(Invited) Toward GeSn Lasers: Light Amplification and Stimulated Emission in GeSn Waveguides at Room Temperature
Authors: Mathews Jay , Li Zairui , Zhao Yun , Gallagher James , Agha Imad , Menendez Jose , Kouvetakis John 
Published: February 2020

The search for Si-based lasers has been ongoing for many years. GeSn alloys represent a possible solution to this problem due to their unique band structure. Pure Ge is an indirect band gap semiconductor, but the difference between the conduction band minima at the L- and Γ-points is only 140 meV. The diamond cubic structure of Sn, α-Sn, is a semimetal. When Sn is substituted for Ge in the Ge lattice, a hybrid band structure is formed, yielding a material with a tunable band gap. For Sn concentrations around 10% or lower, the band structure is still Ge-like with the lowest conduction band minimum at the L-point. However, the energy difference between the L- and Γ-minima reduces with increasing Sn concentration. For higher Sn concentrations, the minimum at the Γ-point becomes the lowest minimum, thereby creating a direct band gap semiconductor.
For the lower Sn concentration regime, where the band gap of GeSn is still indirect, it is still possible to exploit the band structure to achieve direct band gap emission. Even in pure Ge, where the energy difference between the L- and Γ-minima is highest, there is a spillover of conduction band electrons into the Γ-valley, which then transition to the valence band through optical recombination. This effect can be further exploited through the use of n-type doping. The additional electrons from the n-type dopant serve to increase the quasi-Fermi level in the material, thereby filling the L-valley states with electrons. As the material is then injected with new electrons, using either optical or electrical pumping, the new electrons will have a much higher probability of occupying the Γ-valley, where they will undergo optical recombination and emit photons with the direct band gap energy. This effect has been used previously to create optically- and electrically-pumped lasers in tensile strained Ge-on-Si. We now extend this idea to the GeSn system.
In this work, I will discuss efforts to create laser structures that operate at room temperature using GeSn layers with Sn concentrations in the range of 4.4-7.2% that have been highly n-type doped (1019-1020 cm-3) with phosphorus. Waveguides were fabricated from GeSn films grown on Ge-buffered-Si substrates using low pressure chemical vapor deposition (LPCVD) of SnD4 and Ge2H6. The GeSn films were doped in situ using the custom gaseous precursor P(GeH3)3. The waveguides were defined using standard UV photolithography, and then etched using reactive ion etching in a BCl3 plasma. The end facets of the waveguides were mechanically polished to a mirror finish, and then an Al film was sputtered on one facet to form a high-reflector. The Fresnel reflection on the other facet was used as the output coupler.
The GeSn waveguides were optically pumped using a 976 nm CW laser. The waveguide emission from the output coupler was collected using an optical fiber, and the pump wavelength was filtered out using a dielectric filter. The resulting emission was focused onto a LN2-cooled extended InGaAs detector, and the output intensity was measured as a function of input intensity. At low power, the output intensity is proportional to the input intensity, as one would expect from spontaneous emission. As the input intensity is increased, there is an exponential increase in output intensity, indicative of stimulated emission. This stimulated emission was observed in waveguides with Sn concentrations over the entire range tested. As the Sn content is increased, the transition to stimulated emission becomes sharper, thereby indicating the increased efficiency of optical emission due to lowering the difference between the indirect and direct conduction band minima. This is the first observation of stimulated emission in GeSn at room temperature.