Synthesis of 15R polytype of diamond in oxy-acetylene flame grown diamond thin films

Article abstract of DOI:10.1063/1.115841

15R polytype of diamond has been synthesized using a specially designed oxy-acetylene flame system along with 3C diamond and cubic carbon on polycrystalline molybdenum substrates. X-ray diffraction has been used to detect the 15R phase as the dominant phase in these films. Rapid changes in the substrate temperature during the growth process are expected to be responsible for the growth of these phases.

Full text of DOI:10.1063/1.115841

Synthesis of 15R polytype of diamond in oxyacetylene flame grown diamond thin films
R. Kapil, B. R. Mehta, and V. D. Vankar
Citation: Applied Physics Letters 68, 2520 (1996); doi: 10.1063/1.115841
View online: http://dx.doi.org/10.1063/1.115841
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Synthesis of 15R polytype of diamond in oxy-acetylene flame grown
diamond thin films
R. Kapil, B. R. Mehta, and V. D. Vankara)
Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi-110 016, India
͑Received 26 October 1995; accepted for publication 23 February 1996͒
15R polytype of diamond has been synthesized using a specially designed oxy-acetylene flame
system along with 3C diamond and cubic carbon on polycrystalline molybdenum substrates. X-ray
diffraction has been used to detect the 15R phase as the dominant phase in these films. Rapid
changes in the substrate temperature during the growth process are expected to be responsible for
the growth of these phases. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02818-5͔
Among the various polymorphs of carbon, diamond is
the most extensively studied material because of its unique
mechanical, optical, chemical, and electrical properties. Vari-
ous techniques such as hot filament chemical vapor deposi-
tion ͑CVD͒, microwave plasma CVD, and oxy-acetylene
flame method1–3 have been used to grow diamond thin films.
These techniques result in the growth of cubic diamond or a
mixture of cubic diamond with amorphous carbon and a dia-
mondlike carbon material. It has been indicated that the
growth of various polytypes of diamond is possible in the
nonequilibrium conditions. Other phases of diamond, namely
hexagonal 2H, 4H, 6H, 8H, 10H, and rhombohedral 15R,
21R have been theoretically predicted.4–7 The corresponding
infrared and Raman vibrational modes have been proposed
by Spear et al.5 X-ray diffraction data for the hexagonal and
rhombohedral polytypes have been calculated by Ownby
et al.4 Out of the various polytypes mentioned above, a hex-
agonal 2H polytype has been found in natural8 and shock-
compaction-synthesized diamond samples.9 Howard et al.10
and Frenklach et al.11 have been observed 2H and 6H hex-
agonal polytypes in diamond powder synthesized by micro-
wave plasma CVD using oxygen-acetylene and chlorinated
hydrocarbon precursors, respectively. A so-called x-diamond
polytype has also been observed by Rossi et al.12 in diamond
films deposited on a polycrystalline Ta substrate from a mix-
ture of methane and hydrogen by hot filament CVD tech-
nique. The present letter reports the synthesis of rhombohe-
dral 15R polytype of diamond in thin-film samples grown by
the oxy-acetylene flame method.
maintaining an acetylene to oxygen ratio between 1.05 and
1.50, deposition time ϳ60 min, and the speed of rotation
ϳ20–100 rpm. The substrates were positioned at a distance
of about 10 mm from the nozzle head. The temperature of
the substrates as recorded by an optical pyrometer was found
to be ϳ650 °C. The samples were characterized with the
help of a glancing angle x-ray diffractometer using Cu K␣
radiation at a glancing angle of 3°. Substrates were rotated
along an axis perpendicular to the plane of the sample during
the recording of the diffraction pattern. Due to the limitations
of the x-ray diffractometer used, we were constrained to
record the diffraction patterns up to 2⍜ϭ90° only. The sur-
face morphology of the samples was studied by using a scan-
ning electron microscope ͑SEM͒ operated at 21 kV. No over-
layer coating was deposited on the film.
