlation predominantly observed at low temperatures. Therefore one of the immediate challenges is to build FETs from more strongly correlated materials. Furthermore, the strong spin-orbit coupling in *f* electron compounds has recently attracted large attention. It will be intriguing to see how the spin–orbit coupling changes the FET behaviour.

**Advances in science and technology to meet challenges.** Complex-oxide epitaxy has made tremendous
# 4. Complex-oxide field-effect transistors

Hans Boschker

Max Planck Institute for Solid State Research, Stuttgart,
Germany

**Status.** This section of the roadmap focuses on complex-oxide field-effect transistors (FETs). Here, complex oxides are defined as those oxides where the conducting electron/hole systems comprise *d* or *f* electrons. For an overview of oxide transistors using materials with *s* and *p* electrons, such as ITO, ZnO and BaSnO₃, I refer to [16]. The electron systems of the complex oxides are strongly correlated and show rich emergent collective electronic behaviour. Therefore the main motivation to study complex-oxide transistors is to reveal how electronic correlations affect field-effect transistor operation and whether the correlated behaviour facilitates new types of logic devices [71].

Recently, FETs based on complex oxides that actually show current and voltage gain have been realized [72]. The key enabler is the electron system at the LaAlO₃–SrTiO₃ interface, discovered in 2004 by Ohtomo and Hwang [73]. This 2D electron system has a carrier density of $2 \times 10^{14} \text{ cm}^{-2}$ at room temperature and it is therefore possible to completely deplete the electron system with gate voltage smaller than a few V [72]. The electron system has a relatively low electron mobility of $10 \text{ cm}^2 \text{ V}^{-1} \text{ s}^{-1}$ at room temperature, mainly limited by strong electron-phonon scattering.

The state of the art LaAlO₃–SrTiO₃ transistors have a composite gate dielectric comprising a five unit-cell-thick LaAlO₃ layer and a nine unit-cell-thick BaTiO₃ layer (one unit cell is 0.4 nm) [74]. The dielectric constant of LaAlO₃ is 24 and that of BaTiO₃ is 300. This gate stack has a very high specific capacitance and is thick enough to prevent large gate leakage currents. The gate metal is gold and the source and drain contacts to the electron system are made from titanium. Figure 5(a) shows the transfer characteristic of a LaAlO₃–SrTiO₃ transistor. The transistors have been integrated into a ring oscillator circuit, whose output is shown in figure 5(b). Next to the previously discussed top-down lithographically patterned devices, also nanoscale devices have been written with AFM lithography [75].

Other materials that have been investigated for use as phase-transition transistors are VO₂ [76–78] and SmNiO₃ [79]. Recent field-effect studies using electrolyte gating of VO₂ thin films demonstrate a huge switching effect where the channel conductance increases much more rapidly than the gate-induced charge-carrier density [77]. By inducing a metallic state in the surface layer with the gate, metallicity is induced in the entire thin film. This provides a large gain in the operation of transistors. However, it is not yet clear whether this switching behaviour is purely electronic; the field-induced creation of oxygen vacancies has been argued to be intimately related to the large increase of the conductivity [78]. Furthermore, transfer characteristics have not yet been measured, so it is unclear how these transistors compare to semiconductor devices.

**Current and future challenges.** The crucial and as yet unanswered question is whether electronic correlations will facilitate electronics that can outperform standard FETs. With silicon technology reaching its fundamental performance limits, it is uncertain how electronics will advance beyond this stage. Graphene nanoribbon and carbon nanotube transistors receive much attention as a beyond-Si technology, because these materials have a much higher saturation velocity and thereby offer the prospect of faster devices. The physical basis of these devices, however, is unchanged with respect to silicon FETs. Correlated electron systems could facilitate logic devices based on fundamentally different switching mechanisms. In traditional field-effect devices, the applied voltages result in band bending but the band structure itself remains unchanged. In correlated electron systems, applied voltages can change the band structure, possibly leading to different physical limitations for transistor performance. For example, can an electron system that is driven through an intrinsic metal–insulator transition switch faster than 60 mV $\text{dec}^{-1}$, the limit imposed by filling a rigid band structure with a thermal distribution of charge carriers?

The ideal switching device would be a high-density electron system that is switched with a small electric field between an insulating state with a sufficiently large bandgap, stabilized by either direct Coulomb interactions (Mott physics) or by electron-lattice interactions, and a metallic state, ideally with ballistic transport. Therefore one of the challenges is to obtain a material system where the intrinsic metal–insulator transition occurs far above room temperature, but that is still switchable. Interesting candidates are NbO₂ and 2D Mott insulators such as LaTiO₃–LaAlO₃ and SrTiO₃–SrVO₃ heterostructures [80, 81]. Another challenge is to improve the high-field mobility of the complex-oxide materials. Currently mobilities are still too low for ballistic transport, but with a small improvement in materials quality and a drastic reduction of the device dimensions, high-enough mobilities are expected to be possible. Nanoscale devices are not only required to improve the device properties but also to study the underlying physics of the devices. Domain physics plays an important role in the physics of metal–insulator transitions [82]. To find out how fast these devices can switch, devices that incorporate only a single electronic domain are required. Thus, nanoscale patterning and processing of the complex-oxides materials is also a key challenge.

So far, LaAlO₃–SrTiO₃ transistors show characteristics similar to that of canonical semiconductor devices [74, 83], indicating that the effects of electronic correlations are small. The electron system at the LaAlO₃–SrTiO₃ interface has intermediate correlation strength, with effects due to electronic correlation predominantly observed at low temperatures. Therefore one of the immediate challenges is to build FETs from more strongly correlated materials. Furthermore, the strong spin-orbit coupling in *f* electron compounds has recently attracted large attention. It will be intriguing to see how the spin–orbit coupling changes the FET behaviour.

**Advances in science and technology to meet challenges.** Complex-oxide epitaxy has made tremendous
<figure><img src="image_10.png" /><figcaption>**Figure 5.** (a) Transfer curves of a LaAlO₃–SrTiO₃ FET. The on/off ratio is six orders of magnitude and the subthreshold voltage swing is close to 60 mV dec⁻¹. (b) A monolithically integrated LaAlO₃–SrTiO₃ ring oscillator. Upon increasing the supply voltage the output voltage oscillates with a RC time-constant-controlled oscillation frequency of 1.4 kHz. All data was measured at room temperature. Reprinted with permission from [74]. Copyright 2013 Wiley.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 6.** False-colour scanning electron microscopy image of a sub-100 nm gate length LaAlO₃–SrTiO₃ transistor.</figcaption></figure>

advances in recent years. This progress is due not only to improvements in the growth systems and growth monitoring, but also to a better understanding of the ways materials influence each other during heteroepitaxy. Materials properties are modified due to the presence of such phenomena as strain, polar discontinuities, and octahedral connectivity mismatch. This sensitivity offers the unique opportunity to optimize the complex oxides for use in devices. Recent achievements in this area are compositional interface engineering [84], modulation doping [85], oxygen octahedra rotation engineering [86], defect mitigation [74, 87] and improved stoichiometry control [88].

Another crucial development in technology that has been achieved recently is the feature-size reduction of both the correlated-oxide channel materials and the integrated metal structures such as the gates and source and drain contacts [83, 89]. Figure 6 presents a false-colour scanning electron microscopy image of a sub-100 nm LaAlO₃–SrTiO₃ transistor.

The gate length is 60 nm, the channel width is 900 nm and the source and drain contacts are 30 nm separated from the gate. For the characterization of such devices and a discussion about short-channel effects, I refer to [83].

Integration with very high-*K* dielectrics, such as BaTiO₃, does generally not pose a large problem for the complex-oxide FETs. However, the more strongly correlated oxides require a larger charge-carrier-density modulation than that which can be achieved with high-*K* dielectric gate insulators. Here a solution may be found by using liquid electrolytes for gating. Electric-double-layer transistors achieve one order of magnitude larger charge-carrier-density modulation [77, 78]. But it remains to be seen how much electrostatic gating contributes to the induced charge-carrier density as opposed to electrochemical modification of the materials.

**Concluding remarks.** The fabrication and study of complex-oxide field-effect transistors has made tremendous advances, and I expect this progress to continue in years to come. This provides a road towards answering fundamental questions about the physics underlying the performance of FETs using strongly correlated electron systems. Applications will certainly arise if the correlation effects facilitate better FET performance. In the meantime also niche market applications might emerge, especially because of the compatibility of the complex-oxide FETs with the wide spectrum of possibilities for electronics that is offered by the large variety of oxide materials systems.

**Acknowledgements.** The author thanks J Falson, J Mannhart, C Woltmann, and members of the TO-BE Network, especially N Pryds, J Fontcuberta, and F Miletto Granozio, for valuable discussions.
<figure><img src="image_11.png" /><figcaption>**Figure 5.** (a) Transfer curves of a LaAlO₃–SrTiO₃ FET. The on/off ratio is six orders of magnitude and the subthreshold voltage swing is close to 60 mV dec⁻¹. (b) A monolithically integrated LaAlO₃–SrTiO₃ ring oscillator. Upon increasing the supply voltage the output voltage oscillates with a RC time-constant-controlled oscillation frequency of 1.4 kHz. All data was measured at room temperature. Reprinted with permission from [74]. Copyright 2013 Wiley.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 6.** False-colour scanning electron microscopy image of a sub-100 nm gate length LaAlO₃–SrTiO₃ transistor.</figcaption></figure>

advances in recent years. This progress is due not only to improvements in the growth systems and growth monitoring, but also to a better understanding of the ways materials influence each other during heteroepitaxy. Materials properties are modified due to the presence of such phenomena as strain, polar discontinuities, and octahedral connectivity mismatch. This sensitivity offers the unique opportunity to optimize the complex oxides for use in devices. Recent achievements in this area are compositional interface engineering [84], modulation doping [85], oxygen octahedra rotation engineering [86], defect mitigation [74, 87] and improved stoichiometry control [88].

Another crucial development in technology that has been achieved recently is the feature-size reduction of both the correlated-oxide channel materials and the integrated metal structures such as the gates and source and drain contacts [83, 89]. Figure 6 presents a false-colour scanning electron microscopy image of a sub-100 nm LaAlO₃–SrTiO₃ transistor.

The gate length is 60 nm, the channel width is 900 nm and the source and drain contacts are 30 nm separated from the gate. For the characterization of such devices and a discussion about short-channel effects, I refer to [83].

Integration with very high-*K* dielectrics, such as BaTiO₃, does generally not pose a large problem for the complex-oxide FETs. However, the more strongly correlated oxides require a larger charge-carrier-density modulation than that which can be achieved with high-*K* dielectric gate insulators. Here a solution may be found by using liquid electrolytes for gating. Electric-double-layer transistors achieve one order of magnitude larger charge-carrier-density modulation [77, 78]. But it remains to be seen how much electrostatic gating contributes to the induced charge-carrier density as opposed to electrochemical modification of the materials.

**Concluding remarks.** The fabrication and study of complex-oxide field-effect transistors has made tremendous advances, and I expect this progress to continue in years to come. This provides a road towards answering fundamental questions about the physics underlying the performance of FETs using strongly correlated electron systems. Applications will certainly arise if the correlation effects facilitate better FET performance. In the meantime also niche market applications might emerge, especially because of the compatibility of the complex-oxide FETs with the wide spectrum of possibilities for electronics that is offered by the large variety of oxide materials systems.

**Acknowledgements.** The author thanks J Falson, J Mannhart, C Woltmann, and members of the TO-BE Network, especially N Pryds, J Fontcuberta, and F Miletto Granozio, for valuable discussions.
## 5. Oxide electronic materials for resistive switching and TFT applications

Joynarayan Mukherjee¹, M Priyadarshini¹,², Nandita DasGupta² and M S Ramachandra Rao¹

¹ Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology, Madras, Chennai 600036, India

² Microelectronics and MEMS laboratory, Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India

**Status.** Oxide electronic materials offer plenty of opportunities to probe into many fundamental physical properties and provide ample application prospects in opto-electronics, information storage devices, displays, LEDs, fuel cells, batteries and many more. Figure 7(a) shows the assorted functionalities offered by oxide electronic materials. Many metal oxides show resistive switching (RS) behavior, wherein by varying the applied voltage, the resistance of the metal oxide switches from high resistance state (HRS) to a low resistance state (LRS) and vice-versa. Also, metal oxides due to their wide bandgap and high mobility even in amorphous state find applications in thin film transistors (TFTs) which are primarily used as switching devices in displays. Out of the many exciting phenomena oxide materials exhibit, in this article we will focus on the mechanism and the device aspects of resistive switching and TFTs for display applications.

