138
Brazilian Journal of Physics, vol. 27/A, no. 4, December, 1997
Is there a common limiting step in the
activation energy of CVD diamond growth?
E. J. Corat1 , R. C. Mendes de Barros1 2, V. J. Trava-Airoldi1, N. G. Ferreira1,
N. F. Leite1, K. Iha2
;
1: Instituto Nacional de Pesquisas Espaciais,
C.P. 515 - 12201-970 S~ao Jose dos Campos, SP, BRAZIL
2: Instituto Tecnologico de Aeronautica,
12228-900, S~ao Jose dos Campos, SP, Brazil
Received February 3, 1997
In this paper we present an extensive comparison of our determinations of activation energy
for CVD diamond growth with CF4=CH4=H2 and CCl4 =H2 mixtures with most data in the
literature. We found interesting similarities among these data and propose that there is a
common trend in most observations.
I. Introduction
The growth of polycrystalline diamond lms by
chemical vapor deposition (CVD) has been considerably developed in the last few years.[1] The introduction of halogens in the precursor gas mixtures has also
shown some specic characteristics and advantages.[2]
The determination of temperature dependence of
diamond growth rate can give critical information regarding the rate-limiting steps during diamond growth.
Many studies have reported measurements of the dependence of diamond growth with substrate temperature, either for growth of polycrystalline or singlecrystal diamond, using dierent reactors and gas mixtures. Most of these studies found an Arrhenius behavior and determined an activation energy.
In this paper we considered interesting to make a
deeper comparison of most data on activation energy
found in the literature, including our data for growth
with CF4 addition [2] and for growth with CCl4[3].We
normalized all data in order to make this comparison
and plot them together. This procedure shows that the
data of many authors are more correlated than it appears at a rst glance. More than this, the correlation
exists independently of the growth method and the gas
mixture used. This comparison suggests that the lim Contact
iting steps in the surface processes are independent of
the gas system in use.
II. Comparision of Activation Energy Data
Some of the earlier determinations of the activation energy have shown a value in the range 20 ;
30 kcal=mol:[4] However, many other studies have
shown much lower activation energies. We noticed that
are many similarities among the data found in literature. Based on this observation we decided to make a
broader comparison among most available data. There
are experiments made in many dierent reactors, with
several gas mixtures, including C ; H, C ; H ; O,
C ; H ; F and the C ; H ; Cl systems. Also, there
are dierent methods to measure growth rate, dierent
procedures to account growth time and dierent uncertainties on substrate temperature reading. For this
reason we followed a single procedure to compare these
data. We normalized the growth rate by its value at a
substrate temperature of 1000 K; as obtained by each
author (Go = G=G1000K). We arranged this comparison in four sets: a basic set that seems to be in very
good agreement, a second set in which we compare the
data of Snail and Marks with the basic set and, a third
set in which we include our experiments with CF4 addition and CCl4 =H2 mixtures.
Author: EJ Corat Tel: 55 12 3256680, Fax: 55 12 3411869, E-mail: [email protected]
E. J. Corat et al
The basic set is shown in Fig. 1. From Muranaka et
al.[5], we used two sets of data obtained in a MWCVD
reactor. From Yamaguchi et al.[6], we used one set of
data obtained in a HFCVD reactor with a CH4=H2
mixture. They proposed a continuously decreasing apparent activation energy varying from 5 to 1 kcal=mol
with decreasing temperature, in the range 210 ; 700 o C.
The tting of their data is shown by the dashed line in
Fig. 1. Growth rate was measured by the growth of individual particles. From Pan et al.[7], we used one set of
data obtained in a fully chlorine activated reactor. The
gas mixture consisted of CH4, H2 and Cl2 . They measured, "in situ ", the homoepitaxial growth on (110)
faces by a Fizeau Interferometer. They present data
for the full range from 102 to 900 o CFrom their scattered data they found three distinct activation energies.
From Loh and Capelli[8], we used one set of data obtained in an arcjet reactor with a CH4=H2 mixture, in
the range 400 ; 900 o C: From their scattered data they
found two distinct activation energies, 12 kcal=mol in
the range 600 ; 900 o C and 4 kcal=mol in the range
400 ; 600 o C. All the experiments of this basic set of
data have a common characteristic that is interesting
to comment. All of them have enhanced conditions for
the production of radicals.
139
observe a tendency of reduction on activation energy at
lower temperatures. They measured growth rates in an
atmospheric pressure turbulent ame by emission interferometry, for dierent oxygen/acetylene ratio. The
growth rate continued to rise up to nearly 1200 o C.
Other authors have also shown that atmospheric pressure reactors can grow diamond at higher substrate
temperatures than is possible in low-pressure reactors.
Their scattered data are in good agreement with the
basic set for the temperature range 730 ; 950 o C. It
is important to remark that their data points at the
high temperature end seems to be a natural extension
of the basic set. However, at the low temperature end
there is no good agreement. They observed the transition to a lower apparent activation energy at a much
higher temperature than observed in most experiments
in low pressure reactors. This may be a characteristic
of growth at atmospheric pressure.
Figure 2. Comparison of the determination of activation
energy of Snail and Marks with the basic set of Fig. 5. C2 H2 =O2 , Rf = Ro +0:02 and 2 C2 H2 =O2 , Rf = Ro +0:03,
from Snail and Marks [9], + basic set.
Figure 1. Comparison of the dependence of the normalized growth rate (Go ) with substrate temperature. This
is the basic set: 3 C H4 =H2 from Loh and Capelli[8]; 2
C l2 =C H4 =H2 from Pan et al.[7]; C H4 =H2 from Yamaguchi
et al.[6]; 5 C O=H2 and 4 C O=O2 =H2 from Muranaka et
al.[5].
