Ultrafast Dynamics of Photoexcited Bismuth Films.

[摘要] The carrier and lattice relaxation processes following photoexcitation in solidsoccur over time-scales ranging from femtoseconds to nanoseconds. The eventualconversion of the light to lattice heating involves carrier-carrier, carrier-phonon andphonon-phonon interactions. More fundamental understandings of these processesmay lead to advances in thermoelectrics, photovoltaics, and other technologically importantmaterials. Even for bismuth, a well-studied thermoelectric material, detailedinformation on these processes is still unavailable. In this dissertation, I present ultrafastoptical and x-ray studies of photoexcited carrier diffusion and recombination,acoustic phonon generation and propagation and lattice heating and diffusion in thinbismuth films. I model these results to extract information on carrier and thermaltransport.I have measured the carrier and thermal transport properties of photoexcitedbismuth films using ultrafast optical and x-ray techniques for the first time. Thecombination 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 recombinationare density-dependent.Time-resolved x-ray diffraction measures atomic displacements directly, and canbe used as a non-contact probe to study lattice heating and thermal transport inthin films. Here, I employ a grazing incident geometry to investigate the atomicdynamics at various depths. Despite rapid carrier diffusion, I find that the latticeheating occurs near the excited surface. I also use symmetric diffraction to measurethe cooling of the entire film, allowing Kapitza conductance across bismuth/sapphireto be determined.Optical pump-probe experiments is complementary to x-ray diffraction and offeringbetter time-resolution and sensitivity to photoexcited carriers. By comparingresults of conventional and counter-propagating pump-probe geometries, I am ableto discriminate the dynamics of carriers, acoustic phonons, and lattice heating. Atlow excitation, I measure the ambiploar diffusion ,recombination rates and latticethermalization time. I find that the carriers relax by rapidly heating the latticebefore diffusing and ultimately recombining. For higher excitations, the diffusivitydecreases while the recombination rate increases becoming comparable to the rate oflattice heating.

[发布机构] University of Michigan

[效力级别] Pump Probe

[关键词] Bi;Pump Probe;Diffusion;X-ray Diffraction;Recombination;Kapitza Conductance;Physics;Science;Physics