The addition of small quantities of nitrogen to typical hydrocarbon/H2 gas mixtures used during diamond CVD, in both hot filament and microwave reactors, has been shown to lead to enhanced deposition rates,, nitrogen addition appears to affect the growth habit and lead to some modest nitrogen incorporation in substitutional lattice sites.
Previous investigations at Bristol by May et al. focused on diamond CVD in a hot filament reactor using CH4/NH3/H2, CH3NH2/H2 and HCN/H2 gas mixtures. The aims of these studies were to see whether use of alternative nitrogen containing precursors provided a route to enhanced nitrogen incorporation in the grown diamond film, Further aims were to unravel aspects of the chemistry prevailing in H/C/N containing gas mixtures using in-situ molecular beam mass spectrometry (MBMS), to sample the gas-phase composition in the vicinity of the hot filament.
The study observed diamond deposition from CH4/NH3/H2 feedstock gas mixtures, provided the input gas ratio [CH4]:[NH3] ³ 1 was observed. The addition of NH3 was found to reduce deposition rates relative to those found for a simple CH4/H2 gas mixture. CH3NH2/H2 and HCN/H2 gas mixtures were also shown to yield CVD diamond, with low efficiencies comparable to that found using 1:1 mixtures of CH4 and NH3 in H2. Such findings are explicable given the MBMS measurements, which showed substantial conversion of the input carbon to HCN (the thermodynamically favoured product), at the relevant process temperatures. Thus it was proposed that, in the vicinity of the hot filament, HCN acts as a sink for carbon that might otherwise have participated in diamond growth. The findings detailed in this chapter therefore build on the conclusions drawn by May et al.
This chapter describes a combination of experimental and 3-D computer modelling in order to provide a more detailed interpretation of the effects of controlled additions of nitrogen (in the form of N2 and NH3) to CH4/H2 process gas mixtures, in an HF-CVD reactor. The experimental component involves use of REMPI spectroscopy to provide spatially resolved in-situ measurements of H atom and CH3 radical number densities, and served to inspire subsequent Cavity Ring Down Spectroscopy (CRDS) measurements of NH radical column densities with the same gas mixtures, in the same HF-CVD reactor, by J.B. Wills, as a function of process conditions (e.g. feed-stock gas mixing ratio and the temperature, Tfil, of the Ta filament). These REMPI measurements are a natural extension of our investigations of diamond CVD when using CH4/H2 and C2H2/H2 gas mixtures in the same HF-CVD reactor detailed in chapter 3. The results obtained are then compared with the output of a 3-D computer model, specifically tailored to this HF-CVD reactor.
The 3-D computer model has been described in detail in chapter 3, and therefore only changes to the previous modelling procedure will be discussed here. The gas phase chemistry and thermochemical input is provided by the GRI-Mech 3.0 detailed reaction mechanism for C/H/N/O gas mixtures (plus one additional reaction allowing destruction of the species H2CN), but with all reaction steps and species involving O atoms removed (shown in Appendix 6). Comparisons between experimental and theory highlight the very different reactivities of N2 and NH3, in the present HF activated CH4/H2 gas mixtures. These comparisons also help unravel details of the gas-phase chemistry prevailing when using H2/CH4/NH3 process gas mixtures, and reveal clear evidence for a reduced H atom production rate, Q, (described in chapter 3) on the surface of the HF, when NH3 is added to the process gas mixture.
The results of the CRDS study of NH are also shown in this chapter purely to emphasize the accuracy of the 3-D model.
Details of the HFCVD reactor, and the REMPI detection schemes used for spatially resolved measurements of H atom and CH3 radical number densities, have been presented in chapter 2 The H2, CH4, NH3 and/or N2 feedstock gases are metered through separate mass flow controllers, pre-mixed in a manifold, and enter the reactor through a port located above the cradle assembly, so as to maintain an overall flow rate of 100 sccm and total pressure of 20 Torr. and are essentially unchanged, save for accommodating the introduction of N2 and NH3 into the reaction chamber.
Addition of NH3 to a CH4/H2 gas mixture activated by a Ta hot filament was observed to cause a reduction in the Tfil value returned by the two-colour optical pyrometer. No such trend was observed upon addition of a corresponding flow rate of N2. Fig. 4.1 illustrates these effects, via plots of Tfil versus time after the introduction of various partial pressures of NH3 (and N2) to an established 1% CH4 in H2 gas mixture.