**Resistive switching.** The resistive random access memory (RRAM) has gained considerable attention both in academia and industry for its application in non-volatile memory (NVM) devices. RRAM has the advantages of high density, high operation speed, multibit storage, low power consumption and simple device structure [90]. A wide variety of binary and multi-component oxides show resistive switching behavior. Based on the polarity of the applied bias, RRAM can be categorized in two types: (i) unipolar resistive switching and (ii) bipolar resistive switching. Unipolar resistive switching is usually observed in binary oxides such as NiO, CuOₓ, TiO₂, ZrO₂ and Nb₂O₅ whereas bipolar resistive switching has been reported on both binary and ternary oxides like Al₂O₃, Pr₀.₇Ca₀.₃MnO₃, and InGaZnO₄.

**Thin film transistors.** TFTs are the backbone of display technology and are used in different devices like TVs, smartphones, tablets, laptops, e-readers, and wearable displays etc. TFTs using oxide semiconductors were commercialized in flat panel displays for the first time in 2012 and since then they have been used in a plethora of applications. A schematic of how oxide technology has evolved over the years and the tremendous applications offered for the future is shown in figure 7(b). Different metal oxides with constituent cations of Zn, Sn, Ga and In can be used for the channel layer of the TFT. Among different metal oxides, InGaZnO₄ has been the most successful oxide material for use in displays due to its several

advantages like high mobility in amorphous state, low leakage current, low temperature processing and TFTs based on IGZO have commercial advantages like lower power consumption, higher sensitivity, reduction in pixel size and higher resolution [91]. However indium is a rare element with low abundance in Earth's crust. It is also costly and toxic, which limits its long term use.

**Current and future challenges.**

**Resistive switching.** Although RRAM has many advantages there are some issues, such as the large variation in the set and reset voltages, which need to be overcome. It is very important to understand the microscopic origin of the resistive switching behavior for better device performance. The mechanisms such as formation and rupturing of filament and migration of oxygen vacancies in the switching layer play a crucial role in the RS phenomena. Recently, in situ TEM study showed the formation and rupture of conducting filaments during RS operation in ZnO thin films [92]. Though there are many models reported in the literature, more experimental evidence is required to understand the mechanism behind the switching phenomena. Moreover, another important issue regarding the commercialization of RRAM is the large variation in set and reset voltages which leads to switching failures.

**Thin film transistors.** For use in TFTs, binary oxides like ZnO, SnO₂ can primarily be used. However, it is very difficult to deposit these films with amorphous microstructure and they tend to have high density of carriers even in as-deposited state which makes it difficult to modulate the channel conductivity [93]. ZTO on the other hand has amorphous structure and is a promising indium and gallium free material for consideration. One of the problems limiting fabrication of ZTO based TFTs is the difficulty in etching ZTO. ZTO is chemically stable and resists most of the etchants and it becomes more difficult to etch it with increase in tin concentration [94]. Also, ZTO involves higher processing temperature compared to IGZO. Another material considered is TGZO, but it is found to have poorer performance when compared to ZTO. It is more costly due to the presence of gallium in the system and has etching problems [95].

**Advances in science and technology to meet challenges.**

**Resistive switching.** In the early 1960s, resistive switching phenomena in thin oxide layers of Al, Nb, Si and Ti was reported [96]. In 1994, Bloom et al showed the resistive switching behavior in ferroelectric PbTiO₃ [97]. Since then, many materials have been investigated as RRAM candidates. Companies such as Samsung Electronics and Hewlett-Packard have been making serious attempts for the commercialization of RRAM based memory devices. Today, the vital challenge for the RRAM is the storage density which can be overcome through the stacking structure [98]. The other way to increase the storage density is the using of multibit switching property. Multistate resistance can be realized in a unipolar device by tuning the filamentary path [99] and for the bipolar device by varying the defect concentration [100].
# 5. Oxide electronic materials for resistive switching and TFT applications

Joynarayan Mukherjee¹, M Priyadarshini¹,², Nandita DasGupta² and M S Ramachandra Rao¹

¹ Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology, Madras, Chennai 600036, India

² Microelectronics and MEMS laboratory, Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India

**Status.** Oxide electronic materials offer plenty of opportunities to probe into many fundamental physical properties and provide ample application prospects in opto-electronics, information storage devices, displays, LEDs, fuel cells, batteries and many more. Figure 7(a) shows the assorted functionalities offered by oxide electronic materials. Many metal oxides show resistive switching (RS) behavior, wherein by varying the applied voltage, the resistance of the metal oxide switches from high resistance state (HRS) to a low resistance state (LRS) and vice-versa. Also, metal oxides due to their wide bandgap and high mobility even in amorphous state find applications in thin film transistors (TFTs) which are primarily used as switching devices in displays. Out of the many exciting phenomena oxide materials exhibit, in this article we will focus on the mechanism and the device aspects of resistive switching and TFTs for display applications.

**Resistive switching.** The resistive random access memory (RRAM) has gained considerable attention both in academia and industry for its application in non-volatile memory (NVM) devices. RRAM has the advantages of high density, high operation speed, multibit storage, low power consumption and simple device structure [90]. A wide variety of binary and multi-component oxides show resistive switching behavior. Based on the polarity of the applied bias, RRAM can be categorized in two types: (i) unipolar resistive switching and (ii) bipolar resistive switching. Unipolar resistive switching is usually observed in binary oxides such as NiO, CuOₓ, TiO₂, ZrO₂ and Nb₂O₅ whereas bipolar resistive switching has been reported on both binary and ternary oxides like Al₂O₃, Pr₀.₇Ca₀.₃MnO₃, and InGaZnO₄.

**Thin film transistors.** TFTs are the backbone of display technology and are used in different devices like TVs, smartphones, tablets, laptops, e-readers, and wearable displays etc. TFTs using oxide semiconductors were commercialized in flat panel displays for the first time in 2012 and since then they have been used in a plethora of applications. A schematic of how oxide technology has evolved over the years and the tremendous applications offered for the future is shown in figure 7(b). Different metal oxides with constituent cations of Zn, Sn, Ga and In can be used for the channel layer of the TFT. Among different metal oxides, InGaZnO₄ has been the most successful oxide material for use in displays due to its several

advantages like high mobility in amorphous state, low leakage current, low temperature processing and TFTs based on IGZO have commercial advantages like lower power consumption, higher sensitivity, reduction in pixel size and higher resolution [91]. However indium is a rare element with low abundance in Earth's crust. It is also costly and toxic, which limits its long term use.

**Current and future challenges.**

**Resistive switching.** Although RRAM has many advantages there are some issues, such as the large variation in the set and reset voltages, which need to be overcome. It is very important to understand the microscopic origin of the resistive switching behavior for better device performance. The mechanisms such as formation and rupturing of filament and migration of oxygen vacancies in the switching layer play a crucial role in the RS phenomena. Recently, in situ TEM study showed the formation and rupture of conducting filaments during RS operation in ZnO thin films [92]. Though there are many models reported in the literature, more experimental evidence is required to understand the mechanism behind the switching phenomena. Moreover, another important issue regarding the commercialization of RRAM is the large variation in set and reset voltages which leads to switching failures.

**Thin film transistors.** For use in TFTs, binary oxides like ZnO, SnO₂ can primarily be used. However, it is very difficult to deposit these films with amorphous microstructure and they tend to have high density of carriers even in as-deposited state which makes it difficult to modulate the channel conductivity [93]. ZTO on the other hand has amorphous structure and is a promising indium and gallium free material for consideration. One of the problems limiting fabrication of ZTO based TFTs is the difficulty in etching ZTO. ZTO is chemically stable and resists most of the etchants and it becomes more difficult to etch it with increase in tin concentration [94]. Also, ZTO involves higher processing temperature compared to IGZO. Another material considered is TGZO, but it is found to have poorer performance when compared to ZTO. It is more costly due to the presence of gallium in the system and has etching problems [95].

**Advances in science and technology to meet challenges.**

**Resistive switching.** In the early 1960s, resistive switching phenomena in thin oxide layers of Al, Nb, Si and Ti was reported [96]. In 1994, Bloom et al showed the resistive switching behavior in ferroelectric PbTiO₃ [97]. Since then, many materials have been investigated as RRAM candidates. Companies such as Samsung Electronics and Hewlett-Packard have been making serious attempts for the commercialization of RRAM based memory devices. Today, the vital challenge for the RRAM is the storage density which can be overcome through the stacking structure [98]. The other way to increase the storage density is the using of multibit switching property. Multistate resistance can be realized in a unipolar device by tuning the filamentary path [99] and for the bipolar device by varying the defect concentration [100].
<figure><img src="image_12.png" /><figcaption>**Figure 7.** Schematic showing (a) different physical properties and applications of oxide materials. (b) Evolution of thin film transistor technology and its potential future applications.</figcaption></figure>

*Thin film transistors.* Several attempts have been made to etch ZTO using wet chemical methods, unfortunately without a very promising solution. To date the reported wet etching processes show a very narrow process window making it difficult to use them consistently [101]. Costly techniques involving plasma techniques and dry etching using chlorine currently seems to be the only plausible way. Techniques such as solution combustion synthesis [102], deep-ultraviolet (DUV) photochemical activation have recently been explored for reducing the processing temperature. DUV photochemical activation has shown promising results even for room temperature deposition with comparable performance and operational stability as that of high temperature (≥350 °C) annealed devices [103].

*Concluding remarks.* There is plenty of room for using oxide materials in different applications such as logic/rf circuits, memories, radio-frequency identification tags, transparent electronics, medical patches, image sensors and printed electronics. Among other NVMs, RRAM is more promising for next generation memory devices. The microscopic mechanisms need to be investigated to improve the device performance in terms of its size and speed. Oxide materials will continue to play a key role in next generation displays which require that the materials be used are sustainable and cost effective. ZTO is one such material which holds potential to revolutionize the display industry. Advancements in newer etching techniques and methods to process it at lower temperatures are essential to realize the full benefit of this material.
<figure><img src="image_13.png" /><figcaption>**Figure 7.** Schematic showing (a) different physical properties and applications of oxide materials. (b) Evolution of thin film transistor technology and its potential future applications.</figcaption></figure>

*Thin film transistors.* Several attempts have been made to etch ZTO using wet chemical methods, unfortunately without a very promising solution. To date the reported wet etching processes show a very narrow process window making it difficult to use them consistently [101]. Costly techniques involving plasma techniques and dry etching using chlorine currently seems to be the only plausible way. Techniques such as solution combustion synthesis [102], deep-ultraviolet (DUV) photochemical activation have recently been explored for reducing the processing temperature. DUV photochemical activation has shown promising results even for room temperature deposition with comparable performance and operational stability as that of high temperature (≥350 °C) annealed devices [103].

*Concluding remarks.* There is plenty of room for using oxide materials in different applications such as logic/rf circuits, memories, radio-frequency identification tags, transparent electronics, medical patches, image sensors and printed electronics. Among other NVMs, RRAM is more promising for next generation memory devices. The microscopic mechanisms need to be investigated to improve the device performance in terms of its size and speed. Oxide materials will continue to play a key role in next generation displays which require that the materials be used are sustainable and cost effective. ZTO is one such material which holds potential to revolutionize the display industry. Advancements in newer etching techniques and methods to process it at lower temperatures are essential to realize the full benefit of this material.
## 6. Application aspects of ZnO

David J Rogers, Ferechteh H Teherani, Eric V Sandana and Philippe Bove

Nanovation, 8 Route de Chevreuse, 78117 Châteaufort, France

**Status.** A recent study by Nanomarkets estimated a market value of over 500 million € for electronics based on ZnO. This commercial success is based on ZnO's versatile property set combined with low materials cost and an ongoing series of technological developments [104].

