bluuebunny/crossref_metadata_2025 huggingface dataset:
In-Plane ZT Measurements of PbTe/PbSe Superlattice Layers Deposited by Atomic Layer Deposition
Authors: Chen Xin , Linseis Vincent , Lin Pengtao , Zhang Kai , Baumgart Helmut , Reith Heiko
Published: February 2020

The conversion efficiency of the thermoelectric (TE) device is determined by the dimensionless figure of merit ZT, which is expressed as ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. The key strategy to enhance ZT involves the reduction in thermal conductivity k
of TE materials. This led to the synthesis of low-dimensional structured materials such as 2-D superlattice structure, 1-D nanowire, and 0-D quantum dots. The phonon transport was suppressed if the dimension of the film approaching the mean free path of the phonons, resulting in a reduction in thermal conductivity and consequently improves ZT. The direction of the measurement, either in-plane or vertical to the surface, plays an important role in determining TE properties of the anisotropic samples, such as superlattices and quantum well structures. It is essential to measure all the parameters in the same direction and simultaneously to obtain accurate ZT value.

In this work, a novel Si-based test chip was utilized to perform in-plane ZT measurements of PbTe/PbSe superlattice films. Figure 1 (a) shows the pre-fabricated circuit on the chip for measuring simultaneously the Seebeck coefficient, electrical conductivity, and thermal conductivity [1]. The thermoelectric film was deposited in the red square area by using a shadow mask. All the parameters were measured in the temperature range of 120 K to 450 K with a 1 T magnet field. In our work, PbTe and PbSe thin films and superlattice layers were synthesized by Atomic Layer Deposition (ALD) technology directly on the pre-structured Si chip. Figure 1 (b) shows an SEM micrograph of the 30 nm thick PbSe film grown on the chip. The PbTe/PbSe superlattice was then deposited on the PbSe buffer layer using ALD by alternatively depositing PbTe and PbSe layers. A thin Al2O3protection and diffusion barrier layer was applied to avoid oxidation and degradation of PbTe/PbSe films. The period of superlattice structure was varied from 2 nm to 10 nm to observe optimized period for achieving highest ZT value.

Reference:

[1]. Linseis, V., Völklein, F., Reith, H., Woias, P., & Nielsch, K. Platform for in-plane ZT measurement and Hall coefficient determination of thin films in a temperature range from 120 K up to 450 K. Journal of Materials Research,
31(20), 3196-3204, (2016).

Figure 1
Pulsed Laser Deposition of BiCl3 Doped Bi2S3/Cu Nanoparticle Composite Films for Thermoelectric Applications
Authors: Chen Tsung-Han , Chou Wei-Yen , Chen Chun-Hua
Published: February 2020

Thermoelectric (TE) materials capable of directly converting heat into electricity have become a highly potential category in energy materials and are expected to tackle the energy issue of the future. Among a variety of TE materials, lead telluride (PbTe) has been recognized as an outstanding mid-temperature TE material with an applicable TE figure of merit (ZT) of ~1.2. However, due to the toxicity of both Pb and Te, development of alternative mid-temperature TE materials is urgently required. Bismuth sulﬁde (Bi2S3) is specially selected in the present study as the most promising candidate for alternating PbTe mainly because of its potentially high Seebeck coefficient (102−103 μVK-1). Unfortunately, the generally poor electrical conductivity (<102 S/cm) and the resulting low ZT (~0.2) of Bi2S3 severely restrict its applicability. In this work, by controlling the substrate temperature, the volatility of sulfur would lead the compositional deviation from the chemical stoichiometry of Bi2S3 and the electrical conductivity of the sulfur-vacancy introduced Bi2S3-x would thus be improved. Similarly, BiCl3 was also considered and applied as a donor dopant to improve the electrical conductivity. Furthermore, a series of novel BiCl3 doped Bi2S3-Cu nanoparticles layer-by-layer composite films were designed and fabricated through the newly built dual-beam pulsed laser deposition for improving the electric conductivity without seriously sacrificing the Seebeck coefficient.

The prepared highly crystalline intrinsic Bi2S3 films on insulated SiO2/Si substrates exhibit a very high electric conductivity of 638 Scm-1 evidently due to the simultaneously enhanced carrier concentration and mobility, and the Seebeck coefficient of - 453 μVK-1. The resulting power factor is 137 μWcm-1K-1 which is four times higher than the reported highest value (29 μWcm-1K-1) from the chemically prepared Bi2S3 so far. The introduction of BiCl3 into Bi2S3 indeed leads to an enhancement of the electrical conductivity from 170 Scm-1 (before BiCl3 doping) to 450 Scm-1 whereas the Seebeck coefficient decreases from -522 μVK-1 to -221 μVK-1. On the sake of enhancing the electricity conductivity and Seebeck coefficient simultaneously, the Cu nanoparticles were introduced into the BiCl3 doped Bi2S3 film. The heterostructure of Cl doped Bi2S3/Cu played an important role in obtaining the high electrical conductivity of 715 Scm-1, Seebeck coefficient of -413 μVK-1 and the power factor of 122 μWcm-1K-1 which is twice higher than those of the intrinsic Bi2S3 film (49 μWcm-1K-1).
(Invited) Fabrication and Characterization of Copper-Silica-Metal Resistive Switching Devices
Authors: Chen Wenhao , Patadia Arshey , Kozicki Michael N
Published: February 2020

