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- 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.
- ItemRole of hole confinement in the recombination properties of InGaN quantum structures([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Anikeeva, M.; Albrecht, M.; Mahler, F.; Tomm, J. W.; Lymperakis, L.; Chèze, C.; Calarco, R.; Neugebauer, J.; Schulz, T.We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.