Figure 4.1 Variation in pyrometer measured Tfil as a function of time after introducing 0.5% NH3 (¢), 1% NH3 () and 5% NH3 (▲) to a 1% CH4 in H2 gas mixture. The solid lines are best fits to these datasets in terms of equation 4.3 with t = 100 s (0.5% NH3), 21 s (1% NH3) and 6 s (5% NH3). Also shown is the time-dependent recovery of Tfil on the retardation of a 2% NH3 flow (¯) together with a best fit with a time constant of t = 100 s.
In each case, the filament had been run for > 6 hrs with a CH4/H2 gas mixture prior to the NH3 (or N2) addition, and the power supplied to the filament was held constant throughout. Clearly, both the rate and the extent of the temperature drop, DTfil (defined as the difference between Tfil measured prior to any NH3 addition, at t = 0 s, and the asymptotic value found at t = ¥) increase with increasing NH3 fraction, whereas addition of 1% N2 has minimal effect on the observed Tfil. Each of the Tfil versus t trends measured for the various NH3 partial pressures can be described by a function of the form:
Tfil(t) - Tfil(t = ¥) = DTfil exp(-t/t) Equation 4.1
with a time constant, t, that decreases as the NH3 fraction in the input gas feed is increased. Fig. 4.1 also illustrates how this temperature recovers with an apparently invariable time constant when the NH3 flow is shut-off (¯). Prior to any NH3 addition, we foresee the filament as being largely 'carburised', i.e. to be mostly tantalum carbide with a surface that is partially covered by a coating of graphitic carbon. Given knowledge of the input power supplied to the HF (Pinput = 87 W (~12.4 V, 7.0 A) for Tfil(t = 0) = 2473 K in figure 4.1) and estimates of the power expended on H2 dissociation (2 W) and conductive losses (10.7 W) from the 3-D modelling, we can estimate the power radiated by the HF (~74 W). This, in turn, allows estimation of the mean emissivity, e, of the hot filament via Planck's radiation law,
Prad = e s S Tfil4, Equation 4.2
where s is the Stefan-Boltzmann constant and S is the calculated surface area of the filament (0.63 cm2). The emissivities derived, e ~ 0.56 and ~0.61, for the addition of respectively 0% and 1% NH3 fall midway between literature values for the emissivity of TaC (e ~ 0.3) and graphite (e ~ 0.9) at T ~ 2500 K. This implies that, under these conditions, approximately half the filament surface has a graphitic coating.
The observed time dependence of Tfil upon addition of NH3 may be expressed in terms of dynamic competition between the adsorption, desorption and/or etching of the hot filament surface by nitrogen and carbon containing entities. In this scheme, two limiting mechanisms merit consideration. The first assumes that, both N and C containing species are present, and that the former are adsorbed preferentially onto the filament surface. 'N-termination' on the filament is presumed to alter the energy balance and increase the mean emissivity, leading to the observed reductions in Tfil and in Q (detailed in section 4.4). Assuming this is so, the observed time dependence of Tfil (shown in equation 4.1) can then be rationalised by considering a simplified adsorption-desorption / etching kinetic scheme, in which accommodation of a N containing gas phase species on an available surface site, S*, leads to N-termination, i.e. via the dynamic equilibrium,
N + S* ¾ SN. Equation 4.3
The time dependence of, q, the N-terminated surface coverage, will then follow,
q = (1 - exp[-(k5[N] + k-5)t] Equation 4.4
where [N] is the number density of nitrogen containing species near the filament surface, and k5 and k-5 are, respectively, the forward and reverse reaction rate coefficients for the generic gas-surface reaction described in equation 4.3. Fitting the experimental data for addition of 1% and 5% NH3 to a 1% CH4 in H2 gas mixture with
Tfil = 2473 K (at t = 0), shown in figure 4.1, yields values of k-5 ~ 0.01 s-1 and
k5[N] = 0.38 s-1 and 0.157 s-1, respectively. The observed recovery of Tfil when the NH3 flow is discontinued may also be calculated using equation 4.4.
