In Situ Electric-Field-Induced Second Harmonic Generation (EFISH) Spectroscopy of TiO2-Passivated p-GaAs Photoelectrodes.

We employ in situ second harmonic generation (SHG) spectroscopy to investigate the built-in electric field in p-type GaAs semiconductor photoelectrodes. Here, the SHG signal increases linearly above and below the flat band potential, consistent with an increased built-in electric field and band bending under depletion conditions. Both SHG and Mott-Schottky measurements show a shift in the flat band potential upon deposition of a TiO2 protective layer due to TiO2 creating a hole-blocking layer. Measurements performed under illumination show the effects of band flattening both in the SHG measurements and in the photocurrent measurements. We observe an enhancement in the photovoltage of TiO2-passivated p-GaAs electrodes, which is attributed to both the increased electric field and a shift in the flat band potential. Specifically, we observe a 0.2 V shift in the SHG minimum with the addition of the TiO2 layer. Additionally, we probe band flattening in p-GaAs electrodes and show that electric-field-induced second harmonic (EFISH) signals, excited by a 430 nm tunable light-emitting diode (LED), decrease exponentially with an increasing LED power. As the light power rises, the photoinduced charge carrier density in the space-charge region increases, opposing the built-in electric field. This results in a significant reduction of the electric field, which is reflected in the decreased SHG signal. Thus, EFISH spectroscopy enables instantaneous and accurate measurement of the built-in electric field under varying electrochemical potentials and illumination conditions.