Limitations on activation of high dose Ge implants in β -Ga2O3

Among ultrawide bandgap semiconductors, β-Ga2O3 is particularly promising for high power and frequency applications. For devices, n-type concentrations above 1019 cm−3 are required. Ge is a promising alternative n-type dopant with an ionic radius similar to Ga. Homoepitaxial (010) β-Ga2O3 films were implanted with Ge to form 50 and 100 nm box concentrations of 3 × 1019 and 5 × 1019 cm−3, with damage ranging from 1.2 to 2.0 displacements per atom. For lower damage implants, optimized anneals in ultrahigh purity N2 at 950–1000 °C for 5–10 min resulted in an RS of 600–700 Ω/□, mobilities of 60–70 cm2/V s, and a Ge activation of up to 40%. For higher damage implants, activation dropped to 23% with similar mobilities. Ge diffusion, measured by secondary ion mass spectrometry, showed the formation of a Ge “clustering peak” with a concentration exceeding the initial implant following anneals in N2 or O2 at 950–1050 °C. Beyond this peak, minimal Ge diffusion occurred for N2 anneals at 950 °C, but at 1050 °C, non-Fickian diffusion extended to >200 nm. Electrical activation data suggest that clustered Ge is electrically inactive. To understand Ge clustering, several samples were characterized by synchrotron x-ray diffraction. Second-phase precipitates were observed in as-implanted samples, which then fully dissolved after furnace annealing in N2 at 1050 °C. Diffraction peaks suggest that these implant-induced precipitates may be related to a high pressure Pa3¯ phase of GeO2 and may evolve during anneals to explain the Ge clustering. Ultimately, we believe that Ge clustering limits the activation of implanted Ge at high concentrations.