It is also worth mentioning that similar decreases in Tfil have been reported previously in the case of carburised Ta filaments operating at high temperatures (Tfil > 2800 K) and constant Pinput when the hydrocarbon content in CH4/H2 gas mixtures is suddenly reduced. This may seem counterintuitive at first, since a reduction in the number density of gas-phase carbon would be expected to induce a concomitant reduction in the extent of any graphitic overcoat on the filament surface, which should therefore lead to a reduced emissivity and thus cause Tfil to rise. This observation has been explained by realising that the graphitic ‘overcoat’ is considerably less efficient than a 'clean' TaC surface at catalysing H2 dissociation; the observed drop in Tfil is thus attributed to the increase in the fraction of Pinput expended on surface catalysed H2 dissociation.
As shown previously, one effect of NH3 addition is to convert CH4 molecules into HCN in hotter regions of the gas. HCN is a stable molecule in this environment, and thus acts as a 'sink' for gas-phase carbon near the filament surface. Therefore, a second possible model for the time dependence of Tfil upon NH3 addition presumes that, NH3 addition has the effect of reducing the number density of gas-phase carbon available for formation of any graphitic layer on the filament surface. This would have a similar effect to simply reducing the CH4 input flow rate. This, in turn, might then be expected to cause an increase in S*, capable of catalysing H2 dissociation and could thus also give rise to the observed time dependent variations in Tfil. Given our earlier estimates of the fraction of Pinput used in dissociating H2 molecules on the filament surface, at the comparatively low Tfil values used in this work, such an explanation seems incapable of accounting for Tfil reductions of the magnitudes observed upon NH3 addition.
4.3 Gas-phase H atom and CH3 radical relative number densities observed with the addition of NH3 and N2
Relative number densities of H atoms and CH3 radicals were measured by 2+1 REMPI (in an identical manner to that used in chapter 3), as a function both of added NH3 (or N2) and Tfil. Figures 4.2 and 4.3 show experimentally measured [H] and [CH3] values, as a function of added N2 and NH3, for a 1% CH4 in H2 mixture (flow rate = 100 sccm, total pressure = 20 Torr and Tfil = 2573 K), measured at a filament distance d = 4 mm.
Clearly, given the above change in filament characteristics, it was necessary to increase the power supplied to the HF progressively as the NH3 input mole fraction increased in order to maintain a constant Tfil.
Figure 4.2 REMPI measured H atom relative number density, as a function of added NH3 (£) and N2 (p) to a 1% CH4 in H2 gas mixture. The chamber was maintained at 20 Torr and the filament at Tfil = 2573 K. The dashed curve shows the calculated variation in absolute [H] (right hand scale), assuming fixed values of
Q (4.92 ´ 1019 cm-2s-1) for Tnf = 2075 K using the previously established Tgas dependency on d. The solid curve however shows a fit to the experimentally determined NH3 data assuming that Q declines with added NH3 with a function shown in the figure inset. The theoretical and experimentally determined data were vertically scaled so as to match for 1% NH3 addition.
Figure 4.3 Equivalent plot showing REMPI measured CH3 relative number density as a function of added NH3 (£) and N2 (p) to a 1% CH4 in H2 gas mixture. The reactor conditions are identical to that stated for figure 4.2 with measurement also carried out at d = 4 mm. The dashed and solid curves represent the calculated variation in absolute CH3 number density given a fixed Q, and when assuming that Q varies as a function of added NH3 as shown in the inset of figure 4.2.
Interpreting these, and subsequent, experimental data builds on previous modelling of the gas phase chemistry occurring in activated CH4/H2 and C2H2/H2 gas mixtures in this same HF-CVD reactor (Chapter 3). The modelled reactor (with no substrate present) is represented in Cartesian co-ordinates with the z-axis parallel to the direction of feedstock gas flow (input ® output) and perpendicular to the filament, the centre axis, of which, is along y. x is orthogonal to both the filament axis and the direction of gas flow, the point (0, 0, 0) defines the centre of the filament, and d = 0 corresponds to the point (0, 0, 1.5 mm). The model calculation extends to the volume enclosed within
x = ± 16 mm, y = ± 18 mm and z = ± 26 mm, with the gas temperature set to Tgas = 600 K at all boundaries of the numerical grid. The positions (20 mm, 0, 0) and (0, 20 mm, 0) represent the reactor walls while (0, 0, -25 mm) and (0, 0, +25 mm) represent the gas outlet and inlet positions.
