Optical studies of ultrafast carrier dynamics in semiconducting materials

Abstract:

This thesis describes three experiments using pump-probe spectroscopy with picosecond and femtosecond laser pulses to study carrier dynamics in semiconductors. In the first experiment, highly excited gallium arsenide is studied with reflectivity and reflection second-harmonic probes using 160-fs pulses at 623 nm. Above a threshold incident fluence of  0.1 J/ cm2, the second-harmonic signal is observed to fall to zero in 100 fs. This drop shows that the valence electrons undergo a transformation to a centrosymmetric configuration and strongly suggests that the atomic lattice disorders before acquiring appreciable energy from the pulse. With an exponential time of 200 fs the reflectivity rises to a steady high value that is consistent with a metallic molten phase. Tens of picoseconds after excitation, the reflectivity drops considerably from the high value of the liquid and probe light is increasingly scattered out of the plane of incidence. The excitation produces  90-nm-deep pits in the wafer surface that are covered with a layer of solidified droplets. An estimate of the absorption and scattering caused by a cloud of liquid droplets ejected from the surface suggests that ablated material can account for the reflectivity drop. In the second experiment, femtosecond transient absorption spectroscopy is used to study the carrier dynamics of type II GaAs/ AlAs multiple quantum wells. The spectra show a rapid partial recovery in the pump-induced bleaching near the absorption edge that is produced by the rapid scattering of conduction electrons in the Gamma valley of the GaAs layers to the X valleys of the AlAs barrier layers. The scattering time is measured to be 100 fs for an 8-monolayer sample and 400 fs for an 11-monolayer sample. In the third experiment, the picosecond laser melting of silicon is studied using a streak camera to provide spatial and temporal resolution. The reflectivity near Brewsters angle shows the expected eightfold increase following melting. Images of the excited silicon surface emphasize the importance of spatial resolution in nearthreshold experiments.