Chapter 6 - Results for C/H/N Systems

6.1 Introduction

The incorporation of unwanted impurities invariably present in reactant gases remains one of the main drawbacks in CVD diamond growth. The most commonly occurring impurity in both natural and synthetic diamond is nitrogen. Inclusion of small amounts of this element has a noticeable effect on the growth of CVD diamond films and on many of the physical properties of the material,^6.1-6.5 namely its optical transparency and thermal and electrical conductivity. The presence of nitrogen in the reactant gases can also seriously alter the morphology of deposited diamond films which restricts their usefulness for some applications, especially those that require high quality electronic grade diamond.^5.12 Alternatively, however, nitrogen-doped CVD diamond and diamond-like carbon (DLC) films may have very useful semiconducting properties, which are particularly important for microelectronics and field emission display applications.^6.6-6.10 These discoveries have attracted widespread research interest, and prompted many groups to investigate the effects of nitrogen on the growth of CVD diamond films using CH₄/H₂ gas mixtures.^{6.1-6.5,6.11-6.16} Hong et al.^6.17,6.18 also studied the influence of adding nitrogen gas to a more unconventional CH₄/CO₂ gas mixture.

Recent studies have shown that addition of nitrogen to the input gas mixtures causes significant changes in the morphology, the quality and the growth rate of diamond films. For example, Bohr et al.^6.12 found that the growth habit and deposition rate using CH₄/N₂/H₂ mixtures in a hot filament CVD system depended strongly on the ratio of carbon-to-nitrogen in the feed gas; in the region of the [N]/[C] ratio relevant to the present work they observe that small N₂ additions (up to 20% N₂/CH₄) not only increased the growth rate by a factor of ~2, but improved the diamond phase purity, as revealed by laser Raman spectroscopy. Higher N₂ additions (20% to 40% N₂/CH₄) resulted in deterioration of the quality of the diamond films, a reversal of growth rates, and a change in the surface morphology from (111) to predominantly (100) facets. In a somewhat related study, Badzian et al.^6.15 grew diamond using a CH₄/N₂ mixture both in the absence of H₂, and with added H₂. They observed a considerable amount of distortion in the crystal structure of the grown diamond films with N doping, and a reduction in the growth rate with nitrogen addition in the input gas mixture. To date, the precise reaction mechanisms attributable to these observations have not been studied in detail. Although various mechanisms have been proposed involving gas-phase and/or gas-solid reactions,^{6.12,6.15,6.17} it is difficult to draw any parallels between them since very different source gas mixtures were being used. In many cases however, the amount of nitrogen incorporated into the diamond films was found to be very low,^{6.1,6.15,6.17} regardless of the choice of precursors used, which suggests that the growth mechanism is likely to be dominated by gas-phase chemistry rather than gas-solid heterogeneous reactions. In this article we report on the behaviour of nitrogen in hot filament assisted CVD of diamond in terms of the changes in the gas-phase chemistry when nitrogen is present, using various C/N-containing source gases. In-situ molecular beam mass spectrometry^{3.1,5.21,5.22} was used to characterise the gas phase environment, and to determine the mole fractions of the stable gas-phase species prevalent during the CVD process. Such information provides valuable understanding of the reaction mechanisms involved when nitrogen is added to the gas mixtures.

6.2 Experimental Details

(a) Deposition experiments

Table 6.1 below shows the growth conditions used for the C/H/N system. For full experimental details on deposition techniques please refer to Chapter 3 (Sections 3.1 and 3.2).

Pressure 20 Torr

Gases (1) 0.5% CH₄/0.5% NH₃; (2) 0.5% CH₃NH₂;(3) 0.5% HCN and (4) 0.5% CH₄/0.25% N₂.

Total gas flow rate 200 sccm

Substrate type Si (100) substrates (manually abraded)

Substrate temperature ~ 900°C

Filament temperature 2300-2400°C (filament current 6½-6¾ A)

Filament/substrate distance 4 mm

Deposition time 6 hours

Table 6.1. Deposition conditions used for C/H/N system.

For these conditions, and using a gas mixture of 0.5% CH₄ in H₂, typical growth rates of microcrystalline CVD diamond were ~0.32 mm h^-1 (See Section 4.6). In the present study, methane was replaced with a variety of C- and/or N-containing precursor gases, always ensuring a constant C:N ratio of 1:1. The C/N source gases were introduced into the reaction chamber in three different forms: (1) the C and N present in separate small molecules, such as CH₄ + NH₃ (or N₂); (2) the C and N bonded together in the same molecule, as in methylamine (CH₃NH₂); and (3) the C and N present as CºN in hydrogen cyanide gas (HCN). All the precursors were obtained as commercial products except for HCN, which was synthesised by the reaction of NaCN with phosphoric acid (dried by addition of P₂O₅) in vacuo (See Appendix I).

(b) Film analysis

The as-grown films were investigated by scanning electron microscope (SEM) and Auger electron spectroscopy (AES). Laser Raman Spectroscopy (LRS) and secondary ion mass spectroscopy (SIMS) was also carried out on the films produced by CH₄/N₂/H₂ gas mixtures. The composition of films produced by the other C/N precursor gases have been determined using other spectroscopic techniques, such as Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) and is reported elsewhere.^6.19

(c) Gas-phase composition measurements

Gas-phase product distributions were monitored as a function of the filament temperature for all C/N precursor gases. The ionization potentials of all the C-, N- and N/C- containing species that were monitored, together with the electron energy at which they were measured, are shown Appendix II.

When measuring the signal for N₂ species (m/e = 28, I.P. = 15.55 eV) using an electron energy of 30 eV, corrections were made to eliminate signal interference from C₂H₄ (IP = 10.51 eV) and CO (IP = 14.0 eV). Similar corrections were also performed for other species, such as the CH₄ signal (m/e = 16) due to the dissociative ionization of NH₃ to NH₂ (m/e = 16); and the NH₃ signal (m/e = 17) due to fragmentation of methylamine (CH₃NH₂). For full details on the correction procedures, see Section 3.6.

