Modeling of the gas-phase chemistry in C-H-O gas mixtures

Diamond growth from C-H-O gas mixtures

Low-pressure diamond deposition has been achieved using a large range of C, H and O containing gas mixtures. Bachmann et al summarized the results of many deposition experiments involving various gas mixtures and reactor types in the form of an atomic C-H-O phase diagram (Fig. 1). They concluded that the exact nature of the source gases was unimportant for most diamond chemical vapor deposition (CVD) processes and that, at typical process temperatures and pressures, it was only the relative ratios of C, H and O that controlled deposition.

The diagram partitions into three distinct regions associated with: (a) diamond growth, centered on the C-O tie line where the input mole fractions of carbon and oxygen are equal (i.e. [C] = [O]); (b) no growth, lying below the C-O tie line (i.e. [C] < [O]); and (c) non-diamond growth, located above the C-O tie line (i.e. [C] > [O]).

Previous work within our group, concerning diamond growth from CO2/CH4 gas mixtures, has confirmed the presence of three such regions. This work also involved the modeling of gas-phase chemistry for the entire range of CO2/CH4, using the CHEMKIN II computer package. Figure 1 shows a schematic of the Bachmann C-H-O atomic phase diagram with the full range of CO2/CH4 gas mixtures shown as a line cutting the diagram.

Fig. 1. Simplified atomic C-H-O diamond deposition phase diagram. The white area lying above the CO-H tie line is the experimental diamond growth domain. The thick CH4-CO2 tie line corresponds to the full range of CO2/CH4 gas mixtures

Simulations of C-H-O gas phase chemistry

The previous gas-phase chemistry simulations of the CO2/CH4 system (mentioned above) have been repeated but for a large range of binary gas mixtures. All combinations of the hydrocarbons C2H2, C2H3, C2H4, C2H5, C2H6 and CH4, with each of the species, CO, CO2, H2O, OH and HO2 were considered, as well as CH4/H2 and C2H2/H2 mixtures. Simulations were carried out using the SENKIN code which predicts the gas phase species mole fractions for a given reaction scheme, gas temperature, pressure and reaction time. No transport of reactants into or out of the reaction volume is considered.

The simulations were run for the full range of each gas mixture using the following conditions:

Reaction schemeAll relevant C-H-O containing reactions from the GRI-Mech 2.11gas-phase reaction kinetic database
Gas temperature2000 K
Pressure40 Torr
Reaction Time5 s

Table 1. Input conditions for SENKIN simulations

Results

Figure 2 shows the results obtained for the gas mixtures; CO2/H2O, CO2/CH4, CH4/H2, CO/C2H4.

Figure. 2. Calculated species mole fractions for Tgas = 2000 K over the full range of mixing ratios for four different gas mixtures: (a) CO2/H2O; (b) CO2/CH4; (c) CH4/H2; and (d) CO/C2H4. The composition for which [H]/[C2H2] = 0.2 is denoted by a vertical dotted line in (b) and (c).

It can be seen that for the CO2/H2O mixture {which lies in the no growth region of the C-H-O diagram) both [CH3] and [C2H2] are negligible (<10-10 whereas for a gas mixture lying within the non-diamond growth region (i.e. CO/C2H4) the mole fractions of both these species are considerably higher (~ 10-6 and 10-2 respectively). It should be noted that there is a jump of several orders of magnitudein both [CH3] and [C2H2] for all mixtures when crossing the H-CO tie line (i.e. CO2/CH4). CH3 is beleaved to be the species responsible for diamond growth as is confirmed by this result as this line also markes the boundary between the non growth and diamond growth domains (as shown below in Fig. 2).

Figure. 3. C-H-O Phase diagram for diamond CVD. The full lines cutting across the diagram correspond to the four gas mixtures (a) CO2/H2O, (b) CO2/CH4, (c) CH4/H2 and (d) CO/C2H4 illustrated in Fig. 2. The red line indicate the C-O tie line (the no-growth to diamond growth region boundary). Key: solid circles indicate composition where, for any given gas mixture, the [CH3] increases from ~10-10 to ~10-6, outline triangles = composition for which [H]/[C2H2] = 0.2 at Tgas = 2000 K (diamond to non-diamond growth boundary).

The lower boundary of the diamond growth domain (i.e. the no growth-diamond growth boundary) of the C-H-O atomic phase diagram can therefore be defined in terms of [CH3]. It should also be possible to define the upper (diamond-non diamond) boundary in a similar manner. Diamond growth is generally believed to be the result of simultaneous deposition of diamond and graphitic phases with the graphite being etched by atomic H atoms to leave diamond. C2H2 is linked to the deposition of graphitic films therefore the diamond-non diamond growth boundary can be deffined in terms of the ratio [H]/[C2H2]. The value of 0.2 for this ratio gave a boundary in good agreement with experimental results (Table. 2) allowing the regions of the C-H-O atomic phase diagram to be predicted as shown above in Figure. 2.

Table. 1. The position of the no-growth/diamond growth and diamond/non-diamond growth boundaries are predicted for a range of gas mixtures in terms of % hydrocarbon . These boundaries are compared with the results of some of the available deposition experiments:
a) Marinelli et al, J. Appl. Phys. 76, 5702 (1994),
b) Mollart and Lewis, Diamond Relat. Mater. 8, 236 (1999).
c)Bachmannet al, Diamond Relat. Mater. 1, 1 (1991).

we note that the present simulations predict the diamond to non-diamond growth boundary for CH4/H2 gas mixtures to lie at 7% CH4, more than twice the value found experimentally. This may suggest that the criteria governing the boundary between the diamond and non-diamond growth regions may be slightly different for mixtures containing little (or no) oxygen.

Conclusions

The three regions (no growth, diamond growth and non-diamond growth) of the Bachmann diagram have been explored further, via simulation of 38 different input gas mixtures using the CHEMKIN computer package and an assumed gas temperature of 2000 K. The no-growth region is shown to correspond to gas mixtures producing extremely low (<10-10) CH3 mole fractions. Diamond growth is only possible for C, H and O gas mixtures which yield both sufficiently high (~10-6) CH3 mole fractions to enable deposition of diamond and an H atom mole fraction (defined relative to that of C2H2) that is sufficient to etch the non-diamond phases. The non-diamond growth region at Tgas = 2000 K is found to span gas mixtures where [CH3] > 10-7 but [H]/[C2H2] < 0.2, under which conditions deposition is presumed to outpace the etching of non-diamond phases.

Publication

The work presented here has been published in greater detail, as referenced below:

J.R. Petherbridge, P.W. May and M.N.R. Ashfold , "Modeling of the gas-phase chemistry in C-H-O gas mixtures for diamond Chemical Vapor Deposition" J. Appl. Phys., 89 (2001) 5219-23.

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