Positron annihilation in GaAs at low temperatures has been studied with positron-lifetime and diffusion-length measurements. The lifetime results show that in addition to vacancy-type deep traps, positron trapping with a lifetime very close to the bulk value of 230 ps occurs below 200 K. This is observed together with a strong decrease in the positron-diffusion length from 1500 to 800 Å measured with a slow-positron beam. The results give direct evidence on positron localization at shallow traps in GaAs. A Rydberg state around a negative point charge is suggested for the origin of the shallow trap. The detrapping analysis of both lifetime and diffusion-length data yields a binding energy of 43±5 meV for the positron bound to these negative centers. This value is in good agreement with the binding energies of electrons and holes to the shallow levels in GaAs.

• Received 26 August 1988

DOI:https://doi.org/10.1103/PhysRevB.39.5287