6.3 Analysis of the diamond films

C/N Precursor Gas	Growth Rate/ mm h^-1	% CH4 Mole Fraction	% NH3 Mole Fraction	% C2H2 Mole Fraction	% HCN Mole Fraction	% CH3NH2 Mole Fraction
0.5% CH4/ 0.5% NH3	< 0.07	0.017	0.041	0.002	0.102	0.000
0.5% CH3NH2	~ 0.05	0.051	0.031	0.005	0.164	0.009
0.5% HCN	< 0.10	0.023	0.008	0.005	0.157	0.000
0.5% CH4/ 0.25% N2	~ 0.45	0.017	0.061	0.017	0.018	0.028
0.5% CH4	~ 0.32	0.032	-	0.023	-	-

Table 6.2. Diamond growth rates and calculated mole fractions of major stable species measured at filament temperatures (2400°C) for different C/N source gases. Note that the C:N ratio for all precursors is 1:1.

(a) Methane and Ammonia as precursor gas mixture

Scanning electron micrographs (top and cross-sectional view) of films grown on silicon (100) using a gas mixture of 0.5% CH₄/0.5% NH₃ (1:1 ratio) in H₂ are shown in Figure 6.1. The growth rate (See Table 6.2) was calculated from the film thickness, determined from the cross-sectional SEM image, divided by the time of growth (6 hours). The film is non-continuous, being composed of many isolated (111) diamond crystals. With methane-rich gas mixtures (e.g. 0.75% CH₄/0.25% NH₃ in H₂), as expected, diamond was grown (See Figure 6.2), the morphology of which was similar to that observed in standard CVD processes, although as the mole fraction of NH₃ increased the films became more nanocrystalline in nature and the growth rate reduced. As shown in Figure 6.3,^6.19 with ammonia-rich gas mixtures, the Si substrate preferentially reacted with the NH₃ to produce an Si₃N₄ coating, with no carbon. The deposition rate for gas mixtures with C:N ratio of 1:1 is very small (less than 0.07 mm h^-1). A composition diagram showing the types of films produced on Si substrates after CVD as a function of different substrate temperatures and methane:ammonia ratios is shown below in Figure 6.4.

Figure 6.4. Diagram showing the types of films produced on Si substrates after CVD as a function of different substrate temperatures and methane:ammonia ratios. Approximate phase boundaries are included to guide the eye (adapted from Reference 6.19).

(b) Methylamine as precursor gas mixture

The concentration of methylamine (CH₃NH₂) was maintained at 0.5% in H₂ to allow direct comparison with other C/N precursor gases with a carbon-to-nitrogen ratio of 1:1. Deposition under standard CVD conditions produced diamond films (See Figure 6.5), but at a much lower growth rate. (~0.05 mm h^-1)^6.19 compared with that obtained when using 0.5% CH₄ in H₂ (~0.32 mm h^-1). The film composed of mainly isolated (111) faceted crystals.

(c) Hydrogen cyanide as precursor gas mixture

Figure 6.6 shows that using 0.5% HCN in H₂ under standard process conditions produced amorphous films containing both diamond and SiC crystals, but again the deposition rate was low (less than 0.1 mm h^-1).^6.19

Figure 6.1(a). Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 0.5% CH4/0.5% NH3 in hydrogen.	Figure 6.1(b). Scanning electron micrograph showing the cross sectional view of the film.
Figure 6.2. Scanning electron micrograph (SEM) of a polycrystalline diamond film produced on silicon after 6 h growth using methane-rich gas mixtures (0.75% CH4/0.25% NH3 in H2).^6.19	Figure 6.3. Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using ammonia-rich gas mixtures (0.25% CH4/0.75% NH3 in H2).^6.19
Figure 6.5. Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 0.5% CH3NH2 in hydrogen.^6.19	Figure 6.6. Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using 0.5% HCN in hydrogen.^6.19

(d) Methane and nitrogen as precursor gas mixture

The deposition rate and the resulting film quality at optimum growth temperature (2400°C) depended on the choice of C/N precursor used. Inspection of Figure 6.7 reveals that for a 1:1 C/N ratio in the feed gas, a continuous film was grown only by CH₄/N₂ mixtures, showing predominantly (100) facets. Furthermore, the deposition rate is higher than using standard CH₄/H₂ mixtures (Table 6.2). AES analysis of these films indicated that reasonable quality diamond was deposited; however, no nitrogen was detected in the bulk of the films, suggesting very low nitrogen-doping efficiency. This is consistent with the model of film growth predicted by Jin and Moustakas which involves simultaneous deposition and etching.^6.1 SIMS analysis, which is much more sensitive to nitrogen than AES, detected only very small amounts of nitrogen in the films, consistent with results obtained by Hong et al.^6.17

With methane-rich gas mixtures (e.g. 0.75% CH₄/0.125% N₂ in H₂), diamond was grown (See Figure 6.8), the morphology of which was again similar to that observed in standard CVD processes. As shown in Figure 6.9, continuous films were also produced with nitrogen-rich gas mixtures (e.g. 0.25% CH₄/0.375% N₂ in H₂), although at reduced growth rates (See Table 6.3 (a) ).

Figure 6.7 (a). Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 0.5% CH4/0.25% N2 in hydrogen.	Figure 6.7 (b). Scanning electron micrograph showing the cross sectional view of the film.
Figure 6.8 (a). Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using methane-rich gas mixtures (0.75% CH4/0.125% N2 in H2).	Figure 6.8 (b). Scanning electron micrograph showing the cross sectional view of the film.
Figure 6.9 (a). Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using nitrogen-rich gas mixtures (0.25% CH4/0.375% N2 in H2).	Figure 6.9 (b). Scanning electron micrograph showing the cross sectional view of the film.

Gas Flow Rates H2:CH4:N2 (sccm)	N/C	N/C (%)	N2/CH4 (%)	Growth Rate/ mm h^-1	Surface Morphology
(a) 100:0.5:0	0.00	0	0	0.32	C, O, T, (111)
100:0.5:0.25	1.00	100	50	0.45	C, Sq, (100)
100:0.75:0.125	0.34	34	17	0.45	C, O, (111)
100:0.25:0.375	3.00	300	150	0.23	C, O, (111)
(b) 100:1:0	0.00	0	0	0.5	C, O, T, (111)*
100:1:0.2	0.40	40	20	0.69	C, O, (111)
100:1:0.3	0.60	60	30	0.36	C, Sq, (111)
100:1:0.4	0.80	80	40	0.32	C, Sq, (100)
100:1:0.5	1.00	100	50	0.32	C, Sq, (100)

Table 6.3. Deposition rates and crystal morphology of films grown using various H₂/CH₄/N₂ gas mixtures: (a) Total [C] and [N] = 1, but varying the [C]/[N] ratio; (b) 1% CH₄ in N₂/H₂ mixture, the [N] varying between 0.2-0.5%. (Keys used: C = continuous, O = octahedral, T = twinning, Sq = square). * A top-view SEM image of the film is shown in Figure 4.6 (a).

