Transient type-inversion of semi-insulating GaAs under strong illumination at 140 K detected by the nondestructive surface-acoustic-wave technique

Abstract

A nondestructive surface-acoustic-wave (SAW) technique is used to study the behavior of deep trap levels in semi-insulating, Cr-doped GaAs. The importance of these levels in producing high-quality, semi-insulating GaAs wafers is well recognized. The temperature dependence of the free energy of these levels as well as their trapping behavior at different temperatures has been the subject of increasing interest in the past few years. The SAW technique is extremely sensitive in detecting conductivity variations at the surface of the high-resistivity materials and also yields information about surface-conductivity type. At 140 K, high injection level is achieved by illuminating the wafer with a 100 W tungsten lamp. Under the illumination, the surface conductivity is n-type. When the illumination is turned off, the surface becomes p-type for a duration of 1 s. This duration is a function of light intensity and temperature. The type inversion can be attributed to the large-scale capture of electrons by the deep traps, resulting in momentary increase of the effective hole concentration.