Historically, the surface photovoltage (SPV) effect was first reported by W. H. Brattain (a Nobel laureate in physics) as a light-induced voltage change in the out-of-plane direction in his study of point contact transistors. He investigated the SPV effect mostly with extrinsic elemental semiconductors like n-type silicon. Following Brattain’s steps, the majority of SPV studies in the next few decades focus on measuring SPV with capacitive probes without spatial resolution. In the few experiments where probes with high spatial resolution are employed (e.g., scanning tunneling microscope), only extrinsic elemental semiconductors are examined, since intrinsic elemental semiconductors with indirect bandgaps give very weak SPV signal. The strong built-in electrostatic field in extrinsic semiconductors, however, prevents the formation of in-plane SPV charge distributions. Arguably, the three key ingredients in our experiment, including intrinsic semiconductor, direct bandgap (III-V compound) and high spatial resolution have never been put together in past SPV experiments.

Using an electron microbeam probe, we are able to resolve the structure of in-plane SPV charge distributions on intrinsic GaAs surfaces. The photogenerated electrons and holes are spatially separated by the focused laser beam width, which is around 100 µm. We find that the photogenerated electrons are trapped by oxide-induced surface states. Only the photogenerated holes, which are far less mobile, are allowed to move. The long drift distance (sub-millimeter) combined with the low drift speed result in slow photocarrier recombination and extended lifetime for electron-hole pairs. The ensuing long-lived surface photocarriers can potentially be exploited for new designs of highly efficient solar cells.