A series of deposition experiments were performed to explore the effects of adding small amounts of nitrogen (0.2-0.5%) to the standard 1% CH₄ /H₂ gas mixture on the deposition rate and resulting crystal morphology of the diamond films (Table 6.3 (b) ). Figure 6.10 shows the average growth rate as a function of the ratio of [N]/[C] in the gas phase. The maximum growth rate was observed at a [N]/[C] ratio of 40% and decreased for higher N concentrations. It should be noted here that growth rates measurements were unattainable for lower N concentrations (< 40% [N]/[C]) due to the inability of the mass flow controllers to regulate the N₂ gas flow. These trends are consistent with those observed by Refs. 6.12 and 6.16, although the absolute values for the growth rates were different. Scanning electron micrographs of the films grown using the above C/H/N gas mixtures (See Appendix III) revealed that 111 growth facets were observed for low N concentration (up to a [N]/[C] ratio of 60%), although as the concentration of N₂ increased the films showed predominantly (100) facets (See Figure 6.11).

Figure 6.10. The growth rate of diamond film as a function of nitrogen doping concentration.	Figure 6.11. The change in the crystal morphology of the diamond films as a function of [N]/[C].

The red Raman spectra (632.8 nm), as a function of [N]/[C] in the gas phase, are shown in Figure 6.12. The quality of the diamond films are assessed by the 1332 cm-1 Raman line intensity. The best quality films were produces with no added nitrogen. As the [N]/[C] ratio increases the quality of the diamond decreases, consistent with results obtained by Prawer et al.^6.16

Figure 6.12. Raman spectra of diamond films grown with different [N₂] partial pressures. The spectra have been displaced vertically for clarity. The numbers marked on the figure are the values for the [N]/[C] in the gas phase.

6.4 Gas composition versus filament temperature for a variety of C-/N-containing precursor gases in H₂

(a) Methane and Ammonia as source gas mixture

Figure 6.13 shows how the mole fractions of the major carbon and nitrogen containing species gases [CH₄ (m/e=16), NH₃ (m/e=17), C₂H₂ (m/e=26) and HCN (m/e=27)] vary as a function of filament temperature for an initial feedstock of 0.5% CH₄ + 0.5% NH₃ in H₂ measured 4 mm from the filament. The deposition rate under optimum growth conditions is very low (<0.07 mm h^-1), because the dominant gas-phase reactions occurring between the C-containing and N-containing species have the effect of ‘locking up’ the carbon in the form of the stable cyanide product, HCN. The CH₄ concentration steadily decreases with increasing filament temperature, whilst the NH₃ concentration drops sharply at ~1600°C. The absolute concentrations of the two precursor gases in the vicinity of the filament decrease not only as a result of chemical reactions but also because of thermal diffusion effects inherent in multicomponent gas mixtures,^5.25 whereby any temperature gradient induces the heavier species in the mixture to move away from the higher temperature regions. At ~1900°C filament temperature, the HCN concentration increases rapidly, reaching a maximum value at the growth temperature (2400°C), significantly higher than that of either of the two precursor gases. Interesting to note is that very little N₂ was detected; most of the N is locked up either as unreacted NH₃ or HCN (See Table 6.2).

A prerequisite for the formation of HCN is the reaction between the C and N containing precursor gases to create an initial C-N bond. This requires the presence of ^·CH₃ and/or ^·NH₂ radicals. These species could result from pyrolysis of the parent hydride or by H abstraction from methane and ammonia by H atoms created at the filament as a result of the thermal dissociation of H₂. Inspection of the available kinetic data^6.21 suggests that at all filament temperatures pyrolysis of NH₃ is the dominant source of ^·NH₂, and that both of the two reactions mentioned above contribute comparable amounts to the ^·CH₃ yield. Furthermore, the data show that at all relevant temperatures the steady state ^·NH₂ concentration exceeds [^·CH₃] by one or two orders of magnitude. These radicals can then undergo the following reactions:

CH₄ + ^·NH₂ === CH₃NH₂ + H^· (6.1)

^·CH₃ + NH₃ === CH₃NH₂ + H^· (6.2)

both producing methylamine, and hence the vital C-N bond. The sharper decrease observed for the NH₃ concentration at lower temperatures is consistent with our findings that [^·NH₂] >> [^·CH₃] and therefore that reaction (6.1) is the preferred route to methylamine formation. Once formed, the methylamine can either redissociate or, more probably, undergo successive H abstractions to produce HCN:

This reaction is thermodynamically favourable due to the stability of the CºN bond. No gas-phase methylamine or CH₂=NH were detected in these particular experiments, owing to their thermodynamic instability (and thus very low steady state concentration) in the presence of high H atom concentrations, [H]. Qualitatively, the chemistry leading to HCN production is analogous to that for acetylene, which is the most stable hydrocarbon product when using standard hydrocarbon/H₂ mixtures and growth temperatures, regardless of the choice of hydrocarbon precursor used.^3.1 For a CH₄/H₂ gas mixture, the reaction is initiated by the ‘cracking’ of methane (by hydrogen abstraction) to produce ^·CH₃ radicals. In such a case, where no ^·NH₂ radicals are present, there is a greater steady-state [CH₃] and the increased likelihood of methyl recombination, followed by consecutive H abstractions yields larger [C₂H₂]. Since the formation of [C₂H₂] depends on k[CH₃]², the detection of large amounts of gas-phase acetylene in a hot filament CVD reactor is generally taken as an indicator of steady state [CH₃] and thus of fast diamond film growth. Inspection of Figure 6.13 shows that for a CH₄/NH₃ gas mixture, reactions leading to the formation of both HCºCH and HCºN are possible, though acetylene was detected in only very small quantities. This implies that the above reaction scheme (6.1 and/or 6.2, formed by 6.3) is the preferred route, leading to HCN formation. Furthermore, the presence of surplus ^·NH₂ radicals at lower temperatures reduces the effective concentration of CH_x species, thus suppressing diamond deposition.

