<figure><img src="image_42.png" /><figcaption>**Figure 37.** (a) Map showing the EU and associated countries currently participating to TO-BE COST action. (b) Schematic representation of the scientific activities.</figcaption></figure>

of oxygen vacancies in thin films and their role in the filamen-
tary process is an important issue which need to addressed
and understood before these materials can be widely used in a
device [262].

**Ferroelectric tunnel junctions** (FTJs), being the only family of ferroelectric memories to which we restrict here our interest, are rapidly progressing within the larger class of emerging ferroelectric (FE) memories. In FTJs a FE tunnelling layer is typically grown on a metallic oxide back-electrode and covered by a (different) top electrode. Large tunnelling electroresistance (TER) ratios between ON and OFF states, depending on the polarization direction of the FE barrier have been obtained [263]. Typical FE materials employed so far include BaTiO₃, Pb(ZrₓTi₁₋ₓ)O₃ (PZT), and BiFeO₃. Once the use of ferroelectric doped HfO₂ [264] will be demonstrated for application in FTJs, it would offer notable potential advantages. Being based on epitaxial heterostructures, FTJs are at the core of the TO-BE action. The tunneling conductance across the device is determined by the FE polarization direction and by the asymmetry in the screening length of the electrodes. This simple picture has guided the tremendous progress achieved in the last few years. TER values exceeding 10⁴% (at room temperature) have been achieved in micron-size patterned devices and even larger values may be expected by performing modifications to the basic metal/FE/metal structure design. Such strategies include the introduction of slave layers behaving as Mott insulators [265] and the use of appropriate semiconducting electrodes [266] also allowing to modulate the effective barrier thickness [267]. From a more general perspective, additional control parameters of the memory state as light or strain can be exploited. In spite of its' potential, FTJ technology is as yet immature. Scalability below 50 nm, endurance, retention and sample-to-sample variability in patterned devices all need

further investigations. Integration with present semiconduc-
tor technology is made possible by recent progresses on the
growth of epitaxial complex functional oxides on Si [268].
While FTJ fabrication on Si platforms [269, 270] has been
demonstrated, processing is not yet thoroughly CMOS-
compatible and the full potential of integrating FTJ-on-Si has
not been yet exploited [271].

**Concluding remarks.** Beside extending the impressive trend of miniaturization, speed increase and unit cost reduction seen in last decades, oxide-based devices can be the key players within a revolutionary 'more-than-Moore' approach to non-volatile memories. A major example is given by memristors<fnref n="29" /> [272], presently considered as the key components of a future neuromorphic networks technology [273]. The first claim that a device based on resistive switching, based on a ReRAM-like device architecture, could be seen as the 'missing' memristor was reported in 2008 [274]. More recently other kinds of NVMs, including FTJs [275], have been proposed as an alternative to ReRAM for the fabrication of memristive devices. Non-volatile memories appear therefore to be one of the fields where oxide-based technologies may have a large impact, in next decade and beyond. This is an important challenge for oxide electronics that the TO-BE action aims to support in terms of research, networking and dissemination.

***Acknowledgements.*** This work has been partially supported by the TO-BE COST action MP1308. J F acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0496) and MAT2014-56063-C2-1R, and from the Catalan Government (2014 SGR 734). F.M.G. acknowledges support from MIUR through the PRIN 2010 Project ‘OXIDE’.

<footnote n="29">²⁹ Memristors are non-linear passive two-terminal electrical components first postulated on the base of their circuital properties principles, and regardless to their implementation, in 1971 by Leon Chua [272].</footnote>
<figure><img src="image_43.png" /><figcaption>**Figure 37.** (a) Map showing the EU and associated countries currently participating to TO-BE COST action. (b) Schematic representation of the scientific activities.</figcaption></figure>

of oxygen vacancies in thin films and their role in the filamen-
tary process is an important issue which need to addressed
and understood before these materials can be widely used in a
device [262].