The most mature ZnO electronics applications are anti-static layers, electromagnetic interference shielding and optical coatings. Indeed, ZnO is semiconductor with a widely tuneable conductivity and it can be deposited in large-area thin film form at high rates and low temperatures using a number of techniques including sputtering, pulsed laser deposition, chemical vapour deposition and solution based processing. On the optical front, ZnO brings excellent transparency right across the visible spectrum while the plasma frequency can be engineered to reflect environmental IR radiation (e.g. in energy efficient windows) and the bandgap ($E_g$ ~ 3.4eV) blocks UV light (e.g. in welding glasses). UV filtering has been extended to transparent sun creams using ZnO nanoparticles under ~60 nm in diameter that do not scatter visible light. Such nanoparticle manufacturing testifies to the bio-compatibility of ZnO as a basis for the development of nano ZnO based electronics [105].

The largest current electronics application of ZnO is that of varistors for voltage surge protection. Varistors are based on the formation of double Schottky junctions at ZnO grain boundaries in sintered ZnO powders (~10 µm grains) which act as a collection of back-to-back Zener diodes. Due to the high melting point of ZnO (~1975 °C) such varistors can support the heating associated with large current flow. ZnO displaced SiC as the dominant varistor material when it was discovered that doping with metal oxides (of Bi, Pr, Co, Mn...) gives the most non-linear I/V characteristics known so far [106].

Another long standing use of sintered ZnO powders is for the fabrication of gas sensors. These function based on the principle that absorption or desorption of a gas at the surface and grain boundaries changes the conductivity. ZnO was adopted because it was inexpensive and gave a competitive sensitivity combined with high mechanical/chemical stability and low toxicity. Furthermore, a new generation of compact, low-power nanostructured ZnO gas sensors is currently emerging [110]. Thanks to a very large surface-to-volume ratio such devices have shown the capability of sub-parts-per-million (ppm) sensitivity for a number of molecules, including $C_2H_5OH$, CO, H$_2$, H$_2O$, H$_2S$, NH$_3$, NH$_4$, NO$_2$ and O$_3$.

Wurtzite ZnO also offers a polar crystal structure with amongst the highest piezo-responses of any semiconductor. Furthermore, sputtered ZnO films show better quality and adherence to a-SiO$_2$/Si than alternative materials with larger piezo-responses. This capacity for monolithic integration with silicon-based electronic circuitry, along with an amenability to

chemical processing has led to ZnO being adopted for use in surface acoustic wave multiplexing/demultiplexing bandpass filters for telecommunications applications, primarily cellular phones and base stations. In these devices the challenges of processing, contacting and packaging ZnO thin films on an industrial scale has been mastered [107].

ZnO has also been very widely used as a green phosphor in vacuum fluorescent displays based on the efficient oxygen vacancy emission in zinc rich ZnO. Recently, this market has waned, however, due to the advent of novel display technologies.

**Current and future challenges.** Up until the present, the most widely used transparent conducting oxide has been Sn-doped $In_2O_3$ (ITO). Doped ZnO is currently a challenging ITO, however, for applications such as transparent wiring, touch-sensitive panels and transparent electrical contacts (for use in flat panel displays, solar cells—particularly a-Si and CIGS— and LEDs) [108]. This is because it is cheaper, abundant, less toxic, easier to process, shows superior resistance to hydrogen plasmas and can be fabricated at lower temperatures. Moreover, the mobilities and conductivities attainable with degerately doped ZnO (typically with Al, Ga or Si) have become comparable with ITO. The main challenge remains the displacement of the installed base for production of ITO.

Recently, transparent field effect transistors employing amorphous ZnO-alloy channels (typically indium-gallium-zinc-oxide) and electrodes have emerged [108]. These are enhancement mode devices (no power consumption in off state) which have low leakage currents, exhibit impressively high on/off ratios (>10⁸) and have channel mobilities that are more than an order of magnitude higher than current a-Si:H based TFTs. Thus they are being adopted for use as select transistors in active matrix LCD screens, electronic paper and flexible OLED panels. Higher mobility offers increased refresh rates, improved transparencies allow smaller pixels (with reduced aperture ratios) and the wide bandgap reduces light sensitivity (lower noise) and degradation. The materials also have the potential for low temperature, low-cost, wide area, high throughput solution based fabrication. The main challenge is displacement of the installed base for industrial production of a-Si:H based devices. For more on this topic see the sections by section 2 and section 5 in this roadmap.

The use of ZnO as a UV photodetector has received much attention of late, because the bandgap can be tuned from the UVA into the UVC by alloying with Mg while maintaining the wurtzite phase, figure 8 illustrates the alloying potential for bandgap engineering of ZnO in contrast with that for GaN [109].

Current state-of-the art (Mg)ZnO PD are usually based on metal-semiconductor-metal (MSM) architectures. An MSM-PD is made by forming two inter-digited transducers Schottky electrodes on n-type (Mg)ZnO. Such PD exhibit sharp visible wavelength cut-off, low dark currents, high speed operation, linear signal response with optical power and extremely low parasitic capacitance/noise. These characteristics make them attractive for sensing and imaging applications (e.g. aerospace,
## 6. Application aspects of ZnO

David J Rogers, Ferechteh H Teherani, Eric V Sandana and Philippe Bove

Nanovation, 8 Route de Chevreuse, 78117 Châteaufort, France

**Status.** A recent study by Nanomarkets estimated a market value of over 500 million € for electronics based on ZnO. This commercial success is based on ZnO's versatile property set combined with low materials cost and an ongoing series of technological developments [104].

The most mature ZnO electronics applications are anti-static layers, electromagnetic interference shielding and optical coatings. Indeed, ZnO is semiconductor with a widely tuneable conductivity and it can be deposited in large-area thin film form at high rates and low temperatures using a number of techniques including sputtering, pulsed laser deposition, chemical vapour deposition and solution based processing. On the optical front, ZnO brings excellent transparency right across the visible spectrum while the plasma frequency can be engineered to reflect environmental IR radiation (e.g. in energy efficient windows) and the bandgap ($E_g$ ~ 3.4eV) blocks UV light (e.g. in welding glasses). UV filtering has been extended to transparent sun creams using ZnO nanoparticles under ~60 nm in diameter that do not scatter visible light. Such nanoparticle manufacturing testifies to the bio-compatibility of ZnO as a basis for the development of nano ZnO based electronics [105].

The largest current electronics application of ZnO is that of varistors for voltage surge protection. Varistors are based on the formation of double Schottky junctions at ZnO grain boundaries in sintered ZnO powders (~10 µm grains) which act as a collection of back-to-back Zener diodes. Due to the high melting point of ZnO (~1975 °C) such varistors can support the heating associated with large current flow. ZnO displaced SiC as the dominant varistor material when it was discovered that doping with metal oxides (of Bi, Pr, Co, Mn...) gives the most non-linear I/V characteristics known so far [106].

Another long standing use of sintered ZnO powders is for the fabrication of gas sensors. These function based on the principle that absorption or desorption of a gas at the surface and grain boundaries changes the conductivity. ZnO was adopted because it was inexpensive and gave a competitive sensitivity combined with high mechanical/chemical stability and low toxicity. Furthermore, a new generation of compact, low-power nanostructured ZnO gas sensors is currently emerging [110]. Thanks to a very large surface-to-volume ratio such devices have shown the capability of sub-parts-per-million (ppm) sensitivity for a number of molecules, including $C_2H_5OH$, CO, H$_2$, H$_2O$, H$_2S$, NH$_3$, NH$_4$, NO$_2$ and O$_3$.

Wurtzite ZnO also offers a polar crystal structure with amongst the highest piezo-responses of any semiconductor. Furthermore, sputtered ZnO films show better quality and adherence to a-SiO$_2$/Si than alternative materials with larger piezo-responses. This capacity for monolithic integration with silicon-based electronic circuitry, along with an amenability to

chemical processing has led to ZnO being adopted for use in surface acoustic wave multiplexing/demultiplexing bandpass filters for telecommunications applications, primarily cellular phones and base stations. In these devices the challenges of processing, contacting and packaging ZnO thin films on an industrial scale has been mastered [107].

ZnO has also been very widely used as a green phosphor in vacuum fluorescent displays based on the efficient oxygen vacancy emission in zinc rich ZnO. Recently, this market has waned, however, due to the advent of novel display technologies.

**Current and future challenges.** Up until the present, the most widely used transparent conducting oxide has been Sn-doped $In_2O_3$ (ITO). Doped ZnO is currently a challenging ITO, however, for applications such as transparent wiring, touch-sensitive panels and transparent electrical contacts (for use in flat panel displays, solar cells—particularly a-Si and CIGS— and LEDs) [108]. This is because it is cheaper, abundant, less toxic, easier to process, shows superior resistance to hydrogen plasmas and can be fabricated at lower temperatures. Moreover, the mobilities and conductivities attainable with degerately doped ZnO (typically with Al, Ga or Si) have become comparable with ITO. The main challenge remains the displacement of the installed base for production of ITO.

Recently, transparent field effect transistors employing amorphous ZnO-alloy channels (typically indium-gallium-zinc-oxide) and electrodes have emerged [108]. These are enhancement mode devices (no power consumption in off state) which have low leakage currents, exhibit impressively high on/off ratios (>10⁸) and have channel mobilities that are more than an order of magnitude higher than current a-Si:H based TFTs. Thus they are being adopted for use as select transistors in active matrix LCD screens, electronic paper and flexible OLED panels. Higher mobility offers increased refresh rates, improved transparencies allow smaller pixels (with reduced aperture ratios) and the wide bandgap reduces light sensitivity (lower noise) and degradation. The materials also have the potential for low temperature, low-cost, wide area, high throughput solution based fabrication. The main challenge is displacement of the installed base for industrial production of a-Si:H based devices. For more on this topic see the sections by section 2 and section 5 in this roadmap.

The use of ZnO as a UV photodetector has received much attention of late, because the bandgap can be tuned from the UVA into the UVC by alloying with Mg while maintaining the wurtzite phase, figure 8 illustrates the alloying potential for bandgap engineering of ZnO in contrast with that for GaN [109].

Current state-of-the art (Mg)ZnO PD are usually based on metal-semiconductor-metal (MSM) architectures. An MSM-PD is made by forming two inter-digited transducers Schottky electrodes on n-type (Mg)ZnO. Such PD exhibit sharp visible wavelength cut-off, low dark currents, high speed operation, linear signal response with optical power and extremely low parasitic capacitance/noise. These characteristics make them attractive for sensing and imaging applications (e.g. aerospace,
<figure><img src="image_13.png" /><figcaption>**Figure 8.** The relationship between Eg and lattice constant is shown for alloys of wurzite (Al,In)GaN and (Mg,Cd)ZnO. Although UV bandgap engineering of ZnO is typically achieved by such alloying with Mg, deeper UV performance (up to 10.6 eV) can be achieved by alloying with BeO. There are toxicity and handling issues in this case, however.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 9.** The number of annual publications versus year, according to a search of the Scopus database for the term 'zinc oxide' in the abstract, title, or keywords.</figcaption></figure>

automotive, military, petroleum, flame detection and astronomy). The main challenges are to compete with alternative solid state UV PD based on SiC (already available) and (Al) GaN (emerging). However, the insertion of high pass filters is necessary with SiC (bandgap engineering is not possible) and (Al)GaN detectors are difficult/expensive to fabricate.

ZnO is also emerging for biosensing (piezoelectric, electrochemical, optical or field effect transistor based transduction) [111]. Good electrical conductivity, chemical stability, a biomimetic nature, the potential for surface functionalisation and a high isoelectric point (which facilitates immobilization of biomolecules) make ZnO an attractive biosensor matrix. Due to their small dimensions, increased sensing surface and strong binding properties nanostructures offer faster responses and can achieve single-molecule detection. Ongoing trials should pave the way to commercialisation of implantable, miniature point-of-care biosensors compatible with CMOS/MEMS technology for wireless telemedicine.

