(Invited) Direct Band Gap Germanium

Abstract

Germanium is an indirect band gap material. This property is a significant disadvantage to realize an efficient laser emitter with this material. This feature can nonetheless be circumvented by strain engineering of the band structure. It is well known that tensile strain can reduce the energy difference between the conduction band valleys and ultimately lead to a band inversion. With a direct band gap material, one should expect to obtain optical gain with a significantly reduced threshold as compared to unstrained or weakly-strained Ge. Consequently, the integration of a group IV emitter on a silicon chip could become more realistic. In this presentation, we will show that direct band gap germanium can be obtained by transferring tensile strain with nitride stressor layers. We have experimentally determined when the cross-over from indirect to direct band gap occurs using temperature-dependent photoluminescence measurements. This cross-over was found to occur for a biaxial tensile strain of 1.67%, the theoretical values varying between 1.5 and 2% in the literature. We will show that this level of strain transfer is compatible with the fabrication of optical micro-resonators with germanium microdisks. Tensile-strained germanium microdisks with circular Bragg reflectors have been successfully fabricated. They exhibit high quality factors for the quasi-radial modes. These types of structures are ideal candidates to realize microlasers with pure germanium. We will discuss the latest progress in this direction.