Switching in metal-oxide-metal devices has been studied since the late 1960s. A number of mechanisms have been identified to explain the change in resistance in such structures, including the formation of metallic filaments which bridge the insulating layer and provide a low resistance pathway between the electrodes. The low power “cationic” switching of copper-silica-metal devices, where the metal is a relatively inert conductor (e.g., tungsten), has gained attention in recent times as these materials are already present in the back-end-of-line (BEOL) of integrated circuits, making device integration simpler and less expensive than in the case of more “exotic” resistive switching systems. This technology is a variant of programmable metallization cell (PMC) or conductive-bridging random access memory (CBRAM). The switching mechanism involves the transport of copper ions in the oxide and their subsequent reduction to form the conducting metallic pathway. The oxidizable electrode provides ions into the reaction environment to maintain the supply of copper for the growing filament. The switching process is bipolar, in that a bias opposite to that used for writing is required to reverse the filament formation process to return the device to its high resistance state. The copper-silica-metal structures exhibit similar switching characteristics to previously-commercialized devices based on higher chalcogenide glasses, albeit at somewhat lower switching speed for the same programming voltage, while possessing inherently greater CMOS compatibility. Devices have been fabricated using either physical vapor deposition (PVD) or chemical vapor deposition (CVD) of the silica under conditions that allow the film to have a slightly lower density than a typical deposited oxide to promote ion transport and facilitate filament formation. A forming step is required to form a switchable region in the oxide and following this, sub-µs switching at low voltage and current is readily attainable.

In this talk we will report on the operational characteristics of these copper-silica-metal devices and the effect of different processing approaches on individual structures as well as crossbar arrays. We will also show the results of impedance spectroscopy analysis as they pertain to the physical structure of the devices. The applications of this technology range from active and passive non-volatile memories to neuromorphic elements and physical unclonable functions (PUFs) for physical and cyber security and these uses will be highlighted in the talk.
In Operando Characterization of Pt-TaOx-Ta Bipolar Vacancy Change Memories
Authors: Jacobs-Gedrim Robin Bay , Kotula Paul , Mook William Moyer , Bondi Robert James , Lowery Lisa Marie , Goeke Ronald S. , Smith Carl L. , Hughart David R. , James Conrad D. , Marinella Matthew J.
Published: February 2020

Clear and unambiguous experimental observation of the physical processes leading to resistance change during formation and switching of bipolar valence change memories (VCM) remains one of the main remaining challenges in the characterization and understanding of resistive memory devices[1]. Presented here is a Scanning Transmission Electron Microscopy (STEM) Electron Energy Loss Spectroscopy (EELS) study which reveals a valence change filamentary switching mechanism during low-current bipolar resistive switching operation in Pt-TaOx-Ta resistive memory devices. No change in the film stack is observed in High Angle Annular Dark Field (HAADF) STEM during SET or RESET operation, which is consistent with expectations for a low-current operation VCM. A ~50 nm wide filamentary region is observed in EELS maps analyzed with multivariate statistical analysis disappearing on RESET operation and reappearing during SET operation. In this filamentary region, the Ta O2,3 edge is consistent with oxidized Ta but there is a sharp metallic plasmon feature which is blue shifted in accordance with our previous ab initiopredictions for metallic oxides.

[1] Yang Y and Lu W D 2016 Progress in the Characterizations and Understanding of Conducting Filaments in Resistive Switching Devices IEEE Transactions on Nanotechnology
15 465-72
(Invited) Resistance Switching in Silicon-Rich Silica: Electronic, Structural and Photonic Perspectives
Authors: Kenyon Anthony Joseph , Mehonic Adnan , Munde Manveer , Ng Wing Hung , Buckwell Mark , Montesi Luca , Zarudnyi Konstantin , Bosman Michel , Gerard Thomas , Shluger Alex L
Published: February 2020

We present results from a study of resistance switching in silicon-rich silica – ie silicon dioxide containing an excess of silicon. We demonstrate that changes in resistance are the result of field-driven movement of oxygen, and that this can result in large-scale changes in oxide structure and stoichiometry. While resistance switching in oxides has been studied for a number of years as a route to non-volatile electronic memories, the optical response of such systems has remained largely unknown. Here we demonstrate that simple two-terminal MIM devices can be switched between different resistance states using a combination of electrical bias and optical stimulation. This opens up intriguing possibilities for novel classes of sensors.

