The authors report a method to grow high quality strain-relaxed Ge on a combination of low-temperature Ge seed layer on low temperature ultrathin Si0.8Ge0.2 buffer with thickness of 27.3nm by ultrahigh vacuum/chemical-vapor-deposition method without the need to use chemical mechanical polish or high temperature annealing. On 8in. Si wafer, the etch-pit density was 6×106cm−2. The root-mean-square surface roughnesses of Ge epitaxy by atomic force microscopy were 1.4 and 1.2nm for bulk Si and silicon-on-insulator substrates, respectively. Micro-Raman spectroscopy shows extremely uniform distribution of residual strain in the overgrown Ge epitaxy on 8in. wafers.

Si-waveguide-integrated lateral Ge p-i-n photodetectors using novel Si/SiGe buffer and two-step Ge-process are demonstrated for the first time. Comparative analysis between lateral Ge p-i-n and vertical p-Si/i-Ge/n-Ge p-i-n is made. Light is evanescently coupled from Si waveguide to the overlaying Gedetector, achieving high responsivity of 1.16 A/W at 1550 nm with f 3 dB bandwidth of 3.4 GHz for lateral Ge p-i-n detector at 5 V reverse bias. In contrast, vertical p-Si/i-Ge/n-Ge p-i-n has lower responsivity of 0.29 A/W but higher bandwidth of 5.5 GHz at −5 V bias. The higher responsivity of lateral p-i-n detectors is attributed to smaller optical mode overlap with highly doped Ge region as in vertical p-i-n configuration.

The authors report the performance of selective epitaxial Ge ͑400 nm͒ on Si-on-insulator p-in mesa-type normal incidence photodiodes using ϳ14 nm low-temperature Si 0.8 Ge 0.2 buffer without cyclic annealing. At −1 V, very low bulk dark current densities of 1.5-2 mA/ cm 2 were obtained indicating good material quality, and the peripheral surface leakage current densities were 14-19.5 A / cm. For 28 m diameter round photodiode, the highest achieved external quantum efficiencies at −5 V were 27%, 9%, and 2.9% for 850 nm, 1.3 m, and 1.56 m optical wavelengths, respectively. 15ϫ 15 m 2 square photodiode has 3 dB bandwidth ജ15 GHz at −1 V. Good performance was achieved without high-temperature annealing, suggesting easy integration of Ge/ Si photodiode unto existing complementary metal-oxide-semiconductor process.