With almost 8000 publications on ZnO in 2015 (see figure 9) it is impossible to examine all the potential future applications in this section. Some promising emerging topics are memristors, waveguides, extreme (high temperature/field/

radiation/frequency) electronics, nanoelectronics, electron scintillators, piezotronics, piezogenerators, thermoelectrics, dye sensitized solar cells, photoelectrochemical hydrogen generation, actuators, plasmonics, phononics, spintronics and solar cell light/current management. Also of note is that a recent Thomson-Reuters study, recorded more publications dedicated to nanostructured ZnO than to carbon nanotubes. This was attributed to the multifunctional nature of ZnO, the ease of fabricating nanostructures by various techniques (including wide-area, low cost chemical growth) and the extremely large family of nanostructure shapes that can be obtained.

**Advances in science and technology to meet challenges.** A fundamental outstanding challenge for ZnO-based electronics is the achievement of efficient p-n junction devices. Indeed, ZnO is considered as a potentially superior UV emitter to GaN (currently the commercially predominant opto-semiconductor) because of the higher stability of the ZnO free exciton, which has a binding energy of 60 meV (compared to 21 meV for GaN) (see section by J Zuniga-Perez in this roadmap). It has proven difficult, however, to incorporate and activate sufficient acceptor concentrations. Moreover, background donors tend to compensate the acceptors because native defects (O
<figure><img src="image_14.png" /><figcaption>**Figure 8.** The relationship between Eg and lattice constant is shown for alloys of wurzite (Al,In)GaN and (Mg,Cd)ZnO. Although UV bandgap engineering of ZnO is typically achieved by such alloying with Mg, deeper UV performance (up to 10.6 eV) can be achieved by alloying with BeO. There are toxicity and handling issues in this case, however.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 9.** The number of annual publications versus year, according to a search of the Scopus database for the term 'zinc oxide' in the abstract, title, or keywords.</figcaption></figure>

automotive, military, petroleum, flame detection and astronomy). The main challenges are to compete with alternative solid state UV PD based on SiC (already available) and (Al) GaN (emerging). However, the insertion of high pass filters is necessary with SiC (bandgap engineering is not possible) and (Al)GaN detectors are difficult/expensive to fabricate.

ZnO is also emerging for biosensing (piezoelectric, electrochemical, optical or field effect transistor based transduction) [111]. Good electrical conductivity, chemical stability, a biomimetic nature, the potential for surface functionalisation and a high isoelectric point (which facilitates immobilization of biomolecules) make ZnO an attractive biosensor matrix. Due to their small dimensions, increased sensing surface and strong binding properties nanostructures offer faster responses and can achieve single-molecule detection. Ongoing trials should pave the way to commercialisation of implantable, miniature point-of-care biosensors compatible with CMOS/MEMS technology for wireless telemedicine.

With almost 8000 publications on ZnO in 2015 (see figure 9) it is impossible to examine all the potential future applications in this section. Some promising emerging topics are memristors, waveguides, extreme (high temperature/field/

radiation/frequency) electronics, nanoelectronics, electron scintillators, piezotronics, piezogenerators, thermoelectrics, dye sensitized solar cells, photoelectrochemical hydrogen generation, actuators, plasmonics, phononics, spintronics and solar cell light/current management. Also of note is that a recent Thomson-Reuters study, recorded more publications dedicated to nanostructured ZnO than to carbon nanotubes. This was attributed to the multifunctional nature of ZnO, the ease of fabricating nanostructures by various techniques (including wide-area, low cost chemical growth) and the extremely large family of nanostructure shapes that can be obtained.

**Advances in science and technology to meet challenges.** A fundamental outstanding challenge for ZnO-based electronics is the achievement of efficient p-n junction devices. Indeed, ZnO is considered as a potentially superior UV emitter to GaN (currently the commercially predominant opto-semiconductor) because of the higher stability of the ZnO free exciton, which has a binding energy of 60 meV (compared to 21 meV for GaN) (see section by J Zuniga-Perez in this roadmap). It has proven difficult, however, to incorporate and activate sufficient acceptor concentrations. Moreover, background donors tend to compensate the acceptors because native defects (O
vacancies and Zn interstitials) in combination with common background impurities (such as H, Al and Ga) act as shallow donors in ZnO. Recently, many groups have reported significant progress in the development of p-type ZnO [112] allowing the demonstration of light-emitting p-n junctions [113]. The main acceptor dopants adopted are the Group V elements N, P, As, Sb plus the Group Ia elements Li, Na, K and the Group Ib elements Ag, Cu, Au. There are various competing theories as to how the dopants act but experimental results generally show mobilities, conductivities and overall device efficiencies that are relatively disappointing. The recent development of high quality native substrates and the adoption of higher purity sources have helped to suppress defect densities and non-intentional impurity doping respectively and thus reduce donor compensation. Another emerging factor is that Mg alloying could facilitate the task of p-type doping. The two main reasons put forward for this are the shifting

of the conduction band to higher energy acting to reduce the background donor concentration and the Zn vacancy (acceptor) concentration increasing with Mg content. For more on oxide p-n junction devices see the section by M Grundmann in this roadmap.

## Concluding remarks

The broad range of existing and emerging applications described above testifies to the combination of multifunctionality, commercial relevance and manufacturability offered by ZnO. If the efforts towards p-type ZnO continue to advance, we can expect to see this extend to bipolar and complementary (n- and p-type) devices as the basis for brighter and more robust excitonic-based UV emitters (with lower lasing thresholds), ultra-sensitive UV photodetectors and fully-integrated transparent electronics.
vacancies and Zn interstitials) in combination with common background impurities (such as H, Al and Ga) act as shallow donors in ZnO. Recently, many groups have reported significant progress in the development of p-type ZnO [112] allowing the demonstration of light-emitting p-n junctions [113]. The main acceptor dopants adopted are the Group V elements N, P, As, Sb plus the Group Ia elements Li, Na, K and the Group Ib elements Ag, Cu, Au. There are various competing theories as to how the dopants act but experimental results generally show mobilities, conductivities and overall device efficiencies that are relatively disappointing. The recent development of high quality native substrates and the adoption of higher purity sources have helped to suppress defect densities and non-intentional impurity doping respectively and thus reduce donor compensation. Another emerging factor is that Mg alloying could facilitate the task of p-type doping. The two main reasons put forward for this are the shifting

of the conduction band to higher energy acting to reduce the background donor concentration and the Zn vacancy (acceptor) concentration increasing with Mg content. For more on oxide p-n junction devices see the section by M Grundmann in this roadmap.

## Concluding remarks

The broad range of existing and emerging applications described above testifies to the combination of multifunctionality, commercial relevance and manufacturability offered by ZnO. If the efforts towards p-type ZnO continue to advance, we can expect to see this extend to bipolar and complementary (n- and p-type) devices as the basis for brighter and more robust excitonic-based UV emitters (with lower lasing thresholds), ultra-sensitive UV photodetectors and fully-integrated transparent electronics.
# 7. All-oxide photovoltaics

Kevin J Rietwyk and Arie Zaban

Department of Chemistry, Center for Nanotechnology &
Advanced Materials, Bar Ilan University, Ramat Gan 52900,
Israel

**Status.** All-oxide photovoltaics is a promising family of low cost thin film solar cells. With so many metals being abundant and the ability to grow stable, thin film oxides in low vacuum and ambient environments, metal-oxide based solar cells enjoy low fabrications and raw material costs. In addition, metal-oxides typically possess the high chemical stability and low toxicity desired for photovoltaic devices. Due to the various functional properties of metal-oxide compounds, a range of architectures are possible for all-oxide photovoltaics based on conventional photovoltaic, bulk photovoltaic, ferroelectric domain walls, plasmon-enhanced hot-electron injection, nanocomposite materials and even thermoelectricity. The energy diagrams for some of these cells are given in figure 10.

The conventional photovoltaic effect is by far most common, in which, a gradient in the electrochemical potential in the absorbing semiconducting layer is used to separate optically excited electron-hole pair which are collected at different electrodes. The performance of these devices is enhanced by utilising selective contacts to promote limit the transport of carrier species to select electrodes. The highest efficiency achieved for this group of all-oxide solar cells is 5.38% and are based on a Cu₂O|ZnO (pn) heterojunction with a Ga₂O₃ buffer layer [114].

For solar cells based on the bulk photovoltaic effect, a photovoltage is generated in non-centrosymmetric (ferroelectric) crystals due to a polarization induced electric field (depolarisation electric field) which promotes the charge. In a recent *Nature Photonics* paper by Nechache et al [115] an efficiency of 3.3% for single absorber layer and 8.1% for multi-layered solar cell was attained, greatly surpassing the previous record for single layered cells of only 1.25% whilst demonstrating the feasibility of this design.

In an alternate, lateral design that also exploits ferroelectricity, a series of thin layers of a ferroelectric material are grown with regularly spaced conductive, ferroelectric domain walls. Each domain acts like a small cell and the walls produce an electric field in the adjacent domains that separate the charge carriers. While each domain produces a photovoltage only in the order of 10 mV, the domains are linked in series and with hundreds of domains large voltages in excess of 15V can be produced [116].

Plasmon-enhanced hot-electron injection based solar cells are rapidly gaining popularity. The operation depends on optically excited carrier plasmons which decay into hot electron-hole pairs. These carriers then separate at the metal/semiconductor interface and are collected before cooling. The energy of the plasmon is tuned using nanostructures to within the visible light region to ensure optimal current. Barad et al [117] recently developed a method to grow very low cost plasmonic solar cells via the deposition of thin films of silver onto a sprayed pyrolysis deposited TiO₂ layers in which the high surface roughness of the TiO₂ is exploited to form the silver nanostructures.

<figure><img src="image_15.png" /><figcaption>**Figure 10.** Schematic band diagrams of all-oxide photovoltaic cells. (a) Conventional ZnO|Cu₂O heterojunction. (b) Single layer Bi₂FeCrO₆ bulk-photovoltaic cell before (left) and after (right) poling with 10 V showing polarisation induced change in the energetics. (c) Lateral BiFeO₃ cell showing four ferroelectric domains and domain walls (actual cells are typically comprised of hundreds). d) TiO₂|Ag plasmon-enhanced hot-electron injection cell in which hot electrons from optically excited plasmons in the Ag are transported through TiO₂ to an FTO layer, before cooling.</figcaption></figure>

An interesting hybrid technology is solar thermoelectric generators (STEG) in which concentrated solar radiation is used as the heat source for a thermoelectric generator. Since the efficiency is dependent upon the temperature difference across the generator, oxides are an excellent candidate group—commonly exhibiting high-temperature (~1000 K) stability in air while enjoying reasonable thermoelectric properties. Terasaki et al [118] have reported 1.4% efficiency for preliminary BiSrCoO/CaYbMnO uni-couple devices.

**Current and future challenges.** In order for all-oxide photovoltaics to break through into commercial markets, it is necessary to raise the efficiency of the devices. Metal-oxides are an essential building block in modern thin film technology and have proven effective in their role in many photovoltaics and electronic devices as transparent conductors (FTO, ITO, AZO), carrier selective contacts (TiO₂, ZnO, V₂O₅, NiO, WO₃, MoO₃), and ultra-thin buffers layers (Al₂O₃, Ga₂O₃) [119]. The chief shortcoming in all-oxide photovoltaics is a lack of metal-oxides with a suitable suite of properties to perform as ideal light absorbing layers [120].

To achieve high performing solar cells it is important for the absorber layer to possess a range of optical and electronic properties with the most quintessential property being the band gap. Only photons with energy greater than the band gap can be absorbed and contribute to the photocurrent. For traditional devices, there is an additional balancing with the energy of photons that provide the photovoltage, limiting the ideal band gap to 1.34 eV for a single junction solar cell [121]. For other designs, it is essential that the absorbing material possess other key properties such ferroelectrical behaviour or carrier plasmons with appropriate energies.
# 7. All-oxide photovoltaics

Kevin J Rietwyk and Arie Zaban

Department of Chemistry, Center for Nanotechnology &
Advanced Materials, Bar Ilan University, Ramat Gan 52900,
Israel

**Status.** All-oxide photovoltaics is a promising family of low cost thin film solar cells. With so many metals being abundant and the ability to grow stable, thin film oxides in low vacuum and ambient environments, metal-oxide based solar cells enjoy low fabrications and raw material costs. In addition, metal-oxides typically possess the high chemical stability and low toxicity desired for photovoltaic devices. Due to the various functional properties of metal-oxide compounds, a range of architectures are possible for all-oxide photovoltaics based on conventional photovoltaic, bulk photovoltaic, ferroelectric domain walls, plasmon-enhanced hot-electron injection, nanocomposite materials and even thermoelectricity. The energy diagrams for some of these cells are given in figure 10.

