Ultrafast Dynamics of Photoexcited Bismuth Films.

Abstract

The carrier and lattice relaxation processes following photoexcitation in solids occur over time-scales ranging from femtoseconds to nanoseconds. The eventual conversion of the light to lattice heating involves carrier-carrier, carrier-phonon and phonon-phonon interactions. More fundamental understandings of these processes may lead to advances in thermoelectrics, photovoltaics, and other technologically important materials. Even for bismuth, a well-studied thermoelectric material, detailed information on these processes is still unavailable. In this dissertation, I present ultrafast optical and x-ray studies of photoexcited carrier diffusion and recombination, acoustic phonon generation and propagation and lattice heating and diffusion in thin bismuth films. I model these results to extract information on carrier and thermal transport. I have measured the carrier and thermal transport properties of photoexcited bismuth films using ultrafast optical and x-ray techniques for the first time. The combination of laser and x-ray experiments confirms rapid lattice thermalization, leaving an inhomogeneous temperature profile near the surface. At high excitations, the carrier dynamics become nonlinear with the possibility that diffusion and recombination are density-dependent. Time-resolved x-ray diffraction measures atomic displacements directly, and can be used as a non-contact probe to study lattice heating and thermal transport in thin films. Here, I employ a grazing incident geometry to investigate the atomic dynamics at various depths. Despite rapid carrier diffusion, I find that the lattice heating occurs near the excited surface. I also use symmetric diffraction to measure the cooling of the entire film, allowing Kapitza conductance across bismuth/sapphire to be determined. Optical pump-probe experiments is complementary to x-ray diffraction and offering better time-resolution and sensitivity to photoexcited carriers. By comparing results of conventional and counter-propagating pump-probe geometries, I am able to discriminate the dynamics of carriers, acoustic phonons, and lattice heating. At low excitation, I measure the ambiploar diffusion ,recombination rates and lattice thermalization time. I find that the carriers relax by rapidly heating the lattice before diffusing and ultimately recombining. For higher excitations, the diffusivity decreases while the recombination rate increases becoming comparable to the rate of lattice heating.