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Evolution of planar defects during homoepitaxial growth of β-Ga2O3 layers on (100) substrates—A quantitative model
2016, Schewski, R., Baldini, M., Irmscher, K., Fiedler, A., Markurt, T., Neuschulz, B., Remmele, T., Schulz, T., Wagner, G., Galazka, Z., Albrecht, M.
We study the homoepitaxial growth of β-Ga2O3 (100) grown by metal-organic vapour phase as dependent on miscut-angle vs. the c direction. Atomic force microscopy of layers grown on substrates with miscut-angles smaller than 2° reveals the growth proceeding through nucleation and growth of two-dimensional islands. With increasing miscut-angle, step meandering and finally step flow growth take place. While step-flow growth results in layers with high crystalline perfection, independent nucleation of two-dimensional islands causes double positioning on the (100) plane, resulting in twin lamellae and stacking mismatch boundaries. Applying nucleation theory in the mean field approach for vicinal surfaces, we can fit experimentally found values for the density of twin lamellae in epitaxial layers as dependent on the miscut-angle. The model yields a diffusion coefficient for Ga adatoms of D = 7 × 10−9 cm2 s−1 at a growth temperature of 850 °C, two orders of magnitude lower than the values published for GaAs.
Step-flow growth in homoepitaxy of β-Ga2O3 (100)—The influence of the miscut direction and faceting
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.
Static Dielectric Constant of β-Ga2O3 Perpendicular to the Principal Planes (100), (010), and (001)
2019, Fiedler, A., Schewski, R., Galazka, Z., Irmscher, K.
The relative static dielectric constant ℇr of β-Ga2O3 perpendicular to the planes (100), (010), and (001) is determined in the temperature range from 25 K to 500 K by measuring the AC capacitance of correspondingly oriented plate capacitor structures using test frequencies of up to 1 MHz. This allows a direct quantification of the static dielectric constant and a unique direction assignment of the obtained values. At room temperature, ℇr perpendicular to the planes (100), (010), and (001) amounts to 10.2 ± 0.2, 10.87 ± 0.08, and 12.4 ± 0.4, respectively, which clearly evidence the anisotropy expected for β-Ga2O3 due to its monoclinic crystal structure. An increase of ℇr by about 0.5 with increasing temperature from 25 K to 450 K was found for all orientations. Our ℇr data resolve the inconsistencies in the previously available literature data with regard to absolute values and their directional assignment and therefore provide a reliable basis for the simulation and design of devices. © The Author(s) 2019.