Understanding surface reconstructions influenced by hydrogen is essential for controlling growth and materials properties. We use density functional theory to explore the adsorption of hydrogen, gallium, and oxygen adatoms on the Ga2O3 (010) and (110) surface and generate a phase diagram, which shows surface reconstructions as a function of Ga and H chemical potentials. For “bare” surfaces, we find that the ideal unreconstructed surface is low in energy but that reconstructions with Ga and O adatoms can be favorable under more Ga-rich conditions. We question whether such “bare” surfaces can be experimentally observed, since hydrogen is inevitably present during growth, even in ultrahigh-vacuum growth systems. Hydrogen-containing reconstructions are found under H-rich conditions with H being more stable under O-rich conditions. Our comprehensive study of surface reconstructions establishes under which conditions hydrogen will be present on the surface, and how it affects the growth.
Work performed in collaboration with M. Wang and S. Mu and supported by AFOSR.
We have used first-principles calculations based on density functional theory to accurately model the interaction of hydrogen in Ga2O3 and related alloys. Hydrogen is often present during growth or processing of the material, particularly in metal-organic chemical vapor deposition (MOCVD). Our comprehensive study of surface reconstructions establishes under which conditions hydrogen will be present on the surface, and how it affects the growth. For hydrogen inside the material, we calculate diffusion barriers, and the formation of complexes with intentional or unintentional impurities.
Work performed in collaboration with J. L. Lyons, S. Mu, J. B. Varley, M. Wang, and D. Wickramaratne.
We have used first-principles calculations, based on advanced hybrid density functional theory, to accurately model diffusion of point defects and impurities in Ga2O3. Control of doping is crucial for devices: it should be possible to control the carrier concentrations all the way from semi-insulating to highly conductive n-type material. I will discuss impurities used for donor doping, deep acceptors, as well as unintentional contaminants such as carbon and hydrogen. The results provide important guidance for incorporating Ga2O3 into devices.
Ga2O3 is a highly promising material for power electronics, thanks to its large band gap (4.8 eV) and high breakdown voltage. Better control of doping is still an active research topic, both in Ga2O3 and in (AlxGa1-x)2O3 alloys. First-principles modeling, using advanced hybrid functional calculations within density functional theory, can greatly help in resolving experimental puzzles and guiding optimal doping conditions.
Work performed in collaboration with S. Mu, J. L. Lyons, H. Peelaers, J. B. Varley, and D. Wickramaratne.
Point defects are at the heart of the important properties of wide band-gap and oxide semiconductors for power electronics applications, and therefore understanding the details of point defects and their role in determining the properties becomes imperative. Beta-Ga2O3 has received significant attention recently due to its unique advantages, including high breakdown voltage and availability as bulk substrates, which make it a viable candidate for next-generation power device applications. Here we present the first direct microscopic observation of the formation of interstitial-divacancy complexes within beta-Ga2O3 lattice using atomic resolution scanning transmission electron microscopy. We directly observed that cation atoms are present in multiple interstitial sites, and each interstitial atom is paired with two adjacent vacancies. The observed structure of the complexes is consistent with the calculation using density functional theory (DFT), which predicts them to be compensating acceptors. The number of the observed complexes increase as a function of Sn doping concentration, which matches with the increase in the concentration of the trap state at Ec - 2.1 eV measured using deep level optical spectroscopy, which strongly suggests that the defects corresponds to that trap level. Our finding provides new crucial information on the exact origin of the properties of beta-Ga2O3 that has been unobtainable using other methods. The results also provide new important insight on the material’s unique response to the impurity incorporation that can impact their properties, which can ultimately guide the development of growth and doping of new-generation materials for power electronics.
We have used first-principles modeling, based on advanced hybrid functional calculations within density functional theory, to accurately predict properties of point defects and impurities in Ga2O3 and related materials. Point defects act as compensating centers, but they also assist diffusion of impurities. Accurate knowledge of formation energies and migration barriers then allows determining the doping profile. These results provide guidance for incorporating Ga2O3 into devices.
Work performed in collaboration with J. Hwang, A. Janotti, J. L. Lyons, H. Peelaers, and J. B. Varley.
Understanding recombination mechanisms in halide perovskites is of key importance to their applications in photovoltaics and light emission. We perform first-principles calculations to compute the radiative and nonradiative recombination coefficients in the methylammonium lead iodide as well as in other halide perovskites. The computed radiative recombination coefficient is as high as in typical direct-gap semiconductors used in optoelectronics. However, our first-principles calculations of nonradiative rates show that strong Auger recombination will suppress efficiency of light emitters. I will also discuss defect-assisted recombination, a problem closely coupled to the issue of degradation.
Work performed in collaboration with Xie Zhang, Jimmy-Xuan Shen, and Wennie Wang, and supported by DOE.
