Symposium CM
Development and Application of Functional Transparent Conducting and Semiconducting Oxides
ABSTRACTS
CM-1:IL01 Structure and Properties of Amorphous Oxide Semi-conductors
J.E. Medvedeva, Department of Physics, Missouri S&T, Rolla, MO, USA
Unlike Si-based semiconductors, amorphous oxide semiconductors (AOSs) were shown to exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. However, the structure and properties of AOSs are extremely sensitive to deposition conditions, oxygen stoichiometry, and metal composition, rendering the currently available research data scattered and hampering further development. Moreover, given many degrees of freedom in amorphous oxides, defects in AOSs differ fundamentally from those in the crystalline materials. In this talk, complex deposition-structure-property relationships in several multicomponent AOSs will be discussed. Based on a thorough comparison of the results of computationally intensive ab-initio Molecular Dynamics modeling, comprehensive analysis of structural morphology and evolution, and accurate density-functional calculations of the properties with systematic experimental measurements, we will outline a 4-D parameter space that describes the microscopic nature in AOSs, serves as a predictive model to optimize the properties of known AOSs, and helps derive versatile design principles for next-generation transparent amorphous semiconductors.
CM-1:IL02 Reinventing inorganic photochromics
J. Montero, L. Österlund, Division of Solid-State Physics, The Ångström Laboratory, Uppsala University, Uppsala, Sweden
It is well-known that transition metal oxides, such as tungsten oxide, exhibit photochromic properties. The general consensus from previous studies on these materials is that the photochromic behaviour is caused by reaction of oxygen vacancies with hydrogen and/or water molecules of atmospheric origin. This reaction causes oxygen to be released from the oxide lattice upon illumination with light with energy exceeding the optical bandgap of the oxide. In most of the cases, the photochromic contrast is, however, very small in these oxides, and the darkening/bleaching dynamics is very slow. In recent years, it was discovered that oxide-hydrides or oxyhydride materials based on rare-earth elements exhibit pronounced photochromic properties. In these new class of materials, the hydride and oxide anions coexist . These oxyhydrides have been shown to be very stable in air and being able to retain hydrogen even when subjected to relatively high temperatures. Moreover, rare earth oxyhydrides exhibit very good photochromic optical contrast, dynamics and stability and could therefore be attractive alternative to conventional organic photochromic dyes. In this talk, our recent research in the photochromism of oxyhydrides will be discussed.
CM-1:IL03 The Relationship between the Surface Chemistry and Surface Electronic properties of Functional Transparent Semi-conducting Oxides
M. Allen, A. McNeill, L. Carroll, R. Martinez-Gazoni, R. Reeves, A. Downard, MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Canterbury, Christchurch, New Zealand
Transparent semiconducting oxides, such as ZnO, SnO2, In2O3, and Ga2O3, are technologically important materials due to their unusual combination of strong n-type conductivity, high visible transparency, and high breakdown fields. The surface electronic properties of these functional oxides are also interesting as they show fundamental differences in band bending and surface electron density, that have important consequences for the performance of metal-semiconductor contacts and heterojunctions to these materials. These differences are surprising considering that under most conditions, their surfaces are all usually terminated with similar coverages of hydroxyl groups. We have used synchrotron x-ray photoelectron spectroscopy to study the surfaces of ZnO, SnO2, In2O3, and Ga2O3 and to explore the different relationships between their surface hydroxyl coverage and surface band bending. We have also investigated the covalent attachment of organo-phosphonic and aryl-diazonium derived molecules to their surfaces. We will show that the electronic properties of these oxide semiconductors can be selectively modified by controlling their surface hydroxyl density and by covalently attaching selected organic molecules, with important consequences for the fabrication of electronic devices.
CM-1:IL07 Role of Native Defects and Electronic Structure in the Performance of Transparent Conductors
C.G. Van de Walle, Materials Department, University of California, Santa Barbara, CA, USA
I will describe how first-principles calculations are now capable of accurately predicting the performance of transparent conductors. Point defects and impurities can impact the electronic and optoelectronic properties [1-3]. In addition, the large carrier concentrations required for high conductivity can significantly affect optical transparency. Direct absorption to higher-lying conduction bands can take place starting from the carriers in the conduction band [4]. I also address indirect processes assisted by electron-phonon scattering [5,6]. The dependence on wavelength and its physical origins will be discussed. The talk will be illustrated with examples for In2O3, and Ga2O3.
