Microcavity Effects in the Luminescence of GAAS Microcrystals
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MICROCAVITY EFFECTS IN THE LUMINESCENCE OF GAAS MICROCRYSTALS S. JUEN, K. F. LAMPRECHT, R. RODRIGUES, R. A. HOPFEL Institute of Experimental Physics, University of Innsbruck, Technikerstr. 25/4, A-6020 Innsbruck, AUSTRIA.
ABSTRACT Experimental photoluminescence spectra of GaAs microcrystals show pronounced variations compared to the luminescence of bulk GaAs. The observed spectra are explained by spectral enhancement and inhibition of spontaneous emission in a three-dimensional optical resonator formed by a dielectrically confined semiconductor microcrystal. The crystals were produced by pulverization of bulk GaAs, size-separated by sedimentation techniques, and characterized by transmission electron microscopy, electron diffraction and x-ray diffraction.
INTRODUCTION Semiconductor nanostructures exhibiting altered optical and electronic properties due to carrier quantum confinement have been fabricated by various sophisticated technologies. In microscopic structures with dimensions much larger than the bulk exciton Bohr radius, however, the electronic states form already a quasi-continuum, but the electromagnetic modes can be discrete as in optical resonators. The effects of a microscopic resonator on the spontaneous light emission have been of fundamental interest in cavity quantumelectrodynamics and are promising for optoelectronic applicationsI'4. In an optical microcavity the coupling between electronic states and the radiation field is restricted to a small number of electromagnetic field modes. Resonant coupling enhances the spontanous emission rate and ultimately allows zero-threshold laser action by the spatial and spectral overlap of a single field mode with the gain medium 4 . Increased light emission efficiencies and spectral alterations were demonstrated in one- and two-dimensional semiconductor microcavities based on III-V quantum well structures -3. Theoretical calculations predict a drastic reduction of the threshold current for microcavity surface emitting lasers with sub-micron lateral dimensions 5. To our knowledge, semiconductor microcrystals were considered as three-dimensional optical microresonators for the first time in Ref. 6. In this paper we report on the photoluminescence of GaAs microcrystals with dimensions in the range of 0.2 gm to 1.5 gim. The microcrystals are strongly dielectrically confined due to a change in the index of refraction of An - 2.4 at their surface. Anomalies in the measured luminescence spectra are shown to be consistent with the expected light emission from a three-dimensional optical microcavity.
SAMPLE FABRICATION AND CHARACTERIZATION The fabrication process is divided into three main steps: (1) The pulverization of monocrystalline bulk GaAs (semiisolating liquid-encapsulated Czochralsky (LEG) grown) in Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
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de-ionized water yields a colloidal solution. (2) The particle separation process is based on sedimentation and yields a series of colloids with consecutive average crystal sizes. Stand
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