High-performance near-IR photodiodes: A novel chemistry-based approach to Ge and Ge-Sn devices integrated on silicon

Ge/Si heterostructure diodes based on n⁺⁺Si(100)/i-Ge/p-Ge and p⁺⁺Si(100)/i-Ge/n-Ge stacks and intrinsic region thickness of ∼350 and ∼ 900 nm, respectively, were fabricated using a specially developed synthesis protocol that allows unprecedented control of film microstructure, morphology, and purity at complementary metal-oxide- semiconductor compatible conditions. From a growth and doping perspective, a main advantage of our inherently low-temperature (390 °C) soft-chemistry approach is that all high-energy processing steps are circumvented. Current-voltage measurements of circular mesas (60-250 μm in diameter) show dark current densities as low as 6 × 10^-3 A/cm² at -1 V bias, which is clearly improved over devices fabricated under low thermal budgets using traditional Ge deposition techniques. Spectral photocurrent measurements indicate external quantum efficiencies between 30 and 60% of the maximum theoretical value at zero bias, and approaching full collection efficiency at high reverse biases. The above Ge devices are compared to analogous low-temperature-grown (350 ° C) Ge_0.98Sn_0.02 diodes. The latter display much higher dark currents but also higher collection efficiencies close to 70% at zero bias. Moreover, the quantum efficiency of these Ge_0.98Sn_0.02 diodes remains strong at wavelengths longer than 1550 nm out to 1750 nm due to the reduced band gap of the alloy relative to Ge.