We also demonstrate neuromorphic behaviour in our devices. Our electrical measurements show that, under the right conditions, the devices can be made to emulate some of the important behaviours of biological neurons – plasticity, for example. Taken together with the light sensitivity of these resistance switches, this opens up further possibilities for the development of light-triggered neural networks for such applications as pattern recognition and image classification.
(Electronics and Photonics Division Award Address) Compound Semiconductors: A Retrospective
Authors: Buckley D. Noel
Published: February 2020

The very first State-of-the Art Program on Compound Semiconductors (SOTAPOCS I) was called to order at the 166th Meeting of The Electrochemical Society in New Orleans in the Fall of 1984. The first lightwave transmission system was then less than 4 years old and the first commercial cell phone was less than 3 years old. In fact, CW operation of a semiconductor laser at room temperature had been achieved less than 15 years before and even the transistor was still relatively young at 37 years. Our world has changed dramatically in the intervening years and compound semiconductors have played a vital role in the technologies that brought about that change. This talk will look back on some of the highlights and milestones in the development of compound semiconductors, recall some of the events along the way, and reflect on the key role played by The Electrochemical Society.
(Keynote) Navy Application of Silicon Carbide (SiC) Wide Bandgap (WBG) Semiconductors Enabling Future Power and Energy Systems
Authors: Petersen Lynn James
Published: February 2020

Silicon carbide power devices switching greater than 100kHz enable new benchmarks in power converter performance. These converters will enable power systems where all the sources and loads are connected by converters. These new systems are multifunctional and highly-integrated. However, their realization requires research in areas such as: advanced power electronic control across many converters, concepts for distributed storage, and active filtering across many converters.
Comparison of Chirped Superlattices and Linearly-Graded Buffer Layers As Adjustable-Strain Platforms for Metamorphic InAaAs/GaAs (001) Devices
Authors: Chen Xinkang , Islam Md Tanvirul , Kujofsa Tedi , Ayers John E
Published: February 2020

Chirped superlattices are of interest as buffer layers in metamorphic semiconductor device structures, because they can combine the mismatch accommodating properties of compositionally-graded layers with the dislocation filtering properties of superlattices. Important practical aspects of the chirped superlattice as a buffer layer are the surface strain and surface in-plane lattice constant. In this work we study two basic types of InGaAs/GaAs chirped superlattice buffers; type I is compositionally modulated while type II is thickness modulated. For both types, we have studied the equilibrium surface strain and surface in-plane lattice constant, and we have compared the surface strain behavior to linearly-graded buffers having the same top target composition of indium.
(Invited) Optoelectronic Components Based on Semiconducting Transition Metal Dichalcogenides
Authors: Balicas Luis
Published: February 2020

Transition metal dichalcogenides (TMDs) are layered compounds which display a wide range of electronic ground states, ranging from superconductivity, to semi-metallicity to wide gap semiconducting behavior. Here, we will discuss our previous studies on exfoliated semiconducting compounds, which indicate, for example, that field-effect transistors (FETs) based on few atomic layers of these materials (i.e. MoSe2 or WSe2), and their heterostructures, display a great potential for photo-sensing [1,2,3] and photovoltaic applications [4], since these compounds are prone to a particularly strong interaction with light. Other compounds like ReS2 display a gate-voltage controlled metal to insulator transition at room temperature, with the metallic state surviving down to quite low temperatures [5]. Ambipolar FETs based on ReSe2 show promise as voltage inverters [6]. Meanwhile, we find through a detailed structural analysis reveals that it is possible to produce solid solutions of, for example, Mo1-xWxTe2 which leads to a sharp phase-boundary between the semiconducting 2H- and the semi-metallic T
d- phases [7]. Based on a series of recent theoretical predictions, we argue that it might be possible to tune this phase-boundary through a gate voltage perhaps leading to phase change memory elements. Furthermore, given that 2H-MoTe2 display a band gap comparable to Si (≈ 1 eV) and that its semi-metallic counterpart is claimed to display topological properties, we argue that it might possible to produce optoelectronic components characterized by low dissipation if one is capable to harvest the topological nature of its semi-metallic phase.

[1] N. Pradhan et al., ACS Appl. Mater. & Interfaces 7, 22, 12080 (2015).

[2] N. Pradhan et al., 2D Mater. 3, 041004 (2016).

[3] N.R. Pradhan et al., Adv. Electron. Mater. 1, 10, 1500215 (2015)

[4] S. Memaran  et al., Nano Lett. 15, 7532 (2015)

[5] N. Pradhan et al., Nano Lett. 15, 8377 (2015)

[6] N. R. Pradhan (unpublished)

[7] D. Rhodes et al., Nano Lett. 17, 1616 (2017)
(Invited) Novel Electronic Transport in Topological Semimetals
Authors: Yu Wenlong , Jiang Yuxuan , Yang Jeremy , Dun Zhiling , Zhou Haidong , Jiang Zhigang , Lu Ping , Pan Wei
Published: February 2020

Since the discovery of graphene, Dirac systems have attracted considerable attention due to their extraordinary properties. Recently, Dirac semimetals (DSs)-a new quantum state of matter-have been proposed. DSs can be viewed as an analogue of graphene but with linear energy dispersion in all three momentum directions. Due to the distinct electronic properties, DSs have been a fertile playground in discovering novel phenomena. In addition, it is possible to realize topological superconductivity in DSs, which plays a key role in next generation quantum computation. Shortly after theoretical predictions, several materials have been experimentally confirmed to be DSs, such as Cd3As2 and ZrTe5. Here, we report magneto-transport studies on Cd3As2 and ZrTe5. Shubnikov–de Haas oscillations were observed in both materials. A non-trivial Berry’s phase of π was obtained indicating the existence of Dirac fermions. However, angular dependence measurement shows different results in these two materials. Dirac fermions in ZrTe5 show a two-dimensional nature while in Cd3As2 they behave like three-dimensional particles. In addition, we observed supercurrent in superconductor (Al)–DS (Cd3As2)–superconductor (Al) junctions. Realizing topological superconductivity is an essential step towards finding Majorana fermions which are believed to be building blocks for quantum computers.

Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
(Invited) Scalable Planar Fabrication Processes for Chalcogenide-Based Topological Insulators
Authors: Sharma Peter A. , Henry Michael David , Douglas Erica A. , Lima Sharma Ana , Lewis Rupert , Sugar Joshua D , Koirala Nikesh , Salehi Maryam , Oh Seonshik
Published: February 2020

The long spin diffusion lengths predicted for surface transport in topological insulators could lead to numerous applications. Many of the physics experiments used to show promising transport properties were conducted on exfoliated flakes from bulk material, thin films on substrates of limited dimensions, or bulk material, with limited yield. A planar thin film-based technology is needed to make topological insulator devices at a large scale, which is advantageous for applications and could also lead to new device designs. We address two problems related to fabricating chalcogenide-based topological insulator devices on large 3” wafers in the modern microfabrication facility at Sandia National Laboratories using polycrystalline and epitaxial Bi2Te3 films as a prototypical example. (1) Selective inhomogeneous doping was accomplished using ion implantation. Implantation damage and its subsequent mitigation through annealing is characterized. (2) Dielectric layers are often used to manipulate surface potential for elucidating topological surface state transport. We found that the quality of the gate dielectric can show significant degradation with different processing conditions. The leakage current is characterized as a function of temperature. Solutions to these problems are explored and discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
Aluminum Intercalation and High Capacity of Two-Dimensional (2D) Vanadium Carbide (MXene) Cathode in Rechargeable Aluminum Batteries
Authors: VahidMohammadi Armin , Beidaghi Majid
Published: February 2020

Rechargeable aluminum batteries that utilize aluminum metal as the anode are one of the most promising alternative energy storage systems for current Li-ion batteries that suffer from flammability, safety issues, high cost, and limited lithium resources. Aluminum is the most abundant metal in Earth’s crust, offers three-electron redox reactions in electrochemical systems, and has the highest theoretical volumetric capacity of 8040 mAh cm-3 among all metals and reasonably high theoretical gravimetric capacity of 2980 mAh g-1. Also, aluminum metal can be handled in the open air; thus, aluminum batteries benefit from high safety and facile cell fabrication and operation compared to lithium and sodium battery systems [1]. However, development of these battery systems is mostly hindered by a lack of cathode materials that can host aluminum cations and offer high capacities and insertion potentials.

In this presentation, we show reversible intercalation of aluminum in two-dimensional (2D) vanadium carbides (V2C), a member of MXene family of materials, as a potential high-performance cathode material for rechargeable aluminum batteries. MXenes with a general formula of Mn+1Xn (n=1,2, and 3, M is a transition metal, and X is carbon and/or nitrogen) are a family of 2D transition metal carbides and/or carbonitrides that are produced by selective etching of the A layer elements (i.e. Al) from MAX phases (i.e. V2AlC), a large group of layered ternary carbides, nitrides, and carbonitrides [2-4]. The Aluminum battery cells incorporating V2C MXene as cathode operates by electrochemical deposition and dissolution of aluminum at the anode and intercalation/de-intercalation of Al3+ ions in between the layers of 2D nanosheets of MXene cathode. Our study shows that through a careful and proper fabrication of electrode structure, V2C cathodes can deliver reversible capacities more than ~ 200 mAh g-1 at a high charge and discharge rate of 100 mA g-1, comparable to current cathode materials for lithium and sodium-ion batteries. V2C cathode exhibits a high Coulombic efficiency of above 95% and delivers a good rate capability in a 1.7 V potential window. We also show that change in the oxidation state of the transition metal in the MXene structure plays an important role in charge storage mechanism of MXenes as cathode materials for Aluminum batteries.

 

Keywords: 2D, Vanadium carbide, MXenes, Aluminum battery

 

References

1. G. A. Elia, et al., Adv. Mater. 28, 7564-7579 (2016).

2. M. Naguib, et al., ACS Nano. 6, 1322–1331 (2012).

3. B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nat. Rev. Mater.2, 16098 (2017).

4. M. R. Lukatskaya et al., Science. 341, 1502–5 (2013).
Solid State Batteries and the Mechanical Properties of Lithium
Authors: Masias Alvaro , Sakamoto Jeff
Published: February 2020

The current revival of electrified vehicles seen in recent years has been enabled by significant advances in lithium ion batteries. This technology has progressed steadily at approximately 8% per year improvement in specific energy (Wh/kg). The achievement and maintenance of this specific energy growth rate is the result of massive, global progress in battery research, development and manufacturing. However, to promote the rapid displacement of petroleum fuels from the transportation sector, the current evolutionary growth in battery energy may be insufficient.