Quantitative measurements of the same gas-phase species were performed with different CH₄/NH₃ ratios, but still maintaining 1% in H₂. In a methane-rich mixture (e.g. 3:1 C/N ratio) the product distribution versus filament temperature shows similar trends to that observed for a 1:1 stoichiometric mixture, except for a considerable increase in the absolute acetylene concentration at growth temperatures, consistent with higher diamond deposition rates.^3.1,6.19 However, the HCN concentration, and its variation with filament temperature, did not change with increasing [CH₄] in the source gas mixture, because the formation of HCN is limited by the amount of NH₃ in the feed gas. Almost all the NH₃ is converted to HCN, thus enabling any excess methane in the gas-phase to take part in reactions leading to acetylene formation, and hence diamond deposition (See Figure 6.14). In an ammonia-rich mixture (e.g. 1:3 C/N ratio), ^·NH₂ radicals are present in much larger quantities and, as a result, reaction with CH₄ will occur more readily forming CH₃NH₂ and ultimately HCN. The prevalence of reaction (1) is therefore likely to account for the fact that no acetylene was detected at any given filament temperature and, consequently, no diamond deposition occurred (See Figure 6.15). To a lesser extent, ammonia gas itself also has a negative effect on the deposition rate because it is capable of etching diamond from a growing surface during CVD.^6.19 The way in which the concentrations of the major stable gas-phase species vary as a function of filament temperature for the different CH₄/NH₃ ratios is shown in Figure 6.16.

Figure 6.16. Product distribution of (a) acetylene, (b) methane, (c) ammonia and (d) hydrogen cyanide as a function of filament temperature for different CH₄/NH₃ ratios in H₂ as source gas mixtures (3:1, 1:1 and 1:3).

(b) Methylamine as source gas mixture

The product distribution of the major stable species [CH₄, NH₃, C₂H₂ HCN and CH₃NH₂ (m/e=31)] versus filament temperature for a 0.5% CH₃NH₂ initial feedstock in H₂ is shown in Figure 6.17. Here, the methylamine concentration diminishes rapidly at 1300°C, and above 1800°C almost all the CH₃NH₂ has disappeared, either via reaction with H atoms (the reverse of reactions 6.1 and 6.2) producing CH₄ and NH₃, or via H atom abstraction producing HCN. At filament temperatures of ~2000°C the methane and ammonia concentrations reach peak values. Above 2000°C their absolute concentrations decrease because of thermal diffusion effects as well as chemical reactions (6.1) to (6.3) to form HCN. Interesting to note is the sharp rise in the HCN concentration at lower filament temperatures. This may be due to the inherent C-N bond in the methylamine molecule which can convert readily to HCN in the presence of H atoms, since the requirement of C-N bond formation via reactions (6.1) and (6.2) is by-passed. These reactions rely not only on the production of ^·CH₃ and ^·NH₂ radicals, which takes place at relatively high filament temperatures (~1700°C), but also on the relatively low probability step of the two precursor species meeting and reacting together. Competing reactions, such as acetylene formation, are also significantly reduced, due to the ease by which HCN is formed, thus accounting for the low deposition rate observed (~0.05 mm h ¹). In addition, at filament temperatures at and above 2000°C, there is a secondary increase in [HCN] when reactions (6.1) and (6.2) take place, between the C- and N-containing species.

(c) Hydrogen cyanide as source gas mixture

Figure 6.18 shows how the gas composition for an initial feedstock of 0.5% HCN changes versus filament temperature. Compared to ammonia and methylamine, a much more gradual decrease in the HCN concentration is observed as a function of filament temperature, owing to its thermodynamic stability. Thermal diffusion effects account for most of the reduction in the absolute concentration measured as a function of filament temperature. However, the fact that diamond could be deposited using hydrogen cyanide gas shows that some cycling of the carbon must be occurring, and evidence for this is shown in the detection of trace amounts of CH₄, NH₃ and C₂H₂. The deposition rate is again very low (<0.1 mm h^-1) due to the preferential regeneration of HCN by reactions between the C- and N-containing species at higher temperatures.

(d) Methane and nitrogen as source gas mixtures

Figure 6.19 shows the characterisation of the gas-phase environment and its variation with filament temperature for an initial feedstock of 0.5% CH₄/0.25% N₂ in H₂, such that the C:N ratio is again maintained at 1:1 during analysis. The methane and nitrogen concentrations decrease steadily with filament temperature, accompanied by a rise in HCN, C₂H₂, CH₃NH₂ and NH₃ concentrations at ~1700°C. The attenuation of the absolute mole fractions measured for both precursors with increasing filament temperature is again largely a result of thermal diffusion effects. Comparison of the relative concentrations of these major stable species measured at optimum filament temperatures (see Table 6.2) indicates that for a CH₄/N₂ gas mixture, the acetylene concentration is around an order of magnitude greater than that of the other C/N precursor gases, whilst the HCN concentration is reduced by a factor of ~4. The prevalent N-containing species in the gas-phase is NH₃, not HCN. The differences in the species composition can be explained in terms of the effects N₂ molecules have on the gas-phase chemistry and the way in which the C is locked up in the CVD process at different temperatures.

At lower temperatures (~1600°C), unlike CH₄/NH₃ gas mixtures, very few ^·NH₂ radicals will form because the dissociation of the strong NºN bonds requires much more energy. However, ^·CH₃ radicals will arise from a H + CH₄ abstraction reaction as in the normal hydrocarbon/H₂ CVD process, and be free to undergo recombination and H abstraction reactions to form acetylene. The result is that not only the formation of HCN is reduced, (because reactions (6.1) to (6.3) are now terminated), but competing C₁ reactions to produce C₂ species are far less restrained. Thus at lower temperatures, N₂ is seen as being virtually a spectator to the CVD process.