The conventional photovoltaic effect is by far most common, in which, a gradient in the electrochemical potential in the absorbing semiconducting layer is used to separate optically excited electron-hole pair which are collected at different electrodes. The performance of these devices is enhanced by utilising selective contacts to promote limit the transport of carrier species to select electrodes. The highest efficiency achieved for this group of all-oxide solar cells is 5.38% and are based on a Cu₂O|ZnO (pn) heterojunction with a Ga₂O₃ buffer layer [114].

For solar cells based on the bulk photovoltaic effect, a photovoltage is generated in non-centrosymmetric (ferroelectric) crystals due to a polarization induced electric field (depolarisation electric field) which promotes the charge. In a recent *Nature Photonics* paper by Nechache et al [115] an efficiency of 3.3% for single absorber layer and 8.1% for multi-layered solar cell was attained, greatly surpassing the previous record for single layered cells of only 1.25% whilst demonstrating the feasibility of this design.

In an alternate, lateral design that also exploits ferroelectricity, a series of thin layers of a ferroelectric material are grown with regularly spaced conductive, ferroelectric domain walls. Each domain acts like a small cell and the walls produce an electric field in the adjacent domains that separate the charge carriers. While each domain produces a photovoltage only in the order of 10 mV, the domains are linked in series and with hundreds of domains large voltages in excess of 15V can be produced [116].

Plasmon-enhanced hot-electron injection based solar cells are rapidly gaining popularity. The operation depends on optically excited carrier plasmons which decay into hot electron-hole pairs. These carriers then separate at the metal/semiconductor interface and are collected before cooling. The energy of the plasmon is tuned using nanostructures to within the visible light region to ensure optimal current. Barad et al [117] recently developed a method to grow very low cost plasmonic solar cells via the deposition of thin films of silver onto a sprayed pyrolysis deposited TiO₂ layers in which the high surface roughness of the TiO₂ is exploited to form the silver nanostructures.

<figure><img src="image_16.png" /><figcaption>**Figure 10.** Schematic band diagrams of all-oxide photovoltaic cells. (a) Conventional ZnO|Cu₂O heterojunction. (b) Single layer Bi₂FeCrO₆ bulk-photovoltaic cell before (left) and after (right) poling with 10 V showing polarisation induced change in the energetics. (c) Lateral BiFeO₃ cell showing four ferroelectric domains and domain walls (actual cells are typically comprised of hundreds). d) TiO₂|Ag plasmon-enhanced hot-electron injection cell in which hot electrons from optically excited plasmons in the Ag are transported through TiO₂ to an FTO layer, before cooling.</figcaption></figure>

An interesting hybrid technology is solar thermoelectric generators (STEG) in which concentrated solar radiation is used as the heat source for a thermoelectric generator. Since the efficiency is dependent upon the temperature difference across the generator, oxides are an excellent candidate group—commonly exhibiting high-temperature (~1000 K) stability in air while enjoying reasonable thermoelectric properties. Terasaki et al [118] have reported 1.4% efficiency for preliminary BiSrCoO/CaYbMnO uni-couple devices.

**Current and future challenges.** In order for all-oxide photovoltaics to break through into commercial markets, it is necessary to raise the efficiency of the devices. Metal-oxides are an essential building block in modern thin film technology and have proven effective in their role in many photovoltaics and electronic devices as transparent conductors (FTO, ITO, AZO), carrier selective contacts (TiO₂, ZnO, V₂O₅, NiO, WO₃, MoO₃), and ultra-thin buffers layers (Al₂O₃, Ga₂O₃) [119]. The chief shortcoming in all-oxide photovoltaics is a lack of metal-oxides with a suitable suite of properties to perform as ideal light absorbing layers [120].

To achieve high performing solar cells it is important for the absorber layer to possess a range of optical and electronic properties with the most quintessential property being the band gap. Only photons with energy greater than the band gap can be absorbed and contribute to the photocurrent. For traditional devices, there is an additional balancing with the energy of photons that provide the photovoltage, limiting the ideal band gap to 1.34 eV for a single junction solar cell [121]. For other designs, it is essential that the absorbing material possess other key properties such ferroelectrical behaviour or carrier plasmons with appropriate energies.
<figure><img src="image_16.png" /><figcaption>**Figure 11.** Combinatorial development cycle for all-oxide PV.</figcaption></figure>

Once optically excited, for carriers to be collected, the absorber layer must also possess excellent transport properties including high carrier mobilities and a low concentration of trap states and recombination centres. In addition to the internal electric field, these qualities determine the distance carriers can propagate before recombining and as a rule-of-thumb the minority carrier diffusion length should be greater than the optical absorption depth.

Finally, the collection of the carriers from the absorber layer is determined by the interfaces with adjacent layers. For optimal performance, there should be minimal defects at the interfaces and appropriate energy level alignment to ensure transfer of a single carrier species at each interface.

*Advances in science and technology to meet challenges.* While the physical properties of most binary metal-oxides have been extensively investigated, only a fraction of all the possible ternary and even fewer quaternary metal-oxides have been studied. The number of possible chemical compositions and structural phases increases exponentially with the quantity of elements involved, and the combination of both define their set of physical properties. Therefore, it stands to reason that countless metal-oxide compounds with the ideal set of properties for photovoltaic applications exist and have yet to be found. Unfortunately, for complex compounds it is exceedingly difficult to predict the properties of oxides using empirical methods, so it is necessary to perform a large scale search for suitable materials and this requires a high-throughput methodology [122].

The search for new materials can be undertaken using computational materials engineering and/or experimental combinatorial material science. Both methods involve high throughput examination of unknown materials and utilise tools to generate knowledge from the acquired information, which is stored in readily accessible databases along with relevant metadata to enable rapid analysis. The combinatorial material cycle is summarised in figure 11. For the rest of this section we will focus on the experimental efforts, for reasons of brevity.

Compared to their computational counter-parts, there are fewer high-throughput experimental efforts due to the large overhead required to establish the experimental instrumentation. To perform studies, metal-oxide compounds are grown on large substrates (on the order of 50 cm²) using co-depositions of different oxides to produce films with spatially varied chemical composition. The resulting layer is characterised using a series of automated scanning robots that perform local area measurements sequentially in an array of positions across the sample. The selection of suitable analytical techniques is diverse and includes any non-destructive, scanning measurement. The results of each technique can then be related to each other to identify relationships between composition and the various physical properties. The experimental approach enjoys the capability of only producing realistic material structures and can be employed on a wide range of specimens from single material layers up to complete devices.
<figure><img src="image_17.png" /><figcaption>**Figure 11.** Combinatorial development cycle for all-oxide PV.</figcaption></figure>

Once optically excited, for carriers to be collected, the absorber layer must also possess excellent transport properties including high carrier mobilities and a low concentration of trap states and recombination centres. In addition to the internal electric field, these qualities determine the distance carriers can propagate before recombining and as a rule-of-thumb the minority carrier diffusion length should be greater than the optical absorption depth.

Finally, the collection of the carriers from the absorber layer is determined by the interfaces with adjacent layers. For optimal performance, there should be minimal defects at the interfaces and appropriate energy level alignment to ensure transfer of a single carrier species at each interface.

*Advances in science and technology to meet challenges.* While the physical properties of most binary metal-oxides have been extensively investigated, only a fraction of all the possible ternary and even fewer quaternary metal-oxides have been studied. The number of possible chemical compositions and structural phases increases exponentially with the quantity of elements involved, and the combination of both define their set of physical properties. Therefore, it stands to reason that countless metal-oxide compounds with the ideal set of properties for photovoltaic applications exist and have yet to be found. Unfortunately, for complex compounds it is exceedingly difficult to predict the properties of oxides using empirical methods, so it is necessary to perform a large scale search for suitable materials and this requires a high-throughput methodology [122].

The search for new materials can be undertaken using computational materials engineering and/or experimental combinatorial material science. Both methods involve high throughput examination of unknown materials and utilise tools to generate knowledge from the acquired information, which is stored in readily accessible databases along with relevant metadata to enable rapid analysis. The combinatorial material cycle is summarised in figure 11. For the rest of this section we will focus on the experimental efforts, for reasons of brevity.

Compared to their computational counter-parts, there are fewer high-throughput experimental efforts due to the large overhead required to establish the experimental instrumentation. To perform studies, metal-oxide compounds are grown on large substrates (on the order of 50 cm²) using co-depositions of different oxides to produce films with spatially varied chemical composition. The resulting layer is characterised using a series of automated scanning robots that perform local area measurements sequentially in an array of positions across the sample. The selection of suitable analytical techniques is diverse and includes any non-destructive, scanning measurement. The results of each technique can then be related to each other to identify relationships between composition and the various physical properties. The experimental approach enjoys the capability of only producing realistic material structures and can be employed on a wide range of specimens from single material layers up to complete devices.
Experimental efforts would benefit greatly from the devel-
opment and implementation of big data and machine learning
tools. Machine learning algorithms can rapidly enhance data
treatment rates by providing automated background subtrac-
tion and deconvolution of spectral data to identify onsets and
peak locations, width, areas and so on [123]. This information
can be used as metrics for clustering algorithms or datamining
activities that enable high dimensional data analysis to identify
complex trends, such as quantitative-structure-activity rela-
tionship and process-structure-physical property relationship
analyses [124].

**Concluding remarks.** Although efficiencies are currently low, the ability to develop all-oxide photovoltaics from low-cost raw materials and fabrication methods makes them a realistic option for cheap solar cells. Improvement in device performance, comparable to contemporary commercial solar cells, is expected to be achieved via high-throughput investigations into new metal-oxide compounds. However, given the wide range of possible architectures and designs available to this versatile family of materials, the exact form that future all-oxide photovoltaics will adopt is both unpredictable and very exciting.
Experimental efforts would benefit greatly from the devel-
opment and implementation of big data and machine learning
tools. Machine learning algorithms can rapidly enhance data
treatment rates by providing automated background subtrac-
tion and deconvolution of spectral data to identify onsets and
peak locations, width, areas and so on [123]. This information
can be used as metrics for clustering algorithms or datamining
activities that enable high dimensional data analysis to identify
complex trends, such as quantitative-structure-activity rela-
tionship and process-structure-physical property relationship
analyses [124].

**Concluding remarks.** Although efficiencies are currently low, the ability to develop all-oxide photovoltaics from low-cost raw materials and fabrication methods makes them a realistic option for cheap solar cells. Improvement in device performance, comparable to contemporary commercial solar cells, is expected to be achieved via high-throughput investigations into new metal-oxide compounds. However, given the wide range of possible architectures and designs available to this versatile family of materials, the exact form that future all-oxide photovoltaics will adopt is both unpredictable and very exciting.
# 8. Perovskite-type materials for energy converters

Angelika Veziridis and Anke Weidenkaff

Materials Chemistry, Institute for Materials Science, University of Stuttgart, DE-70569 Stuttgart, Germany
E-mail: weidenkaff@imw.uni-stuttgart.de

**Status.** Perovskites are a unique class of compounds due to their wide-ranging electronic, chemical and mechanical properties. Their composition can be varied on a large scale (subject to the Goldschmidt tolerance factor) without substantial change of the crystal structure. This flexibility allows the incorporation of a vast range of elements modifying the attractive physical and chemical properties. Thus, substitution materials free of critical elements can be created, which is a prerequisite for device application. They have a particularly huge potential for many energy conversion processes due to their insulating, p- or n-type semiconducting, metallic or even superconducting transport properties. Ferroelectric, piezoelectric, catalytic or thermoelectric materials are also known in this family [125].

Major challenges such as global warming and the finite nature of fossil fuels can only be met by the development of renewable resources and new energy conversion and storage technologies. For this purpose, nothing short of temperature- and oxidation-resistant, non-toxic compounds based on abundant, noncritical and inexpensive elements are required. Perovskite-type transition metal oxides (figure 12) do meet these conditions and are already widely used for energy conversion processes. Applications include but are not limited to thermoelectrics, photovoltaics, photoelectrocatalysis and fuel cells.

