Electron-Hole Competition in InP:Fe -- The Role of Multiple Defects

The photorefractive effect has now been identified and studied in many semi-insulating III-V compound semiconductors [1]. In general, it can be well described by a Single Defect Model (SDM) with one or two types of charge carriers. However, discrepancies arise between theory and the experimental results due to the presence of additional defects. In understanding the role of multiple deep defects in the photorefractive effect, temperature plays a key role. Each different defect has a specific energy position within the bandgap of the material. Thermal emission of carriers trapped at defect sites is a strong (exponential) function of defect energy and the sample temperature. The thermal relaxation of nonequilibrium defect occupancies, and the subsequent effect on the photorefractive effect, can be studied by monitoring the two-wave mixing (2WM) gain and four-wave mixing (4WM) diffraction efficiency as functions of temperature, providing a natural tool to characterize the material. We find that multiple defect levels in InP:Fe lead to dramatic changes in the photorefractive behavior. Reduced gain reported at room temperature can be shown to arise from an additional defect in InP other than isolated Fe.