Effect of increasing thickness on tensile-strained germanium grown on InGaAs buffer layers

We have investigated the optical properties of tensile-strained germanium grown on InGaAs buffer layers as a function of film thickness and buffer layer composition. We study the dependence of the photoluminescence as a function of the strain amplitude and degree of relaxation which are also monitored by X-ray diffraction and Raman spectroscopy. We show that 0.75 biaxially strained germanium can be obtained up to a thickness of 150 nm, a value sufficiently high to allow confinement of the spontaneous emission in a guiding structure. For large thicknesses (>200 nm) and large indium content in the buffer layer, a partial relaxation of the film is observed characterized by a large in-plane anisotropy of the germanium lattice. In this case, a difference of strain magnitude deduced either by microphotoluminescence spectra or by X-ray or Raman measurements is reported. We explain this difference by the sensitivity of microphotoluminescence to the local properties of the material. This study provides guidelines in order to achieve high optical quality and high biaxial tensile strain in Ge films with thicknesses compatible with optical waveguiding.