We investigate the selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on prepatterned silicon substrates by reduced pressure chemical vapor deposition. A vertical p-i-n Si0.1Ge0.9 diode with Ge/Si0.15Ge0.85 quantum wells in the intrinsic region is selectively grown in holes in a SiO2 mask. We find perfect growth selectivity and very low dependence on size or arrangement of the mask holes. The fabricated p-i-n diode shows very low reverse leakage current and high breakdown voltage, suggesting good epitaxy quality. The quantum-confined Stark effect in this quantum-well system is observed for wavelengths >1.5 μm at room temperature.