Improved thin film quality and photoluminescence of N-doped epitaxial germanium-on-silicon using MOCVD

This paper addresses one of the key issues in the scientific community of Si photonics: thin-film quality and the light emission properties of band-engineered n + Germanium-on-Silicon (Ge-on-Si). Compared to the traditional delta doping approach, which was utilized in the first electrically-pumped Ge-on-Si lasers, we offer an n + Ge-on-Si thin film with better material quality and higher carrier injection efficiency grown by metal-organic chemical vapor deposition (MOCVD). The impacts of thermal cycle annealing and Si substrate offcut on the thin film quality were investigated, including surface roughness, strain, threading dislocation density, Si-Ge interdiffusion, and dopant diffusion. It was revealed that: 1) MOCVD overcomes the outdiffision issue of n-type dopants by having the dopant peaks at the bottom of the Ge films; 2) the characterization of the light emission properties of these MOCVD n + Ge-on-Si samples (1.0 × 10 19 cm −3 doped) compared to delta-doped ultra-high vacuum chemical vapor deposition (UHVCVD) Ge, showing comparable photoluminescence (PL) spectral intensity at 1/4 of the doping level; 3) Detailed PL spectral analyses showed that population inversion from the direct gap transition has been achieved, and the injected electron density in the direct Γ valley is comparable to that of the delta-doped sample even though the n-type doping level is 75% less; and 4) Experimental evidences that Si-Ge interdiffusion has a much larger impact on PL intensity than threading dislocation density in the range of 10 8 -10 9 /cm 3 . These results indicate that MOCVD n + Ge is very promising to reduce the threshold of Ge gain media on Si notably.