The Si-based light source has long been considered to be the “holy grail” of Silicon Photonics. Many efforts have been made to achieve light sources on Si platform, such as Si nanocrystals, erbium-doped Si, band-engineered Ge, GeSn alloy, Ge Zener-Emitter andⅢ-Ⅴmaterial on Si. Among those techniques, the band-engineered Ge has attracted great interest due to its Si-compatible fabrication process and ability to be electrically driven. The idea of band-engineered Ge was originally proposed by a MIT group, conceiving that the energy difference between the Γ and L points can be reduced by introducing tensile strain and the remaining electronic states can be further filled by extrinsic electrons from n-type doping so that more injected electrons can occupy the Γ valley where efficient radiative recombination occurs. A variety of strain techniques have been explored including SiNx stressor and suspended microstructures. The highest reported tensile strain values, to the best of our knowledge, are 4.9% and 1.9% in the case of uniaxially and biaxially tensile stressed, respectively.
On the other hand, it is widely accepted that the lasers using low dimensional quantum structures as gain media have lower threshold current density due to the modified density of state and scaling law. Therefore, Ge/SiGe quantum well has received intense research interest as a potential Si-based light source. However, considering the band structure of Ge/SiGe quantum well is different from that of direct bandgap material, it is necessary to theoretically compare the threshold current density of the uniaxially tensile stressed Ge/SiGe quantum well laser and bulk Ge laser.
In this paper, we propose and design uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers with the stress along <100> direction. The micro-bridge structure is adapted for introducing uniaxial stress in Ge/SiGe quantum well. To enhance the fabrication tolerance, full etched circular gratings with high reflectivity bandwidths of ~500 nm are deployed in laser cavities. We compare and analyze the density of state, the number of states between Γ- and L-points, the carrier injection efficiency and the threshold current density for the uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers. Simulation results show that the threshold current density of the Ge/SiGe quantum well laser is much higher than that of the bulk Ge laser even combined with high uniaxial tensile stress owing to the larger number of states between Γ- and L- points and extremely low carrier injection efficiency. Electrical transport simulation reveals that the reduced effective mass of hole and the small conduction band offset cause the low carrier injection efficiency of the Ge/SiGe quantum well laser. Our theoretical results imply that unlikeⅢ-Ⅴmaterial, uniaxially tensile stressed bulk Ge outperforms Ge/SiGe quantum well with the same strain level and is a promising approach for Si-compatible light sources.
The paper is published on Optical Express (Vol. 25, Issue 22, 2017.doi: 10.1364/OE.25.026714). This work is supported by the National Natural Science Foundation of China under Grant No. 61435004.
Fig.1. Schematic diagrams of the uniaxially tensile stressed (a) bulk Ge laser and (b) Ge/SiGe quantum well laser. (c) Cross section view of the straight waveguide. (d) Top view of the circular grating. Reflection coefficient spectra of the fundamental TE-mode for the gratings of (e) bulk Ge laser and (f) Ge/SiGe quantum well laser.