**Current and future challenges.** Thermoelectricity (TE) is the direct conversion of thermal energy into electricity and vice versa. Therefore, TE is a versatile technology to improve many technical and industrial processes, which for thermodynamic reasons discharge heat (e.g. fuel cells (figure 13(a)) [126]). The challenges regarding the widespread implementation of this technology lie in the high cost and low efficiency of TE systems. The TE conversion efficiency increases with increasing temperature gradients across the material due to higher Carnot efficiencies. But the currently best-performing TE materials (Bi- and Pb-containing chalcogenides) lack stability at higher temperatures due to decomposition, vaporization and melting.

The interest in perovskite-type oxides derives from their temperature stability, their correlated electronic structures enabling additional band structure tuning, the cost-effective and non-toxic constituents. Particularly manganese and titanium oxides are considered promising n-type materials while cobalt oxides have been identified as suitable p-type materials with a reasonably high TE performance to build all oxide thermoelectric converters (figure 13(b)). This is due to both a suitable crystal and electronic structure enabling adjustment of TE-relevant transport properties.

Using the example of SrTiO₃, the possibility of Ti⁴⁺ → Ti³⁺ transition metal reduction provides the basis for increasing the

charge carrier concentration and mobility by appropriate A- and B-site substitution and deficiency, respectively. In addition, a well-defined defect chemistry induced by the synthesis conditions also ensures effective phonon scattering reducing the counterproductive thermal conductivity. In a comparative study of different A-site-substituted SrTiO₃ compounds a maximum high-temperature ZT value of 0.42 was measured [127].

Moreover, nano-structuring using well-established and scalable soft-chemistry synthesis techniques combined with stepwise sintering further improves the TE-properties of perovskite-type oxides. This was revealed by B-site substituted CaMnO₃-δ compounds resulting in peak ZT values >0.2 at 1160 K [128].

**Photovoltaics** (PV) is probably one of the most widespread sustainable energy conversion technologies. The efficiency depends on the ability of the PV material to absorb solar light, to separate electron and hole pairs, and to transport the carriers, which makes demands on the band gap, a built-in electric field and the carrier diffusion length.

In recent years, the class of intrinsic semiconductors has become an alternative to conventional PV materials. Their most important advantages is that the thickness of the active materials can be reduced lowering the internal resistance of the cells. In a minimum of time, solar cells based on hybride halide perovskites have made very rapid progress.

Meanwhile, the search for suitable, more stable inorganic perovskite oxides continued, mainly because of their comparatively higher robustness to moisture. The multiferroic BiFeO₃ (BFO) is the most intensely studied compound for this purpose due to its solar light-suited band gap and the internal electric field. But in contrast to their hybrid counterparts, the conversion efficiency of BFO-based both single-layer devices and heterojunctions currently remains below 10% [129]. The multifunctional behavior of BFO is due to a slight strain in the structure. In order to improve and model the material, a better understanding of how external and internal stimuli affect the properties such as carrier concentration and recombination is required in the first place.

<figure><img src="image_17.png" /><figcaption>**Figure 12.** Ideal cubic perovskite crystal structure: the B cation (brown) is octahedrally coordinated with six X anions (lilac) forming an infinite array of corner-sharing BX₆ octahedra, The A atom (dark blue) is located in the center of eight octahedra.</figcaption></figure>
# 8. Perovskite-type materials for energy converters

Angelika Veziridis and Anke Weidenkaff

Materials Chemistry, Institute for Materials Science, University of Stuttgart, DE-70569 Stuttgart, Germany
E-mail: weidenkaff@imw.uni-stuttgart.de

**Status.** Perovskites are a unique class of compounds due to their wide-ranging electronic, chemical and mechanical properties. Their composition can be varied on a large scale (subject to the Goldschmidt tolerance factor) without substantial change of the crystal structure. This flexibility allows the incorporation of a vast range of elements modifying the attractive physical and chemical properties. Thus, substitution materials free of critical elements can be created, which is a prerequisite for device application. They have a particularly huge potential for many energy conversion processes due to their insulating, p- or n-type semiconducting, metallic or even superconducting transport properties. Ferroelectric, piezoelectric, catalytic or thermoelectric materials are also known in this family [125].

Major challenges such as global warming and the finite nature of fossil fuels can only be met by the development of renewable resources and new energy conversion and storage technologies. For this purpose, nothing short of temperature- and oxidation-resistant, non-toxic compounds based on abundant, noncritical and inexpensive elements are required. Perovskite-type transition metal oxides (figure 12) do meet these conditions and are already widely used for energy conversion processes. Applications include but are not limited to thermoelectrics, photovoltaics, photoelectrocatalysis and fuel cells.

**Current and future challenges.** Thermoelectricity (TE) is the direct conversion of thermal energy into electricity and vice versa. Therefore, TE is a versatile technology to improve many technical and industrial processes, which for thermodynamic reasons discharge heat (e.g. fuel cells (figure 13(a)) [126]). The challenges regarding the widespread implementation of this technology lie in the high cost and low efficiency of TE systems. The TE conversion efficiency increases with increasing temperature gradients across the material due to higher Carnot efficiencies. But the currently best-performing TE materials (Bi- and Pb-containing chalcogenides) lack stability at higher temperatures due to decomposition, vaporization and melting.

The interest in perovskite-type oxides derives from their temperature stability, their correlated electronic structures enabling additional band structure tuning, the cost-effective and non-toxic constituents. Particularly manganese and titanium oxides are considered promising n-type materials while cobalt oxides have been identified as suitable p-type materials with a reasonably high TE performance to build all oxide thermoelectric converters (figure 13(b)). This is due to both a suitable crystal and electronic structure enabling adjustment of TE-relevant transport properties.

Using the example of SrTiO₃, the possibility of Ti⁴⁺ → Ti³⁺ transition metal reduction provides the basis for increasing the

charge carrier concentration and mobility by appropriate A- and B-site substitution and deficiency, respectively. In addition, a well-defined defect chemistry induced by the synthesis conditions also ensures effective phonon scattering reducing the counterproductive thermal conductivity. In a comparative study of different A-site-substituted SrTiO₃ compounds a maximum high-temperature ZT value of 0.42 was measured [127].

Moreover, nano-structuring using well-established and scalable soft-chemistry synthesis techniques combined with stepwise sintering further improves the TE-properties of perovskite-type oxides. This was revealed by B-site substituted CaMnO₃-δ compounds resulting in peak ZT values >0.2 at 1160 K [128].

**Photovoltaics** (PV) is probably one of the most widespread sustainable energy conversion technologies. The efficiency depends on the ability of the PV material to absorb solar light, to separate electron and hole pairs, and to transport the carriers, which makes demands on the band gap, a built-in electric field and the carrier diffusion length.

In recent years, the class of intrinsic semiconductors has become an alternative to conventional PV materials. Their most important advantages is that the thickness of the active materials can be reduced lowering the internal resistance of the cells. In a minimum of time, solar cells based on hybride halide perovskites have made very rapid progress.

Meanwhile, the search for suitable, more stable inorganic perovskite oxides continued, mainly because of their comparatively higher robustness to moisture. The multiferroic BiFeO₃ (BFO) is the most intensely studied compound for this purpose due to its solar light-suited band gap and the internal electric field. But in contrast to their hybrid counterparts, the conversion efficiency of BFO-based both single-layer devices and heterojunctions currently remains below 10% [129]. The multifunctional behavior of BFO is due to a slight strain in the structure. In order to improve and model the material, a better understanding of how external and internal stimuli affect the properties such as carrier concentration and recombination is required in the first place.

<figure><img src="image_18.png" /><figcaption>**Figure 12.** Ideal cubic perovskite crystal structure: the B cation (brown) is octahedrally coordinated with six X anions (lilac) forming an infinite array of corner-sharing BX₆ octahedra, The A atom (dark blue) is located in the center of eight octahedra.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure 13.** (a) Hexis SOFC cell stack and (b) thermoelectric oxide modules (TOM) made of perovskite-type oxides.</figcaption></figure>

The economic advantage of perovskite oxide-based PV modules might be their simpler processing and absence of environmentally hazardous heavy metals. Instead of challenging well-established products, a cooperative approach in terms of tandem cells appears more promising [130].

Direct conversion of solar into chemical energy is accomplished by means of photoelectrocatalysis. This is particularly interesting for the production of hydrogen as storage medium in a sustainable energy management system. But up to now there are no materials or device setups offering sufficient efficiency and stability for direct solar water splitting.

Perovskites based on the abundant element titanium can be an alternative. They are a natural choice to replace pristine or doped TiO₂ allowing for band gap tuning by anionic along with simultaneous cationic composition modification. Partial anionic substitution of oxygen for nitrogen leads to the formation of oxynitride perovskites, a group of compounds with interesting physicochemical properties attributed to the resulting changes in the electronic band- and defect structures [131]. Synergetic effects with ferroelectric or piezoelectric properties might be an asset.

The water splitting process involves separation of charges and their fast transport to the device surface. Therefore, the absence of anionic vacancies is crucial because they act as recombination centers. Two basic concepts are used to avoid an additional formation of vacancies inside the perovskite crystal structure due to charge neutrality: Partial substitution of trivalent (e.g. La³⁺) and five/six-valent (e.g. Ta⁵⁺, Mo⁶⁺) cations for the A- and B-site cations, respectively, or partial substitution of equal amounts of nitrogen and fluorine for oxygen [132].

Another challenging task is the further processing of synthesized phases to a photoelectrochemical device with lasting high performance. Especially, the preparation of dense bulk samples to ensure a high mobility of generated charge carriers is suffering from limited thermal stability of the oxynitrides.

The advantages of solid oxide fuel cells (SOFC) converting chemical into electrical energy are their high efficiency and fuel flexibility. In recent years, considerable efforts have been made to lower the operating temperature for cost and stability reasons. This virtually requires the development of a whole new set of tailor-made materials with adequate reactive and regenerative properties in a broader temperature range.

Perovskite-type La/Sr/Mn oxides are already the most commonly used SOFC cathode material. Replacing Mn for

Co/Fe improves the electric conductivity and catalytic efficiency at lower temperatures. But cation segregation at elevated temperatures under electrochemical polarization can considerably impair the cathodic performance. Additional long-term surface poisoning by impurities (e.g. Cr evaporating from steel interconnects) might lead to degradation [133].

Perovskites have also been explored as solid-state electrolytes with high oxygen conductivities and low exchange resistance. Unfortunately, the studied LaGaO₃-based materials form secondary phases with the used cathode materials, as high oxygen mobility is inevitably associated with weaker bonds and hence lower stability. Accordingly, they are also sensitive to the reducing conditions at the anode leading to decomposition or unwanted electron conduction. These side effects might be controlled by using composites of perovskites and the conventional electrolyte yttrium-stabilized zirconia (YSZ). In addition, this approach also reduced grain boundary resistance and increased the ionic conductivity [134].

Several Mo-containing double-perovskite phases have been proposed as anode material in the low temperature range. The challenges are similar to those of the cathode materials: stability at reducing conditions, electrolyte compatibility and specific resistance.

Perovskites have also been suggested as interconnect materials needed for stack formation. LaCrO₃-based compounds are particularly promising owing to their sintering properties, electronic conductivity and mechanical strength.

***Advances in science and technology to meet challenges.*** The characteristics of perovskite-type compounds depend on their electronic structure. For the design of material properties geared to a defined application, the ‘3 D strategy’ has proven the treatment of choice: **Density of states (electronic band structure)**, **Defects (traps, in-gap states)** and **Dynamics (catalysis, regeneration)**.

The density of states (DOS) is a function of the crystal lattice, spin states, dimensionality of the crystallites (quantum confinement) and correlation effects. It determines the chemical potential (Fermi energy, *E*<sub>F</sub>) as well as the band gap and thus the transport properties of the material. Defects exist in every solid and govern the so-called ‘Realstruktur’ with the catalytic sites. They can dramatically modify the electronic properties as they add states to the band gap. This ‘out-of-equilibrium’ situation is the fundamental driving force for all energy conversion and catalytic processes. Eventually, the
<figure><img src="image_19.png" /><figcaption>**Figure 13.** (a) Hexis SOFC cell stack and (b) thermoelectric oxide modules (TOM) made of perovskite-type oxides.</figcaption></figure>

The economic advantage of perovskite oxide-based PV modules might be their simpler processing and absence of environmentally hazardous heavy metals. Instead of challenging well-established products, a cooperative approach in terms of tandem cells appears more promising [130].

