J. Phys. Chem. 1993,97, 11787-1 1796
11787
Heterogeneous Reactions o f H Atoms and CH3 Radicals with a Diamond Surface in the 300-1133 K Temperature Range Lev N. Krasnoperov*vt Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia
Ilia J. Kalinovski, Hae-Nuh Chu,: and David Gutman' Department of Chemistry, The Catholic University of America, Washington, D.C. 20064 Received: June 14, 1993; In Final Form: August 17, 19938 The heterogeneous reactions of H atoms and CH3 radicals with a polycrystalline diamond surface have been studied using discharge flow and pulsed excimer-laser photolysis techniques coupled with photoionization mass spectrometry. A random walk Monte Carlo solution of the three-dimensional diffusion equation was used to interpret the experimental data and to obtain the collisional efficiency for removal of hydrogen atoms by the diamond surface (yw)over the temperature range 300-1 119 K. The result expressed in Arrhenius form is yw(H on diamond) = 10-3.4i0.3 10°.29M.15 exp(-(6020 f 470)callRT). The second term is assigned to reaction 1, surface H atom abstraction by gas-phase H atoms, followed by the fast recombination reaction 2 of gas-phase H atom with the surface sites created by reaction 1. The experimentally measured ywyielded the rate constant of reaction 1 (assuming the surface concentration of H atom, [H,] = 1.8 X 1015molecule cm-2, and k2 >> kl): kl = 10-10.06i0.15exp(-(6680 f 470)callRT) cm3 molecule-' s-l. This expression for the rate constant is in good agreement with that for the gas-phase abstraction of tertiary hydrogen atoms. The CHs(g) radical decay above the diamond film was studied over the temperature range 739-1 133 K. The measured collision efficiency for the removal of CH3(g) radicals by the diamond surface in this temperature range is yw(CH3on diamond) = 10°.1i0.7 exp(-(10640 f 3420)cal/RT), which, if interpreted as reaction 3 (surface H atom abstraction by gas-phase methyl radicals) with subsequent fast reaction (4) of gas-phase methyl radical with the surface sites created by reaction 3, yields k3 = 10-10.8*0.7exp(-(11540 f 3420)cal/RT) cm3 molecule-' s-l. The absolute value of k3 over the temperature range studied (derived assuming the same surface density of hydrogen atoms) is 6 times higher than that of the analogous gas-phase processes.
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most CVD processes. Reactions 1 and 2 constitute a catalytic cycle for H atom recombination which is the main loss mechanism While the chemical vapor deposition (CVD) of synthetic for hydrogen atoms under CVD condition^.^"^ In addition, these diamond films has become a well-established technology,l-l the two reactions provide an effective mechanism for heat transfer gas/surface chemical kinetics responsible for diamond film growth to the ~ u b s t r a t e , ~and ~ - ~this * heating must be taken into account are still not well understood. Theoretical modeling of the when modeling diamond growth processes. Reaction 3, while elementary kinetic steps involved in the CVD process has less important as a source of surface sites, is possibly a significant demonstrated clearly that hydrogen atoms and methyl radicals sink for methyl radicals which must be considered when modeling are two important intermediates in the chemistry of diamond diamond growth processes involving this radical. Reaction 4 is film growth.12-26Experimentaldetection of these b~termediateS~~-~ now a widely accepted initial step leading to tetrahedral carbon and the ability of kinetic modeling to reproduce aspects of the growth on the diamond surface. observed behavior of these intermediates in CVD processes There have been essentially no kinetic studies which have confirm the importance of gas-phase as well as gas/surface isolated reactions 1-4 for quantitative study. In the cases of 'elementary" steps involving hydrogen atoms and methyl radicals, reactions 1 and 3, in lieu of available kinetic information, it has including their removal of surface hydrogen atoms ( H a ) from been the practice in CVD modeling studies to transfer gas-phase the diamond substrate to create active sites (4)as well as their kinetic parameters for H atom abstraction processes to the attachment to these active sites: comparable gaslsurface processes to obtain the rate constants (or reaction probabilities) needed for such heterogeneous steps H(g) + H S H2(g) + (1) in CVD mechanism~.'3.~~822 In the current paper, new experiments are described in which H(g) S H S (2) the kinetics of reactions 1 and 3 have been isolated and studied over an extended temperature range to obtain quantitative CH3(g)+ H S - C H , ( g ) + S (3) measures of gas/surface reaction probabilities. Both reactions were studied in a heatable tubular reactor coupled to a photoionization mass spectrometer. H atom loss acrossdiamond plates CH3(g) S C H 3 S (4) located along the bottom of the reactor was measured. Diffusion of H atoms to the diamond surface was an important rateReaction 1 is considered to be the principal step responsible for determining factor. Therefore, a random walk Monte Carlo producing active surface sites under the conditions existing in solution of the three-dimensionalkinetics/diffusionproblem was t On leave. Current address: Department of Chemistry, The Catholic developed and used to obtain the kinetic parameters of reaction University of America, Washington, D.C. 20064. 1 from the measured attenuation of H atom concentrations by t Present address: Engineering Research Center for Plasma-Aided Manthe diamond plates. In separate experiments,CH3 radicals were ufacturing, University of Wisconsin, Madison, WI 53706. produced above the diamond plates using UV laser photolysis. *Abstract published in Advance ACS Abseacrs, October 1, 1993.
