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transport proteries Wolfe, J.P. Smith, Leigh Morris 2011-05-17T16:00:03Z 2011-05-17T16:00:03Z 10000-01-01 1988 "This thesis describes work on the thermodynamics and transport properties of photoexcited carriers in bulk and two-dimensional semiconductors. Two major topics are addressed: I. Excitonic Phase Diagram in Si: Evidence for Two Condensed Phases At low temperatures and sufficient densities, free excitons in Si and Ge undergo a simultaneous gas-liquid and insulator-metal transition into droplets of electron-hole liquid. Some previous theoretical and experimental studies have suggested that, under certain values of density and temperature, there may be separate metal-insulator and liquid-gas transitions. In the present paper, we examine the difficult trans-critical region for electron-hole liquid formation in unstressed Si using time-and space-resolved photoluminescence spectroscopy. Using the latest models for the luminescence of electron-hole plasma and small excitonic complexes (EC), we have succeeded in characterizing the complicated luminescence spectra both above and below the liquid-gas critical temperature (Tc(LG) ~ 24.5 K) with a relatively small number of free parameters. We find that the EHP density is remarkably independent of temperature and particle density, providing evidence for a second condensed phase of electron-hole III plasma. The new condensed liquid has a density of about one-tenth that of the ground-state electron-hole liquid (EHL), and is observed both above and below the EHL critical temperature. An excitonic phase diagram for silicon is described which includes two condensed plasmas. A triple point at 18.5 K is observed where the electron-hole liquid coexists with the lower density condensed plasma (CP) and excitonic gas. Above this temperature the CP is observed at all temperatures up to a second critical point at 45 5K. II. Motion of Photoexcited Carriers in GaAs/AlxGal-xAs Multiple Quantum Wells-Anomalous Confinement at High Densities We have measured the in-plane motion of photoexcited carriers in semiconductor quantum wells with 5 {lm spatial and 10 ps temporal resolution and have discovered several surprising results. The effective diffusivity of the carriers at densities below n =2 x 1011cm-2 is found to depend upon excitation level, possibly indicating defect-limited diffusion or phonon-wind effects. Above this density the spatial profiles exhibit two distinct components with widely differing diffusivities. This remarkable behavior may be understood with consideration of the interactions of non-equilibrium phonons with the photoexcited carriers. We postulate that the slowly diffusing component represents carriers which are ""thermally confined"" to a phonon hot spot, while the rapidly moving component is driven by the flux of non-equilibrium phonons away from the excitation region." Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-05-17T16:00:03Z No. of bitstreams: 1 1988_smith.pdf: 5813548 bytes, checksum: 7c779f0e23a56fdbd321bb67b6485c3f (MD5) Made available in DSpace on 2011-05-17T16:00:03Z (GMT). No. of bitstreams: 1 1988_smith.pdf: 5813548 bytes, checksum: 7c779f0e23a56fdbd321bb67b6485c3f (MD5) Previous issue date: 1988 Restriction data tranferred 2014-07-01T11:14:30-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-05-17T16:00:03Z Item is restricted indefinitely. Thesis U of I Only 3502360 http://hdl.handle.net/2142/23916 en 1988 Leigh Morris Smith excitonic phase diagram Si Ge condensed phases motion of photoexcited carriers GaAs multiple quantum wells anomalous confinement high densities thermodynamics I. Excitonic phase diagram in Si: evidence for two condensed phases ; II. Motion of photoexcited carriers in GaAs/Alx̳Ga1̳-̳x̳As multiple quantum wells : anomalous confinement at high densities Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.