Carrier-Driven Disordering in Semiconductors: Time-Resolved X-Ray Diffraction and Density Functional Perturbation Theory Investigations.

Time-resolved x-ray science has opened the door to a previously inaccessible experimental world. Now the possibility of imaging ultrafast events with atomic spatial resolution is a reality. This dissertation highlights these new experimental techniques and uses them to study the effects of carrier photo-excitation in semiconductors using both time-resolved x-ray diffraction and time-resolved x-ray absorption spectroscopy. I have probed the ultrafast atomic disordering in InSb after intense photoexcitation with ultrafast x-ray diffraction measurements at the Sub-Picosecond Pulse Source (SPPS), The results indicate that three disordering regimes exist, depending on the photoinduced carrier density. At lower carrier densities, disordering occurs via a thermal mechanism, occurring on a picosecond time scale with the dominant relaxation mechanism being the transfer of energy from hot carriers to the lattice. At intermediate carrier density values, the potential energy surface flattens, allowing the atoms to move with the inertial room temperature velocities for approximately 500 fs at which point other processes take over including thermal energy transfer, atomic collision, and diffusion. At higher carrier densities, it is observed that accelerated atomic disordering occurs, indicating the formation of a repulsive potential energy surface. These experimental observations are in contrast with previous theoretical work and therefore, I have performed calculations using Density Functional Perturbation Theory (DFPT) to more clearly outline the role of excited carriers in lattice destabilization. The calculations show that with increasing carrier density the transverse acoustic modes soften and the lattice destabilizes first in the (100) direction (X point) with 3.7% of the valence band electrons excited into the conduction band. Increasing the carrier density leads to the entire transverse acoustic mode becoming unstable, indicating a repulsive interatomic potential. A model has been developed for converting the lattice dynamics calculations into predicted diffraction intensities, leading to theoretical verification of the three disordering regimes that were observed experimentally. I have also studied carrier excitation in Cu2O using time-resolved x-ray absorption spectroscopy (XAS) at the Cu L3 edge and the O K edge using 70 ps x-ray pulses at the Advanced Light Source. After photoexcitation of carriers above the band gap, changes in the XAS spectrum are monitored at 70 ps time delay. By probing the two different atomic sites, changes in the XAS spectrum can be extracted that are both atomic and angular momentum specific. It is observed that the energetic shift in the XAS spectra after photoexcitation of Cu2O is much less than the band gap. The relative change in absorption is observed to be 50% larger at the Cu site than at the O site and there is no integral change in absorption in the transient spectra. The desire to understand the mechanism that underlies chemical, physical and biological transformations motivates the majority of time-resolved studies. While ultrafast laser spectroscopy has greatly enhanced our understanding of dynamical phenomena, many important and interesting processes remain unexplained. Ultrashort pulses of x-rays provide the ability to access atomic and electronic structure with a detail and clarity absent from most optical spectroscopy measurements. It was time-resolved x-ray diffraction using the high per pulse intensity of SPPS that allowed the disordering mechanism in highly excited InSb to be clearly defined in contrast with previous work using optical probes and laser plasma-generated x-ray probes....