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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 \sim350 and \sim900~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^\circC) soft-chemistry approach is that all high-energy processing steps are circumvented. Current–voltage measurements of circular mesas (60–250 \mum in diameter) show dark current densities as low as 6 \times 10^-3~A/cm^2 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^\circC) Ge{0.98}Sn0.02 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 Ge0.98Sn0.02 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. [ABSTRACT FROM PUBLISHER]