High-resolution photo-induced transient spectroscopic and modulated photo-current techniques were compared with regard to their possible application to the investigation of defect levels in semi-insulating monocrystalline materials. These techniques complemented each other, and their possibilities were demonstrated by measurements of the trap parameters for a given sample of semi-insulating Cr-doped or undoped material. A possible model for the properties of the Cr defect was deduced from the results. Both techniques were useful for the determination of the trap activation energy, escape attempt-frequency and order of magnitude estimate of the trap density, in the case of the modulated photo-current technique. By using the statistics of correlated states, it was shown that the ground state of the Cr defect was located at about 0.44eV from the valence band. This explained why the Cr2+/Cr3+ and Cr4+/Cr3+ levels were observed at low and high fluxes, respectively. The measurements revealed differences between the defect structures of Cr-doped and undoped material. In the former material, levels which were related to Cr4+/Cr3+ and Cr2+/Cr3+ predominated. In the latter case, the predominant traps were attributed to O-related centres and to EL12 centres. The common defect which was detected in both materials was EL6.

Investigation of Defect Levels in Semi-Insulating Materials by Modulated and Transient Photocurrent - Comparison of Methods C.Longeaud, J.P.Kleider, P.Kaminski, R.Kozlowski, M.Pawlowski, J.Cwirko: Semiconductor Science and Technology, 1999, 14[9], 747-56