At higher temperatures (>1600°C), addition of N₂ results in a slightly higher deposition rate (Table 6.2) but of poorer quality diamond (as determined by LRS). This is consistent with the results obtained by Jin and Moustakas,^6.1 when using similar amounts of N₂ in the feed gas. We believe that this might be explicable if N₂ is acting as a catalyst, effectively scavenging H atoms and returning them to the gas system as H₂. This will be a multi-step process, such as:

(M = N₂/H₂)

Analogous hydrogenation reactions could occur to produce N₂H₄, and the weak N-N bond will then be susceptible to fission and NH₃ formation. All of these reaction steps are reversible, but would require H abstraction, initiated by yet another H atom to occur. For example:

The net effect of such a catalytic scheme would be that the introduction of only a small amount of N₂ (<0.5%) in the system could lead to removal of a significant amount of H atoms, thereby slowing all the subsequent gas-phase chemistry. Thus all the reactions which depend upon high H atom concentrations would be reduced (e.g. reactions 6.1-6.3), resulting in the observed depletion in HCN compared to the amounts seen with the other gas additions. The unusually high methylamine concentration observed at high filament temperatures could also be explained by the fact that the attenuation of H atom concentration reduces the effective H abstraction reactions of existing methylamine species, thus allowing higher steady state concentrations.

Quantitative measurements of the same gas-phase species were performed with different CH₄/N₂ ratios, but still maintaining 1% in H₂. In a methane-rich mixture (e.g. 3:1 C/N ratio) the product distribution versus filament temperature shows similar trends to that observed for a 1:1 stoichiometric mixture, except for an increase in the absolute acetylene (and ethylene) concentration at growth temperatures. However, the HCN and CH₃NH₂ concentrations and their variation with filament temperature, did not change with increasing [CH₄] in the source gas mixture, presumably because the formation of these species depends on the amount of N₂ in the feed gas. In a nitrogen-rich mixture (e.g. 1:3 C/N ratio), a decrease in [CH₃], [C₂H₂] and [C₂H₄] were observed, consistent with lower deposition rates. In addition the HCN, and CH₃NH₂ concentration, and their variation with filament temperature, did not change with increasing [N₂] in the source gas mixture, because the formation of these species also depends on the amount of CH₄ in the feed gas. The way in which the concentrations of the major stable gas-phase species vary as a function of filament temperature for the different CH₄:N₂ ratios is shown in Figure 6.22.

Figure 6.22. Product distribution of (a) acetylene, (b) methane, (c) ammonia, (d) hydrogen cyanide, (e) methyl radical, (f) nitrogen, (g) methylamine, and (h) ethylene, as a function of filament temperature for different CH₄/N₂ ratios in H₂ as source gas mixtures (3:1, 1:1 and 1:3).

Additional MBMS measurements were performed which showed that increasing the N₂ concentration (for a given C/H₂ mixture) further suppresses HCN, while allowing the presence of increasing concentrations of both NH₃ and CH₃NH₂. This theory is in good agreement with observations of Hong et al.^6.17 who found that the amount of atomic hydrogen decreased relative to the increase of nitrogen.

These observations are also compatible with the proposed gas phase sequence (6.4) - (6.6) involving catalytic destruction of H by N₂. The rates for reactions (6.4) and (6.5) at standard growth temperatures are 10¹²-10¹³ cm³ mol^-1 s^-1,^6.23 which are similar in magnitude to that for most of the other nitrogen gas-phase reactions in this system,^6.24 which would suggest that reactions (6.4) and (6.5) might take place readily under the process conditions. Indeed, Bozzelli and Dean^6.23 state that when the H atom mole fraction is high, i.e. under high temperature reaction conditions, N₂H formation from H + N₂ is very rapid. Thus we could anticipate equilibrium, or near equilibrium concentrations of N₂H which might be high enough to undergo bimolecular reactions with other important radical species, in our case hydrogen atoms, ^·H. The driving force in this rapid cycling of N₂ ® N₂H ® N₂ is the very strong and stable NºN bond. However, inspection of Figure 6.19 shows that less thermodynamically favourable side reactions do occur to produce NH₃, CH₃NH₂ and ultimately HCN.

As a final piece of supporting evidence for the H atom depletion mechanism we observe that increasing N₂ concentrations lead to diamond films of poorer quality, as evidenced by LRS. This is consistent with there being a reduction in the H atoms required to etch away the non-diamond phases on the film surface. Hong et al.^6.17 also found that increasing the N₂ concentration reduces the growth rate, partly as a result of the additional etching caused by NH₃, but probably also because of a decrease in available atomic H.

All of this speculation relies on the assumption of a purely gas phase mechanism and a sufficient stability for the (as yet) poorly characterised N₂H intermediate. However, preliminary measurements of H atom concentrations using laser spectroscopy techniques^6.25 in a similar hot filament CVD reactor suggest negligible depletion of [H]. If correct, we would need to consider alternative explanation(s) for the observations. These are likely to involve heterogeneous chemistry, initiated by N₂ decomposition on the filament surface, or, possibly, on the (cooled) growing diamond surface to produce N atoms. A subsequent reaction such as:

in the gas phase or on the surface could be followed by further reduction to NH₃ whilst HCN formation could arise (in part at least) by H atom abstraction of CN terminating groups on the surface:

which has been proposed as one route to C₂H₂ formation.^6.26

6.5 Conclusions

The effects of nitrogen on the CVD diamond growth mechanism have been investigated using in-situ molecular beam mass spectrometry. The deposition rate at optimum growth conditions depends critically on the choice of C/N precursor used, and the origin of the carbon-containing species. The reactions occurring in the gas-phase seem to lead predominantly to the formation of HCN, (except for CH₄/N₂ gas mixtures). The stability of this species precludes most of the cycling of carbon during the CVD process, resulting in low rates of diamond deposition. Thermodynamic equilibrium calculations^5.16 confirm that HCN production is highly favoured in H/C/N gas mixtures at high gas processing temperatures.

