Defects in Photo-Assisted CBE-Grown GaAs
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DEFECTS IN PHOTO-ASSISTED CBE-GROWN GaAs.
PETER J. GOODHEW, R. BEANLAND AND T. FARRELL Department of Materials Science and Engineering, The University of Liverpool, P.O. Box 147, Liverpool L69, 3BX, England.
ABSTRACT Excimer laser light has been used to achieve the maximum growth rate of GaAs in a chemical beam epitaxy system when temperatures were more than a hundred degrees below the normal growth temperature. Secondary electron and transmitted electron microscopy of material grown using laser assistance shows the presence of surface ripples aligned with crystallographic directions. Layers grown at the lowest temperatures using a high 2fluence of excimer laser light contain a high density of small dislocation tangles (>1011 cm- ). Lower fluences have no effect on the microstructure of the material.
INTRODUCTION In the CBE growth of GaAs using triethylgallium (TEG) and cracked arsine (AsH 3 ) precursors, the maximum growth rate Rmax occurs in the range 500-550'C[ 1]. Photoassistance during growth allows a recovery of Rmax at lower substrate temperatures and thus allows localised growth to be achieved. Enhanced low temperature CBE growth has been observed with CW Ar+ (518 nm)[ 2' 31 and pulsed excimer (198 and 308 nm) 14-6] laser assistance. The growth mechanisms associated with laser assistance are ascribed to pyrolysis and/or photolysisl[l, although other mechanisms such as photocatalysis have been proposed[81. In this paper we present a SEM and TEM study of three samples, having various degrees of XeCl excimer laser assisted growth. The laser light had a wavelength 308 nm, with a pulse duration of 12 ns and a repetition rate of 20 Hz. These pulses periodically raised the temperature of the outermost atomic layers of the substrate to at least that where Rmax occurred, although the change in average sample temperature was negligible. EXPERIMENTAL A diode laser (670 nm) and the XeCI excimer laser (Lumonics model 861T) were aligned such that their light passed through the pyrometer port of the growth chamber and was incident normally upon the same region of an (001) GaAs substrate[6]. With the excimer laser off, GaAs and AlxGalixAs layers were grown at low temperature. The AlxGal-xAs layers gave a change in optical reflectivity with overlayer thickness that was measured from the intensity of the reflected light of the diode laser. This dynamical optical reflectivity (DOR) measurement allowed the growth rate to be obtained in real time. The final layer grown was GaAs; once the growth rate for this layer was established, the excimer laser was switched on. An immediate increase in growth rate was observed from the DOR measurement. It was found that the maximum growth rate was achieved by samples A and C. The experimental conditions are described in table 1. The enhancement factor is the ratio of growth rates measured by DOR during and prior to excimer laser irradiation, and was confirmed by post growth cleaved edge TEM measurements. Bulk samples were examined using the secondary electron detector and the scanning fa
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