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Title:I. Excitonic Phase Diagram in Silicon: Evidence for Two Condensed Phases. II. Motion of Photoexcited Carriers in Gallium-Arsenide/aluminum(x)gallium(1-X)arsenide Multiple Quantum Wells--Anomalous Confinement at High Densities
Author(s):Smith, Leigh Morris
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Condensed Matter
Abstract: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. Photoluminescence experiments of excitons in unstressed silicon are presented which indicate the existence of a new non-degenerate condensed phase of plasma. This new liquid has a density one-tenth that of the ground state electron-hole liquid and is observed both above and below the liquid-gas critical point ($\approx$24.5K). A new phase diagram of excitons in silicon is presented which includes these two condensed plasmas. Consistent with the Gibbs phase rule, a triple point at 18.5 K is inferred from the luminescence data as the only temperature where the exciton gas, condensed plasma (CP) and electron-hole liquid (EHL) coexist. The low density condensed plasma persists up to a second critical point at 45 $\pm$ 5K, above which the photoexcited carriers are observed to continuously decay into a partially ionized excitonic gas.
II. We have measured the in-plane motion of photoexcited carriers in semiconductor quantum wells with 5 $\mu$m spatial and 10 ps temporal resolution and have discovered several surprising results. The effective diffusivity of the carriers at densities below n = 2 $\times$ 10$\sp $cm$\sp{-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.
Issue Date:1988
Description:166 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
Other Identifier(s):(UMI)AAI8823253
Date Available in IDEALS:2015-05-13
Date Deposited:1988

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