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Photovoltaic Effect in a III-IV-V Compound Semiconductor
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DOI：10.1002/pip.70141.png)
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Abstract
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We have studied the potential of (GaAs) 1 − x $$ {}_{1-x} $$ (Ge 2 $$ {}_2 $$ ) x $$ {}_x $$ 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.
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