Making Germanium, an Indirect Band Gap Semiconductor, Suitable for Light-Emitting Devices

Germanium (Ge) is a group-IV indirect band gap semiconductor but the difference between its direct and indirect band gap is only 140 meV. It has been shown that when Ge is subjected to a tensile strain and a heavy n-doping level, room-temperature photoluminescence (PL) can be greatly enhanced. Among these two factors, achieving a heavy n-doping level in Ge (i.e., electron concentrations higher than 1 × 1019 cm−3) is a challenge since the solubility of most group-V elements (P, As, Sb) in Ge is very low. We report here Ge growth on silicon substrates using molecular beam epitaxial (MBE) technique. To enhance the n-doping level in Ge, a specific n-doping process based on the decomposition of the GaP compound has been implemented. The GaP decomposition allows producing P2 molecules, which have a higher sticking coefficient than that of P4 molecules. We show that phosphorus doping at low substrate temperatures followed by flash thermal annealing are essential to get a high doping level. We have obtained an activate phosphorus concentration up to 2 × 1019 cm−3 and room-temperature PL measurements reveal an intensity enhancement up to 50 times. This result opens a new route for the realization of group-IV semiconductor optoelectronic devices.