Strain-induced band gap shrinkage in Ge grown on Si substrate

Band gap shrinkage induced by tensile strain is shown for Ge directly grown on Si substrate. In Ge-on-Si pin diodes, photons having energy lower than the direct band gap of bulk Ge were efficiently detected. According to photoreflectance measurement, this property is due to band gap shrinkage. The origin of the shrinkage is not the Franz–Keldysh effect but rather tensile strain. It is discussed that the generation of such a tensile strain can be ascribed to the difference of thermal expansion between Ge and Si. Advantages of this tensile Ge for application to photodiode are also discussed.