Anti-phase domain-free GaAs on Ge substrates grown by molecular beam epitaxy for space solar cell applications

Elimination of anti-phase domains (APDs), threading dislocations and uncontrolled interface diffusion are critical considerations for achieving maximum design flexibility and high efficiency in multi-bandgap III-V solar cells on Ge. In this paper, we identify critical growth steps to eliminate each of these problems and present an optimum molecular beam epitaxy (MBE) growth procedure which yields APD-free, near-dislocation-free GaAs/Ge with greatly suppressed interdiffusion in both the GaAs overlayer and Ge substrate. For solid source MBE, elimination of APDs requires a double-stepped, clean Ge surface and a prelayer consisting of a complete monolayer of either As or Ga. Correct conditions can be observed and maintained by real-time in-situ monitoring to ensure reproducibility. Initiating growth at low temperature with migration enhanced epitaxy virtually eliminates Ge diffusion into GaAs and Ga diffusion into Ge, while As diffusion into Ge is substantially suppressed.