Amongst candidate next generation battery approaches, solid state (SS) is one of the most versatile and promising technologies. SS batteries can enable improvements in specific energy mainly by allowing for the use of high energy (due to its low electronegativity and high capacity) lithium metal anodes. A SS separator of comparable dimensions as a conventional polyolefin separator could lead to ~50% improvement in cell energy density by enabling metallic lithium anodes [1]. However, elemental lithium is pyrophoric & difficult to stabilize owing to its strongly reactive nature, and as a result its use as a reliable rechargeable battery electrode has proven challenging despite research efforts for more than 50 years. The development of a SS electrolyte material that could introduce lithium metal anodes would provide rechargeable batteries with a step change improvement in specific energy, leading to longer range EVs due to lighter/smaller batteries.

The research and selection of a solid state material requires balancing many different and often competing requirements simultaneously, which no one material can satisfy currently. Additionally, some SS electrolytes have shown a tendency to allow for lithium dendrites to penetrate their structure once a critical charge density (CCD) has been reached [2]. The role lithium's mechanical properties play in the dendrite growth mechanism is poorly understood and is the focus of this work. The literature features limited reports of lithium's elastic modulus, with tension values as low as 1.9, commonly referencing bending values of 5.0 and single crystal calculations as high as 11.5 GPa [3,4,5]. This work will explore further the mechanical properties of lithium in context of the actual SS battery environment where additional properties such as creep may dominate.

REFERENCES

[1] McCloskey, B. et al. J Phys. Chem. Lett. 6 (2015) 4581-4588.

[2] Sharafi, A. et al. J. Power Sources, 302 (2016) 135-139.

[3] Schultz, R. Technical Report FERMILAB-TM-2191. (2002).

[4] Bridgeman, P. Proc Am Acad Arts Sci., 57 (1922) 41-66.

[5] Robertson, W. et al. Phys. Rev., 117 (1960) 440-442.
Synergistic Effects from Substrate and Electrolytes on Li Deposition
Authors: Lochala Joshua Andrew , Wu Bingbin , Tarverne Tyler , Xiao Jie
Published: February 2020

Li metal has been considered the “holy grail” anode materials. Li metal has the most negative potential with an extremely high theoretical specific capacity of 3860 mA h g−1.[1] It is, however, plagued with multiple challenges which need solutions before the realization of this material. Li metal has two main failure mechanisms, one is the random growth of detrimental dendrites, which can lead to rapid thermal runoff, i.e. Galaxy Note 7.[2] The other issue that arises upon cycling is pulverization of lithium metal through none uniform plating, leading to an increase in solid electrolyte formation(SEI) and thereby the increase in voltage polarization within the cell.[3] Both Li failure mechanisms are rooted in the uneven distribution of SEI layers whose properties vary depending on the location of SEI formed.[4] The initial surface chemistry on Li metal influences SEI formation in the very beginning. In this work, electrochemical deposition of Li metal on specific substrate has been performed to understand the nucleation and growth of Li crystals. Electrolyte and substrate have been identified to have synergistic effects on the morphology and efficiency of deposited Li. The fundamental mechanisms that dictates the smooth Li deposition will be discussed.

References:

1) Xu, W., Wang, J., Ding, F., Chen, X., Nasybulin, E., Zhang, Y. and Zhang, J.G., 2014. Lithium metal anodes for rechargeable batteries. Energy & Environmental Science, 7(2), pp.513-537.

2) Cohen, Y.S., Cohen, Y. and Aurbach, D., 2000. Micromorphological studies of lithium electrodes in alkyl carbonate solutions using in situ atomic force microscopy. The Journal of Physical Chemistry B, 104(51), pp.12282-12291.

3) Lu, D., Shao, Y., Lozano, T., Bennett, W.D., Graff, G.L., Polzin, B., Zhang, J., Engelhard, M.H., Saenz, N.T., Henderson, W.A. and Bhattacharya, P., 2015. Failure mechanism for fast‐charged lithium metal batteries with liquid electrolytes. Advanced Energy Materials, 5(3).

4) Aurbach, D., Zinigrad, E., Cohen, Y. and Teller, H., 2002. A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions. Solid state ionics, 148(3), pp.405-416.
(Invited) Designing Symmetric and Asymmetric Morphology in Silicon Nanowires to Encode Advanced Electronic and Photonic Functionality
Authors: Cahoon James F
Published: February 2020

Silicon nanowires have been widely pursued as a technology for electronic, photonic, and renewable energy devices. Nevertheless, the implementation of nanowire technology has been limited by the extent to which the composition and morphology of wires can be controlled during synthesis. Here, we describe a bottom-up method—termed ENGRAVE (Encoded Nanowire GRowth and Appearance using VLS and Etching)—to modulate nanowire shape along the growth axis with sub-10 nanometer spatial resolution. Rapid modulation of phosphorus or boron incorporation during vapor-liquid-solid (VLS) growth encodes a high-resolution pattern of dopants in the wires. Subsequent solution processing is then used to selectively etch the wires, creating high resolution structures such as bow-ties, gratings, sawtooths, fractals, nanorods, sinusoids, and nanogaps. The capability to encode arbitrary morphology opens the door to new properties and functionality in the wires. For instance, we demonstrate how symmetric and periodic morphology can be used to design tunable photonic modes, enabling the coupling and waveguiding of light. In addition, we show how emergent electronic properties can be encoded through asymmetric morphology. The results highlight the broad set of new technologies that can be enabled through the design of precise sub-10 nanometer morphology in silicon nanowires.
(Invited) Band Gap Engineering and Optoelectronic Performance of Indium Gallium Arsenide Nanobelts
Authors: Zhang Xuehong , Yang Tiefeng , Pan Anlian
Published: February 2020

