Dual-beam transient surface photo-voltage measurements were performed on undoped semi-insulating material. It was found that, above 270K, the surface photo-voltage recovery transients which followed a bias pulse reflected near-surface conductivity changes. These were in turn controlled by surface interface-state thermal emissions. Due to the absence of a strong surface electric field, the emitted carriers were not immediately removed from the near-surface region. Re-capture of the emitted carriers was shown to be responsible for non-exponential conductivity and reciprocal surface photo-voltage transients. This behavior was considered to be characteristic of relaxation-type semiconductors with near-surface ungated structures. At temperatures below 150K, the photo-induced transition of EL2 from its ground to its EL2* metastable state was shown to change the effective electron and hole mobilities and to increase the surface photo-voltage signals which immediately followed the bias pulse. Thermally induced EL2* recovery above 120K decreased the surface photo-voltage signal from its maximum value. This decay rate could fitted to a single exponential curve, and an activation energy of 0.32eV and a pre-exponential constant of 1.9 x 1012/s were obtained. This was attributed to the thermal recovery rate of EL2*.

Q.Liu, H.E.Ruda: Physical Review B, 1997, 55[16], 10541-8