Novel Free-Carrier Pump-Probe Analysis of Carrier Transport in Semiconductors

We have developed a pump-probe configuration to measure the carrier lifetime using the transient free-carrier density. The free-carrier absorption varies as lambda(2) Delta n/mu, where lambda is 10.6 mu m in this paper. We measure the transient photoconductive decay that is proportional to Delta n (*) mu. The data product gives Delta alpha (*) Delta sigma similar to lambda(2) Delta n(t)(2). The mobility variation is nullified by multiplying the data from the two parallel measurements. From the product data, both Delta n(t) and mu(Delta n) can be determined. A large increase in Delta alpha and decrease in mu are observed and caused by space-charge effects in regions of high injection. These data show the unexpected and remarkable result that the lifetime is relatively constant up to an injection level of about three times the doping level. However, the mobility decreases by about a factor of six over the same injection range.