Investigation of Lasing in Highly Strained Germanium at the Crossover to Direct Band Gap

TL;DR

This study demonstrates a method to analyze lasing in highly strained germanium microstructures, revealing the conditions and limitations for achieving lasing at the crossover to a direct band gap, with implications for Si-compatible laser development.

Contribution

It introduces a novel measurement approach for determining gain and losses in strained Ge lasers, and elucidates the temperature and strain conditions limiting germanium lasing.

Findings

Lasing observed at low temperatures and specific tensile strain levels.

Parasitic intervalence band absorption limits lasing at higher temperatures.

N-doping reduces material loss but does not extend lasing regime.

Abstract

Efficient and cost-effective Si-compatible lasers are a long standing wish of the optoelectronic industry. In principle, there are two options. For many applications, lasers based on III-V compounds provide compelling solutions, even if the integration is complex and therefore costly. However, where low costs and also high integration density are crucial, group-IV-based lasers - made of Ge and GeSn, for example - could be an alternative, provided their performance can be improved. Such progresses will come with better materials but also with the development of a profounder understanding of their optical properties. In this work, we demonstrate, using Ge microbridges with strain up to 6.6%, a powerful method for determining the population inversion gain and the material and optical losses of group IV lasers. This is made by deriving the values for the injection carrier densities and the cavity losses from the measurement of the change of the refractive index and the mode linewidth, respectively. We observe a laser threshold consistent with optical gain and material loss values obtained from a tight binding calculation. Lasing in Ge - at steady-state - is found to be limited to low temperatures in a narrow regime of tensile strain at the crossover to the direct band gap bandstructure. We explain this observation by parasitic intervalence band absorption that increases rapidly with higher injection densities and temperature. N-doping seems to reduce the material loss at low excitation but does not extend the lasing regime. We also discuss the impact of the optically inactive carriers in the L-valley on the linewidth of group IV lasers.