Optical spectroscopy of strained germanium micro-structures

Kévin Guilloy, Institute for Nanosciences and Cryogenics, Université Grenoble Alpes

Abstract:
Despite the indirect nature of its band gap, germanium is a strong candidate as a laser source for silicon photonics. The application of tensile strain is known to reduce the energy difference between its direct and indirect band gaps. We report here several micro-fabrication processes allowing to stress germanium micro-crystals. We carried out Laue X-ray micro-diffraction at the BM32 beamline of ESRF, showing that the longitudinal strain can reach 5 %. Raman scattering experiments reveal that the relation between strain and Raman shift is nonlinear above about 2 % strain. Additionally, we performed photocurrent, photoluminescence and electro-absorption spectroscopy in order to measure the direct band gap of these micro-objects. The strain dependence of the band edges was observed to be significantly different than predicted using the deformation potential theory, when the strain reaches several percent. We compared the obtained experimental results to theoretical calculations fully supporting the observed trends.