Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity.

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Surface photovoltage in undoped n-type GaN

Near-band gap transitions in the surface photovoltage spectra for GaAs, GaP and Si surfaces

Transient response in n-InP PEC cells and effect of surface modification

The role of band structure in the photovoltaic effect is examined. Photovoltage is predicted to be sensitive to Van Hove singularities. This prediction is verified experimentally. Spectra of GaAs, GaP, and GaN at above-band-gap photon energies are compared with calculated band structure showing that band features, such as conduction band valleys and valence band peaks, have a clear manifestation in photovoltage spectra, for both direct and indirect transitions. This effect facilitates a band structure probe. This probe reveals experimental evidence on the 2H-GaN valence band structure.

W. A. Orr

Papers

GaP photovoltage transients