Limited-thickness epitaxy of semiconductors down to room temperature

We use TEM of thin layers to study the low-temperature limit to semiconductor molecular beam epitaxy (MBE). In Si, Ge/Si and GaAs MBE we show that at low temperature limited-thickness epitaxy occurs: layers initially grow epitaxially, and the amorphous phase forms beyond some epitaxial thickness, h_epi. TEM and doping experiments show that the defect density always remains low in these films, and growth on H-terminated Si suggests that in Si, at least, segregation of residual H in the MBE chamber is not responsible: this limited-thickness epitaxy is probably linked to surface increasing roughness. Ge marker layers are used to define the surface morphology in Si(100) growth at low temperature, and TEM measurements confirm a significant increase in surface roughness just before the breakdown of epitaxy. We conclude that increasing surface roughness arising from growth at temperatures where surface diffusion processes are very slow changes the kinetics for nucleation of amorphous zones on the crystalline surface, and is thus responsible for the final breakdown of epitaxy.