Ultra-High Boron In Situ Doping of Germanium Layers Grown on Si(100) Substrates via Low-Temperature UHV-CVD Methods

Ge2H6-based ultrahigh-vacuum chemical vapor deposition (UHV-CVD) has rarely been explored for the fabrication of hyper-doped Ge:B, despite its potential for low-temperature and CMOS-compatible epitaxy. Here, we demonstrate that low precursor flow rates and sub-Torr growth pressures have enabled exceptionally high B incorporation in monocrystalline Ge. Across the investigated sample series, the maximum total B concentration has reached 9.42 × 1021 cm−3, the highest active carrier concentration has reached 1.6 × 1021 cm−3, and the minimum resistivity obtained was 2.56 × 10−4 Ω·cm. These extrema were obtained from different samples grown under similar UHV-CVD conditions. The use of a Ge buffer reduced lattice-mismatch-related degradation and improved both crystalline quality and electrical conductivity. SIMS quantification of the total boron incorporation ratio was supported by RBS analysis, while TEM showed a coherent Ge:B/i-Ge interface at 11.2% B without amorphization. A specific contact resistivity of 2.35 × 10−9 Ω·cm2 was obtained based on selected highly-doped Ge:B samples. These results demonstrate the capability of UHV-CVD for producing hyper-doped Ge:B and identify the balance among B incorporation, electrical activation, and structural integrity as the key criterion for process optimization.