Low dimensional III-V compound semiconductors, such as GaAs, InAs, and their ternary alloys, exhibit unique optical and electronic properties. Their abilities of efficient carrier transport, light emission, and detection means they can be used in integrated transistors, lasers, light-emitting diodes, and detectors for infrared communications [1-3]. Very recently, intense research has been directed toward the fabrication and application of the III-V nanoplates/sheets owing to their realization of multiterminal electronic devices such as Hall bars, quantum point contacts, and so forth. However, the influence of growth condition on the formation of these nanostructures is still unclear. Furthermore, these nanoplates/sheets generally suffer from small and irregular morphology, demonstrating the difficulties in the controlled growth. Importantly, realizing the continuously tunable composition and heterojunction of these nanostructures remains great challenges, particularly for the technologically important InGaAs. It's well known that InGaAs is of great interest for high-performance device in electronics and optoelectronics [4,5]. Unfortunately, the transistors and photodetectors based on low dimensional InGaAs nanostructures still need improvement due to their low on/off ratio, poor photoresponsivity, and so on. To address these problems, we have realized the controlled growth of high-quality single-crystal InGaAs nanobelts with fully tunable composition/bandgap via an orientation selective vapor growth. Density functional theory calculations reveal that the surface energy variation induced by the chemical potential of arsenic (μ
As) is critical for the formation of these nanobelts. When configured into transistors, the InGaAs nanobelt devices exhibit impressive electrical properties with the turn on/off ratio of 2×107, better than any value reported for a InGaAs nanowire device to date. The broadband phototransistor (covering 405-1,550 nm) were further fabricated with an ultrahigh photoresponsivity of >8000 A/W and an external quantum efficiency of >106 %, both of which are much higher than those of all the reported InGaAs nanowire photodetectors. These results suggest that the novel InGaAs nanobelts may open up new opportunities for various applications in integrated electronics, optoelectronics, and quantum electronics.

References:

[1] Xuehong Zhang, Xiao Wang, Anlian Pan et

al
., Adv
.
Mater., 2017, 1604431.

[2] Xinliang Chen, Xuehong Zhang, Anlian Pan et al., Nanotechnology, 2016, 27, 50.

[3] Huang Tan, Xuehong Zhang, Anlian Pan et al., Nano-Micro Lett., 2016, 8, 29-35.

[4] Liang Ma, Xuehong Zhang, Anlian Pan et al., Semicond. Sci. Technol., 2015, 30, 105033.

[5] Liang Ma, Xiujuan Zhuang, Anlian Pan et al., Nano Lett., 2014, 14，694–698.

[6] Pinyun Ren, Qinglin Zhang, Anlian Pan et al., Adv. Mater., 2014, 26, 7444-7449.
(Invited) Colloidal Perovskite Quantum Wells for High-Color-Purity Optoelectronics
Authors: Shih Chih-Jen
Published: February 2020

Pure green light emitting diodes (LEDs) are essential to realize an ultra-wide color gamut in the next-generation displays, as is defined by the Rec. 2020 standard. However, because the human eye is more sensitive to the green spectral region, it is not yet possible to achieve an ultra-pure green electroluminescence (EL) with sufficiently narrow bandwidth that covers >95% of the Rec. 2020 standard in the CIE 1931 color space. Here, we synthesize quantum-confined, two dimensional (2D) formamidinium lead bromide (FAPbBr3) hybrid perovskites and demonstrate efficient, ultra-pure green EL for the first time. Through the dielectric-quantum-well (DQW) engineering, the quantum-confined 2D FAPbBr3 perovskites exhibit a high exciton binding energy of 162 meV, resulting in a high photoluminescence quantum yield (PLQY) of ~92% in the spin-coated films. Our preliminary LED devices show a maximum current efficiency of 10.0 cd A-1, comparable to the highest values reported in the perovskites LEDs. More importantly, we report the color coordinates of (0.168, 0.773), which covers 97% of the Rec. 2020 standard in the CIE 1931 color space, and represents the “greenest” LEDs ever reported. We further demonstrate large-area (3 cm2) and ultra-flexible (bending radius of 2 mm) LEDs based on the 2D perovskites.
(Invited) Rectifiers, Mos Diodes and LEDs Made of Fully Porous GaN Produced by Chemical Vapor Deposition
Authors: Carvajal Joan J. , Mena Josue , Aixart Jordi , Díaz Francesc , Aguiló Magdalena
Published: February 2020

GaN is an important wide band-gap semiconductor in electronics and optoelectronics. In its porous form is particularly interesting for developing optoelectronic devices with improved efficiency, such as LEDs with enhanced efficiency and sensors with enhanced sensitivity.

