Epitaxial Cu₂O Thin Films Deposited from Solution: the Enabling Role of Cu Diffusion into the GaAs Substrate

Cuprous oxide (Cu₂O) thin films were chemically deposited from a solution onto GaAs(100) and (111) substrates using a simple three-component solution at near-ambient temperatures (10-60 °C). Interestingly, a similar deposition onto various other substrates including Si(100), Si(111), glass, fluorine-doped tin oxide, InP, and quartz resulted in no film formation. Films deposited on both GaAs(100) and (111) were found alongside substantial etching of the substrates. The etching of GaAs(100) was uneven, resulting in pyramid-like vacancies, while for GaAs(111), the etching was more even and resulted in a flat interface. X-ray diffraction measurements indicated highly preferential (110) growth of Cu₂O regardless of GaAs substrate orientation, while TEM and a selected area of electron diffraction pointed out epitaxial growth on both substrates. X-ray photoelectron spectroscopy confirmed the diffusion of copper ions into the GaAs up to depths of 20 nm and the formation of intermediate phases at the interface. Raman spectroscopy indicated high structural quality of the films and showed good agreement with TEM and XRD results and Raman shifts corresponding to Cu₂O, with no frequencies typical of CuO. The GaAs substrate appears to play a critical and unusual role in the deposition of Cu₂O thin films on GaAs, which allows for growth of Cu₂O in a previously unreported mechanism.