Low Pressure Diamond Growth Using a Secondary Radical Source
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LOW PRESSURE DIAMOND GROWTH USING A SECONDARY RADICAL SOURCE TERTrU I. HUKKA, ROBIN E. RAWLES, AND MARK P. D'EVELYN Department of Chemistry and Rice Quantum Institute, Rice University, Houston, TX 77251-1892 ABSTRACT
A novel method for chemical vapor deposition and atomic layer epitaxy using radical precursors under medium vacuum conditions is being developed. Fluorine atoms are generated
by thermal dissociation in a hot tube and abstract hydrogen atoms from precursor molecules injected immediately downstream of the source, generating radicals with complete chemical specificity. The radical precursors are then transported to the growing substrate surface under nearly collision-free conditions. To date we have grown diamond films from CC13 or CH 3 radicals together with atomic hydrogen, generated by injecting CHC13 or CH4 and H2 into the F atom stream at reactor pressures between 10-4 and 10-2 Torr. This approach should be ideal for low-temperature growth and atomic layer epitaxy: growth rates remain relatively high because activation energies for radical reactions are typically small and because the cycle times for atomic layer epitaxy can be reduced to the msec range by fast gas-stream switching, and contamination and segregation are minimized by keeping the surface "capped" by chemisorbed intermediates. INTRODUCTION Radical chemistry offers a number of potentially very important advantages for chemical vapor deposition and atomic layer epitaxyI of thin films but has been largely ignored. The attainment of low growth temperatures is a major goal of current research-interdiffusion is greatly reduced and the effects of differential thermal expansion coefficients in complex layered materials are minimized. Radical precursors enable the dangling bonds on the substrate surface to remain capped during the entire growth process-reducing contamination, segregation, interdiffusion, and three-dimensional island formation-since radicals can create vacant surface sites by simply abstracting the capping adsorbed atoms. Maintaining a terminated surface appears to be absolutely essential to the growth of diamond films. 2 Once a radical precursor has chemisorbed on the substrate surface, the resulting surface species is more likely to decompose (yielding growth) than to simply desorb (yielding nothing). This preference for decomposition over desorption is the most likely reason why CH 3 is a more effective growth precursor for diamond than is C2H 2 .3 ,4 A further consequence of the "stickiness" of radical precursors is that nucleation densities may be greatly enhanced, particularly for materials with extreme properties such as diamond. Finally, radical precursors may specifically enhance atomic layer epitaxy (ALE) processes. ALE involves alternating exposures to different gas-phase reactants, with approximately one monolayer deposited per cycle. 1 Development of ALE processes for group IV materials such as silicon and diamond has proved to be very challenging,", 5 and radical reactants may provide the key to the development of
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