TEMPERATURE DEPENDENCE OF SLOW MINORITY CARRIER DE-TRAPPING IN CZOCHRALSKI SILICON WITH HIGH CONCENTRATIONS OF THERMAL DONORS

摘要

The slow de-trapping characteristics of defects related to thermal donors have been studied with transient photoconductance measurements in Cz monocrystalline silicon wafers. The temperature dependent photoconductance decay curves were analyzed to extract de-trapping time constants for a range of Cz-grown wafers. These included an as-cut wafer with a moderate concentration of pre-existing thermal donors, a wafer with a high concentration of thermal donors (High TD sample) which was annealed at 450°C for 72 hours, and a sample with no thermal donors (No TD sample) annealed at 650°C for 30 minutes to destroy the pre-existing thermal donors. The de-trapping time constants were measured at four temperatures (12°C, 30°C, 60°C and 80°C) using photoconductance decay measured up to 10 minutes after the illumination was turned off. We find that the de-trapping time constants of the as-cut sample are higher than those of the High TD sample for all the temperatures studied. Also, in both samples, the de-trapping time constants decrease with temperature, indicating that trapped minority carriers are released back into the bands more rapidly at higher temperatures. De-trapping was not observed in the samples without thermal donors. We also estimate the energy level of the significant traps using Arrhenius plots and found that in n-type Cz silicon, the as-cut wafers mostly contain traps close to the valence band, whereas the High TD sample has much deeper traps.