ZnS-GaP Solid Solution Thin Films with Enhanced Visible-Light Photocurrent

A high photocurrent, particularly under visible-light wavelengths, is critical for developing a semiconductor photoelectrode for efficient solar-to-hydrogen conversion. Here, we demonstrate a ZnS-GaP solid solution thin film grown on a silicon substrate by pulsed laser deposition, where the growth conditions are tailored to promote intermixing throughout the entire film thickness. The photocurrent density of this solid solution film reaches a maximum of ∼27 μA/cm2 at ∼0.9 V bias, which is ∼3 times higher than that of a comparable multilayered ZnS-GaP film, where ZnS and GaP form distinct layers. In addition, the solid solution film shows up to 50 times stronger photosensitivity compared to the multilayered film. Examination of the local atomic structure and nanoscale chemistry of the solid solution thin film using transmission electron microscopy and energy-dispersive X-ray spectroscopy techniques revealed the formation of quaternary solid solution (Ga,Zn)(P,S) and ternary (Ga,Zn)Sb phases, as well as some trace amounts of binary GaSy. These phases have previously been shown to have a direct band gap in the energy range of visible light. We thus attribute the enhanced photocurrent and photosensitivity in the solid solution film to the presence of the aforementioned phases as well as defects.