**Ferroelectric tunnel junctions** (FTJs), being the only family of ferroelectric memories to which we restrict here our interest, are rapidly progressing within the larger class of emerging ferroelectric (FE) memories. In FTJs a FE tunnelling layer is typically grown on a metallic oxide back-electrode and covered by a (different) top electrode. Large tunnelling electroresistance (TER) ratios between ON and OFF states, depending on the polarization direction of the FE barrier have been obtained [263]. Typical FE materials employed so far include BaTiO₃, Pb(ZrₓTi₁₋ₓ)O₃ (PZT), and BiFeO₃. Once the use of ferroelectric doped HfO₂ [264] will be demonstrated for application in FTJs, it would offer notable potential advantages. Being based on epitaxial heterostructures, FTJs are at the core of the TO-BE action. The tunneling conductance across the device is determined by the FE polarization direction and by the asymmetry in the screening length of the electrodes. This simple picture has guided the tremendous progress achieved in the last few years. TER values exceeding 10⁴% (at room temperature) have been achieved in micron-size patterned devices and even larger values may be expected by performing modifications to the basic metal/FE/metal structure design. Such strategies include the introduction of slave layers behaving as Mott insulators [265] and the use of appropriate semiconducting electrodes [266] also allowing to modulate the effective barrier thickness [267]. From a more general perspective, additional control parameters of the memory state as light or strain can be exploited. In spite of its’ potential, FTJ technology is as yet immature. Scalability below 50 nm, endurance, retention and sample-to-sample variability in patterned devices all need

further investigations. Integration with present semiconduc-
tor technology is made possible by recent progresses on the
growth of epitaxial complex functional oxides on Si [268].
While FTJ fabrication on Si platforms [269, 270] has been
demonstrated, processing is not yet thoroughly CMOS-
compatible and the full potential of integrating FTJ-on-Si has
not been yet exploited [271].

**Concluding remarks.** Beside extending the impressive trend of miniaturization, speed increase and unit cost reduction seen in last decades, oxide-based devices can be the key players within a revolutionary ‘more-than-Moore’ approach to non-volatile memories. A major example is given by memristors<fnref n="29" /> [272], presently considered as the key components of a future neuromorphic networks technology [273]. The first claim that a device based on resistive switching, based on a ReRAM-like device architecture, could be seen as the ‘missing’ memristor was reported in 2008 [274]. More recently other kinds of NVMS, including FTJs [275], have been proposed as an alternative to ReRAM for the fabrication of memristive devices. Non-volatile memories appear therefore to be one of the fields where oxide-based technologies may have a large impact, in next decade and beyond. This is an important challenge for oxide electronics that the TO-BE action aims to support in terms of research, networking and dissemination.

**Acknowledgements.** This work has been partially supported by the TO-BE COST action MP1308. J F acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0496) and MAT2014-56063-C2-1R, and from the Catalan Government (2014 SGR 734). F.M.G. acknowledges support from MIUR through the PRIN 2010 Project ‘OXIDE’.

<footnote n="29">²⁹ Memristors are non-linear passive two-terminal electrical components first postulated on the base of their circuital properties principles, and regardless to their implementation, in 1971 by Leon Chua [272].</footnote>
<table><caption>**Table 1.** Synoptic table summarizing and evaluating different novel memory technologies, as reported in the ITRS-2013 ERD.</caption><thead><tr><th rowspan="2">Parameters</th><th colspan="3">Prototypical</th><th colspan="6">Emerging</th></tr><tr><th>P1<br>FeRAM</th><th>P2<br>STT-MRAM</th><th>P3<br>PCRAM</th><th>E1.1<br>Conducting<br>bridge</th><th>E1.2<br>Bipolar<br>filamentary</th><th>E1.3<br>Unipolar<br>filamentary</th><th>E1.4<br>Bipolar<br>interface effect</th><th>E2<br>Emerging FE<br>memories</th><th>E3-E4-E5<br>Mott, carbon,<br>molecular</th></tr></thead><tbody><tr><td>Scalability</td><td>+</td><td>++</td><td>+++</td><td>+++</td><td>+++</td><td>+++</td><td>+++</td><td>++</td><td>?</td></tr><tr><td>MLC</td><td>+</td><td>+</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>+</td><td>?</td></tr><tr><td>3D integration</td><td>+</td><td>++</td><td>+++</td><td>++</td><td>+++</td><td>+++</td><td>+++</td><td>+</td><td>?</td></tr><tr><td>Fabrication cost</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>?</td></tr><tr><td>Retention</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>++</td><td>+</td><td>++</td><td>?</td></tr><tr><td>Latency</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>?</td></tr><tr><td>Power</td><td>+++</td><td>++</td><td>+</td><td>+++</td><td>++</td><td>+</td><td>++</td><td>+++</td><td>?</td></tr><tr><td>Endurance</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>+</td><td>++</td><td>++</td><td>?</td></tr><tr><td>Variability</td><td>+++</td><td>+</td><td>++</td><td>++</td><td>+</td><td>+</td><td>+++</td><td>++</td><td>?</td></tr></tbody></table>

