Kinetics of plasmas and melting induced in silicon and germanium by nanosecond laser pulses

We have performed time-resolved infrared reflectivity measurements at 5.3 and 10.6 μm to determine the plasma and melting kinetics of intrinsic, crystalline silicon and germanium following excitation by 25-ns 0.53- and 1.06-μm laser pulses. Below the threshold of melting, the maximum non-equilibrium plasma density is 1020 cm-3 in both materials for etched samples. For germanium, the results were compared with a numerical modeling of the plasma which employed only known optical and thermal properties. The solid-state plasma kinetics were found to be consistent with conventional models using the usual generation, diffusion, and recombination rates of lower-density plasmas. The only unusual feature is the apparent observation, in some circumstances, of reduced plasma diffusion or possibly confinement in the presence of the strong temperature-induced band-gap gradients. Above the threshold for melting we have taken advantage of the large skin depth (800 Å) at infrared wavelengths to determine melt-front kinetics for depths up to 1000 Å. Results of this contactless technique are in good agreement with earlier measurements of Galvin et al. [Phys. Rev. B. 27, 1079 (1983)] who pioneered the use of a dc conductivity technique in melt-front measurements.