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0022-3654/93/2097- 11787$04.00/0
0 1993 American Chemical Society
Krasnoperov et al.
11788 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Quadrupole Mass Filter
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Figure 1. Experimental apparatus showing the heatable flow reactor (containing diamond plates) coupled to a photoionization mass spectrometer (the distance between diamond plates is exaggerated). The position of the diamond plates shown is for the measurements of CH3 radical decay. A different positioning was used in the experiments of H atom decay (see text).
The decay of these radicals was monitored in time-resolved experiments to obtain a measure of the reaction probability of CH3 radicals on the diamond substrate used. These experiments and the results obtained are reported here. Harris and Weiner recently reported the first determination of the H atom destruction probability on diamond.39 The study was conducted at a single temperature, 1200 K. The technique used, which was quite different than what was employed here, involved measurement of the heat released by the catalytic recombination of H atoms on a thermocouple tip coated with a diamond film. They reported that the H atom loss probability on collision with a diamond surfaceis 0.1 at 1200K. Interpretation of these experiments also involved taking diffusion of H atoms to the very active surface into account. There have been no prior studies of reaction 3.
reactor in the heated zone. In the experiments on H atom decay, the diamond plates were placed upstream of the gas-sampling hole, the distance between the end of the last diamond plate and the sampling hole being 3 cm. In the experimentson CH3 radical decay, two diamond plates were always used and were placed so that the sampling hole was above the last plate (it is this configuration that is shown in Figure 1). The end of this last diamond plate was 1 cm beyond the gas sampling orifice. The vacuum-UV radiation used for photoionization in the ion source of the quadrupole mass spectrometer is provided by atomic resonances l a m ~ s . 4H ~ atoms and H2 molecules were detected at m / z = 1 and 2, respectively, using radiation from a neon resonance lamp with a collimated hole structure Ywindow”as the photoionization source (photon energy 16.5 eV). A hydrogen resonance lamp with a MgF2 window was used to detect CM3 radicals (hv = 10.2 eV, m / z = 15). Experimental Section The temperature in the reactor was measured and controlled by a chromel-alumel thermocouple located inside the reactor A. Experimental Apparatus. The basic experimental appaalong the axis 2 cm downstream from the sampling hole. ratus, which consists of a heatable tubular flow reactor coupled to a photoionization mass spectrometer, has been d e ~ c r i b e d . ~ ~ Temperature profiles in the reaction zone were measured in a separate set of experiments using flow conditions identical to Although the apparatus was initially developed for studying those used in the kinetics experiments. homogeneous gas-phase reactions, it was later modified to study The gases used were obtained from Matheson (He, 99.995%; heterogeneous catalytic processes as well.42 The modification of H2, 99.99%) and Aldrich ((CH&CO, >99%). Acetone was the apparatus used in the current study is shown in Figure 1. degassed before use; the other gases were used as provided. The main reactor consists of a quartz tube, 2.2-cm i.d., with nichromeheating tape wrapped along 20 cm of its length. Zirconia To keep the heterogeneous recombination of hydrogen atoms ceramic was used as a thermoinsulator outside the reactor which on the inner surface of the quartz reactor insert at a minimum, made it possible to heat the reactor in vacuum up to 1200 K using the insert was frequently treated according to the procedure moderate power. A concentric quartz tubular insert (not shown developed by Sepehrad et al.44and used by Martin in a flow-tube in Figure l), 1.8-cm i.d. (and nearly 2.2-cm o.d.), slides into the diamond growth study.4O The procedure consists of soaking the main reactor and constitutes the actual quartz wall which is in insert in 10 M aqueous NaOH solution for approximately 15 h, contact with the flowinggas. This insert, which is easily removed multiple rinsings with distilled water, a 15-h contact with 10 M without removing the main tubular reactor, was frequently H N 0 3solution, and finally multiple rinsings with distilled water extracted and its surface passivated (see below) to reduce H atom following by drying. As was shown b e f ~ r e and ~ ~ as v ~was found recombination on the exposed quartz walls of the reactor. in the current study, this procedure provides long-lived passivation Gas in the reactor was sampled continuously from a small of the quartz surface to H atom recombination up to the highest conical hole (with orifice 0.43 mm in diameter) in the wall of the temperature (1 133 K) used in this study, reactor located 7 cm upstream from the end of the heated zone. Hydrogen atoms were produced by a microwave discharge in The sampled gas is formed into a beam by a conical skimmer a Pyrex discharge tube located at the upstream end of the tubular (with a 1-mm-diameter orifice) before flowing through the ion reactor (See Figure 1): source of the photoionization mass spectrometer that is used to discharge monitor the concentrations of stable and labile species in the H, 2H (5) reactor. One or two plates (1.5 X 6.0 cm) of CVD diamond (Norton Helium/hydrogen gas mixtures were flowed through this tube. diamond TE 114 or TE 145) were placed on the bottom of the The molecular hydrogen concentration was varied to change the
