Diamond growth using a new low-pressure combustion technique is reported. A large-area hydrogen/oxygen flame is used as the source of atomic hydrogen. Methane diluted in hydrogen is injected into the flame near a heated silicon substrate, on which diamond crystallites nucleate and grow. This technique is potentially capable of large-area film growth, since atomic hydrogen can be generated uniformly over arbitrarily large areas

Glumac, N. G.

Goodwin, D. G.

English

Caltech Authors - Main

Diamond growth in a novel low pressure flame 
N. G. Glumac and D. G. Goodwin 
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, 
CaIifrnia 9112.5 
(Received 7 February 1992; accepted for publication 26 March 1992) 
Diamond growth using a new low-pressure combustion technique is reported. A large-area 
hydrogen/oxygen flame is used as the source of atomic hydrogen. Methane diluted in hydrogen 
is injected into the flame near a heated silicon substrate, on which diamond crystallites nucleate 
and grow. This technique is potentially capable of large-area film growth, since atomic hydrogen 
can be generated uniformly over arbitrarily large areas. 
Since the recognition in the early 1980’s of the impor- 
tance of atomic hydrogen for diamond growth at low pres- 
sure, various thermal and plasma techniques have been 
used to produce H atoms.“” With most of these techniques, 
however, it is difficult to generate atomic hydrogen over a 
large area, which limits the size of the diamond film which 
can be grown. In addition, many of these techniques in- 
volve hot cathodes or filaments which introduce metal con- 
taminants into the film. In this letter, we report the suc- 
cessful growth of diamond using a large-area, clean, low- 
pressure hydrogen/oxygen flame as an atomic hydrogen 
source. 
The experimental setup is shown schematically in Fig. 
1. A premixed hydrogen/oxygen flame is stabilized on a 
6-cm-diam flat-flame burner at a pressure (typically) of 40 
Torr. The burner is the same as we have used previously 
for diamond growth in low-pressure acetylene/oxygen 
flames3 The burner surface consists of a water-cooled cop- 
per plate drilled with an array of closely packed 1 mm 
diam holes. 
The flame is run fuel-rich ($= 1.25), to avoid exces- 
sive oxygen in the post-flame region. Numerical simula- 
tions of this flame indicate that the primary reaction zone 
extends less than 1 cm from the burner surface. Beyond the 
reaction zone, mole fractions of H, OH, and O2 are esti- 
mated to be 3%, 2%, and 0.5%, respectively. (Including 
the effect of the cool boundary layer above the substrate 
would considerably reduce the OH and O2 mole fractions 
at the substrate.) 
A small, heated silicon substrate is held in the flame 
approximately 5 cm from the burner surface, well beyond 
the primary reaction zone. The substrate temperature is 
controlled by mounting it on a resistivity heated molybde- 
num strip. A methane/hydrogen mixture is injected 
through a small (1.7 mm ID) ceramic tube immediately 
above the substrate. 
Under appropriate conditions, well-faceted diamond 
crystals grow near the injection tube exit. The deposit 
shown in Fig. 2 resulted after 7 h at 46 Torr, with injection 
of 200 seem of 0.5% methane in hydrogen. The substrate 
temperature was estimated to be 1020 “C using a single- 
color pyrometer and correcting for the substrate emissiv- 
ity. The region in which diamond is found extends from 3 
to 5 mm beyond the injector, and is 3 mm wide at its widest 
point. Outside this region, no carbon deposit is seen. Some 
morphology variation across the deposit is observed, with 
particle size increasing with distance from the injector. 
Also, particles near the center of the deposit show more 
secondary nucleation than those near the edge. This may 
indicate a beneficial effect of O2 or OH near the interface 
between the flame and the gas jet. 
The micro-Raman spectrum for a sample grown under 
