Investigation of the negative peak in photoinduced transient spectra of semi-insulating gallium arsenide

A common observation in the photoinduced transient (PITS) spectra of semi-insulating GaAs is the appearance of a negative peak, which is anomalous in that both electron and hole traps should give rise to positive peaks. In this paper, it is shown that the negative peak can be explained in terms of charge exchange with the GaAs surface and only occurs in material which displays particular types of current-voltage and current-temperature characteristics. The dependence of this peak on the processing effects of surface passivation, etching, and baking and polishing has been investigated and its sensitivity to variations in incident light intensity is demonstrated. A new variation of PITS, namely gated-PITS has been employed. This technique suppresses the negative peak in the spectrum, allowing transients corresponding to emission from EL2 to be detected in particular undoped liquid encapsulated Czochralski GaAs substrates for the first time.