*Note:* Freely rearranged from [261]. The number of plus marks is intended to quantify how successfully of the single device types have been meeting specific parameters.

## References

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[2] Sarkar T, Gopinadhan K, Zhou J, Saha S, Coey J M D, Feng Y P, Ariando and Venkatesan T *ACS Appl. Mater. Interfaces* **7** 24616

[3] Chen Y Z *et al* *Nat. Commun.* **4** 1371

[4] Li C *et al* 2015 *Nano Lett.* **15** 2568

[5] Lin P T, Wessels B W, Imre A and Ocola L E 2008 *Integrated Photonics and Nanophotonics Research and Applications* (Optical Society of America) paper *IWF4*

[6] Elfimov I S, Yunoki S and Sawatzky G A 2002 *Phys. Rev. Lett.* **89** 216403

[7] Rusydi A *et al* 2012 *Phil. Trans. R. Soc. A* **370** 4927

[8] Lü W M *et al* 2016 *Nat. Commun.* **7** 11015

[9] Xu X, Randorn C, Efstathiou P and Irvine J T S 2012 *Nat. Mater.* **11** 595

[10] Asmara T C *et al* Unpublished

[11] Krishnan-Kutty V *et al* Unpublished

[12] Barquinha P, Pereira L, Martins R and Fortunato E 2012 *Transparent Oxide Electronics: from Materials to Devices* (New York: Wiley)

[13] Wager J F 2016 *Inf. Disp.* **32** 16–21

[14] Displaybank, Nanomarkets, Touch Display Research and IDTechEx

[15] Kim S J *et al* 2014 *Japan. J. Appl. Phys.* **53**

[16] Fortunato E, Barquinha P and Martins R 2012 *Adv. Mater.* **24** 2945–86

[17] Liu A *et al* 2014 *ACS Appl. Mater. Interfaces* **6** 17364–9

[18] Liu G X *et al* 2015 *Adv. Funct. Mater.* **25** 2564–72

[19] Wager J F, Yeh B, Hoffman R L and Keszler D A 2014 *Curr. Opin. Solid State Mater.* **18** 53–61

[20] Kim M-G, Kanatzidis M G, Facchetti A and Marks T J 2011 *Nat. Mater.* **10** 382–8

[21] Avis C and Jang J J. *Mater. Chem.* **21** 10649–52

[22] Ong B S, Li C, Li Y, Wu Y and Loutfy R 2007 *J. Am. Chem. Soc.* **129** 2750–1

[23] Meyers S T, Anderson J T, Hung C M, Thompson J, Wager J F and Keszler D A 2008 *J. Am. Chem. Soc.* **130** 17603–9

[24] Yang C, Hong K, Jang J, Chung D S, An T K, Choi W-S and Park C E 2009 *Nanotechnology* **20** 465201

[25] Lee C Y, Lin M Y, Wu W H, Wang J Y, Chou Y, Su W F, Chen Y F and Lin C F 2010 *Semicond. Sci. Technol.* **25** 105008