Through chemical vapour deposition (CVD) [1], we have shown that it is possible to produce nanoporous GaN without any etching or chemical post-growth treatment, with the porosity being present only on the (0001) face of the material. Low resistivity ohmic Pt and Au metallic contacts were demonstrated on porous n-type GaN by the formation of intermetallic seed layers through the vapour-solid-solid (VSS) mechanism [2]. Also, we have been able to develop p-type porous GaN by doping with Mg, with a charge carrier concentration of the order of 1018 cm-3 [3]. By tuning the concentration of Mg, introduced as Mg2N3 in the CVD system, it has been possible to form a polycrystalline high-k oxide between an ohmic metallic alloy interlayer contact and the porous GaN, while maintaining a clean interface, that allowed to fabricate a MOS-type diode on silicon in a single growth regime [4].

Through the careful selection of the substrate it has also been possible to produce porous GaN epitaxial layers [5] that allow for the fabrication of high quality partially and fully porous GaN rectifying p-n junctions, through a two step CVD process, and show their behaviour as diodes with effective uniform conduction under a green technology [6]. Here, we will also present the recent results we obtained in the light emission of these structures. Thus, these porous junctions have potential applications in high brightness unencapsulated LEDs with enhanced light emitting properties and high surface area sensors with improved sensitivity.

[1] Carvajal & Rojo, Crystal Growth Des., 9 (2009) 320

[2] Bilousov et al., ACS Appl. Mater. Interfaces 4 (2012) 6927

[3] Bilousov et al., Appl. Phys. Lett. 103 (2013) 112103

[4] Bilousov et al., Chem. Mater. 26 (2014) 1243

[5] Bilousov et al., CrystEngComm 16 (2014) 10255; Bilousov et al., ACS Appl. Mater. Interfaces 6 (2014) 17954

[6] Carvajal et al., ECS Transactions 66 (2015) 163

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(Invited) Fabrication of AlGaN/InGaN/GaN Quantum Wires for Electronic and Optoelectronic Applications
Authors: Leszczynski Mike , Sarzynski Marcin , Grzanka Ewa , Czernecki Robert , Smalc Koziorowska Julita , Domagala Jarek
Published: February 2020

AlGaInN epi structures are used for manufacturing white LEDs, UV, blue and green laser diodes (LD) and high-power/high-frequency transistors. In this work, we will show our proprietary and patented technology of lateral patterning in which we fabricate GaN substrates with locally variable off-cut (variable atomic step width). Such substrates are used for InGaN and AlGaN growth of 3-dimensional epi structures. The GaN off-cut influences a number of AlGaInN characteristics: Indium incorporation (smaller for a higher off-cut), Ga and N vacancies concentration, critical conditions for relaxation, decomposition rate, and some others. Therefore, using the GaN substrates after such lateral patterning, we may fabricate nano-wires which would be used for electronic device manufacturing. Unfortunately, AlGaInN epitaxial layers contain a large number of defects (dislocations, trench defects, In-clusters, and some others) which disturb the step-flow. We will show a comparison of results obtained using different GaN crystals: thick layers on sapphire and SiC (dislocation density higher than 108 cm-2), bulk crystals grown by various methods having dislocation density between 107 cm-2 and 104 cm-2. Then, we will present how the growth conditions (temperature, pressure, flows of reactants, presence of hydrogen in the carrier gas) influence the nanowire formation. One of the big issues in nitride technology is InGaN decomposition during the p-type growth at temperatures higher than those used for InGaN growth. Therefore, part of the presentation will be devoted to that issue, in particular, how lateral patterning may influence this decomposition.
(Invited) New Approaches for Shrinking the Performance Gap for GaN Power Devices
Authors: Shur Michael , Simin Grigory
Published: February 2020

The expected performance of GaN and SiC based power devices far exceeds that of Si power transistors, but the gap between the expected and achieved performance is much larger for these wide band gap materials. In these paper, we discuss new approaches for shrinking this performance gap for GaN power devices. They include using the quantum well channel designs that lead to the electron wave function penetration into wide band gap cladding layers with the commensurate increase in the breakdown voltage while keeping the advantage of a high mobility in the device channel. The gate edge engineering (beyond just using field plates) that optimizes the voltage distribution in the drain-to-gate spacing could be combined with a low conducting passivation for smoothing or even eliminating the sharp maximum of the electric field in the vicinity of the gate and field plate edges. Additional contacts in the drain-to-gate spacing for the field control and variable doping implants should allow for further optimization. The perforated channel designs could alleviate both the parasitic series resistance problem and the heat dissipation problem. Extending the gate perforations into the drain-to-gate region allows for a considerable reduction of the switching RC constant with a commensurate decrease in power dissipation. The ultimate design should use the lateral-vertical structures. The AlInN/AlN/GaN technology is uniquely poised for the breakthrough in high temperature performance. We predict that the combination of these approaches will dramatically shrink the performance gap firmly establishing GaN as a superb material for power applications.
(Invited) Novel Implantation Processing and Characterization for Scalable GaN Power Devices
Authors: Goorsky Mark S. , Bai Tingyu , Li Chao , Tadjer Marko J. , Hobart Karl D , Anderson Travis J , Hite Jennifer K , Feigelson Boris
Published: February 2020

Novel ion implantation processing has been introduced to enable p-type doping for high performance vertical GaN p-n power devices. Multicycle Rapid Thermal Annealing (MRTA) leads to state-of the-art p-type doping (> 8% dopant ionization); high n-type doping can also be achieved. The implantation process is also compatible with a tapered implanted junction terminal extension approach to achieve effective edge termination necessary to provide breakdown voltages over 90% of the theoretical maximum value at minimal cost and processing complexity.