The dashed lines in figures 4.2 and 4.3 show absolute H atom and CH3 radical number densities predicted by 3-D simulations, assuming, the experimental values for Tfil and the gas pressure, a net H atom production rate at the filament
(Q = 4.92 ´ 1019 cm-2 s-1), a near filament gas temperature (Tnf = 2075 K) and a d dependence for Tgas consistent with our previous measurements. In both figures, the measured [H] and [CH3] values appear insensitive to the addition of N2. This result can be readily understood by inspecting the calculated temperature (and thus position) dependent interconversion rates and number densities for the various participating species. In the case of a 1%CH4/1% N2/H2 gas mixture, N2 is deduced to play the role of an essentially inert spectator, with a predicted fractional dissociation [N]/[N2] < 10-7 even at Tgas ~ 2000 K (i.e. as attained in the immediate vicinity of the HF). Further simulations, in which this ratio was arbitrarily increased (such as could conceivably apply in the event of catalysed N2 dissociation on the filament surface or, in the case of microwave activation, as a result of non-thermal plasma chemistry), indicate that the [N]/[N2] ratio would have to be at least 100 times higher in order to cause a measurable reduction in [H] or [CH3].
However, the addition of NH3 into the hot filament activated 1% CH4 in H2 gas mixture, clearly changes the gas-phase chemistry. The calculated z dependent interconversion rates and number densities for all major species present in a hot filament activated
1% CH4 / 1% NH3 / H2 gas mixture, displayed in figure 4.4, show this nitrogen containing precursor to be intimately involved in the high temperature gas-phase chemistry. Temperature and pressure dependent rate constants, k(T,P), for the numerous elementary steps underpinning the H/C/N gas phase chemistry are taken from the GRI-Mech 3.0 reaction mechanism. The addition of the highly exothermic reaction,
H + H2CN ® H2 + HCN, Equation 4.5
which was originally excluded from the GRI-Mech database due to its overall minor role in combustion analysis, curtailed the calculated build-up of unstable H2CN in the cooler regions of the reaction chamber.
Figure 4.4 Plots showing the semi-logarithmic calculated rates, R (left-hand scale), for (a) the series of hydrogen shift reactions, NHx + H NHx-1 + H2, and (b) other selected reactions dealing with nitrogen containing species. Both plots are shown as a function of distance z (with x = y = 0) for a chamber operating with 1% CH4 / 1% NH3 / H2 and Tfil = 2573 K. The R values were calculated assuming Q = 2.0 ´ 1019 cm-2 s-1, Tnf = 2100 K and a z dependent temperature profile as shown in (a) (right-hand scale).
The calculated species number densities, clearly show HCN acting as a carbon ‘sink’ in both the hot and cool regions of the chamber. This is shown in figure 4.5 together with other relevant species.
The experimental results, shown in figures 4.2 and 4.3, and the calculated rate constants and species number densities, shown in figure 4.4, are for a constant Tfil, gas pressure and flow rate. As the dashed lines in these figures show, no single value of Q is capable of fitting all data recorded as a function of added NH3. To match the experimental observations it is necessary to assume Q to be NH3 dependent, i.e. to assume that N-termination of the HF surface causes not just an increase in emissivity but also a reduction in the mean H atom production rate, Q.
This much improved agreement is illustrated by the solid lines in figures 4.2 and 4.3, which demonstrates that it is possible to obtain a reasonable fit with the experimentally measured [H] and [CH3] versus Tfil dependencies, if Q for the filament, maintained at a constant Tfil, declines with the percentage of added NH3. The deduced functional form of Q(NH3; Tfil = 2600 K) is shown in the inset to figure 4.6. Such a dependence may be considered plausible given the previous conclusions that NH3 addition leads to modification ('N-termination') of the filament surface and of its emissivity.
4.4 Gas-phase H atom and CH3 radical number densities in a 1%CH4/1%NH3/H2 gas mixture as a function of Tfil
Figure 4.6 compares H atom relative number densities measured 4 mm from the filament surface as a function of Tfil for a 1%CH4/H2 input gas mixture, with and without 1% added NH3, while figure 4.7 shows the corresponding plots for the measured CH3 radical relative number densities.
Figure 4.6 Calculated [H] and measured relative H atom number density as a function of Tfil. Both the calculation and the measurements were carried out using a 1% CH4 / H2 gas mixture, with and without the addition of 1% NH3, at 20 Torr. REMPI measurements were carried out at d = 4 mm.
Figure 4.7 Calculated [CH3] and measured relative CH3 number density as a function of Tfil. Both the calculation and the measurements were carried out using the same conditions as described in figure 4.6.
