Improved Thin Film Quality and Photoluminescence of N-Doped Epitaxial Germanium-on-Silicon using MOCVD

Abstract: Ge-on-Si structures in-situ doped with phosphorus or arsenic via metal organic chemical vapor deposition (MOCVD) were investigated. Surface roughness, strain, threading dislocation desnity, Si-Ge interdiffusion, dopant diffusion, and photoluminescence were characterized to study the impacts of defect annealing and Si substrate offcut effects on the Ge film quality and most importantly, the light emission properties. All samples have a smooth surface (roughness < 1.5 nm), and the Ge films have a small tensile strain of 0.2%. As-grown P and As-doped Ge films have threading dislocaiton densities from 2.8e8 to 1.1e9 cm^-2 without defect annealing. With thermal cycling, these values reduced to 1-1.5e8 cm^-2. The six degree offcut of the Si substrate was shown to have little impact. In contrast to delta doping, the out-diffusion of dopants has been successfully suppressed to retain the doping concentration upon defect annealing. However, the photoluminescence intensity decreases mostly due to Si-Ge interdiffusion, which also causes a blue-shift in the emission wavelength. Compared to a benckmarking sample from the first Ge laser work doped by delta doping method in 2012, the as-grown P or As-doped Ge films have similar photoluminescence intensity at a 25% doping concentration and smoother surface, which are promising for Ge lasers with better light emission efficiencies.