For a 1:1 C:N ratio in the feed gas, continuous films were produced after 6 hours deposition only by CH₄/H₂/N₂ gas mixtures. Incorporation of nitrogen in the grown diamond films was very low, consistent with the conclusions of Jin and Moustakas,^6.1 who calculated a theoretical value for the doping efficiency of nitrogen in diamond of around 10^-4. At lower temperatures N₂ simply acts as a spectator to the CVD process, as evidenced by the significant increase in the C₂H₂ concentration and reduction in the HCN concentration in the gas-phase compared to other N source gas additions. At optimum filament temperatures (~2400°C), addition of N₂ to a CH₄/H₂ gas mixture leads to higher deposition rates of poor quality diamond films (determined by LRS). We suggest that this might be explicable if N₂ is acting as a catalyst for the destruction of H atoms, thereby reducing the etching rate of non-diamond phases on the film surface. However, recent laser spectroscopy analysis suggests that the [H] concentration is relatively insensitive to addition of trace N₂; thus we also consider whether the observations may reflect gas-surface heterogeneous chemistry. In either case, addition of a tiny amount of N₂ to the hot filament CVD process will affect not only the chemistry occurring under standard deposition conditions, but the growth rate, the morphology and the quality of the resulting diamond films. As a consequence it is clear that the diamond CVD process is likely to be more dependent on gas purity and ultimate vacuum than has often been appreciated.

6.6 References

6.1 S. Jin, and T.D. Moustakas, Appl. Phys. Lett., 65, 403 (1994).

6.2 L. Bergman, M.T. McClure, J.T. Glass, and R.J. Nemanich, J. Appl. Phys., 76 3020 (1994).

6.3 R. Samlenski, C. Haug, R. Brenn, C. Wild, R. Locher, and P. Koidl, Appl. Phys. Lett., 67, 2798 (1995).

6.4 E. Boettger, A. Bluhm, X. Jiang, L. Schafer, and C.-P. Klages, J. Appl. Phys.,77 6332 (1995).

6.5 J. Mort, M.A. Machonkin, and K. Okumura, Appl. Phys. Lett., 59, 3148 (1991).

6.6 K. Okano, S. Koizumi, S.R.P. Silva, and G.A.J. Amaratunga, Nature, 381, 140 (1996).

6.7 M.W. Geis, J.C. Twichell, N.N. Efremow, K. Krohn, and T.M. Lyszczarz, Appl. Phys. Lett., 68, 2294 (1996).

6.8 G.A.J. Amaratunga and S.R.P. Silva, Appl. Phys. Lett., 68, 2529 (1996).

6.9 S.R.P. Silva, B. Rafferty, G.A.J. Amaratunga, J. Schwan, D.F. Franceschini, and L.M. Brown, Diamond and Relat. Mater., 5, 401 (1996).

6.10 P. Ball, Nature, 381, 116 (1996).

6.11 S.Jin and T.D. Moustakas, Appl. Phys. Lett., 63, 2354 (1993).

6.12 S. Bohr, R. Haubner, and B. Lux, Appl. Phys Lett., 68, 1075 (1996).

6.13 L. Locher, C. Wild, N. Herres, D. Behr, and P. Koidl, Appl. Phys. Lett., 65, 34 (1994).

6.14 G.Z. Cao, J.J. Schermer, W.J.P. van Enckevort, W.A.L.M. Elst, and L.J. Giling, J. Appl. Phys., 79 1357 (1996).

6.15 A. Badzian, T. Badzian, and S.-T Lee, Appl. Phys Lett., 62, 3432 (1993).

6.16 S. Prawer, T.L. McCormick, W.B. Alexander, L.E. Seitzman, and J.E. Butler, in preparation.

6.17 T-M. Hong, S-H Chen, Y-S Chion and C-F Chen, Thin Solid Films, 270, 148 (1995).

6.18 T-M. Hong, S-H Chen, Y-S Chion and C-F Chen, Appl. Phys Lett., 67, 2149 (1995).

6.19 P.W. May, P.R. Burridge, C.A. Rego, R.S. Tsang, M.N.R. Ashfold, K.N. Rosser, R.E. Tanner, D. Cherns and R. Vincent, Diamond and Relat. Mater., 5, 354 (1996).

6.20 R.S. Tsang, C.A. Rego, P.W. May, in preparation.

6.21 S.M. Hwang, T. Higashihara, K.S. Shin and W.C. Gardiner, Jr., J. Phys. Chem., 94, 2883 (1990).

6.22 H. Ellis (ed.), Nuffield Advanced Science Book of Data 4^th edn., Longman, 1986.

6.23 J.W. Bozzelli and A.M. Dean, Int. J. Chem. Kinet., 27, 1097 (1995).

6.24 J.A. Miller, M.D. Smooke, R.M. Green and R.J. Kee, Comb. Sci. and Tech., 34, 149 (1983).

6.25 S.A. Redman and M.N.R. Ashfold, unpublished results.

6.26 J.E. Butler and R.L. Woodin, Phil. Trans. R. Soc. Lond. A, 342 209 (1993).

---

6.7 Appendix

(I) Synthesis of Hydrogen Cyanide

All the precursors examined in the present study were obtained as commercial products except for HCN, which was synthesised by the reaction of NaCN with phosphoric acid in vacuo:

NaCN _(s) + H₃PO_{4 (l)} (excess) ® HCN _(g) + Na₃PO_{4 (l)}

The phosphoric acid was first dried by gradual addition of phosphorus pentoxide (P₂O₅) until the liquid became viscous. Phosphoric acid was used in preference to other acids to allow a sensible rate of reaction. The reaction was executed using vacuum line techniques at a pressure of ~0.001 Torr (See Figure II). The dried H₃PO₄ was introduced into round-bottomed flask #1 containing the NaCN (1g, 0.02 moles) via a dropping funnel. The reaction occurred spontaneously releasing HCN gas, which passed through the vacuum line to round-bottomed flask #2. Attached to this flask was a “cold finger” (immersed in liquid nitrogen) which was used to condense the HCN gas. When the reaction mixture stopped effervescing, and the reaction had completed, the round-bottomed flask was isolated from the vacuum system to allow the HCN to revert to a gas. Assuming the reaction went to completion, the expected yield for HCN would be 0.45 dm³ at room temperature and pressure.

Figure I. Vacuum line apparatus used for the synthesis of HCN.

(II) Ionization potentials (taken from Reference 3.19) and the user selected electron energies of the various gas-phase species monitored.