The diamond films were deposited using a ‘‘specially
designed’’ oxy-acetylene flame deposition setup described
elsewhere.13,14 In this setup substrates were mounted on a
water-cooled substrate holder rotating with respect to the
oxy-acetylene flame. It was possible to vary the rotation
speed of the substrate holder and the distance between the
nozzle and the substrate which resulted in uniform and large
area deposition of diamond thin films. Polycrystalline
molybdenum discs of ϳ1–2 mm thickness were used as sub-
strates for diamond deposition. Molybdenum substrates were
initially polished with very fine grade SiC polishing paper
and finally with 2–5 ␮m diamond paste. The substrates were
degreased in isopropyl alcohol and ultrasonically cleaned in
trichloroethylene and acetone. Samples were deposited by
FIG. 1. ͑a͒ SEM micrograph of the diamond thin-film sample. ͑b͒ Cross-
sectional SEM micrograph.
a͒
Electronic mail: vdvankar@physics.iitd.ernet.in
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2520 Appl. Phys. Lett. 68 (18), 29 April 1996 0003-6951/96/68(18)/2520/3/$10.00 © 1996 American Institute of Physics
137.149.200.5 On: Tue, 25 Nov 2014 11:22:41

cal x-ray diffraction ͑XRD͒ pattern of the samples. The in-
terplanar spacings, d͑Å͒ and I/I0 values for the various ob-
served peaks are given in Table I. Theoretically calculated
XRD data for 15R diamond polytype from Ref. 4 have also
been given in the table for comparison. The ASTM data for
the 3C diamond, cubic carbon, molybdenum carbide
(Mo2C), and molybdenum are also presented in Table I.
The strongest peak ͑No. 10͒ observed in the XRD data
for our sample occurs for dϭ2.101 and has been assigned to
the ͑104͒ plane of the 15R diamond. The calculated intensity
(I/I0) of this line is 79.51. The most prominent line (I/I0
ϭ100) in the calculated data for the 15R polytype occurs at
dϭ2.0593 ͑hklϭ0015). The corresponding peak at our
sample has been observed for dϭ2.065. Several other XRD
peaks ͑Nos. 9, 13, 14, 15, 20, 24͒ match very well with the
calculated XRD data for the 15R structure ͑Table I͒. The
XRD data show that the 15R diamond polytype is the domi-
nant phase in the sample.
FIG. 2. X-ray diffraction pattern of the sample.
Figure 1͑a͒ shows a typical microstructure exhibiting
dense and uniform growth of the films. Cross-sectional SEM
micrograph of one of these films is shown in Fig. 1͑b͒. The
micrograph shows that columnar growth has occurred
throughout the thickness of the films. Figure 2 shows a typi-
Furthermore, it is observed that the XRD peaks ͑Nos.
11,24͒ correspond to 3C diamond peaks at dϭ2.060 (I/I0
ϭ100) and 1.261 ͑25͒, respectively. These peaks coincide
TABLE I. d-spacings and I/I0 values of the peaks in the observed, calculated, and standard data.
a
b
d
Peak
no.
Observed
d(Å)/(I/I0)
Diamond ͑3C͒
d(Å)/(I/I0)
Diamond ͑15R͒
Cubic Carbonc
d(Å)/(I/I0)
Mo2C ͑3H͒
Moe
d(Å)/(I/I0)
d(Å)/(I/I0)(hkl)
d(Å)/(I/I0)
1.
2.
3.
4.
5.
6.
7.
8.
3.206/49
2.794/25
2.704/2
2.612/2
2.415/2
2.366/4
2.279/8
2.230/49
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3.208/50
2.770/100
-
-
-
-
-
-
-
-
-
-
-
-
2.600/20
2.467/100
-
-
-
-
-
2.370/30
2.280/100
-
-
2.225/100
-
2.1788/0.10/͑101͒
-
9.
10.
11.
