Temperature Programmed Desorption of Hydrogen and Deuterium from CVD Diamond Samples
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TEMPERATURE PROGRAMMED DESORPTION OF HYDROGEN AND DEUTERIUM FROM CVD DIAMOND SAMPLES MICHELLE T. SCHULBERG, GLENN D. KUBIAK, and RICHARD H. STULEN Sandia National Laboratories, Division 8342, Livermore, CA 94551 ABSTRACT The desorption kinetics of H2, HD, and D2 from a CVD-grown diamond sample have been measured using temperature programmed desorption. The sample was exposed to hydrogen and deuterium at - 100 K. Molecular hydrogen does not react with the diamond surface but atomic hydrogen chemisorbs. The sample is then heated to 1500 K at a rate of 6 K/s. The hydrogen desorption spectrum from these polycrystalline specimens is surprisingly simple, consisting of a single peak near 1270 K for all coverages studied. The width of the desorption peak is - 100 K. Neither the width nor the peak maximum shift with increasing hydrogen coverage. There is no isotope effect to the desorption. No other desorption products (water, small hydrocarbons) are detected. Kinetic parameters can be extracted from these spectra, and these can be compared to hydrogen desorption from natural diamond single crystal surfaces. These parameters can be used to model diamond growth processes. INTRODUCTION In recent years, rapid progress has been made in the field of chemical vapor deposition (CVD) diamond growth. Despite this success, the details of the atomic scale mechanisms for carbon addition to the diamond lattice remain unknown. Extensive modeling efforts' 2 3 have considered various possibilities and have made reasonable predictions of growth rates. These models consider gas phase and surface species and their reactions. Kinetic parameters for many of these gas-surface reactions are not known, however, and therefore must be estimated from those of gas-phase analogues. Atomic hydrogen is central to most CVD reactors. A common element of hot filaments, microwave, RF, and DC plasmas, combustion flames, and other diamond growth methods is the activation of molecular hydrogen to generate atoms. This key role for atomic hydrogen makes it essential to have accurate measurements of its reaction rates with other compounds present under growth conditions. The complex environment of a reactor chamber makes it impossible to measure the rate of a single reaction in situ, however, and therefore individual steps must be studied under single-collision conditions. This study describes such measurements of the kinetics of hydrogen desorption from CVD diamond surfaces. CVD-grown samples are polycrystalline, with surfaces composed of many facets and grain boundaries. Desorption from a CVD diamond surface therefore encompasses desorption from all of these sites. For this reason, it will be instructive to compare kinetic parameters obtained from polycrystalline surfaces with those of low-index planes of natural diamond single crystals. Finally, an estimate can be made as to the relative importance of hydrogen abstraction and desorption to create open sites on the diamond lattice at growth temperatures. EXPERIMENTAL DETAILS The CVD diamond samples were free-sta
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