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Strain and Absorption Coefficient of Finite Ge Structures on Si

A finite structure of Ge under tensile stress was investigated theoretically and experimentally focusing on applications to near-infrared photodetectors on (001) Si. We calculated the direct band gap energy of strained Ge between the conduction band and the heavy/light-hole valence band via the k·p theory. Three types of in-plane stresses were considered, i.e., a biaxial stress and uniaxial stresses along the <100> and <110> directions. On the basis of the direct band gap change, absorption spectra due to the direct transitions were calculated. The calculated absorption spectra showed that the biaxial stress is more effective than the uniaxial stresses in terms of the absorption red-shift, which increases the detection wavelength range. Localized strain measurements revealed that a selectively grown Ge mesa on (001) Si maintains a biaxial strain caused by the thermal expansion mismatch when its width is larger than 1 µm. A uniaxial stress probably develops owing to the strain relaxation in a finite Ge structure smaller than 1 µm. The application of Ge finite structures to waveguide photodetectors is discussed.