2.164/5
2.101/100
2.065/21
-
-
-
2.1627/4.50/͑012͒
2.1017/79.51/͑104͒
2.0593/47.77/͑015͒
2.0593/100/͑0015͒
-
-
-
-
-
-
-
-
-
-
-
-
2.060/100
-
12.
13.
14.
1.979/4
1.920/2
1.901/2
-
-
-
-
-
-
1.961/50
-
-
-
-
-
-
-
-
1.9575/31.13/͑107͒
1.9011/4.42/͑018͒
-
-
-
1.844/50
-
-
-
-
1.750/16
-
-
-
1.7241/0.66/͑0111͒
-
-
-
15.
16.
17.
18.
19.
20.
21.
22.
1.611/9
1.596/14
1.574/9
1.526/1
1.439/6
1.397/4
1.386/16
1.333/1
-
1.285/15
1.259/5
-
1.202/1
1.181/1
1.134/12
1.112/6
-
-
-
-
-
-
-
-
1.6080/1.37/͑1013͒
-
-
-
-
-
-
-
-
1.600/50
-
1.574/21
1.5522/0.17/͑0114͒
1.4465/0.28/͑1016͒
1.3969/3.66/͑0117͒
-
1.485/50
-
1.386/͑400͒
1.345/͑322͒
1.503/12
-
-
-
-
-
-
-
-
f
-
-
-
1.349/18
1.303/2
-
1.269/16
-
1.255/8
1.183/2
1.140/2
-
f
-
-
1.3041/20.69/͑1019͒
-
-
-
-
-
-
23.
24.
-
1.285/39
1.261/25
1.2610/60.19/͑110͒
1.2610/9.57/͑0120͒
-
1.1811/9.79/͑1022͒
1.1440/6.59/͑0123͒
-
-
-
-
-
-
-
-
-
-
25.
26.
27.
28.
f
1.132/͑422͒
-
-
1.1127/11
aASTM 6-0657.
bReference 4.
dASTM 11-680.
eASTM 4-089.
fhkl values calculated using the lattice parameter aϭ5.545 Å as in Ref. 15.
cASTM 18-311.
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Appl. Phys. Lett., Vol. 68, No. 18, 29 April 1996 Kapil, Mehta, and Vankar 2521
137.149.200.5 On: Tue, 25 Nov 2014 11:22:41

with the calculated data for the other polytypes as well.4
However, no other line of other polytypes matches with the
experimentally observed data. Hence, their presence in our
samples is ruled out.
rapid quenching of the substrate during diamond growth is
known to enhance the formation of noncubic diamond
phases.4,12 The rapid changes in the substrate temperature
during growth in our case is expected to contribute to the
synthesis of 15R polytype. Study of the dependence of
growth of 15R and other diamond polytype phases as a func-
tion of deposition parameters is in progress.
In summary, a new phase of diamond which matches
very well with the theoretically calculated data for the 15R
structure of diamond has been observed. X-ray diffraction
results show that this phase is the most dominant phase in the
coatings obtained by the oxy-acetylene flame method.
The authors are thankful to Professor K. L. Chopra for
his valuable suggestions and interest. One of the authors ͑R.
Kapil͒ is grateful to University Grants Commission ͑India͒
for providing a research fellowship. Authors are also thank-
ful to Department of Science and Technology for providing a
research grant.
Six other peaks ͑Nos. 1, 2, 5, 12, 16, 21͒ with d
ϭ3.206 (I/I0ϭ49), 2.794 ͑25͒, 2.415 ͑2͒, 1.979 ͑4͒, 1.596
͑14͒, 1.386 ͑16͒ have been assigned to the cubic carbon
phase. This phase has been observed by Aust et al.15 during
resistance measurements on graphite as a function of pres-
sure. Transformation of graphite to cubic carbon phase has
been observed to take place at a pressure greater than 150
kbar. Some of the peaks ͑marked by f) shown in Table I have
not been reported earlier. However, they are consistent with
the other ͑hkl͒ values calculated using the lattice parameter
aϭ5.545 Å as in Ref. 15.
