High-concentration Ge doping of (001) β-Ga2O3 by halide vapor phase epitaxy beyond the practical Si doping limit

High-concentration n-type doping of (001) β-Ga2O3 grown by halide vapor phase epitaxy (HVPE) was investigated using Si and Ge dopants. While Si doping caused severe surface degradation when [Si] exceeded approximately 6 × 1018 cm–3, Ge doping produced homoepitaxial layers without surface degradation up to [Ge] = 1.9 × 1019 cm–3. X-ray diffraction measurements revealed that the degraded Si-doped regions contained in-plane rotated twin domains and exhibited broadened X-ray rocking curves, whereas the heavily Ge-doped layers maintained single crystallinity with crystalline quality comparable to that of the substrate. Hall-effect measurements showed that the carrier concentration (n) closely followed [Ge], indicating nearly complete electrical activation of the incorporated Ge donors. The electron mobility and bulk resistivity followed the previously reported carrier-concentration dependence for HVPE-grown Si-doped (001) β-Ga2O3. A minimum bulk resistivity of 11.6 mΩ cm was obtained. Furthermore, Ti/Pt/Au contacts fabricated on the Ge-doped layers exhibited a specific contact resistivity as low as 9.6 × 10–6 Ω cm2 for n = 1.3 × 1019 cm–3, comparable to values reported for more heavily Si-implanted β-Ga2O3. These results demonstrate that Ge enables heavily doped n-type (001) β-Ga2O3 layers to be grown by HVPE beyond the practical Si doping limit while preserving excellent crystalline quality. This capability enables monolithic growth of a thick drift layer together with heavily doped contact layers in a single HVPE growth run, providing a promising route toward lower-loss, lower-cost, and higher-performance β-Ga2O3 vertical power devices. Ge enables high-concentration n-type doping in HVPE-grown (001) β-Ga2O3 while preserving crystal quality, providing a practical route to low-resistance contact layers for high-performance, low-cost power devices.