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Title: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
Author(s):Smith, Leigh Morris
Doctoral Committee Chair(s):Wolfe, J.P.
Department / Program:Physics
Subject(s):excitonic phase diagram
condensed phases
motion of photoexcited carriers
multiple quantum wells
anomalous confinement
high densities
transport proteries
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. 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.
Issue Date:1988
Genre:Dissertation / Thesis
Rights Information:1988 Leigh Morris Smith
Date Available in IDEALS:2011-05-17
Identifier in Online Catalog:3502360

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