Mechanism of diamond film growth by hot-filament CVD: Carbon-13 studies

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I. INTRODUCTION

Interest in the growth of diamond films by lowpressure chemical vapor deposition techniques has burgeoned in recent years.1"3 While considerable progress has been achieved in solving various technological problems in diamond film growth, further development is hampered by a primitive level of fundamental mechanistic understanding. The critical role of atomic hydrogen is widely acknowledged, but the relative importance of (i) modification of the gas phase pyrolysis chemistry, (ii) maintenance of a hydrogen-terminated, nonreconstructed diamond substrate, (iii) abstraction of chemisorbed hydrogen to create reactive sites on the diamond surface, (iv) preferential etching of graphite, and (v) the role of other species such as oxygen remain topics of uncertainty and controversy. Several reaction mechanisms have been proposed, involving the methyl radical,4'5 the methyl radical cation,4'5 or acetylene6'7 as the primary carbon-containing growth precursor. The available information on these questions is limited. To date only a handful of mechanistic8"19 or kinetic modeling studies12"15-20 have been reported. While numerous types of growth chemistry have been used successfully, the oldest approach and the one still in most widespread use involves dilute mixtures of CH4 in H 2 , activated by means of a hot filament21 or a microwave discharge.22 This is also the approach about which the most is known, and in this paper we will focus exclusively on hot-filament CVD with mixtures of hydrocarbons in hydrogen. From in situ probing of the gas phase above the growing substrate by both laser spectroscopy9"11 and mass spectrometry12"15 together with modeling studies of the gas phase chemistry,12"15'20 it appears that under typical hot-filament CVD condiJ. Mater. Res., Vol. 5, No. 11, Nov 1990

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tions, the only potential growth precursors present in high enough concentrations to account for the observed growth rates are methane, acetylene, and the methyl radical. While the work of Angus and co-workers3'23'24 has demonstrated that growth from CH4 is possible without filament or plasma activation, the rate is much less than that achieved in typical hot-filament CVD conditions, and it is very unlikely that CH4 itself contributes significantly to diamond film growth at substrate temperatures below 950 CC. However, evidence for the predominance of CH3 or C 2 H 2 as the growth precursor has been lacking. The approach taken in the present work is to use carbon-13 isotopic labeling to distinguish the carbon atoms in different growth precursors. Diamond films are grown with mixtures of the labeled growth precursors, and by comparing the 13C mole fraction of the resulting diamond film to that of the various gas phase species, the incorporation efficiency of these species can be determined. The conceptual steps in our analysis are as follows: (i) The frequency of the one-phonon Raman scattering spectrum of the films is determined as a function of 13C mole fraction, so that th

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