An experimental investigation of the isotope effect in the CVD growth of diamonds

Mat. Res. Bull., Vol. 24, pp. 733-738, 1989. Printed in the USA. 0025-5408/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press ple.

AN EXPERIMENTAL INVESTIGATION OF THE ISOTOPE EFFECT IN THE CVD GROWTH OF DIAMONDS H. B. Vakil, W. F. Banholzer, R. J. Kehl, and C. L. Spiro General Electric Research and Development Center Schenectady, NY, 12301 (Received March 14, 1989; Communicated b y J . F . Aekerman)

ABSTRACT An experimental investigation of the effect of isotopic substitution of hydrogen by deuterium was carried out for the chemical vapor deposition (CVD) of diamond films to determine whether the rate determining step involves a CH or a C-C bond. Results show that if such a comparison is carried out in a proper manner the growth rate with deuterium is slower by the expected isotopic factor of the square root of 2, indicating that most likely a C-H bond is involved in the rate limiting step. Additionally, it was found that the kinetics of the atomic species generation were equally affected by the isotopic switch, strongly suggesting that the rate limiting step for that process involves the metal-H bond. MATERIALS INDEX:

diamonds, isotopes

Introduction Low pressure Chemical Vapor Deposition (CVD) growth of diamonds under metastable conditions has been the subject of many investigations in the last decade in what has proven to be an exciting area of research. The diversity of processes that have been reported to have produced diamonds at low pressures include thermal filament assisted growth; plasma-assisted growth with RF, microwave, and DC plasmas; laser assisted growth; and ion beam assisted growth. Rather than cite individual publications to these, we will simply refer to some excellent reviews that have been published recently [1-3]. It has been generally recognized that the growth rate is limited by a heterogeneous step rather than by vapor phase transport. There have been numerous proposed mechanisms 733

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for the preferential growth of the diamond phase over graphite, however there is a great shortage of experimental evidence to test the various mechanistic hypotheses. This is somewhat understandable since for even the simplest case of the thermal, hot filament CVD process, it is difficult to carry out carefully tailored experiments to probe the mechanistic details without affecting key experimental parameters. While some recent thermodynamic investigations [4] have provided some insights into the nature of the chemical species that could account for the observed growth rates, the details of the rate limiting step remains a mystery. Some of the proposed mechanisms involve many isolated steps, starting with the generation of a vacant surface site via hydrogen recombination, followed by a multiple step incorporation of an impinging hydrocarbon species such as acetylene [5]. One of the problems with the various proposed mechanistic hypotheses is that they do not identify a specific rate limiting step that could be tested against observed growth kinetics, and thereby, help provide the needed evidence for their validity. The goal of this study was to provide experimental data which would aid in elucidating the correct mechanism. In order to keep experimental variables to a minimum, a simple hot-filament CVD process was chosen for the study. The basic aim of the study was a direct measurement of the isotope effect on diamond film growth rate. Deuterium and deuterated methane were substituted for hydrogen and methane respectively. In-situ kinetic measurements were made using an electro-balance. Hydrogen isotope effect has often been used to gain insights into reaction pathways [6]; its use in this study is slightly different in that we were interested in determining whether the heterogeneous rate-limiting step involved a C-C bond or a C-H bond. The premise was that if a C-C bond was controlling the mechanism there would be no change in rate upon isotopic substitution. If a C-H bond was involved one would observe a rate decrease on the order of the square root of two in accordance with the appoximate change in the reduced mass from a substitution of a C-H by a C-D bond. It should be noted that at the very high temperatures, typically 1200K to 1300K for the filament process, we are not concerned with the thermodynamic isotope effect arising from zero-point energy differences but more with the changes in C-H vibration frequencies [6]. In the remainder of this paper we will describe the experimental apparatus, then discuss some of the subtleties of the filament process that have a bearing on the requirements for a meaningful comparison between the isotopic rates, and conclude with some interpretations of the results.

Experimental Apparatus The experiments were carried out using a six-way stainless cross with a tube ID of 4.6 cm (2" OD), with ports fitted with electrical and gas feed-throughs, a quartz window on one side for pyrometric measurement of the filament and substrate temperatures, an electro-balance (Cahn, Model 2000), a sampling tube for mass spectrometry, and vacuum system. The reactor pressure was controlled at 10 torr using a capacitance manometer as a sensor with a feed-back to a butterfly valve at the exit. The mass-spec sample was withdrawn roughly 2 cm from the filament using a differentially pumped system controlled at 2.5 torr with a 25 micron orifice between it and the high vacuum chamber. A .015" diameter tungsten filament wire coiled with 12 turns (1/8" ID) and 1" overall length after coiling was used. The substrate was a nickel bead of roughly 6 m m diameter suspended

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from the electro-balance. The filament was powered by a DC power supply at roughly 400W (16A, 25V). The filament temperature was monitored by a two color pyrometer through the quartz window and the substrate temperature was checked by a single band pyrometer. The reactor feed gases were controlled by mass flow controllers calibrated separately for hydrogen, deuterium, and methane. The typical operating conditions were a hydrogen/deuterium flow of 100 sccm and a methane/CD 4 flow of 1.5 sccm.