However, there are other data that do not agree very
well with this basic set and it is necessary to comment
on. In Fig. 2 we compare the two data sets of Snail
and Marks[9] to the basic data set. This comparison
is important because Snail and Marks were the rst to
The third set is shown in Fig. 3. Our data for activation energy in HFCVD with CF4 addition[2] and with
CCl4=H2 mixture [3] are compared with the basic set.
We also compare the three data sets of Fox et al.[10].
Even thought their lowest growth temperature was only
600oC, all of their data show a tendency to follow a similar behavior. In this data we found a good agreement
with the basic set at the high temperature end. However, at the low temperature end the growth rates are
much lower. Schimidt et al.[11] described a qualitative
analysis that seems very similar to this behavior. They
demonstrated that the lowest temperature for growth
of crystalline diamond is very dependent on lament
140
Brazilian Journal of Physics, vol. 27/A, no. 4, December, 1997
to substrate distance and related this to the necessity
of high radical concentration to enhance growth at low
temperature. Probably we may still explain the behavior of this data set at the low temperature end by the
lack of the necessary concentration of radicals.
Figure 3. Comparison of data sets that do not agree
with the basic set at the low temperature end.4 C H4 =H2 ,
5 C F4 =H2 and 3 C H3 F =H2 , from Fox et al.[10]; 2
C H4 =C F4 =H2 from Corat et al.[2]; C C l4 =H2 from Corat
et al.[3] ; + basic set.
III. Discussion
Independently of the interpretation each author
have attributed to their data we have to consider that
the good agreement among them, as shown in Fig.
1, is really impressive. The largest scattering is at
the lower temperature end, mainly represented by the
abrupt change in Pan et al. data. The continuous line
in Fig. 1 is representative of a continuously decreasing
apparent activation energy, tted to the complete set
of data. The apparent activation energy probably decreases continuously, as proposed before by Yamaguchi
et al.[6], rather than having two (or three) ranges of
distinct activation energies. This means that the probable mechanisms responsible for the reduction of the
apparent activation energy at low temperatures gradually increases its contribution. Some authors have discussed the eect of surface diusion in low temperature growth[11]. The higher inclusion of non-diamond
phases at low temperatures may also be explained by
a decrease of surface diusion. Accordingly, the gradual increase of the inuence of surface diusion at low
substrate temperatures appears to be an important process to determine the apparent activation energy in this
range.
Also, there is the possible inuence of gas-surface
interactions. The etching of sp2 carbon is known to depend on temperature. This means that the ability of
atomic hydrogen in etching non-diamond phases may
be considerably reduced at lower temperatures. The
growth of non-diamond phases and the re-nucleation
of diamond on it may also be a mechanism that contributes for the reduction of activation energy of diamond growth at low temperatures.
Most experimental observations have shown the
crescent inuence of non-diamond phases at lower temperatures. Both of the above proposed mechanisms
propitiate the growth of non-diamond phases. The necessity of a high concentration of radicals in diamond
growth at low temperature may be related to the maintenance of a considerable rate of transformation of sp2
into sp3 carbon, in a process of diamond re-nucleation.
IV. Conclusions
We presented a broad comparison of most data
available in the literature. This comparison suggests
that, at least in the range from around 600 ; 900o C,
there is a common process. Most experiments agree
quite well in this range. The exceptions are the earlier
experiments and the determination in HFCVD operating at typical conditions with CH4=H2 mixtures. In
this temperature range the reported activation energies
are around 9 ; 14 kcal=mol, depending on the interpretation each author conferred to their scattered data.
Our analysis of the whole set of data in this range, considering a linear t to all data points, gives an activation
energy of 9:7 kcal=mol. It is very plausible to attribute
this value to a mechanism that depends in single surface
vacant sites. The process of hydrogen abstraction from
diamond surface have been considered to have an activation energy of 7 kcal=mol and the carbon source from
the gas phase also have a dependence on substrate temperature (4 kcal=mol for CH3 in HFCVD[12]). Probably hydrogen abstraction is the rate limiting step in
this temperature range. It is very important to remark
once more that the activation energy in this temperature range seems to be independent of growth method
and gas mixtures.
The comparison of a basic set of data shows a good
agreement also in the low temperature end, indicating that probably the same mechanism, or mechanisms,
E. J. Corat et al
are responsible for the reduction of activation energy.
It seems to be independent of the existence of dierent growth species, as the proposed CH2Cl, or the
need of special surface etchants, as chlorine or uorine
atoms. The same eect is also observed from conventional CH4=H2 mixtures, provided there is a high concentration of atomic hydrogen.
The comparison presented in this paper indicates
that the inuence of halogens and oxygen in the growth
process is related to the formation of a gas phase with
larger radical concentration. Muranaka et al.[5] have
shown that high concentrations of oxygen are responsible for high concentrations of atomic hydrogen. Corat
et al.[2] have proposed that CF4 addition in HFCVD
may create excess atomic hydrogen. Ferreira et al.[13]
have identied by actinometry an apparent increase of
the atomic hydrogen in CF4=H2 plasma. From our
analysis of the eect of chlorine in the present paper
we concluded that Cl and H atoms probably compete
in surface H abstraction. Therefore, we infer that high
concentrations of H atoms are essential for low temperature growth. Even though the various gas additives may have particular interactions with the growth
surface it seems that the production of excess atomic
hydrogen is sucient for growth enhancement.
This research was supported in part by Fundac~ao de
Amparo a Pesquisa do Estado de S~ao Paulo (FAPESP)
under contract No. 93/4690-6. One of us (RCMB) acknowledges FAPESP for a scholarship.
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