Silicon photonics is regarded as a critical technology that can achieve photonic integrated circuits (PIC) with lower cost and higher integration density. Apart from chip-scale optical interconnect and optical communication in popular S(1460–1530 nm), C (1530–1565 nm) and L (1565–1625 nm) bands, silicon photonics has recently found it applications in mid-infrared band for short distance optical communication as well as spectroscopy, chemical and biological sensing. The past two decades have witnessed a rapid development in silicon photonics which involves most of the building blocks such as hybrid III-V laser, modulator, germanium detector and a variety of passive components. However, a CMOS-compatible on chip light source which enables high-volume fabrication is still a missing piece.Even though the hybrid III-V laser exhibits desirable performance, the scalability of this technique is inferior to a fully CMOS-compatible approach. The strain engineering of Germanium is a promising approach to achieve a CMOS-compatible light source, however, the low threshold electrically driven laser using highly tensile strained Ge has not yet been demonstrated even theoretically.
In this paper, we propose a CMOS-compatible Distributed Bragg Reflector (DBR) laser based on the Ge micro-bridge structure. Our design combines three critical elements for an efficient electrically pumped CMOS-compatible laser: highly uniaxial tensile stressed Ge, low loss optical resonator and heterojunction for carrier injection. The DBR laser has the following three advantages. First, optical gain of Ge is remarkably improved due to the decreased energy difference between direct and indirect gap. Second, the passive region is naturally transparent to the light emitted by the active region since the band gap of the passive region is larger than that of the active region, which enables simple fabrication of the device. Third, the smaller band gap of the active region also forms a horizontal quasi-heterojunction which is beneficial to carrier injection. The threshold characteristics and the internal quantum efficiency of the DBR laser is simulated. As a crucial factor in an electrically pumped laser, the effects of nonradiative recombination including SRH recombination and Auger recombination on the threshold current density and the internal quantum efficiency are discussed. The effects of strain and n-type doping on the laser performance are also investigated. Simulation results signify that the proposed Ge DBR laser can be a promising candidate for silicon-based light source.
The paper is published on Optical Express (Vol. 25, Issue 6, 2017.doi: 10.1364/OE.25.006497). This work is supported by the National Natural Science Foundation of China under Grant No. 61435004.
Fig.1. (a) Schematic of the micro-bridge structure for introducing high uniaxial tensile stress in Ge (not to scale).(b) Straindistribution of the micro-bridge in the x-y plane with a distance of 150nm away from the bottom of Ge layer. (c) Schematic of the DBR laser based on the micro-bridge structure (not to scale).
