Investigation of the Kinetics of Surface-Limited thin Film Growth of SiGe Alloys by CVD of Si 2 H 6 /Ge 2 H 6 Mixtures
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INVESTIGATION OF THE KINETICS OF SURFACE-LIMITED THIN FILM GROWTH OF SiGe ALLOYS BY CVD OF Si 2 Hd/Ge 2 H 6 MIXTURES J. W. SHARP- AND GYULA ERES** *Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37896-2200 **Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 37831-6056 ABSTRACT The kinetics of surface-limited thin film growth of SiGe alloys was investigated by time-resolved surface differential reflectometry. The source gas, mixtures of disilane and digermane in ratios from 1:1 to 6:1 in helium carrier gas, was delivered to a heated Si(001) substrate by a fast-acting pulsed molecular jet valve. The adsorption and desorption kinetics were determined from the surface differential reflectance signal obtained using a polarized, high-stability HeNe probe laser. Thin film growth was studied in the temperature range of 400-6001C on Si(001) substrates. Preferential incorporation of digermane into the film produces an alloy composition that depends upon but does not mirror the gas composition. For all gas mixtures, there is a strong temperature dependence of the rate at which the adsorption layer decomposes into film plus by-product. The kinetic data and the alloy compositions provide a basis for deducing some of the characteristics of the reaction sequence that leads to SiGe alloy thin film growth. INTRODUCTION Potential device applicationsl, 2 have led to an increasing effort in SiGe thin film growth. 3 -5 One of the most interesting recent developments in this area is the demonstration that growth rates for Si-rich SiGe alloy chemical vapor deposition (CVD) can be more than an order of magnitude greater than for pure Si growth under similar (low-pressure) conditions. 6 Because H2 desorption is frequently the rate-limiting step in the growth of Si,7-9 it was inferred that the incorporation of Ge into a Si-rich film accelerates growth by increasing the hydrogen desorption rate.6 Along with others, we are attempting to unravel the 10 12 intricacies of H 2 desorption from SiGe surfaces. Ning and Crowell performed temperature-programmed desorption (TPD) experiments with Si(001) substrates that were partially coated with Ge and saturated with deuterium. 10 For zero Ge coverage, their TPD spectrum has the expected peaks at 670 K and 780 K that correspond respectively to D 2 desorption from SiD 2 and SiD units. For non-zero Ge coverages, these two peaks shrink and shift to lower temperatures, and a third peak arises at 590 K. The assignment of this new peak to D 2 desorption from Ge sites 10 is supported by TPD spectra of H 2 desorption from Ge(111) and Ge(001) surfaces. 13 H 2 TPD spectra showing similar dependence on surface composition have been reported for Si(001) surfaces exposed to germane and digermane.11 Here, we report isothermal H2 desorption studies performed during low pressure CVD of SiGe thin films. EXPERIMENTAL The films were grown in a turbo-pumped UHV chamber with a base pressure of -3 x 10-9 Torr and an H 20 partial pressure of 1 x 10- 9 Torr. The pumping time
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