similar conditions (p=36 Torr) is shown in Fig. 3. This 
spectrum was acquired from a single 5 pm diam crystal, 
and clearly shows a sharp diamond 1332 cm-’ peak, in 
addition to a broad luminescence background. The broad 
feature at 1540 cm- ’ is commonly seen in CVD diamond 
Raman spectra, and is indicative of the presence of a small 
amount of amorphous carbon.2 
Both substrate temperature and the composition of the 
injection gas have a large influence on the deposit charac- 
teristics. At a substrate temperature of 900 “C, only round 
ball-like carbon deposits resulted. Using an injection gas 
mixture of 5% methane in hydrogen also resulted only in 
ball-like deposits, but these were distributed over most of 
the substrate, in contrast to the diamond deposited with 
0.5% methane, which was confined to the area around the 
injection tube. The trends with substrate temperature and 
gas composition appear to be qualitatively similar to those 
commonly observed for hot-filament growth. 
For diamond growth to occur using this method, rapid 
diffusional mixing between the gas jet and the atomic hy- 
drogen in the flame must occur. We may estimate the time 
Premixed 
H2 and 02 
FIG. 1. Schematic of experimental setup. 
2695 Appl. Phys. Lett. 60 (Zl), 25 May 1992 0003-6951/92/212695-02$03.00 $51 1992 American institute of Physics 2695 
Downloaded 15 Jan 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
10000 r 
FIG. 2. Diamond particles on the substrate near the injection tube after FIG. 3. Micro-Raman spectrum of a 5 pm diam crystal grown at p=36 
7 h of growth. Run conditions were p=46 Torr, r,= 102O”C!, and the Torr. The 1332 cm’.’ diamond line is clearly seen, along with a broad 
injected gas was 0.5% methane in hydrogen. feature near 1540 cm-’ associated with nondiamond carbon. 
required for H to diffuse through the gas jet as tzd2/4D 
where d is the jet diameter and D the diffusion coefficient. 
Taking the diffusion coefficient of atomic H to be 350 
cm2/s (Ref. 4) and assuming dz2 mm results in a diffu- 
sion time of approximately 30 ps. Since the initial jet ve- 
locity is roughly 300 m/s, this corresponds to a distance 
several millimeters downstream from the injection tube. 
The observed onset of growth is consistent with this order- 
of-magnitude-estimate. 
The downstream edge of the deposit may correspond 
to the time required for oxidants (OH or 0,) to reach the 
substrate. The diffusion coefficient of O2 under these con- 
ditions is five times smaller than that of H, so there exists 
a region on the substrate with adequate H and hydrocar- 
bon concentrations for growth, but where conditions are 
sufficiently reducing to avoid oxidation of the film. 
so00 
E 
5 
8 
6000 
4000 
WAVENUMBER SHIFT 
Developing this technique for large-area film growth 
will require engineering injectors capable of providing 
larger, more uniform gas flow than the small tube used in 
this work. Proper design of the gas injector will require an 
understanding of the flow and mixing between the flame 
and gas jet. Studies to examine this question are currently 
in progress. 
We would like to thank Dr. 3. E. Butler at NRL for 
performing the Raman analysis. We also gratefully ac- 
knowledge financial support for this work from the Na- 
tional Science Foundation and Norton Co. 
‘J. C. Angus and C. C. Hayman, Science 241, 913 (1988). 
’ W. .4. Yarbrough and R. Messier, Science 247, 688 ( 1990). 
3N. G. Glumac and D. G. Goodwin, Thin Solid Films (in press). 
4R. J. Kee, G. Dixon-Lewis, J. Warnatz, M. E. Coltrin, and J. A. Miller, 
Technical Reports SAND 86-8246, Sandia National Laboratories 
(1986). 
2696 Appl. Phys. Lett., Vol. 60, No. 21, 25 May 1992 N. G. Glumac and D. G. Goodwin 2696 
Downloaded 15 Jan 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Diamond growth in a novel low pressure flame

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Diamond growth in a novel low pressure flame

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