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Heterogeneous Reactions of H and CH3 with Diamond amount of atoms produced. The inner wall of the tube was treated with a 5% aqueous solution of H3B03 (three cycles of rinsingdrying at 200 "C)before each series of experiments.& B. ExperimenW Study of Heterogeneous Loss of Hydrogen Atoms 011 the Quartz Reactor Surface at Ambient Temperature. The heterogeneous recombination rate constant of H atoms on the conditioned quartz surface of the reactor insert at room temperature was measured to verify the effectiveness of the passivation technique used. H atom concentrations were measured as a function of atom contact time with the reactor walls (before gas sampling). To vary the contact time of H atoms, which are produced at a fixed location upstream from the sampling orifice, the linear flow velocity of the gas in the main tubular reactor was varied between 63 and 191 cm/s by changing the main helium camer gas flow. The total gas pressure in the system was kept constant (2.20 Torr). The flow rates of Hz and He through the discharge were also kept constant, which provided a constant mole fraction of HZ dissociated (and hence initial upstream H-atom concentration) as the flow velocity (reaction time) in the main reactor was varied. For each flow velocity used, the ion signals (S) of H+ and H2+ were measured with the microwave dischargeon and off. The logarithm of the ratio ASH/ S H , , ~(S~,~~-SH,~ff)/SH,,~nplotted ~' vs contact time is a straight line. The slope of this line provides the heterogeneous wall recombination rate constant of hydrogen atoms on the treated wall of the quartz reactor insert, k, 2 s-* (which corresponds to a wall reaction probability of yw = 1.8 X l e 5 ) at room temperature. C. Experimental Study of Heterogeneous Loss of Hydrogen Atoms on a DiamondSurface. H atom heterogeneous loss on the diamond surfacesused was measured as a function of temperature between 300 and 1119 K. The basic experiment consisted of measuring the H atom and Hz ion signals at the sampling point in the flow reactor with and without the diamond plate(s) in the reactor as well as with and without the microwave discharge which produced the hydrogen atoms upstream from the diamond plates in the reactor. As mentioned above, the gas sampling orifice was located 3 cm downstream from the end of the last plate (this plate alignment is not the one illustrated in Figure 1). All experimentswere conductedbetween 1.68 and 1.87 Torr total gas pressure. At each temperature, a set of measurements was performed keeping all flow parameters constant (including [H]o, flow rates, and the flow velocity). The H+ and Hz+ion signalswere measured without thediamond plate(s) present (the blank experiment) and with the diamond plate(s) present. In both configurations, ion signals were measured with the microwave discharge both on and off (the difference between the ion signal with and without the discharge on is designed by AS). In each set of experiments,two blankexperiments were conducted,one before the diamond plate(s) were inserted and again after they were removed. Measurements were typically reproducible within f10-15%. The experimental results obtained from such a set of experiments conducted between 300 and 1119 K are plotted in Figure 2. Shown in Figure 2a and 2b are two measures of the H atoms which survived the flow through the region between the gas discharge and the gas sampling point: -ASH,/SH2,0ff (the remaining fractional loss of HZat the sampling point) and ASH/ S H , (which , ~ ~ is proportional to the remaining H atom concentration in the gas flow when it reaches the orifice). With the diamond plate(s) absent, there is only a small decrease (3040%) of the H atom concentration at the sampling point when the temperature is raised from room temperature to the highest temperature of this study. The smallness of the decrease is due in part to the fact that, as the temperature is raised, the gas flow velocity does not remain constant but rather increases from 11.6 m s-l at 300 K to 38.4 m s-* at 1119 K. The H atom loss rate constant on quartz increased from 2 s-1 at 300 K to 70 s-1 at 1119 K. With the diamond plate(s) present, the residual H atom
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11789
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Figure 2. Experimental data on H atom decay on the diamond surface. Open circles: blank experiment (no diamond inside the reactor). Filled circles: two diamond plates (CVD Norton diamond TE 114, 1.5 X 6.0 cm, total length 12 cm) inside the reactor. (a) Dependence of the relative on temperature. (b) H atom signal, depletion of Hz signal, -ASH~/SH~,~K, normalized to signal of molecular hydrogen, as a function of temperature. (c) The ratio of H atom signal and molecular hydrogen depletion as a function of temperature. Gas mixture: 0.98% Hz in He. Total pressure changes with temperature from 1.68 Torr at 300 K to 1.87 Torr at 11 19 K. Flow velocity is 1194 cm/s at 300 K and increases with temperature to 3834 cm/s at 11 19 K.
concentration drops drastically with increasing temperature. At 1119 K, it is ca. 1/200 that which was recorded at 300 K (with two diamond plates inside the reactor). Above 700 K, the remaining fractional loss of HZwas too small to measure with the diamond plates present (see Figure 2a). Above this temperature, the diamond surface activity was so great that practically all the H atoms had catalytically recombined on the plates to regenerate H2, leaving the residual H2 depletion too small to measure (
