Temperature-Dependent Evolution of Chemisorbed Digermane in Ge Thin Film Growth
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TEMPERATURE-DEPENDENT EVOLUTION OF CHEMISORBED DIGERMANE IN Ge THIN FILM GROWTH DJULA ERES AND J. W. SHARP* Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6056 * Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996
ABSTRACT The formation and evolution of chemisorbed digermane layers in context with germanium thin film growth was investigated by time-resolved surface reflectometry. Modulation of the source gas supply made possible the separation and independent study of the temperature dependence of the adsorption and desorption processes. The regeneration of active sites by molecular hydrogen desorption was identified as the rate-limiting step at low substrate temperatures. A dynamic method of thin film growth was demonstrated by repetitively replenishing the active film growth sites regenerated between two successive source gas pulses. The film growth rate was shown to be related to the substrate temperature and the delay time between successive source gas pulses. INTRODUCTION The potential for thickness control at the monolayer level has inspired much new research into the understanding of surface chemical reactions of silicon and germanium-containing compounds [1,2]. Of particular interest are self-limiting growth mechanisms that can be externally monitored and controlled. Various diagnostic techniques that allow nonintrusive in situ probing are an integral part of the surface chemistry studies. This paper describes time-resolved studies of hydrogen-covered semiconductor film growth surfaces conducted by utilizing a nonintrusive optical probe. The hydrogen coverage is a consequence of using hydrogenated source gas molecules for thin film growth. As a side product of the surface chemical reaction, hydrogen hinders further film growth by occupying the active surface sites. Since in conventional CVD growth techniques, hydrogen desorption occurs concurrently with film growth, steady state measurements are not adequate to study the desorption and the film growth processes. It is necessary to turn to a dynamic approach in which thin film growth is studied as a result of a time-varying flux of source molecules. An additional advantage of using a pulsed gas source is that homogeneous components to film growth are excluded because all unreacted gas is immediately pumped away. Detection and monitoring of the adsorption layers is achieved by an optical technique that is based on the relationship between surface reflectance and the electronic structure of the surface. The changes in the optical absorption of the surface produced by chemisorption-induced bond formation are revealed through measurements of the intensity of reflected light. The laser light utilized as a probe is nonintrusive and is compatible with a wide range of growth environments. We demonstrate for the first time a dynamic method of germanium thin film growth termed "digital epitaxy." Digital epitaxy is defined as thin film growth that occurs in sub-monolayer increments from a multi
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