Work performed in collaboration with J. Hwang, A. Janotti, H. Peelaers, J. Speck, and J. B. Varley. [1] J. B. Varley et al., Appl. Phys. Lett. 97, 142106 (2010). [2] J. B. Varley et al., J. Phys. Condens. Matter 23, 334212 (2011). [3] H. Peelaers et al., APL Materials 7, 022519 (2019). [4] H. Peelaers and C. G. Van de Walle, Appl. Phys. Lett. 111, 182104 (2017). [5] H. Peelaers and C.G. Van de Walle, Phys. Rev. B 100, 081202 (2019). [6] H. Peelaers, E. Kioupakis, and C. G. Van de Walle, Appl. Phys. Lett. 115, 082105 (2019).
CM-1:IL09 Sub-band-gap Absorption in TCOs
H. PEELAERS, Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
Transparent conducting oxides (TCOs) are a technologically important class of materials used in optoelectronic devices, as TCOs balance two conflicting properties: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higher-lying conduction band states, or by an indirect process, for example mediated by phonons or charged impurities. We used advanced first-principles calculations to elucidate the importance of these absorption processes in several TCO materials, including the widely used In2O3 [1] and Ga2O3 [2,3] a material with promising applications in high-power devices and UV photodetectors.
Work in collaboration with E. Kioupakis and C. G. Van de Walle, and supported by DOE and AFOSR. [1] H. Peelaers, E. Kioupakis, and C. G. Van de Walle, Appl. Phys. Lett. 115, 082105 (2019). [2] H. Peelaers and C.G. Van de Walle, Appl. Phys. Lett. 111, 182104 (2017). [3] H. Peelaers and C.G. Van de Walle, Phys. Rev. B 100, 081202 (2019).
CM-2:IL02 Amorphous Oxide Semiconductor Resistive Switching Materials and Devices
J. Deuermeier, M. Pereira, E. Carlos, C. Silva, R. Martins, E. Fortunato, A. Kiazadeh, i3N/CENIMAT, Department of Materials Science, NOVA School of Science and Technology and CEMOP/UNINOVA, NOVA University Lisbon, Campus de Caparica, Caparica, Portugal
Artificial intelligence (AI) has become ubiquitous in our daily lives, but little do we notice its immense environmental impact. The reason is that traditional computer architectures are terribly inefficient for AI. Hence, novel dedicated AI hardware systems are being developed at a fast pace. Their key-enabling components are resistive switching devices – or memristors. These are two-terminal structures which change their resistance according to the (usually) electric stimulus. Different criteria need to be fulfilled depending on the neural network architecture, but a common requirement is an analog control over multiple resistance levels. Numerous material systems are being researched for this purpose, but only few are compatible with transparent and/or flexible electronics. ZnO-based amorphous oxide semiconductors are among the most promising candidates due to transparency and low-temperature fabrication. They can even be ink jet printed. Hence for the near future, we expect to see large-area-electronics (LAE) with seamlessly embedded memristor-based hardware AI. This can effectively mitigate the environmental impact of the rising Internet-of-Things (IoT) era, especially for wearables.
CM-2:IL04 Searching for New Transparent Conducting and Electride Materials using High-throughput Computational Screening
G. Hautier, Thayer school of Engineering, Dartmouth College, Hanover, NH, USA
Essential materials properties can now be assessed through ab initio methods. When coupled with the exponential rise in computational power, this predictive power provides an opportunity for large-scale computational searches for new materials. We can now screen thousands of materials by their computed properties even before the experiments. This computational paradigm allows experimentalists to focus on the most promising candidates, and enable researchers to efficiently and rapidly explores new chemical spaces.
In this talk, I will present how this approach has been used to identify unexpected new p-type transparent conducting materials as well as electrides. I will outline the high-throughput methodology and the newly discovered materials focusing especially on materials for which experimental follow-up studies have been performed. I will finish my talk discussing some of our new results on small polaronic materials indicating that this class of material that has been so far dismissed for transparent conducting applications could lead to better performances than previously thought.
