Jialin Jiang and Junqiang Sun*
Silicon is an attractive platform for photonic integration circuits due to its expected low-cost and mass-production. Silicon photonics has its applications spanning from high-capacity and energy-efficient optical interconnect to spectroscopy, chemical and biological sensing. Although most of the building blocks for silicon photonics, such as passive components, photo detector and modulator have been demonstrated, the on-chip CMOS-compatible light source still remains a barrier which hinders the implementation of an ideal photonics chip. Germanium is considered to be a promising candidate for Si-based light source due to its unique pseudo-direct gap band structure. Theoretical and experimental works have shown that Ge can be turned into a direct-gap material and the light-emitting efficiency can be dramatically enhanced with the help of tensile strain and n-type doping. In addition, quantum structures such as quantum wells, wires and dots also provide a potential to lower the high threshold current in the state-of-art bulk Ge laser.
In this paper, we first evaluate the optical gain of the  uniaxial tensile strained and n+-doped Ge/GeSi quantum well. The model is established on the basis of 8 Band k∙p method which is an efficient approach to study the optical and electrical properties of semiconductor. As the main loss in heavily doped and highly injected material, Free Carrier Absorption (FCA) is considered by the modified Drude-Lorentz model. The impacts of tensile strain and n-doping on the band structure, carrier occupation, polarization dependent gain and FCA loss are discussed. Net peak gain and transparency carrier density at various strain values and doping concentrations are also obtained. A net peak gain up to 2061 cm-1 for TE-polarized light is predicted at realistic strain value and doping concentration, indicating that the proposed QW can be a promising candidate for Si-based light source.
The paper is published on Optical Express (Vol. 24, Issue 13, 2016.doi: 10.1364/OE.24.014525). This work is supported by the National Natural Science Foundation of China under Grant No. 61435004.
Fig. 1. Neat peak gain for TE-polarized light as a function of (a) tensile strain and doping concentration with an injected carrier density of , (b) tensile strain and injected carrier density with a doping concentration of , (c) doping concentration and injected carrier density with a strain value of 4%.