Direct conversion of solar into chemical energy is accomplished by means of photoelectrocatalysis. This is particularly interesting for the production of hydrogen as storage medium in a sustainable energy management system. But up to now there are no materials or device setups offering sufficient efficiency and stability for direct solar water splitting.

Perovskites based on the abundant element titanium can be an alternative. They are a natural choice to replace pristine or doped TiO₂ allowing for band gap tuning by anionic along with simultaneous cationic composition modification. Partial anionic substitution of oxygen for nitrogen leads to the formation of oxynitride perovskites, a group of compounds with interesting physicochemical properties attributed to the resulting changes in the electronic band- and defect structures [131]. Synergetic effects with ferroelectric or piezoelectric properties might be an asset.

The water splitting process involves separation of charges and their fast transport to the device surface. Therefore, the absence of anionic vacancies is crucial because they act as recombination centers. Two basic concepts are used to avoid an additional formation of vacancies inside the perovskite crystal structure due to charge neutrality: Partial substitution of trivalent (e.g. La³⁺) and five/six-valent (e.g. Ta⁵⁺, Mo⁶⁺) cations for the A- and B-site cations, respectively, or partial substitution of equal amounts of nitrogen and fluorine for oxygen [132].

Another challenging task is the further processing of synthesized phases to a photoelectrochemical device with lasting high performance. Especially, the preparation of dense bulk samples to ensure a high mobility of generated charge carriers is suffering from limited thermal stability of the oxynitrides.

The advantages of solid oxide fuel cells (SOFC) converting chemical into electrical energy are their high efficiency and fuel flexibility. In recent years, considerable efforts have been made to lower the operating temperature for cost and stability reasons. This virtually requires the development of a whole new set of tailor-made materials with adequate reactive and regenerative properties in a broader temperature range.

Perovskite-type La/Sr/Mn oxides are already the most commonly used SOFC cathode material. Replacing Mn for

Co/Fe improves the electric conductivity and catalytic efficiency at lower temperatures. But cation segregation at elevated temperatures under electrochemical polarization can considerably impair the cathodic performance. Additional long-term surface poisoning by impurities (e.g. Cr evaporating from steel interconnects) might lead to degradation [133].

Perovskites have also been explored as solid-state electrolytes with high oxygen conductivities and low exchange resistance. Unfortunately, the studied LaGaO₃-based materials form secondary phases with the used cathode materials, as high oxygen mobility is inevitably associated with weaker bonds and hence lower stability. Accordingly, they are also sensitive to the reducing conditions at the anode leading to decomposition or unwanted electron conduction. These side effects might be controlled by using composites of perovskites and the conventional electrolyte yttrium-stabilized zirconia (YSZ). In addition, this approach also reduced grain boundary resistance and increased the ionic conductivity [134].

Several Mo-containing double-perovskite phases have been proposed as anode material in the low temperature range. The challenges are similar to those of the cathode materials: stability at reducing conditions, electrolyte compatibility and specific resistance.

Perovskites have also been suggested as interconnect materials needed for stack formation. LaCrO₃-based compounds are particularly promising owing to their sintering properties, electronic conductivity and mechanical strength.

***Advances in science and technology to meet challenges.*** The characteristics of perovskite-type compounds depend on their electronic structure. For the design of material properties geared to a defined application, the ‘3 D strategy’ has proven the treatment of choice: **Density of states (electronic band structure)**, **Defects (traps, in-gap states)** and **Dynamics (catalysis, regeneration)**.

The density of states (DOS) is a function of the crystal lattice, spin states, dimensionality of the crystallites (quantum confinement) and correlation effects. It determines the chemical potential (Fermi energy, *E*<sub>F</sub>) as well as the band gap and thus the transport properties of the material. Defects exist in every solid and govern the so-called ‘Realstruktur’ with the catalytic sites. They can dramatically modify the electronic properties as they add states to the band gap. This ‘out-of-equilibrium’ situation is the fundamental driving force for all energy conversion and catalytic processes. Eventually, the
dynamics of catalytic and energy conversion processes play a
vital role for the performance of materials.

Control of the material properties by means of the elemen-
tal composition presupposes the precise knowledge of the
above-mentioned correlations. Additionally, also the synthe-
sis method has a marked influence on the properties through,
for instance, density, surface area and microstructure of the
compound. Only the appropriate consideration of all these
aspects allows development of improved and competitive
oxide materials.

***Concluding remarks.*** Better energy conversion processes require materials with desired properties tailored to the respective application. The versatile perovskite structure ABX$_{3}$ is a suitable basis for this purpose. Its remarkable stability provides the opportunity to use most elements of the periodic table as well as organic ions on the A-, B- and X-sites.

In view of a target-oriented approach, this plethora of pos-
sible combinations demands theoretical models to predict
the material properties under operating conditions. In order
to minimize the risk of degradation in harsh environments,
appropriate measures such as coatings, (nano-)composites or
modified microstructures have to be taken to ensure a reason-
able service life. Therefore, a better knowledge of the mech-
anisms involved in surface and transport reactions (along
with improved operando characterization and modelling tech-
niques) is the essential precondition to design materials with
better performance and stability for sustainable energy con-
verters of the future.

The flexibility of perovskite-type oxides facilitates to com-
ply apart from conversion efficiency with additional very
important constraints such as abundance, toxicity and environ-
mental benignity. In the end, they have to be cost competitive
with conventional technologies.
dynamics of catalytic and energy conversion processes play a
vital role for the performance of materials.

Control of the material properties by means of the elemen-
tal composition presupposes the precise knowledge of the
above-mentioned correlations. Additionally, also the synthe-
sis method has a marked influence on the properties through,
for instance, density, surface area and microstructure of the
compound. Only the appropriate consideration of all these
aspects allows development of improved and competitive
oxide materials.

***Concluding remarks.*** Better energy conversion processes require materials with desired properties tailored to the respective application. The versatile perovskite structure ABX$_{3}$ is a suitable basis for this purpose. Its remarkable stability provides the opportunity to use most elements of the periodic table as well as organic ions on the A-, B- and X-sites.

In view of a target-oriented approach, this plethora of pos-
sible combinations demands theoretical models to predict
the material properties under operating conditions. In order
to minimize the risk of degradation in harsh environments,
appropriate measures such as coatings, (nano-)composites or
modified microstructures have to be taken to ensure a reason-
able service life. Therefore, a better knowledge of the mech-
anisms involved in surface and transport reactions (along
with improved operando characterization and modelling tech-
niques) is the essential precondition to design materials with
better performance and stability for sustainable energy con-
verters of the future.

The flexibility of perovskite-type oxides facilitates to com-
ply apart from conversion efficiency with additional very
important constraints such as abundance, toxicity and environ-
mental benignity. In the end, they have to be cost competitive
with conventional technologies.
# 9. Superconductivity in oxides: from processing to applications

Miryala Muralidhar and Masato Murakami

Superconductivity Research Laboratory, Graduate School of Science and Engineering, Shibaura Institute of Technology, 3-7-8 Toyosu, Koto-ku, Tokyo 135-8548, Japan

**Status.** High temperature copper-oxide superconductors (HTS, cuprates) possess significant potential for various high-field applications due to their high superconducting transition temperature ($T_c$) and upper critical field ($B_{c2}$) values. However, the giant relaxation causes that the above limit of applicability of HTS materials is reduced from $B_{c2}$ to $B_{irr}$, the irreversibility field. Due to rather strong anisotropy of cuprates, their irreversibility field is angularly dependent and reaches the lowest value just for magnetic field parallel to the c-axis, the configuration particularly important for many applications. The relaxation and thus also $B_{irr}$ value are controlled by the operating temperature and by effectiveness of pinning at this temperature. Anyway, cuprates can trap magnetic fields by an order of magnitude higher than the best classical hard magnets and are therefore promising as permanent magnets. In this roadmap, recent development in processing, production, flux pinning and applications of cuprates is presented and discussed.

In the polycrystalline form the high-$T_c$ materials can carry only low critical current densities, due to grain boundaries (weak links) and crystal anisotropy. The numerous attempts to improve the critical current density have resulted in development of a variety of processing techniques among which the most successful one is the top seeded melt-texturing [135, 143]. In this process, the superconducting phase (REBa$_2$Cu$_3$O$_y$, RE = yttrium or a rare earth) is formed by a peritectic reaction of RE$_2$BaCuO$_5$ (2:1) with the liquid phase (BaCuO$_2$ and CuO). The growth process of the superconducting phase starts from a suitable seed crystal and is accelerated by means of fine homogeneously dispersed 2:1:1 phase in the liquid phase; rests of the not reacted 2:1:1 phase serve as pinning precipitates dispersed in the superconducting phase. Especially the materials with light rare earths, LREBa$_2$Cu$_3$O$_y$ (LRE: Nd, Sm, Gd, NEG etc), produced by means of the oxygen controlled melt growth technique show rather high irreversibility field (>5 T, H//c-axis) and critical current density ($J_c$) of the order of $10^6$ A cm$^{-2}$ at 77 K [136]. Moreover, due to tendency of LRE ions to exchange positions with Ba in the crystallographic lattice, a special point-like disorder is formed in the LRE-123 materials that contributes to formation of the peak effect at moderate magnetic fields [137]. The beauty of melt textured LRE-123 materials is that a superconducting pellet can be magnetized to a high external magnetic field, and then, a large part of the field is trapped in the pellet upon removing the external field. The field trapping capability of melt-textured materials depends on the critical current density and the single grain size [138] (figure 14, left). Thus, an improvement of the critical currents, in particular at high magnetic fields and at high temperatures above LN$_2$ are crucial for the successful introduction of these new magnets to the market.

**Current and future challenges.** The electromagnetic performance of LREBa$_2$Cu$_3$O$_y$ has reached the level required for industrial applications and bulk superconducting magnets will be able in a close future to enter the market as basic parts of various utilities. While at moderate magnetic fields nanoscopic point-like defects play an important role, at low magnetic fields rather large secondary phase particles are effective. A successful refinement of the secondary phase particles up to nanometer level (figure 14, middle left) results in a considerable increase of super-current density. The (Nd,Eu,Gd)Ba$_2$Cu$_3$O$_y$ 'NEG-123' samples exhibit not only the highest $J_c$ ever reported for bulk samples but also enable (for the first time) levitation at liquid oxygen temperature (90.2 K) (figure 14, right) [139]. Another milestone in the LRE-123 materials' performance was reached by doping the melt-textured (Nd$_{0.33}$Eu$_{0.33}$Gd$_{0.33}$) Ba$_2$Cu$_3$O$_y$ + 35 mol% Gd$_2$BaCuO$_5$ (70 nm in size) composite by tiny amounts of nanometer-sized MoO$_3$ or Nb$_2$O$_5$ particles. The doping led to a spontaneous formation of clouds of extremely small (≤10 nm) (Nd,Eu,Gd)$_2$BaCuXO$_5$ particles, $X = Mo, Nb$ (figure 15, left). As a result, the self-field super-current at 77 K was, respectively, two- and three-fold higher than before, (figure 15, middle left) [140].

A correlated pinning disorder originating from chemical composition variation in (Nd,Eu,Gd)Ba$_2$Cu$_3$O$_y$ led to an enhancement of flux pinning at high magnetic fields. It manifested itself as a lamellar structure composed of clusters of a normal and NEG/Ba-rich material in the NEG-123 matrix (figure 15, middle right). The high magnification TEM and STM showed that the nano-twins are in fact planes composed of aligned clusters of 3–4 nm in size, aligned with and filling channels between regular twin boundaries. Such a new pinning structure led to a significant improvement of $B_{irr}$ up to 15 T at 77 K and H//c-axis [141, 142], which is the highest irreversibility field reported so far for high $T_c$ superconductors at 77 K (figure 15, right). The most challenging task is to transfer this technology to the second generation of magnetically levitated vehicles (MAGLEVs), where operation cost reduction is highly demanded. For this purpose, it is necessary to push the technology further by fabricating good-quality large-size samples via batch production.