CM-2:IL05 Electrochromism : Combination of Materials and Devices Architectures
A. ROUGIER, ICMCB, Pessac, France
Electrochomism, EC, refers as a modulation of the optical properties under an applied voltage. This technology find applications in buildings and automobile industry by controlling light and heat transfer through windows for transmissive devices while colour changes in reflective devices offer great interest in the field of displays and printed electronics. The optimization of the EC properties results from a combination of both materials and device architecture. Herein, we will demonstrate through various examples using oxides, polymers or hybrids, the benefit of adjusting this combination on the EC performance including colour modulation, durability, switching time... For instance, starting from the state of the art five layers device, lowering of the energy consumption was achieved by decreasing the number of layers allowing activation through mobile phones while independently of the device configuration hybrids mixing oxides and polymers offers a new strategy for colour modulation [1,2].
[1] A. Danine et al., Toward Simplified Electrochromic Devices Using Silver as Counter Electrode Material, ACS Appl. Mater. and Interfaces, 11, 34030, 2019 [2] D. Levasseur et al., Color Tuning by Oxide Addition in PEDOT:PSS-Based Electrochromic Devices’ Polymers, 11,1, 179, 2019
CM-2:IL06 Epitaxial Copper Iodide Thin Films grown by Pulsed Laser Deposition
M. Lorenz, Universität Leipzig, Felix Bloch Institute for Solid State Physics, Leipzig, Germany
Copper iodide is a wide-bandgap, naturally p-type semiconductor with high transparency and high hole mobility [1]. Although DC-sputtered CuI thin films using an external iodine vapour source reached unprecedented electrical p-type conductivity, and high rectification ratio of junctions with n-ZnO, and large Seebeck coefficient [2], a reproducible control of structure and electrical parameters is difficult. Pulsed Laser Deposition has proven to be a surprisingly simple and efficient growth method for high-quality CuI thin films with controlled hole concentration, without the requirement of an extra iodine supply [3]. Various selenium doping concentrations were obtained using an elliptically segmented target [4]. From a study of CuI films with various capping layers we found evidence for oxygen being a dominant shallow acceptor in CuI [5]. We acknowledge the financial support from the DFG Research Unit FOR 2857.
[1] M. Grundmann et al., phys. stat. sol. (a) 210, 1671 (2013) [2] C. Yang, M. Lorenz et al., PNAS 113, 12929 (2016); and Sci. Rep. 6, 21937 (2016); and Nat. Commun. 8, 16076 (2017) [3] P. Storm, M. Lorenz et al., APL Mater. 8, 091115 (2020) [4] P. Storm, M. Lorenz et al., Phys. Status Solidi RRL 2100214 (2021) [5] P. Storm, M. Lorenz et al., APL Mater. 9, 051101 (2021).
CM-2:IL07 Ga2O3-based Heterostructures for Various Polymorphs
M. GRUNDMANN, Felix Bloch Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany
We investigate heterostructures of alumina and gallia and their alloy (Al,Ga)2O3 in the alpha-, beta- and kappa-phases. Compared to hexagonal semiconductors, the symmetry for trigonal crystals is reduced. Regarding the elastic properties, this leads to the occurrence of shear strains in heteroepitaxy and a difference between a- and m-planes [1-3]. We discuss also plastic strain relaxation mechanisms [4] and derive a universal relation for the orientation of slip lines from prismatic glide [5]. Optically, hexagonal and trigonal materials are uniaxial. This means that particular care must be taken when evaluating Raman scattering for the determination of the Raman tensor [6]; for thin films, also the layer thickness must be taken into account [7]. Further complications regarding the dielectric function and Raman scattering have been solved by us for biaxial materials, in particular beta-Ga2O3 [8-10].