[26] Faber H, Hirschmann J, Klaumünzer M, Braunschweig B, Peukert W and Halik M 2012 *ACS Appl. Mater. Interfaces* **4** 1693–6

[27] Chang Y-J, Lee D-H, Herman G S and Chang C-H 2007 *Electrochem. Solid-State Lett.* **10** H135

[28] Seo S-J, Choi C G, Hwang Y H and Bae B-S 2008 *SID Symp. Dig. Tech. Pap.* **39** 1254–7

[29] Park S K, Kim Y-H, Kim H-S and Han J-I 2009 *Electrochem. Solid-State Lett.* **12** H256

[30] Seo S-J, Hwang Y H and Bae B-S 2010 *Electrochem. Solid-State Lett.* **13** H357

[31] Lim J H, Shim J H, Choi J H, Joo J, Park K, Jeon H, Moon M R, Jung D, Kim H and Lee H J 2009 *Appl. Phys. Lett.* **95** 2009–11

[32] Yang Y-H, Yang S S and Chou K-S 2010 *IEEE Electron Device Lett.* **31** 969–71

[33] Lee D-H, Chang Y-J, Herman G S and Chang C-H 2007 *Adv. Mater.* **19** 843–7

[34] Choi C G, Seo S-J and Bae B-S *Electrochem. Solid-State Lett.* **11** H7

[35] Park K-B, Seon J-B, Kim G H, Yang M, Koo B, Kim H J, Ryu M-K and Lee S-Y 2010 *IEEE Electron Device Lett.* **31** 311–3

[36] Banger K K, Yamashita Y, Mori K, Peterson R L, Leedham T, Rickard J and Sirringhaus H *Nat. Mater.* **10** 45–50

[37] Kim Y-H, Heo J-S, Kim T-H, Park S, Yoon M-H, Kim J, Oh M S, Yi G-R, Noh Y-Y and Park S K 2012 *Nature* **489** 128–32

[38] Kim H S, Byrne P D, Facchetti A and Marks T J J. *Am. Chem. Soc.* **130** 12580–1

[39] Park S *et al* 2015 *Adv. Funct. Mater.* **25** 2807–15

[40] Lee D-H, Chang Y-J, Stickle W and Chang C-H 2007 *Electrochem. Solid-State Lett.* **10** K51

[41] Hwang Y H, Jeon J H, Seo S-J and Bae B-S 2009 *Electrochem. Solid-State Lett.* **12** H336–9

[42] Hwang Y H, Jeon J-H and Bae B-S *Electrochem. Solid-State Lett.* **14** H303–5

[43] Hennek J W, Kim M G, Kanatzidis M G, Facchetti A and Marks T J 2012 *J. Am. Chem. Soc.* **134** 9593–6

[44] Seo J-S, Jeon J-H, Hwang Y H, Park H, Ryu M, Park S-H K and Bae B-S 2013 *Sci. Rep.* **3** 1–9
<table><caption>**Table 1.** Synoptic table summarizing and evaluating different novel memory technologies, as reported in the ITRS-2013 ERD.</caption><thead><tr><th rowspan="2">Parameters</th><th colspan="3">Prototypical</th><th colspan="6">Emerging</th></tr><tr><th>P1<br>FeRAM</th><th>P2<br>STT-MRAM</th><th>P3<br>PCRAM</th><th>E1.1<br>Conducting<br>bridge</th><th>E1.2<br>Bipolar<br>filamentary</th><th>E1.3<br>Unipolar<br>filamentary</th><th>E1.4<br>Bipolar<br>interface effect</th><th>E2<br>Emerging FE<br>memories</th><th>E3-E4-E5<br>Mott, carbon,<br>molecular</th></tr></thead><tbody><tr><td>Scalability</td><td>+</td><td>++</td><td>+++</td><td>+++</td><td>+++</td><td>+++</td><td>+++</td><td>++</td><td>?</td></tr><tr><td>MLC</td><td>+</td><td>+</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>+</td><td>?</td></tr><tr><td>3D integration</td><td>+</td><td>++</td><td>+++</td><td>++</td><td>+++</td><td>+++</td><td>+++</td><td>+</td><td>?</td></tr><tr><td>Fabrication cost</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>?</td></tr><tr><td>Retention</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>++</td><td>+</td><td>++</td><td>?</td></tr><tr><td>Latency</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>++</td><td>++</td><td>+++</td><td>?</td></tr><tr><td>Power</td><td>+++</td><td>++</td><td>+</td><td>+++</td><td>++</td><td>+</td><td>++</td><td>+++</td><td>?</td></tr><tr><td>Endurance</td><td>+++</td><td>+++</td><td>++</td><td>++</td><td>++</td><td>+</td><td>++</td><td>++</td><td>?</td></tr><tr><td>Variability</td><td>+++</td><td>+</td><td>++</td><td>++</td><td>+</td><td>+</td><td>+++</td><td>++</td><td>?</td></tr></tbody></table>

