Direct and indirect band gaps in Ge under biaxial tensile strain investigated by photoluminescence and photoreflectance studies

Germanium is an indirect semiconductor which attracts particular interest as an electronics and photonics material due to low indirect-to-direct band separation. In this work we bend the bands of Ge by means of biaxial tensile strain in order to achieve a direct band gap. Strain is applied by growth of Ge on a lattice mismatched InGaAs buffer layer with variable In content. Band structure is studied by photoluminescence and photoreflectance, giving the indirect and direct bands of the material. Obtained experimental energy band values are compared with a $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ simulation. Photoreflectance spectra are also simulated and compared with the experiment. The obtained results indicate direct band structure obtained for a Ge sample with $1.94%$ strain applied, with preferable $\mathrm{\ensuremath{\Gamma}}$ valley to heavy hole transition.