Precursor Gas	Chemical Formula	Ionization Potential (eV)	User Selected Electron Energy (eV)
Methyl radical	CH3	9.84	13.5
Methane	CH4	12.98	14.8
Ethylene	C2H4	10.51	14.8
Acetylene	C2H2	11.41	16.8
Ammonia	NH3	10.15	14.8
Methylamine	CH3NH2	8.97	16.8
Hydrogen cyanide	HCN	13.73	16.8
Nitrogen	N2	15.55	30.0

(III) SEM Photo Library

Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 1% CH4 and 0.2% N2 in hydrogen.	A close up view of the film surface. The white spots are Si debris found on the surface after the substrate was cleaved in preparation for the SEM analysis.
Scanning electron micrograph showing the cross sectional view of the film.	SEM of the filament surface after 6 h growth using input gas mixtures of 1% CH4 and 0.2% N2 in hydrogen. When the [N]/[C] ratio in the gas phase increases, the condition of the filament gradually deteriorates, as shown in the subsequent SEM photos. This may be due to etching reactions occurring between the filament and NH3, which become more pronounced at higher [N]/[C] ratios, as more NH3 is formed in the gas phase (revealed by MBMS analysis).
Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 1% CH4 and 0.3% N2 in hydrogen.	A close up view of the film surface.
Scanning electron micrograph showing the cross sectional view of the film.	SEM of the filament surface after 6 h growth using input gas mixtures of 1% CH4 and 0.3% N2 in hydrogen.
Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 1% CH4 and 0.4% N2 in hydrogen.	A close up view of the film surface.
Scanning electron micrograph showing the cross sectional view of the film.	SEM of the filament surface after 6 h growth using input gas mixtures of 1% CH4 and 0.4% N2 in hydrogen.
Scanning electron micrograph (SEM) of a film produced on silicon after 6 h growth using input gas mixtures of 1% CH4 and 0.5% N2 in hydrogen.	A close up view of the film surface.
Scanning electron micrograph showing the cross sectional view of the film.	SEM of the filament surface after 6 h growth using input gas mixtures of 1% CH4 and 0.5% N2 in hydrogen. The dark grey nodules present on the filament surface are currently being analysed; but the initial guess is that these nodules could be tantalum nitride, formed as a result of reactions between the Ta filament and NH3.

(IV) Experimental Data

The following experimental values have been converted from raw MBMS data into species mole fractions as a function of filament temperature for the various precursor gases examined in this chapter. The species concentration are presented with no correction being made as a result of thermal diffusion.

0.5% CH₄ & 0.5% NH₃ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -6% (DISCRIM), -20% (DELTAM), -40% (RES’N), 2600V (SEM), 3.0V (CAGE), 140 mA (EMISS).

MS Pressure = 2.7x10^-6 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH4 (14.8eV)	NH3 (14.8eV)	C2H4 (14.8eV)	C2H2 (16.8eV)	HCN (16.8eV)
23	0.500	0.500	0.004	0.000	0.000
1300	0.310	0.326	0.003	0.000	0.006
1450	0.270	0.271	0.002	0.000	0.005
1540	0.244	0.244	0.003	0.000	0.006
1660	0.227	0.202	0.002	0.000	0.009
1750	0.197	0.131	0.004	0.000	0.012
1870	0.172	0.081	0.005	0.000	0.031
1990	0.129	0.064	0.007	0.000	0.061
2050	0.119	0.064	0.006	0.000	0.070
2130	0.094	0.057	0.008	0.000	0.092
2240	0.060	0.052	0.009	0.001	0.103
2330	0.026	0.046	0.007	0.002	0.107
2420	0.014	0.040	0.007	0.002	0.101
2520	0.006	0.041	0.007	0.002	0.097

0.75% CH₄ & 0.25% NH₃ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -6% (DISCRIM), -20% (DELTAM), -40% (RES’N), (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 2.3x10^-6 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH4 (14.8eV)	NH3 (14.8eV)	C2H4 (14.8eV)	C2H2 (16.8eV)	HCN (16.8eV)
23	0.750	0.250	0.000	0.000	0.000
1440	0.463	0.152	0.000	0.000	0.005
1610	0.378	0.109	0.000	0.000	0.008
1740	0.337	0.045	0.000	0.000	0.021
1900	0.297	0.034	0.000	0.000	0.056
2020	0.241	0.029	0.002	0.000	0.077
2100	0.199	0.028	0.003	0.000	0.091
2180	0.169	0.026	0.006	0.004	0.100
2240	0.149	0.026	0.007	0.005	0.110
2300	0.125	0.021	0.006	0.008	0.106
2400	0.090	0.013	0.009	0.014	0.101
2480	0.056	0.011	0.007	0.014	0.109
2560	0.032	0.012	0.006	0.014	0.106

0.25% CH₄ & 0.75% NH₃ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -6% (DISCRIM), 0% (DELTAM), -40% (RES’N), 2600V (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 2.0x10^-6 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH4 (14.8eV)	NH3 (14.8eV)	C2H4 (14.8eV)	C2H2 (16.8eV)	HCN (16.8eV)
23	0.250	0.750	0.000	0.000	0.000
1290	0.194	0.565	0.000	0.000	0.000
1470	0.156	0.484	0.000	0.000	0.000
1550	0.152	0.415	0.000	0.000	0.000
1640	0.152	0.367	0.000	0.000	0.000
1730	0.146	0.281	0.000	0.000	0.000
1790	0.132	0.143	0.000	0.000	0.006
1900	0.100	0.112	0.000	0.000	0.027
2000	0.087	0.089	0.004	0.000	0.037
2070	0.062	0.069	0.000	0.000	0.038
2210	0.042	0.051	0.004	0.000	0.050
2290	0.020	0.041	0.004	0.000	0.042
2420	0.006	0.022	0.005	0.000	0.040
2520	0.006	0.018	0.001	0.000	0.038