The XRD peaks ͑Nos. 8, 17, 23, 28͒ are assigned to the
molybdenum, and are due to the substrate. The remaining
peaks ͑Nos. 4, 6, 7, 25͒ match very well with the ASTM data
for Mo2C ͑3H͒ phase and confirm the Mo2C ͑3H͒ phase in
our film. The formation of the Mo2C phase at the substrate
film interface during the initial stages of diamond growth is
expected due to the reaction between molybdenum and car-
bon. Presence of Mo2C at the interface has been confirmed
by glancing angle x-ray diffraction measurements recorded
at various glancing angles.
1 K. E. Spear, J. Am. Ceram. Soc. 72, 171 ͑1989͒.
2 J. C. Angus, Y. Wang, and M. Sunkara, Annu. Rev. Mater. Sci. 21, 221
͑1991͒.
3 L. M. Hanssen, K. A. Snail, W. A. Carrington, J. E. Butler, S. Kellogg,
and D. B. Oakes, Thin Solid Films 196, 271 ͑1991͒.
4 P. D. Ownby, X. Yang, and J. Liu, J. Am. Ceram. Soc. 75, 1876 ͑1992͒.
5 K. E. Spear, A. W. Phelps, and W. B. White, J. Mater. Res. 5, 2277
͑1990͒.
Some of the peaks ͑Nos. 19, 22, 24, 26, 27͒ in our
sample are close to the standard data for more than one of the
phases. However, this does not affect the overall conclusions
drawn earlier. Peak No. 3 is unidentified.
6 C. E. Holcombe, Calculated X-Ray Diffraction Data for Polymorphic
Forms of Carbon, Report No. Y-1887, Oak Ridge Y-12 Plant, July 23,
1973.
7 A. W. Phelps, W. Howard, W. B. White, K. E. Spear, and D. Huang,
Mater. Res. Soc. Symp. Proc. 162, 213 ͑1989͒.
Intermittent deposition and a possible quenching of the
substrate during growth may be responsible for the growth of
15R polytype. Because of the substrate rotation, different
regions of the substrate pass through different deposition and
no deposition zones of the flame, repeatedly. As the substrate
moves through the flame area, the temperature of the sub-
strate surface in direct contact with the flame is expected to
increase and deposition of the film takes place. The substrate
temperature is expected to fall significantly when this portion
comes out of the flame ͑no deposition zone͒. This cycle is
then repeated with the substrate rotation. In CVD techniques,
8 P. D Ownby and R. W. Stewart, in ASM Engineered Materials Handbook
͑ASM International, Materials Park, OH, 1992͒, Vol. 4.
9 F. P. Bundy and J. S. Kasper, J. Chem. Phys. 46, 3437 ͑1967͒.
10 W. Howard, D. Huang, J. Yuan, M. Frenklach, K. E. Spear, R. Koba, and
A. W. Phelps, J. Appl. Phys. 68, 1247 ͑1990͒.
11 M. Frenklach, R. Kematick, D. Huang, W. Howard, K. E. Spear, A. W.
Phelps, and R. Koba, J. Appl. Phys. 66, 395 ͑1989͒.
12 M. Rossi, G. Vitali, M. L. Terranova, and V. Sessa, Appl. Phys. Lett. 63,
2765 ͑1993͒.
13 R. Kapil, B. R. Mehta, and V. D. Vankar, J. Mater. Sci. ͑to be published͒.
14 R. Kapil, B. R. Mehta, and V. D. Vankar, Bull. Mater. Sci. ͑to be pub-
lished͒.
15 R. B. Aust and H. G. Drickamer, Science 140, 817 ͑1963͒.
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2522 Appl. Phys. Lett., Vol. 68, No. 18, 29 April 1996 Kapil, Mehta, and Vankar
137.149.200.5 On: Tue, 25 Nov 2014 11:22:41