MRTA, with ambient control (N2 ambient at 200 psi), pulsed annealing up to ~1400 °C, and an appropriate dielectric encapsulant, is utilized to further improve activation efficiency and, ultimately, p-n junction performance through selective ion implantation and processing optimization. Here, the focus is on MRTA activation of Mg+ implantation, with comparison to the potential effectiveness of other p-type dopants, to assess the impact of the crystalline nature of the substrate and to optimize the encapsulant mask, in order to promote the highest dopant activation. This fundamental study of the role of defects in material produced by GaN epitaxial deposition is expected to lead to a pathway to high activation efficiency through a mechanistic understanding of the relationship between processing methods and performance. The critical link between defects and implant activation will be discussed through the use of novel structural materials characterization techniques as well as standard electrical and optical techniques. Electron microscopy defect analysis, determining the role of implanted and annealed defects, and providing localized strain measurements helps assess defect evolution as a function of implant and annealing parameters. X-ray based reciprocal space mapping as a function of the implant and subsequent annealing will be shown to correlate strain and defect formation with dopant activation in order to parameterize the MRTA process non-destructively. This information leads to the production of p-n junction devices that are scalable, provide high surge currents, and high ideality factors. Understanding point defect concentrations, dislocation types and concentrations, and residual stress will lead to effective p-type dopant activation and high device performance.
(Invited) In Situ/Operando Characterization of Electrocatalysts Using Ambient Pressure XPS
Authors: Crumlin Ethan J
Published: February 2020

What is really happening at an electrochemical interface? This is what I want to reveal. The interface is where the true magic of electrochemistry occurs and yet we know so little about it. The reason for our lack of understanding exists in the challenges that need to be overcome to probe an interface. It’s often buried underneath a gas or liquid phase making it difficult for traditional surface science techniques to be used, while those that can easily work in gas or liquid environments are often probing bulk properties. Many researchers have worked actively to overcome these challenges. Their effort has yielded significant advances to probe the interface under in situ and operando conditions. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is one of the promising techniques developed that is capable of providing this information. This talk will introduce APXPS and how it has been utilized to provide new insight into the solid/gas electrochemical interface for PEM fuel cells. Then I will share the progress our group has made extending this technique into the tender X-ray regime (2 keV to 6 keV) to probe solid/liquid electrochemical interfaces under operando conditions. This will include several studies detailing our investigations to directly probe the electrochemical double layer (EDL), and insight into how platinum and oxide electrodes behave undergoing oxygen evolution reaction. Gaining new insight to guide the design and control of future electrochemical interfaces.
Oxygen Transport Effects of Cobalt Cation Contamination of Ionomer Thin Films in Proton Exchange Membrane Fuel Cells
Authors: Braaten Jonathan , Kongkanand Anusorn , Litster Shawn
Published: February 2020

Proton exchange membrane fuel cells (PEMFCs) are a promising automotive energy conversion technology for reducing the dependence on petroleum and reducing greenhouse gas emissions. However, the high cost of the platinum (Pt) electrocatalyst responsible for increasing the rate of the oxygen (O2) reduction reaction (ORR) in the cathode is one of the primary inhibitors to this technology’s mass market penetration. In order for PEMFCs to become cost competitive with internal combustion engine technologies, the electrocatalyst cost must be significantly reduced. To address this issue, the use of Pt alloys such as Pt3Co has been explored. Alloys such as Pt3Co have been shown to have higher activities than pure Pt electrocatalysts,(1) reducing the mass of the expensive Pt. However, the utilization of Pt3Co poses contamination risks to the ionomers used in PEMFCs due to cobalt’s (Co) propensity to dissolve out of the catalyst, migrate to the ionomer, and replace protons on the sulfonic acid sites(1). Cai et al.(2) demonstrated the effects of this risk through the testing of membrane electrode assemblies (MEAs) containing Co doped ionomers, which showed a decrease in proton conductivity through the ionomer. In addition, Kongkanand et al.(3) have shown that voltage losses at high current densities for low-Pt loaded electrodes can be attributed to localized O2 transport resistances in the thin films of ionomer covering the electrocatalyst surface. Thus, the effects of dissolved Co on the local O2 transport through these thin films of ionomer are of particular interest.

 

The purpose of this work is to experimentally characterize the effects of Co contamination on the localized O2 transport properties of ionomer thin films. To study these effects, we modified an existing fuel cell to allow for the testing of gas (O2) transport through ionomer thin films in a well-controlled environment, similar to the setup used by Liu et al.(4) The setup allows for the films’ transport resistance to be measured under non-polarized (uniform Co distribution) and interface free conditions, in order to isolate the effect of the Co contamination. In addition, the setup allows for the control of the ionomer sample’s temperature and relative humidity, to evaluate the effects of each on the O2 transport resistance of the doped thin films. To test the effects of differing levels of Co cation contamination during PEMFC operation, we take O2 transport resistance measurements for various amounts of controlled Co ion doping. Our preliminary results have indicated significant increases in O2 transport resistances for high Co contamination levels, consistent with our hypotheses derived from the results mentioned above(2, 3).

 

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

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