Methodology of Experiments When comparing rates of diamond growth with an isotopic substitution, one has to decide at the outset what is to be held constant. Based on the pioneering research by Langmuir on tungsten filament power consumption with hydrogen vs. other molecules [7], it is clear that the generation of atomic hydrogen is a major source of energy consumption at the conditions for the CVD diamond growth. It is reasonable to assume that this additional energy consumption to produce atomic hydrogen is very important for the diamond growth, since it determines the flux of a critical atomic species at the surface. Therefore, the highest priority was to ensure that an equal flux of atomic species is maintained during the comparison. Since the filament reactions - hydrogen splitting in particular - are likely to be affected by the isotopic substitution (experimental evidence for this is a rise in filament temperature when hydrogen is replaced by deuterium), a proper comparison is to maintain a constant power rather than a constant filament temperature. The rise in filament temperature at constant power (approximately 150C) effectively compensates for the reduction in the rate of splitting deuterium to force roughly an equal amount of power to be absorbed in creating atomic species. By contrast, if the filament temperature is kept constant, the diamond film growth rate with deuterium is likely to be decrease for two reasons: a) the flux of atomic species is lower and b) since recombination of atomic species is a major source of energy transfer to the substrate, the substrate temperature is lower, thus affecting the heterogeneous reaction rate. Another important item to keep in mind when making comparisons of growth kinetics is that the growth kinetics are based on apparent surface area and not the true film surface area, which is highly dependent on the film morphology. Consequently, it is very important that a stable film morphology be established prior to the isotopic switch and that the rate data during isotopic switch be acquired over a short period of time, lest the morphology and the true surface area change. The results reported here were acquired in this manner with an emphasis on reproducing the original rate upon switching back to hydrogen. The rate data reported here were gathered in the following manner: First, a diamond film was grown over a sufficiently long period of time using hydrogen and methane so that the thickness of the film was greater than the surface crystallite size of roughly 50 microns. During this initial growth period the growth rate reached a constant value over a long period of time. The filament temperature as indicated by a two color pyrometer was constant at roughly 2400C based on calibration against a standard tungsten lamp. It should be noted that the uncertainties about the optical properties of the carburized filament imply that this is a relative measure, and not an absolute one. The switch to the deuterated gases was made and the rates at constant power and at constant filament temperature were

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measured over a period of four hours. A return to the original rate was confirmed by switching back to the original feed gases. The total growth during the isotopic switch was kept small compared to a typical crystallite size of 50 microns. The major results of the isotopic substitution were repeated to ensure reproducibility. Experimental Results The growth rates are reported on a normalized basis with the hydrogen and methane rate as the basis (100%). A typical mass spectrum during this growth is shown in Fig. 1 indicating the two major features of methane peaks at a.m.u.'s 13 to 16, and acetylene at a.m.u. 26. The spectrum after the isotopic switch is shown in Fig. 2, clearly depicting the IOE-07 I

I I I I I I I I I

I II

IIIII

III

III

III

IIIII

IIII

Relative Intensities £-08

-

..... ,.+I ....... Io

20

, A............ 30

40

Atomic Mass Units FIGURE 1 Mass Spectrum with a Reactor Feed of 1.5% CH 4 in H 2 even peaks in both the C a and the C 2 range. The mass spectrum obtained when D 2 and CH 4 were fed to the reactor indicates that the feed methane is totally decomposed by the filament, giving rise primarily to peaks characteristic of deuterated methane and acetylene, with odd a.m.u, peaks at reduced levels from the hydrogen. The normalized growth rates are shown below: Base Equal

rate

with

power

H 2 and

with

CH 4

D 2 and CD 4

Constant filament temperature w i t h D 2 a n d CD 4

100% 71% 32%

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1.01[-07 I

Relative Intensities

l I

I |

I I I I I 1 II

I i i i i i I l l l i l l l l l l l

E-08

. . . .

o

IO

20

. . . . . . .

30

40

Atomic Mass Units FIGURE 2 Mass Spectrum with a Reactor Feed of 1.5% CD 4 in D 2

The observed rate depression at constant power is fortuitously close to that predicted by the isotopic effect and it should be noted that there are uncertainties in the experiment that exceed the degree of agreement. On the other hand, the large decrease in the growth rate at constant filament temperature attests to the contributions due to a reduced atomic species flux and to a lower substrate temperature. The degree to which an introduction of a small amount of hydrogen enhances the growth rate is a subject of on-going studies and beyond the scope of this study. Conclusions The decrease in growth rate associated with substituting deuterium for hydrogen while maintaining constant power suggests that a C-H bond is involved in the rate limiting step for diamond growth. The results at constant filament temperature demonstrate that the generation of atomic species at the filament also shows a pronounced isotopic effect, indicating that the hydrogen splitting reaction is most likely limited by a heterogeneous step involving a metal-hydrogen bond. Acknowledgements The authors wish to thank the management at G E Research and Development and at G E Superabrasives for their permission to publish this work, and to acknowledge the helpful discussions that one of us (HBV) had with Drs. Douglas McKee and James Bray.

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References [1]

DeVries, R.C., Annual Rev. Mater. Sci., 17:161 (1987)

[2]

Badzian, A.R., DeVries, R.C., Mat. Res. Bull., vol 23, 385 (1988)

[3]

Angus, J.C., Hayman, C.C., Science vol 241, 913 (1988)

[4]

Sommer, M., Muk, K., Smith, F.W., paper presented at Diamond Technology Initiative Symposium, Crystal City, VA (1988)

[5]

Frenklach, M., Spear, K.E., J. Mater. Res., vol 3, No. 1,133 (1988)

[6]

Ozaki, A., Isotopic Studies of Heterogeneous Catalysis, Ch 6, Kodansha Ltd. and Academic Press, NY (1977)

[7]

The Collected Works of Irving Langmuir, ed. C.G. Suits, Pergamon Press, vol 1, p 103 (1960). Langmuir, I., J. of Am. Chem. Soc., vol 36, No 8, 1708 (1914)