0.5% CH₃NH₂ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -4% (DISCRIM), -20% (DELTAM), -40% (RES’N), 2600V (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 8x10^-7 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH4 (14.8eV)	NH3 (14.8eV)	CH3NH2 (16.8eV)	C2H2 (16.8eV)	HCN (16.8eV)
23	0.000	0.000	0.500	0.000	0.000
960	0.008	0.023	0.193	0.001	0.041
1230	0.009	0.035	0.144	0.001	0.064
1420	0.018	0.042	0.103	0.001	0.082
1540	0.032	0.041	0.066	0.001	0.163
1700	0.055	0.051	0.044	0.002	0.163
1810	0.088	0.058	0.025	0.001	0.157
1930	0.114	0.062	0.019	0.002	0.152
2030	0.131	0.057	0.013	0.002	0.141
2100	0.113	0.049	0.009	0.002	0.156
2175	0.090	0.044	0.004	0.002	0.155
2300	0.072	0.034	0.006	0.003	0.165
2400	0.051	0.031	0.009	0.005	0.164
2500	0.033	0.030	0.007	0.006	0.172

0.5% HCN in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -4% (DISCRIM), -20% (DELTAM), -40% (RES’N), 2600V (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 8x10^-7 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH4 (14.8eV)	NH3 (14.8eV)	C2H2 (16.8eV)	HCN (16.8eV)
23	0.000	0.000	0.000	0.500
1040	0.002	0.002	0.001	0.304
1410	0.002	0.002	0.001	0.255
1590	0.013	0.003	0.001	0.217
1660	0.020	0.004	0.002	0.191
1830	0.032	0.008	0.002	0.177
1940	0.034	0.007	0.002	0.155
2040	0.029	0.007	0.002	0.141
2100	0.026	0.007	0.002	0.136
2180	0.027	0.006	0.003	0.138
2300	0.024	0.007	0.004	0.144
2420	0.022	0.008	0.005	0.159

0.5% CH₄ & 0.25% N₂ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -3% (DISCRIM), -20% (DELTAM), -40% (RES’N), (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 8x10^-7 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH3 13.5eV	CH4 14.8eV	NH3 14.8eV	C2H4 14.8eV	C2H2 16.8eV	HCN 16.8eV	CH3 NH2 16.8eV	N2 30.0eV
25	0.000	0.500	0.000	0.000	0.000	0.000	0.000	0.250
1340	0.009	0.178	0.000	0.002	0.000	0.002	0.003	0.075
1525	0.011	0.150	0.001	0.002	0.000	0.002	0.004	0.053
1570	0.011	0.138	0.004	0.002	0.000	0.002	0.003	0.041
1650	0.012	0.123	0.011	0.002	0.000	0.002	0.003	0.032
1769	0.012	0.102	0.016	0.003	0.002	0.008	0.005	0.018
1830	0.013	0.087	0.026	0.003	0.003	0.007	0.005	0.015
1895	0.012	0.079	0.028	0.004	0.005	0.008	0.007	0.012
2010	0.017	0.063	0.033	0.005	0.008	0.010	0.008	0.007
2098	0.017	0.056	0.0423	0.007	0.011	0.013	0.010	0.010
2200	0.023	0.044	0.045	0.007	0.013	0.013	0.010	0.007
2297	0.025	0.031	0.053	0.009	0.015	0.016	0.016	0.006
2370	0.026	0.023	0.063	0.009	0.018	0.019	0.024	0.003
2450	0.021	0.008	0.057	0.009	0.017	0.018	0.033	0.001

0.75% CH₄ & 0.125% N₂ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -3% (DISCRIM), -20% (DELTAM), -40% (RES’N), 2600V (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 9x10^-7 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH3 13.5eV	CH4 14.8eV	NH3 14.8eV	C2H4 14.8eV	C2H2 16.8eV	HCN 16.8eV	CH3 NH2 16.8eV	N2 30.0eV
24	0.000	0.750	0.000	0.000	0.000	0.000	0.000	0.125
900	0.002	0.375	0.000	0.002	0.000	0.003	0.001	0.075
980	0.002	0.340	0.000	0.002	0.000	0.003	0.001	0.073
1260	0.004	0.306	0.000	0.003	0.000	0.003	0.001	0.046
1400	0.005	0.270	0.001	0.003	0.000	0.003	0.001	0.037
1430	0.006	0.245	0.001	0.004	0.000	0.004	0.001	0.032
1565	0.006	0.229	0.002	0.005	0.000	0.004	0.001	0.027
1650	0.008	0.228	0.004	0.004	0.000	0.004	0.003	0.023
1690	0.012	0.224	0.004	0.004	0.000	0.005	0.002	0.019
2215	0.018	0.109	0.008	0.008	0.017	0.012	0.005	0.009
2320	0.024	0.067	0.011	0.012	0.021	0.014	0.006	0.000
2400	0.028	0.046	0.015	0.011	0.026	0.019	0.017	0.000
2560	0.032	0.032	0.038	0.012	0.027	0.0255	0.039	0.000

0.25% CH₄ & 0.375% N₂ in H₂ at 20 Torr vs. Filament Temperature

MS Probe Parameters: -3% (DISCRIM), -20% (DELTAM), -40% (RES’N), (SEM), 3.0V (CAGE), 140mA (EMISS).

MS Pressure = 8x10^-7 Torr

Species Mole Fractions vs. Filament Temperature

Fil. Temp (°C)	CH3 13.5eV	CH4 14.8eV	NH3 14.8eV	C2H4 14.8eV	C2H2 16.8eV	HCN 16.8eV	CH3 NH2 16.8eV	N2 30.0eV
23	0.000	0.250	0.000	0.000	0.000	0.000	0.000	0.375
900	0.002	0.160	0.000	0.001	0.000	0.002	0.002	0.249
1323	0.003	0.113	0.002	0.002	0.000	0.003	0.002	0.187
1503	0.004	0.096	0.001	0.002	0.000	0.003	0.004	0.132
1602	0.005	0.078	0.000	0.003	0.000	0.002	0.003	0.120
1805	0.007	0.056	0.000	0.003	0.000	0.007	0.003	0.086
1890	0.008	0.049	0.001	0.002	0.001	0.009	0.003	0.071
2000	0.011	0.043	0.003	0.004	0.003	0.008	0.004	0.061
2107	0.012	0.032	0.007	0.003	0.004	0.010	0.006	0.049
2210	0.015	0.021	0.012	0.004	0.003	0.012	0.008	0.046
2308	0.018	0.011	0.027	0.005	0.003	0.019	0.020	0.023
2400	0.020	0.002	0.034	0.005	0.002	0.024	0.041	0.027
2450	0.022	0.001	0.061	0.007	0.001	0.026	0.065	0.014