As shown in previous experiments in Chapter 3 (Figure 3.6), the H atom number density increases almost exponentially as a function of increasing Tfil. The observation that the decrease in the H atom number density, with the addition of NH3, continues throughout the filament temperature range is also shown. As in figure 4.2, model calculations based purely on gas-phase chemistry, together with the assumption of a constant Q, suggest that [H] should increase upon addition of NH3. In order to obtain convergence between experimental observation and the model calculations it is necessary to assume that the NH3 induced modification of Q is itself a function of the filament temperature. As figure 4.6 shows, a reasonable match between experiment and theory can be obtained if the ratio Q(1%NH3)/Q(0%NH3) is assumed to vary with Tfil. Figure 4.7 shows, in accord with previous findings, that [CH3] rises smoothly with increasing Tfil for the standard 1% CH4/H2 mixture, but that addition of 1% NH3 causes a marked decrease in [CH3], most notably at high Tfil. Both trends are reproduced well by the 3D model calculations but, again, better agreement with the trends measured in the presence of 1%NH3 can be obtained by assuming that Q(1%NH3)/Q(0%NH3) is Tfil dependent.
The value of Q may be seen as a function of factors attributed to Tfil and the NH3 fraction. Let us consider both factors in terms of H production on the filament surface. Q may be related to Tfil via the equation,
Q(Tfil) ~ exp(-DHdiss/RTfil) Equation 4.8
4.5 NH radical number densities and spatial profiles in 1% CH4 / x% NH3 / H2 gas mixtures.
Analysis of the modelling results of the z dependent species concentrations and elementary reaction rates, detailed in figure 4.5, indicate that the number density of NH radicals in the vicinity of the hot filament (>1012 cm-3), should be sufficient to permit their detection by a sensitive absorption method like CRDS. Unlike REMPI, such measurements offer a route to absolute column densities, and thus provide a further test of the absolute values of Q, and their dependence on Tfil and [NH3], returned by the gas phase chemistry modelling. Such measurements have been undertaken by J.B. Wills6. As with the REMPI measurements, number densities were determined as a function of radial distance from the lower edge of the coiled filament, and as a function of input NH3 mole fraction. Figure 4.8 compares the measured and predicted filament distance, d, dependence of NH radicals in a hot filament activated 1% CH4 / 5% NH3 / H2 gas mixtures with Tfil = 2473 K, monitored via the R1(2) and R1(3) transitions of the NH(A-X) origin band.
Figure 4.8 Measured () and calculated(¾) d dependence of the NH absorbance. The measured data were obtained by monitoring (a) the R1(2) and (b) the R1(3) transitions of the NH(A-X) origin band.
Naturally the NH absorbance, and thus [NH], is found to scale essentially linearly with the input NH3 mole fraction. The level of agreement (to within ~20% in the implied absolute number densities) provides strong support for the validity of the foregoing modelling of the gas phase chemistry of H/C/N containing mixtures in a HFCVD reactor, and the spatially resolved absolute number densities returned by the simulations for all major species.
The results presented here, and elsewhere, provide a reasonably self-consistent picture of the gas phase transformations occurring in a HF CVD reactor operating at typical modest filament temperatures when trace amounts of NH3 are added to a hydrocarbon/H2 gas mixture. Input CH4 molecules pass through a multi-step sequence of reactions en route to stable products C2H2 and HCN, thereby depleting the number densities of CHx species both near the HF and in the surrounding volume, extending out to distances where the substrate would normally be positioned in a diamond growth experiment. HCN also acts as a sink for input NH3 molecules. The studies detailed here show that NH3 additions to an activated CH4/H2 gas mixture lead to reductions in the CHx number density in the vicinity of the HF in two ways. Firstly, addition of NH3 introduces additional, purely gas-phase, CHx loss processes. Secondly, NH3 additions are deduced to cause modifications of the HF surface, increasing its emissivity and thus lowering Tfil (for a given input power) and reducing its efficiency for catalysing H2 dissociation even if Tfil is maintained constant.
Additional confirmation of the REMPI measurements and the modelling results was provided by CRDS analysis of NH radicals, which afforded knowledge of the radical radial number density dependence as a function of the distance from the filament.
It is worth noting that the modelling study predicts CN radical concentrations at typical filament – substrate distances to be < 109 cm-3. If, as thought, nitrogen incorporation into a diamond film is carried out via CN addition into a vacant surface site, this may provide an explanation for low nitrogen doping efficiencies.
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