Pressure-dependent photoluminescence study of band structure in germanium

Germanium (Ge) has long been regarded as a promising laser material for silicon photonics due to its quasi-direct bandgap to make up for the deficiency of indirect bandgap silicon, but its energy band structure under pressure remains puzzling. Here, we study the pressure-dependent photoluminescence (PL) of Ge compressed in a diamond anvil cell to reveal its energy band structure up to 3.89 GPa. Unlike the earlier reported results studied by absorption, the effect of high pressure on bandgaps can be studied for the same sample and the determinations of bandgap positions are not influenced by the sample thickness and the interference pattern. The PL peak related to X–Γ bandgap transition was observed with pressure coefficient of −12.4 ± 2.1 meV/GPa. We unambiguously show that the L- and Γ-valleys move upward while the X–valley moves downward in energy with increasing pressure, with a Γ–X crossover observed at an onset pressure of 0.74 GPa, and then a L–X crossover takes place near 2.85 GPa. These findings provide experimental evidence for identification of the band structure of Ge, deepening the understanding of pressure-induced bandgap modification and conduction band valley crossover.