Surface States for Photoelectrodes of Gallium Phosphide (GaP) with Surface-Specific Electronic Spectra and Phase Measurements

Gallium phosphide (GaP) photoelectrodes havereceived tremendous attention owing to their applications inphotocatalysis and photoelectrocatalytic reduction of CO2. Surfaceelectronic states of GaP are important in such catalysis applications.However, knowledge of surface states of GaP under ambientconditions is lacking. Here, we combined azimuth-dependentelectronic sum-frequency generation (ESFG) spectroscopy withphase measurements to investigate the surface states for n-type andp-type GaP(100) semiconductors. ESFG spectroscopic studiesenabled us to identify three surface states of the GaP crystals underambient conditions. These experiments have also shown that all ofthe spectral features come from surface contributions for both then-type and p-type GaP(100) crystals and that both surface dipoles and surface charges were responsible for the electronic transitionsof isotropic and anisotropic components. Combined with azimuth-dependent phase measurements, surface charges were found toaccount for the isotropic surface ESFG components: negative for n-type and positive for p-type GaP(100). Finally, we conducted athorough theoretical analysis of surface and bulk contributions for azimuth-dependent ESFG responses. With these spectral andphase signatures, we have further quantified surface and bulk contributions along different orientations for the n-type and p-typeGaP(100) crystals.