[1] doi:10.1002/pssb.202100104 [2] doi:10.1002/pssb.202000323 [3] doi:10.1557/s43578-021-00375-3 [4] doi:10.1039/d1ma00204j [5] doi:10.1063/1.5140977
[6] doi:10.1103/PhysRevLett.116.127401 [7] doi:10.1063/5.0060198 [8] doi:10.1038/s41598-020-73970-9 [9] doi:10.1103/PhysRevA.103.053510 [10] doi:10.1038/srep35964
CM-2:IL08 Epitaxial Growth Parameters for Phase Selection of Ga2O3 Epilayers
R. FORNARI, P. Mazzolini, F. Mezzadri, University of Parma, Parma, Italy; M. Bosi, L. Seravalli, IMEM-CNR Institute, Parma, Italy
Gallium Oxide (Ga2O3) is a wide bandgap semiconductor of great interest for high power electronics and for UVC solar-blind detectors. It may crystallize in different phases, namely alpha, beta, gamma, delta, epsilon (more recently referred to as kappa), all of them metastable except the thermodynamically stable monoclinic beta−Ga2O3. Recently, there has been a growing interest around metastable phases, as they possess very peculiar properties, like for instance the spontaneous ferroelectricity of the kappa phase, that can potentially be exploited to generate a 2D electron gas at the interface of heterostructures. In this presentation we focus on the growth of Ga2O3 thin films via metal-organic vapour phase epitaxy on c-plane sapphire at different substrate temperatures, using water and trimethyl-gallium as precursors. A review of own and literature growth parameters is presented, which allows tailoring the growth process in view of the desired Ga2O3 polymorph. The experimental data are discussed in terms of growth thermodynamics (chemical potential of nutrient and crystalline phase) and kinetics (surface energy of nuclei of different phases). It will be shown that the interplay between kinetics and thermodinamic is decisive for the nucleation and stabilization of a certain phase.
CM-2:IL10 Solution Processed ITO Thin Films for Optoelectronic Applications
R.A. GERHARDT, N. Xia, S. Sethuraman, S. Joshi, Georgia Institute of Technology, Atlanta, GA. USA
Indium tin oxide (ITO) has been the premier transparent electrode for a number of years. Because its cost keeps increasing, there have been many attempts to supplant its use but none of the replacement materials can achieve the same degree of transparency and/or electrical conductivity necessary for many optoelectronic applications. Additionally, most research and industrial applications relies on vacuum deposited thin films which must be patterned according to the needs of the application. This results in wasting the material during the deposition and also during the patterning. On the other hand, solution processed films can be deposited via a number of different methods such as inkjet printing which can be deposited in a ready-made configuration minimizing waste. However, most attempts to use solution processed films have resulted in inconclusive results because it is difficult to control their thickness, properties and microstructure. In this talk, we will review all of the different ways that we have made ITO thin films and compare them to other work in the literature and conclude with a recipe that we have found leads to homogeneous films with excellent properties that can be used in LCDs, LEDs as well as solar cells.
CM-2:IL11 Combinatorial Epitaxial Growth and High-throughput Physical Property Screening of Monoclinic, Orthorhombic and Rhombohedral Ternary Group-III Sesquioxide Alloys
H. von Wenckstern, M. Kneiß, A. Hassa, C. Sturm, D. Splith, M. Lorenz, M. Grundmann, Felix-Bloch-Institut für Festkörperphysik, Universität Leipzig, Leipzig, Germany
The discovery of new materials and the exploration of the phase space of alloys with known constituents have been greatly accelerated by high-throughput experimental and computational screening methods. On the experimental side, high-throughput screening requires sample sets with systematic variation of a growth parameter such as growth temperature and pressure and, most importantly, the composition of the material flux impinging on the substrate. Combinatorial pulsed laser deposition (C-PLD) enables a lateral spatial control of the chemical flux composition and the generation of thin film materials libraries with a continuous lateral compositional distribution that can be analyzed with spatially resolved physical property screening methods for high-throughput characterization. We have prepared material libraries with continuous compositional distribution for monoclinic, orthorhombic and rhombohedral ternary alloys of (In,Ga,Al)2O3 by C-PLD and discuss structural, phonon mode, optical and electrical properties as a function of composition for the different polymorphs. Further, formation and properties of Schottky barrier diodes will be discussed. Finally, growth of heterostructures and superlattices in the orthorhombic modification with sharp interfaces will be addressed.
CM-3:IL01 Transparent Conducting Materials for High Efficiency Solar Cells
M. Morales-Masis, MESA+ Institute, University of Twente, Enschede, The Netherlands
Transparent conducting oxides (TCO) with low UV to NIR absorption, high lateral conductivity and low contact resistance with device layers are crucial to avoid parasitic absorption and electrical losses in solar cells (SCs). Fulfilling simultaneously these requirements represents a challenge for developing optimum TCOs with device-compatible deposition processes . H- and Zr-doped In2O3 are among the best performing TCOs for SCs, due to their high electron mobility (> 70 cm2/Vs) achieved at low deposition temperatures (< 200 °C), wide band gap (3.5 - 4 eV) and low sub-bandgap absorption. These properties are strongly linked to a low density of defects and passivated grain boundaries. Here we compare these TCOs, from their fundamental properties to their integration in perovskite SCs. We furthermore discuss possibilities for indium reduction by developing ultrathin TCOs with high mobility, as well as new alternative indium-free (Sn-based) TCOs developed by applying material design and controlled synthesis from amorphous up to buffer-induced crystalline growth. Finally, we discuss how lessons learned in the TCO field are applied to develop new non-oxide p-type transparent contacts and halide perovskite absorber materials.
