Near-Surface Optical Detection of CH 3 During Diamond Growth
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NEAR-SURFACE OPTICAL DETECTION OF CH 3 DURING DIAMOND GROWTH
E. J. CORAT AND D. G. GOODWIN Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125 ABSTRACT Resonance-enhanced multiphoton ionization (REMPI) has been used to detect the methyl radical CH 3 within 0.5 mm of the substrate during CVD diamond growth. A strong dependence of the CH3 REMPI signal near the surface on substrate temperature is observed, which is not seen further from the surface. Below 1000 K, the observed temperature dependence may be characterized by an activation energy of approximately 4 ± 1 kcal/mole. The cause of the methyl depletion at low temperatures is not yet clear, but may be due to either gas-phase recombination near the surface or surface chemistry. The same qualitative behavior is observed for different substrate-filament distances and for gas compositions from 0.5% to 5% CH4 in H2. INTRODUCTION Recent experiments have shown that the methyl radical CH3 is the primary diamond growth species under typical conditions [1-5]. While several mechanisms have been proposed for diamond growth from methyl [6-9], there is at present little experimental evidence that could serve to confirm or refute any proposed mechanism, beyond identification of the growth species. Also, in addition to the mechanism for sp3 carbon formation, a complete picture of CVD diamond growth requires an understanding of the kinetics of sp 2 carbon deposition and removal by atomic hydrogen. This is particularly critical for development of low-temperature growth processes, for which the sp 2 content of the film becomes a problem. A useful means of examining the growth kinetics is to measure the temperature dependence of the growth rate and sp2 /sp 3 ratio. Two studies have recently reported measurements of the temperature dependence of the diamond growth rate in hot-filament systems [10, 11]. To properly interpret the results of these studies, it is necessary to know whether (and by how much) the growth species concentration at the substrate varies with temperature. Non-intrusive probing of CH3 immediately above the substrate is needed to answer this question. Also, both numerical models [12] and experiment [13] indicate that very near the substrate surface, surface chemistry may alter the gas-phase composition. By probing the gas environment very near the surface, it may be possible to obtain additional information on the surface kinetics of growth, which is difficult to obtain in other ways. Here we examine the effect of substrate temperature on the gas-phase concentrations of CH3 close to the substrate for typical hot-filament diamond growth conditions. Resonance-enhanced multiphoton ionization (REMPI) is used as a non-intrusive probe of the methyl concentration near the substrate. This method has been used previously to detect methyl in hot-filament diamond growth [14], and has excellent spatial resolution and sensitivity. EXPERIMENTAL The hot filament reactor consists of a 2.0 inch OD five-way cross with water-cooled wa
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