The use of spatially-dependent carrier capture rates for deep-level-defect transient studies

The spatial dependence of the free carrier capture rate for the defect states located near the edge of the depletion layer is investigated. A theoretical treatment is presented here to illustrate the strong variation of the capture rate near the depletion edge with distance. If a transient capacitance technique is used to observe this capture transient, computer-calculated data confirm that the resulting signal comes only from the capture of electrons by those states located near the point at which the Fermi level and the deep level intersect. The capture of electrons by the states that are located in regions other than near the intersection point does not contribute to the detectable signal. Thus the transient signal measures the empty state concentration in a narrow region. Since the electric field in the narrow region can be accurately chosen by the use of different reverse pulse heights, the empty state concentration at the region can be controlled by the emission at different field strengths. The electric field dependence of the emission rate can be measured directly from the capture transient capacitance. Detailed comparisons of the calculated and experimentally measured data show good agreement. Possible applications using this technique in study of deep-level defects are outlined.