CM-3:IL02 Resolving the Metal Insulator Transition Mechanism for Nb Oxides for Memristor Applications
WEI-CHENG LEE, L.F.J. Piper, Department of Physics, Applied Physics, and Astronomy, Binghamton University, Binghamton, NY, USA
Neuromorphic computing, a new computing architecture mimicking the function of a mammal’s neuron system, has been extensively studied and considered to be the most promising new computing scheme, but its development is severely hindered by lack of a crucial component, a commercializable, nanoscale memristor. In short, the memristor is a material that can adjust its resistivity based on the history of current running through it, and the fundamental mechanism of such novel memory effect from quantum materials remains unclear. In this talk, I will present some of our team work on microscopic understanding of memristors made from transition metal oxides NbO2 and Nb2O5. For NbO2, we offer strong evidences from X-ray spectroscopy as well as the density-functional theory to show that the metal-to-insulator transition is mainly due to the dimerization of Nb atoms driven by Peierls instability. We propose that the observed memristive effect in NbO2 is attributed to the resistivity switch induced by the modulation of Nb-Nb dimer distance due to the local Joule-heating. For Nb2O5, it is found to exhibit memristive behavior only in amorphous phases, for which we propose a new mechanism of obtaining a nanoscale memristor by leveraging Anderson localization states in amorphous materials.
CM-3:IL03 Flexible and Autonomous Electronics using Sustainable Thin Film and Nanostructured Oxides
A. Rovisco, A. SantA, M. PEREIRA, J. MARTINS, R. Branquinho, R. Igreja, E. FORTUNATO, R. MARTINS, P. Barquinha, i3N/CENIMAT, Department of Materials Science, NOVA School of Science and Technology and CEMOP/UNINOVA, NOVA University Lisbon, Campus de Caparica, Caparica, Portugal
While indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) are nowadays assumed as a crucial technology to fabricate active matrix backplanes for high-resolution displays, it is imperative to seek for sustainable routes to push the boundaries of oxide electronics. One dream is to conceive a technologic platform to seamless integrate autonomous electronic functionalities into everyday objects. At CENIMAT we are exploring zinc-tin oxide (ZTO) towards that end. In 2018 we reported for the first time flexible amorphous ZTO TFTs processed at only 180 °C [1]. Controlled H incorporation during ZTO sputtering and integration with an engineered high-κ multicomponent dielectric [2] were critical to achieve performance metrics similar to IGZO TFTs and to enable digital and analog circuit blocks. Another approach has been exploring ZTO at nanoscale, by using hydrothermal synthesis to grow different Nanostructures [3, 4], particularly ZnSnO3 nanowires (NW), which have been extensively characterized and directed to multiple applications, such as pH sensors, photocatalysis, memristors and energy harvesters [5]. For the later, composite nanogenerators combining piezo and triboelectric effects show output voltage, current, and instantaneous power density of 120 V, 13 μA, and 230 μW·cm-2, respectively [6]. We also show how NW alignment achieved by nanoimprinted seeds can significantly boost the performance of transistors and energy harvesters.
[1] C. Fernandes et al., Advanced Electronic Materials, Vol. 4, 1800032 (2018)
[2] J. Martins et al., submitted to Electronic Materials (2020)
[3] A. Rovisco et al., ACS Applied Nano Materials, Vol. 1, 2986 (2018)
[4] A. Rovisco et al., Nanomaterials, Vol. 9, 1002 (2019)
[5] A. Rovisco et al., Hydrothermal Synthesis of Zinc Tin Oxide Nanostructures for Photocatalysis, Energy Harvesting and Electronics, in Novel Nanomaterials, Intech (2020). Accepted (ISBN 978-1-83881-025-2)
[6] A. Rovisco et al., ACS Applied Materials and Interfaces, Vol. 12, 18421 (2020