**Advances in science and technology to meet the challenges.** Recent success in batch production of melt-textured LREBa$_2$Cu$_3$O$_y$ superconductors has indicated the way to cost reduction in production of these materials. For a successful application of the bulk cuprate superconducting super-magnets the mechanical strength is also an important issue. The mechanical performance of the ceramic material has been improved by silver addition and by epoxy reign impregnation. As a result, these bulk magnets can survive at low temperatures and even a very high Lorenz force. During crystallographic transformation from non-superconducting tetragonal to the superconducting orthorhombic phase, a number of micro-cracks is formed in the lattice, especially in the (a,b)-plane direction. When the superconductor traps a high magnetic field, a large electromagnetic force acts on the material. As the cracks are the weakest spots of the lattice, there
# 9. Superconductivity in oxides: from processing to applications

Miryala Muralidhar and Masato Murakami

Superconductivity Research Laboratory, Graduate School of Science and Engineering, Shibaura Institute of Technology, 3-7-8 Toyosu, Koto-ku, Tokyo 135-8548, Japan

**Status.** High temperature copper-oxide superconductors (HTS, cuprates) possess significant potential for various high-field applications due to their high superconducting transition temperature ($T_c$) and upper critical field ($B_{c2}$) values. However, the giant relaxation causes that the above limit of applicability of HTS materials is reduced from $B_{c2}$ to $B_{irr}$, the irreversibility field. Due to rather strong anisotropy of cuprates, their irreversibility field is angularly dependent and reaches the lowest value just for magnetic field parallel to the c-axis, the configuration particularly important for many applications. The relaxation and thus also $B_{irr}$ value are controlled by the operating temperature and by effectiveness of pinning at this temperature. Anyway, cuprates can trap magnetic fields by an order of magnitude higher than the best classical hard magnets and are therefore promising as permanent magnets. In this roadmap, recent development in processing, production, flux pinning and applications of cuprates is presented and discussed.

In the polycrystalline form the high-$T_c$ materials can carry only low critical current densities, due to grain boundaries (weak links) and crystal anisotropy. The numerous attempts to improve the critical current density have resulted in development of a variety of processing techniques among which the most successful one is the top seeded melt-texturing [135, 143]. In this process, the superconducting phase (REBa$_2$Cu$_3$O$_y$, RE = yttrium or a rare earth) is formed by a peritectic reaction of RE$_2$BaCuO$_5$ (2:1) with the liquid phase (BaCuO$_2$ and CuO). The growth process of the superconducting phase starts from a suitable seed crystal and is accelerated by means of fine homogeneously dispersed 2:1:1 phase in the liquid phase; rests of the not reacted 2:1:1 phase serve as pinning precipitates dispersed in the superconducting phase. Especially the materials with light rare earths, LREBa$_2$Cu$_3$O$_y$ (LRE: Nd, Sm, Gd, NEG etc), produced by means of the oxygen controlled melt growth technique show rather high irreversibility field (>5 T, H//c-axis) and critical current density ($J_c$) of the order of $10^6$ A cm$^{-2}$ at 77 K [136]. Moreover, due to tendency of LRE ions to exchange positions with Ba in the crystallographic lattice, a special point-like disorder is formed in the LRE-123 materials that contributes to formation of the peak effect at moderate magnetic fields [137]. The beauty of melt textured LRE-123 materials is that a superconducting pellet can be magnetized to a high external magnetic field, and then, a large part of the field is trapped in the pellet upon removing the external field. The field trapping capability of melt-textured materials depends on the critical current density and the single grain size [138] (figure 14, left). Thus, an improvement of the critical currents, in particular at high magnetic fields and at high temperatures above LN$_2$ are crucial for the successful introduction of these new magnets to the market.

**Current and future challenges.** The electromagnetic performance of LREBa$_2$Cu$_3$O$_y$ has reached the level required for industrial applications and bulk superconducting magnets will be able in a close future to enter the market as basic parts of various utilities. While at moderate magnetic fields nanoscopic point-like defects play an important role, at low magnetic fields rather large secondary phase particles are effective. A successful refinement of the secondary phase particles up to nanometer level (figure 14, middle left) results in a considerable increase of super-current density. The (Nd,Eu,Gd)Ba$_2$Cu$_3$O$_y$ 'NEG-123' samples exhibit not only the highest $J_c$ ever reported for bulk samples but also enable (for the first time) levitation at liquid oxygen temperature (90.2 K) (figure 14, right) [139]. Another milestone in the LRE-123 materials' performance was reached by doping the melt-textured (Nd$_{0.33}$Eu$_{0.33}$Gd$_{0.33}$) Ba$_2$Cu$_3$O$_y$ + 35 mol% Gd$_2$BaCuO$_5$ (70 nm in size) composite by tiny amounts of nanometer-sized MoO$_3$ or Nb$_2$O$_5$ particles. The doping led to a spontaneous formation of clouds of extremely small (≤10 nm) (Nd,Eu,Gd)$_2$BaCuXO$_5$ particles, $X = Mo, Nb$ (figure 15, left). As a result, the self-field super-current at 77 K was, respectively, two- and three-fold higher than before, (figure 15, middle left) [140].

A correlated pinning disorder originating from chemical composition variation in (Nd,Eu,Gd)Ba$_2$Cu$_3$O$_y$ led to an enhancement of flux pinning at high magnetic fields. It manifested itself as a lamellar structure composed of clusters of a normal and NEG/Ba-rich material in the NEG-123 matrix (figure 15, middle right). The high magnification TEM and STM showed that the nano-twins are in fact planes composed of aligned clusters of 3–4 nm in size, aligned with and filling channels between regular twin boundaries. Such a new pinning structure led to a significant improvement of $B_{irr}$ up to 15 T at 77 K and H//c-axis [141, 142], which is the highest irreversibility field reported so far for high $T_c$ superconductors at 77 K (figure 15, right). The most challenging task is to transfer this technology to the second generation of magnetically levitated vehicles (MAGLEVs), where operation cost reduction is highly demanded. For this purpose, it is necessary to push the technology further by fabricating good-quality large-size samples via batch production.

**Advances in science and technology to meet the challenges.** Recent success in batch production of melt-textured LREBa$_2$Cu$_3$O$_y$ superconductors has indicated the way to cost reduction in production of these materials. For a successful application of the bulk cuprate superconducting super-magnets the mechanical strength is also an important issue. The mechanical performance of the ceramic material has been improved by silver addition and by epoxy reign impregnation. As a result, these bulk magnets can survive at low temperatures and even a very high Lorenz force. During crystallographic transformation from non-superconducting tetragonal to the superconducting orthorhombic phase, a number of micro-cracks is formed in the lattice, especially in the (a,b)-plane direction. When the superconductor traps a high magnetic field, a large electromagnetic force acts on the material. As the cracks are the weakest spots of the lattice, there
<figure><img src="image_19.png" /><figcaption>**Figure 14.** A photo graph of batch processed as grown single grain Gd-123 samples prepared by the top seeded met growth process (left); nanometer sized Gd-211 secondary phase dispersion in NEG-123 matrix (left middle); permanent Fe-Nd-B magnet stably levitating over an NEG-123 material at 90.2 K (right middle); a magnetized NEG-123 pellet suspended below another NEG-123 permanent magnet (right).</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 15.** TEM image of the NEG-123 sample with 35 mol% Gd-211 and 0.1 mol% Nb₂O₅ (left); the critical current density performance for the same material around 77 K, H//c-axis (left, middle); STM image and critical current density for (Nd₀.₃₃Eu₀.₃₈Gd₀.₂₈)Ba₂Cu₃Oγ with 5 mol% of 211 (right middle and right).</figcaption></figure>

is the highest probability that just at the cracks the material will break. Epoxy resin can penetrate the cracks in the bulk superconductor so as the new class of superconducting supermagnets is able to trap magnetic fields as high as 17 T at 29 K (about 1.5 T at 77 K) without damage [144]. Taking advantage of the new technology, a compact, lightweight, and mobile permanent high-T_c magnet system was designed using the Gd-123 rings.

Another important application is in medicine; it is magnetic drug delivery system (MDDS) constructed and tested on rats and pigs with encouraging results. The drug delivery can be controlled by the superconducting magnetic force in the body and as a result, a high concentration of the drug is delivered to a targeted diseased tissue. In this way the drug toxicity to the normal tissue can be significantly reduced. The drugs delivery to the tissues located deep inside the body requires very strong small-size magnets. Only the superconducting ones can fulfil such a task. The industrial production of oxide superconductors already occurs, together with their first industrial applications. We believe that nano-structured cuprate superconducting super-magnets are able to open

a new path for variety of industrial applications like MRI, NMR, drug-delivery systems in medical applications, water cleaning, space, railway, and naval transport systems, UPS systems etc.

**Concluding remarks.** Nano-structured LRE-123 superconducting super-magnets are suitable for industrial super-magnet applications, capable of making rather dramatic changes in daily life. The technology opens the way for more practical, economical and reliable production, if we can find ways of how to control the matrix chemical ratio and/or the size and distribution of the initially added nano-meter size particles in the final compound. The high critical current density and irreversibility field at around liquid nitrogen temperature indicate that the nano-structured LRE-123 are the right materials for the next generation of superconducting super-magnets of top quality.

**Acknowledgment.** The paper was supported by Grant-in-Aid FD research budget code: 112261, Shibaura Institute of Technology (SIT).
<figure><img src="image_20.png" /><figcaption>**Figure 14.** A photo graph of batch processed as grown single grain Gd-123 samples prepared by the top seeded met growth process (left); nanometer sized Gd-211 secondary phase dispersion in NEG-123 matrix (left middle); permanent Fe-Nd-B magnet stably levitating over an NEG-123 material at 90.2 K (right middle); a magnetized NEG-123 pellet suspended below another NEG-123 permanent magnet (right).</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 15.** TEM image of the NEG-123 sample with 35 mol% Gd-211 and 0.1 mol% Nb₂O₅ (left); the critical current density performance for the same material around 77 K, H//c-axis (left, middle); STM image and critical current density for (Nd₀.₃₃Eu₀.₃₈Gd₀.₂₈)Ba₂Cu₃Oγ with 5 mol% of 211 (right middle and right).</figcaption></figure>

is the highest probability that just at the cracks the material will break. Epoxy resin can penetrate the cracks in the bulk superconductor so as the new class of superconducting supermagnets is able to trap magnetic fields as high as 17 T at 29 K (about 1.5 T at 77 K) without damage [144]. Taking advantage of the new technology, a compact, lightweight, and mobile permanent high-T_c magnet system was designed using the Gd-123 rings.

Another important application is in medicine; it is magnetic drug delivery system (MDDS) constructed and tested on rats and pigs with encouraging results. The drug delivery can be controlled by the superconducting magnetic force in the body and as a result, a high concentration of the drug is delivered to a targeted diseased tissue. In this way the drug toxicity to the normal tissue can be significantly reduced. The drugs delivery to the tissues located deep inside the body requires very strong small-size magnets. Only the superconducting ones can fulfil such a task. The industrial production of oxide superconductors already occurs, together with their first industrial applications. We believe that nano-structured cuprate superconducting super-magnets are able to open

a new path for variety of industrial applications like MRI, NMR, drug-delivery systems in medical applications, water cleaning, space, railway, and naval transport systems, UPS systems etc.

**Concluding remarks.** Nano-structured LRE-123 superconducting super-magnets are suitable for industrial super-magnet applications, capable of making rather dramatic changes in daily life. The technology opens the way for more practical, economical and reliable production, if we can find ways of how to control the matrix chemical ratio and/or the size and distribution of the initially added nano-meter size particles in the final compound. The high critical current density and irreversibility field at around liquid nitrogen temperature indicate that the nano-structured LRE-123 are the right materials for the next generation of superconducting super-magnets of top quality.

**Acknowledgment.** The paper was supported by Grant-in-Aid FD research budget code: 112261, Shibaura Institute of Technology (SIT).