We have studied the effect of hydrogen annealing on the surface roughness of germanium (Ge) layers grown by chemical vapor deposition on silicon using atomic force microscopy and cross-sectional high resolution scanning electron microscopy (HR-SEM). Our results indicate a strong reduction of roughness that approaches 90% at 825°C. The smoother Ge surface allowed for the fabrication of metal-oxide-semiconductor capacitors using germanium oxynitride ͑GeO x N y ͒ as the gate dielectric. Electrical quality was studied using high frequency capacitance-voltage characteristic of epi-Ge showing negligible hysteresis. We discuss the results in terms of Ge-H cluster formation, which lowers the diffusion barrier, allowing for higher diffusivity and surface mobility. The temperature dependence shows tapering off for temperatures exceeding 800°C, indicating a barrier reduction of ϳ92 meV.

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We demonstrate extremely efficient germanium-on-silicon metal-semiconductor-metal photodetectors with responsivities (R) as high as 0.85 A/W at 1.55 microm and 2V reverse bias. Ge was directly grown on Si by using a novel heteroepitaxial growth technique, which uses multisteps of growth and hydrogen annealing to reduce surface roughness and threading dislocations that form due to the 4.2% lattice mismatch. Photodiodes on such layers exhibit reverse dark currents of 100 mA/cm2 and external quantum efficiency up to 68%. This technology is promising to realize monolithically integrated optoelectronics.

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