Heterogeneous Integration of Device Grade Epitaxial Germanium on Silicon Platform using Laser-Induced Crystallization for Optoelectronic Applications
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 >20 % efficient solar cells were obtained as 3-5 μm and >100 μm 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. This efficiency can be improved by ~2% by using a p-type base and ~ 3% by the addition of an AlGaAs window layer. However, to grow thin-film GaAs with grain size> 100 μm on polycrystalline germanium (as a virtual substrate), grains of comparable sizes are needed. Hence, as per these simulation results, the goal of fabricating high-efficiency polycrystalline thin-film GaAs solar cells on the demonstrated polycrystalline LIC-Ge seems difficult. Therefore, our focus shifted to epitaxial LIC-Ge obtained on films with thickness < 300 nm. The growth of high-quality epitaxial germanium on silicon is a challenge owing to the 4.2 % lattice mismatch between germanium and silicon. Processes employed to get epitaxial germanium (epi-Ge) over silicon include ultra-high vacuum chemical vapor deposition (CVD), molecular beam epitaxy (MBE), metal-organic CVD, reduced pressure CVD, and liquid phase epitaxy (LPE). These processes require complex equipment involving high and ultra-high vacuum, extremely careful surface preparation, and graded buffer, which increase cost and constrain the application space for epi-Ge. In this work, epitaxial germanium is obtained using LIC in ambient conditions and at room temperature. Yet the films show excellent crystallinity with a rocking curve full width at half maximum of only 0.20°, i.e., an implied threading dislocation density (TDD) of 6 x 108 cm-2. The epitaxy is demonstrated at the wafer-scale by rastering the laser spot across a 2ʺ wafer. To enable device applications, MSM (Metal Semiconductor Metal) photodetectors which find use in next-generation silicon photonics and as IR (Infra-Red) sensors, were fabricated on epitaxial LIC-Ge on Si films using Cr-Au as metal contacts. The detectors were found to be responsive in the IR range from 1100 nm to 1700 nm with high responsivity. TiO2 interlayer was used between semiconductor and metal to reduce dark current.