*Note:* Freely rearranged from [261]. The number of plus marks is intended to quantify how successfully of the single device types have been meeting specific parameters.

## References

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[2] Sarkar T, Gopinadhan K, Zhou J, Saha S, Coey J M D, Feng Y P, Ariando and Venkatesan T *ACS Appl. Mater. Interfaces* **7** 24616

[3] Chen Y Z *et al* *Nat. Commun.* **4** 1371

[4] Li C *et al* 2015 *Nano Lett.* **15** 2568

[5] Lin P T, Wessels B W, Imre A and Ocola L E 2008 *Integrated Photonics and Nanophotonics Research and Applications* (Optical Society of America) paper *IWF4*

[6] Elfimov I S, Yunoki S and Sawatzky G A 2002 *Phys. Rev. Lett.* **89** 216403

[7] Rusydi A *et al* 2012 *Phil. Trans. R. Soc. A* **370** 4927

[8] Lü W M *et al* 2016 *Nat. Commun.* **7** 11015

[9] Xu X, Randorn C, Efstathiou P and Irvine J T S 2012 *Nat. Mater.* **11** 595

[10] Asmara T C *et al* Unpublished

[11] Krishnan-Kutty V *et al* Unpublished

[12] Barquinha P, Pereira L, Martins R and Fortunato E 2012 *Transparent Oxide Electronics: from Materials to Devices* (New York: Wiley)

[13] Wager J F 2016 *Inf. Disp.* **32** 16–21

[14] Displaybank, Nanomarkets, Touch Display Research and IDTechEx

[15] Kim S J *et al* 2014 *Japan. J. Appl. Phys.* **53**

[16] Fortunato E, Barquinha P and Martins R 2012 *Adv. Mater.* **24** 2945–86

[17] Liu A *et al* 2014 *ACS Appl. Mater. Interfaces* **6** 17364–9

[18] Liu G X *et al* 2015 *Adv. Funct. Mater.* **25** 2564–72

[19] Wager J F, Yeh B, Hoffman R L and Keszler D A 2014 *Curr. Opin. Solid State Mater.* **18** 53–61

[20] Kim M-G, Kanatzidis M G, Facchetti A and Marks T J 2011 *Nat. Mater.* **10** 382–8

[21] Avis C and Jang J J. *Mater. Chem.* **21** 10649–52

[22] Ong B S, Li C, Li Y, Wu Y and Loutfy R 2007 *J. Am. Chem. Soc.* **129** 2750–1

[23] Meyers S T, Anderson J T, Hung C M, Thompson J, Wager J F and Keszler D A 2008 *J. Am. Chem. Soc.* **130** 17603–9

[24] Yang C, Hong K, Jang J, Chung D S, An T K, Choi W-S and Park C E 2009 *Nanotechnology* **20** 465201

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[27] Chang Y-J, Lee D-H, Herman G S and Chang C-H 2007 *Electrochem. Solid-State Lett.* **10** H135

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