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- ItemExperimental electronic structure of In2O3 and Ga2O3(Bristol : IOP, 2011) Janowitz, C.; Scherer, V.; Mohamed, M.; Krapf, A.; Dwelk, H.; Manzke, R.; Galazka, Z.; Uecker, R.; Irmscher, K.; Fornari, R.; Michling, M.; Schmeißer, D.; Weber, J.R.; Varley, J.B.; Van De Walle, C.G.Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ∼1019 cm-3 (In2O3) and ∼1017 cm-3(Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross.
- ItemCobalt as a promising dopant for producing semi-insulating β -Ga2O3crystals: Charge state transition levels from experiment and theory(Melville, NY : AIP Publ., 2022) Seyidov, Palvan; Varley, Joel B.; Galazka, Zbigniew; Chou, Ta-Shun; Popp, Andreas; Fiedler, Andreas; Irmscher, KlausOptical absorption and photoconductivity measurements of Co-doped β-Ga2O3 crystals reveal the photon energies of optically excited charge transfer between the Co related deep levels and the conduction or valence band. The corresponding photoionization cross sections are fitted by a phenomenological model considering electron-phonon coupling. The obtained fitting parameters: thermal ionization (zero-phonon transition) energy, Franck-Condon shift, and effective phonon energy are compared with corresponding values predicted by first principle calculations based on density functional theory. A (+/0) donor level ∼0.85 eV above the valence band maximum and a (0/-) acceptor level ∼2.1 eV below the conduction band minimum are consistently derived. Temperature-dependent electrical resistivity measurement at elevated temperatures (up to 1000 K) yields a thermal activation energy of 2.1 ± 0.1 eV, consistent with the position of the Co acceptor level. Furthermore, the results show that Co doping is promising for producing semi-insulating β-Ga2O3 crystals.
- ItemStep-flow growth in homoepitaxy of β-Ga2O3 (100)—The influence of the miscut direction and faceting(Melville, NY : AIP Publ., 2019) Schewski, R.; Lion, K.; Fiedler, A.; Wouters, C.; Popp, K.; Levchenko, S.V.; Schulz, T.; Schmidbauer, M.; Bin Anooz, S.; Grüneberg, R.; Galazka, Z.; Wagner, G.; Irmscher, K.; Scheffler, M.; Draxl, C.; Albrecht, M.We present a systematic study on the influence of the miscut orientation on structural and electronic properties in the homoepitaxial growth on off-oriented β-Ga2O3 (100) substrates by metalorganic chemical vapour phase epitaxy. Layers grown on (100) substrates with 6° miscut toward the [001⎯⎯] direction show high electron mobilities of about 90 cm2 V−1 s−1 at electron concentrations in the range of 1–2 × 1018 cm−3, while layers grown under identical conditions but with 6° miscut toward the [001] direction exhibit low electron mobilities of around 10 cm2 V−1 s−1. By using high-resolution scanning transmission electron microscopy and atomic force microscopy, we find significant differences in the surface morphologies of the substrates after annealing and of the layers in dependence on their miscut direction. While substrates with miscuts toward [001⎯⎯] exhibit monolayer steps terminated by (2⎯⎯01) facets, mainly bilayer steps are found for miscuts toward [001]. Epitaxial growth on both substrates occurs in step-flow mode. However, while layers on substrates with a miscut toward [001⎯⎯] are free of structural defects, those on substrates with a miscut toward [001] are completely twinned with respect to the substrate and show stacking mismatch boundaries. This twinning is promoted at step edges by transformation of the (001)-B facets into (2⎯⎯01) facets. Density functional theory calculations of stoichiometric low index surfaces show that the (2⎯⎯01) facet has the lowest surface energy following the (100) surface. We conclude that facet transformation at the step edges is driven by surface energy minimization for the two kinds of crystallographically inequivalent miscut orientations in the monoclinic lattice of β-Ga2O3.