The unique properties of Ga2O3 and related oxides enable applications as transparent conductors and in power electronics. Ga2O3 has a large band gap (4.8 eV) but can also be highly n-type doped. A thorough understanding of its properties, combined with knowledge of how to control them, is crucial to improving materials quality and enabling further applications. I will show how first-principles modeling, using advanced hybrid functional calculations within density functional theory, can accurately predict band structure [1], properties of point defects [2,3] and impurities [4], and transport [5]. Combining Ga2O3 with In2O3 [6] or Al2O3 allows tuning the atomic and electronic structure. We determine the preferential crystal structures as a function of alloy composition, along with values for band gaps and band alignment. These results provide guidance for incorporating Ga2O3 into devices.
Work performed in collaboration with H. Peelaers, J. B. Varley and Y. Kang.
[1] H. Peelaers and C.G. Van de Walle, Phys. Status Solidi B 252 (2015), 828.
[2] J. B. Varley et al., Appl. Phys. Lett. 97 (2010), 142106.
[3] J. B. Varley et al., J. Phys. Condens. Matter 23 (2011), 334212.
[4] H. Peelaers and C. G. Van de Walle, Phys. Rev. B 94 (2016), 195203.
[5] Y. Kang et al., J. Phys.: Condens. Matter 29 (2017), 234001.
[6] H. Peelaers et al., Phys. Rev. B 92 (2015), 85206.
Nitride semiconductors are the key materials for solid-state lighting. Point defects may act as compensating centers, charge traps, or radiative or nonradiative recombination centers. Unintentional impurities often play an equally important role; for instance, carbon that is unavoidably incorporated during metal-organic chemical vapor deposition can act as a source of yellow luminescence. Theoretical advances now enable us to calculate the energetics as well as electronic and optical properties of point defects with unprecedented accuracy. We have developed a first-principles methodology to determine nonradiative carrier capture coefficients. Accurate calculations of electron-phonon coupling, combined with results for defect formation energies and charge-state transition levels, enable the calculation of nonradiative capture rates for electrons and holes and the evaluation of Shockley-Read-Hall coefficients. This approach allows us to identify specific defects that play a key role in limiting the efficiency of nitride semiconductor devices.
---Work performed in collaboration with A. Alkauskas, C. Dreyer, A. Janotti, J. Lyons, J. Shen, J. Speck, and D. Wickramaratne, and supported by DOE and NSF.
Using hybrid density functional theory, we investigate the influence on electronic structure of common defects and impurities in tungsten oxide (WO3). As an easily reducible perovskite with the A-site atom missing, high concentrations of foreign dopants and oxygen deficiencies are possible. Our calculations show that both oxygen vacancies and alkali dopants are shallow donors, and we explore the physical origins for this behavior. In particular, we examine whether oxygen vacancies can give rise to localized states or small polarons. Our results show that in crystalline material no such charge localization occurs. We discuss how these results impact electrical conductivity and optical properties.
Optical spectroscopy is a powerful approach for detecting defects and impurities in ZnO, an important electronic material. However, knowledge of how common optical signals are linked with defects and impurities is still limited. The Cu-related green luminescence is among the best understood luminescence signals, but theoretical descriptions of Cu-related optical processes have not agreed with experiment. Regarding native defects, assigning observed lines to specific defects has proven very difficult. Using first-principles calculations, we calculate the properties of native defects and impurities in ZnO and their associated optical signals. Oxygen vacancies are predicted to give luminescence peaks lower than 1 eV; while related zinc dangling bonds can lead to luminescence near 2.4 eV. Zinc vacancies lead to luminescence peaks below 2 eV, as do the related oxygen dangling bonds. However, when complexed with hydrogen impurities, zinc vacancies can cause higher-energy transitions, up to 2.3 eV. We also find that the Cu-related green luminescence is related to a (+/0) deep donor transition level.
Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO2 (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO2. In addition, we present new results for the reflectivity of pure TiO2.
We describe the use of first-principle calculations to address problems related to defects, doping, and band- structure engineering in III-V nitrides. For n-type doping it is found that nitrogen vacancies are too high in energy to be formed during growth, but silicon and oxygen readily incorporate as donors. The properties of oxygen, including DX-center formation, support it as the main cause of unintentional n-type conductivity. For p-type doping we find that the solubility of Mg is the main factor limiting the hole concentration in GaN. We discuss the beneficial effects of hydrogen during acceptor doping. Compensation of acceptors by nitrogen vacancies may occur, becoming increasingly severe as x increases in AlxGa1-xN alloys. Acceptors other than Mg are also investigated. Finally, we discuss our first-principles investigations of the atomic and electronic structure of heterojunction interfaces between the III-nitrides, and provide values for band lineups.
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