Excitons in cuprous oxide: high mobilities and strain confinement

Abstract

This thesis presents two studies of the spectral and spatial properties of excitons in the semiconductor cuprous oxide (CU0) at 2liquid helium temperatures. The lowest lying member of the yellow exciton series, the paraexciton, is the main focus of these studies. The technique cf time-resolved luminescence imaging was used to measure the transport properties of the paraexcitons. In addition, these excitons were bpatially confined in a strain-induced parabolic potential well where their thermodynamic properties and interparticle kinetics could be studied. Chapter 3 describes the technique used to measure the diffusion constant, D, and the drift mobility, 11, of paraexdtons, as well as a theoretical model to explain the unusual results. .An externally applied inhomO'::feIl-"'Ous stress gradient was used to provide the motive force for the mobility measurement. Very large values of D (~ 1000 2 7 2 cm Is) and 11 (-10cm /eV-s) were measured at a bath temperature of 1.2 K. Both these parameters exhibited anomalous temperature dep.:=.'ldences as well as a drastic dependence on the externally applied stress. A calculation of low temperature deformation-potential scattering of excitons with acoustic phonons, iv making none of the usual approximations which are valid at higher temperatures, resulted in good agreement between experiment and theory. The stress dependence is explained by an increased coupling to transverse acoustic phonons with increased applied stress. Chapter 4 explains the strain confinement technique, along with a derivation of the expected luminescence lineshapes and spatial profiles of excitons confined in the resulting parabolic potential well. Experimental results for paraexcitons using low power laser excitation are given and compared to theory. The discouraging result that the paraexcitons do not seem to thermalize in the well within their lifetime is obtained. Interparticle kinetics of the excitons i~ examined under increasing laser excitation yielding a significant sublinear dependence of the paraexciton density on absorbed laser power. Time-resolved measurements showed a very slow thermalization taking place, with lattice heating occurring at high laser intensities.