Negative photoconductance in 150 GHz transients from irradiated p-type Cz-Silicon

Boron doped (p-type) silicon wafers of the same type are irradiated with gamma, proton and chlorine ion beams. This causes radiation damage in the form of migration of vacancies, traps to photoelectrons. We use time-resolved millimeter wave pump-probe spectroscopy (150 GHz CW probe signal) and 532 nm ultrafast laser as pump source with variable fluence. Upon studying the transient response of the detector probe voltage as function of the pump-probe delay period we note a good positive (absorption) photoconductance peak and soon after recombination of photocarriers there occurs a negative photoconductance (NPC) transient. We consistently find that the NPC lasts for about 36 microseconds and this study points out that the positive to NPC peaks for each laser fluence varies with the type of radiation damaged samples.2 MeV proton beam damage create damage that trap carriers very effectively, and enhances the resistivity of the silicon wafer from 15 Ohms to 150 Ohms. The Chlorine ion damaged silicon responds consistently to the 150 GHz probe beam and correlates strongly with the laser fluence.