Photovoltaic Effect in a III‐IV‐V Compound Semiconductor

ABSTRACT

We have studied the potential of (GaAs)(Ge) alloys as a photovoltaic material. Solar cells and single crystal films have been grown by molecular beam epitaxy on GaAs (001) substrates. Even with a moderate Ge content (6.5%), a large reduction of the band gap is obtained (from 1.42 to 1 eV). Intense room temperature luminescence is found for n‐type samples. Both n‐type and p‐type samples have high carrier densities, high compensation, and consequently low mobilities. X ray diffraction results indicate a large positive deviation from Vegard's law. Density functional modeling suggests that this deviation from Vegard's law is evidence of the formation of a true alloy, rather than a nanoscale mixture of GaAs and Ge domains. The latter possibility is also discarded by transmission electron microscopy. Energy dispersive x‐ray spectroscopy indicates that films have As deficiency, even when using a 14 to 1 V/III flux ratio, presumably due to the presence of Ge at the surface inhibiting As incorporation. Cross‐sectional scanning tunneling microscopy reveals the presence of vacancies and antisites. It is also found that Ge can substitute both Ga and As sites, with a prevalence of the former sites in n‐type samples. Heterojunction solar cell devices were fabricated with 0.76‐V open circuit voltage and an internal quantum efficiency above 40% at energies below the bandgap of the GaAs emitter.