Heterogeneous Integration of Device Grade Epitaxial Germanium on Silicon Platform using Laser Induced Crystallization for Optoelectronic Applications

Abstract

Integration of germanium on silicon platform has applications in III-V photovoltaics, integrated silicon photonics, high-speed transistors, etc. In this work, we have used laser-induced crystallization (LIC) to obtain crystalline germanium (LIC-Ge) on silicon platform. LIC is a fast, scalable, and low-thermal budget process than CVD or furnace-based processes. Here, we have explored the effect of film thickness, scanning velocity, and laser intensity on the nature of crystallization and grain size. The annealing set-up includes just the laser and the sample stage. The samples are processed in ambient conditions and at room temperature. The grown films are characterized using SEM, XRD, TEM, and Raman measurements. For film thicknesses greater than 300 nm, polycrystalline Ge with maximum grain size in the range of 400 nm - 600 nm was obtained. Polycrystalline LIC-Ge films were obtained on SiO2/Si and TiN/Si substrates also. The resulting smaller grain sizes are a result of superfast cooling happening during the nanosecond pulsed laser annealing, as explained through the COMSOL simulation of the process. Germanium acts as a virtual substrate for growing thin-film GaAs. To enable device application for these polycrystalline germanium films, we aimed for low-cost high efficiency polycrystalline thin-film GaAs solar cells. To get a preliminary idea, solar cells with varying grain sizes and base thicknesses were simulated in two dimensions using SILVACO ATLAS software. GaAs film thickness and minimum grain size requirements for gt20 % efficient solar cells were obtained as 3-5 and#956;m and gt100 and#956;m respectively. The GaAs layers must also be free from contamination, with an electron recombination lifetime of at least 10 ns. This lifetime approximately corresponds to a defect density in the range of 1012-1013 cm-2. These numbers can be used as guidelines by materials growth specialists to fabricate better thin-film GaAs solar cells...