Infrared reflectivity probing of thermal and spatial properties of laser-generated carriers in germanium

We report on the thermal and spatial properties of laser-generated carriers in single-crystal intrinsic germanium as determined from infrared (10.6 μm) reflectivity studies. Densities up to 5×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$ are achieved using fundamental (1.06 μm) and second-harmonic (0.53 μm) 80-nsec pulses from a $Q$-switched Nd: glass laser. The carriers are probed using a 10.6-μm 100-μsec C${\mathrm{O}}_2$ laser beam which is electronically synchronized to the glass laser. For a carrier density of ∼${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$, characteristic of the plasma resonance at 10.6 μm, the reflectivity attains a minimum value; for higher carrier densities the reflectivity is enhanced beyond its intrinsic value (0.36) to values as high as 0.95, depending on the peak surface density. Such transient, laser-induced changes in the reflectivity of semiconductors were first investigated by Galkin and co-workers, who interpreted the reflectivity minimum value in terms of damping of the plasmon resonance by carrier-assisted free-carrier absorption processes. Subsequently Vakhnenko and coworkers offered an interpretation solely in terms of spatial inhomogeneity of the laser-generated carriers. Here we report detailed experimental and theoretical investigations of this transient reflectivity phenomenon in which we have determined the influence of excitation laser wavelength and intensity, lattice temperature, plasma inhomogeneity effects, and surface quality. For a density of ∼${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$ our results indicate that intervalence-band absorption and phonon-assisted free-carrier absorption are the dominant damping mechanisms of the plasmon resonance and that spatial inhomogeneity of the carrier density plays a minor, albeit measurable, role in determining the value of the reflectivity minimum. The applications of these results to diagnostics of laser-induced semiconductor plasmas, including pulsed laser annealing and the performance of laser-activated semiconductor reflection switches, are discussed.