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 λ² Δn/μ, where λ is 10.6 μm in this paper. We measure the transient photoconductive decay that is proportional to Δn * μ. The data product gives Δα * Δσ~ λ² Δn(t)². The mobility variation is nullified by multiplying the data from the two parallel measurements. From the product data, both Δn(t) and μ(Δn) can be determined. A large increase in Δα and decrease in μ 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.