Enhancement of (100) texture in diamond films grown using a
temperature gradient
E. Titusa, A.K. Sikderb, U. Paltnikara, M.K. Singha, D.S. Misraa,*
a
Department of Physics, Indian Institute of Technology, Powai, Mumbai 400076, India
Center for Microelectronics Research, University of South Florida, Tampa, FL 33620, USA
b
Abstract
Diamond films with dominant (100) texture were grown with a temperature gradient across the Si (100) substrates using hot
filament chemical vapor deposition technique. Deposition was carried out with 0.8% CH4 in balance hydrogen at an average
substrate temperature of 880 8C. The deposition pressure was varied between 20–120 torr. Films were characterized using X-ray
diffraction (XRD), scanning electron microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). XRD shows very
strong (400) reflection in all the samples. SEM results show a smooth diamond surface comprised of (100) platelets. As the
(100) diamond plates were grown on top of the (100) oriented silicon substrate the faces are more or less aligned parallel with
the substrate surface, resulting in a relatively smoother diamond surfaces. FTIR studies show novel features in the films.
Quantitative analysis was carried out to measure the H content in the films.
Keywords: Temperature gradient; Textured diamond films; FTIR spectroscopy; Hydrogen content
1. Introduction
During the past few years, interest in oriented diamond films synthesized by chemical vapor deposition
techniques has grown enormously w1,2x. For practical
applications in optics and electronics, large area singlecrystal diamond films on silicon or other suitable substrates are very desirable. However, the lattice mismatch
between the substrate and the film, dominance of the
secondary nucleation and the lack of layer by layer
growth has generally resulted in the polycrystalline
diamond films on non-diamond substrates w3x. On the
other hand, (100) ‘texture’ developed in the films during
the growth when seeded with the help of bias enhanced
nucleation (BEN). In BEN, the normal procedure of
treating the substrate by rubbing against diamond powder is avoided and oriented nuclei of (100) diamond or
SiC are generated on the substrate by biasing it. Since
the first reports of the deposition of oriented films by
bias enhanced nucleation w4x, the majority of studies
were concentrated on the improvement of the texture
and size of the (100) oriented crystals. For instance,
Jiang et al. w5,6x synthesized an oriented diamond film
on Si substrate via BEN. Wolter et al. w7x reported that
oriented diamond films were deposited via in situ
carburization prior to BEN, and Chen et al. w8x reported
the synthesis of oriented diamond films on a Si substrate
by hot filament chemical vapor deposition (HFCVD).
Wild et al. w9x found that the successive control of the
growth faces, {100} at the initial stage and {111} later
stage, was important for the synthesis of highly oriented
films. These studies have contributed significantly in
the optimization of the parameter window for the deposition of (100) oriented diamond films with much
improved structure and properties.
The crystallinity of diamond films is strongly dependent on the orientation of the nuclei formed during the
BEN. Li et al. w10x investigated the role of the BEN by
means of single wavelength ellipsometry, and reported
that the BEN consisted of four steps: carburization,
etching, nucleation, and the growth of the nuclei. Yugo
et al. w11x observed the cross-section of a diamond film
and found that the diamond formed on an amorphous
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interlayer and also directly on the Si substrate. In another
study it was reported that the interfacial strain was
released by a non-diamond carbon phase that was
formed between Si and diamond w12,13x. Plitzko et al.
w14x however, reported that diamond film was synthesized directly on a Si substrate using bias-assisted
microwave plasma chemical vapor deposition
(MPCVD). Recently, Saada et al. w15x synthesized
highly oriented diamond films on silicon substrate by
the MPCVD technique using an ultra short bias
enhanced nucleation process.
In the present study, a new innovative method of
using a temperature gradient has been employed for the
growth of (100) textured diamond films by HFCVD.
The temperature gradients ranging from 50 to 150 8C
cmy1 are generated on the growth surface of the
substrates using a thin film chromium heater deposited
on the back surface of the substrates. The substrates are
treated by diamond powder prior to deposition. A
dominant (100) texture is observed in the diamond
films. The grain size and the density of the (100) grains
vary sensitively as a function of the gradient. We report
the details of the morphological and structural analyses
of the samples by SEM, XRD and FTIR spectroscopy.
Some interesting features observed in the textured diamond samples are reported.
2. Experimental
The deposition of the films was carried out in a
HFCVD apparatus whose details are discussed elsewhere
w16x. Mirror polished p-type Si (100) wafers were used
as substrate. The substrates were treated with 2 mm
diamond powder and cleaned ultrasonically in an acetone bath. Prior to the deposition of the thin film heater,
the substrates were dipped in HF followed by de-ionized
water cleaning in order to remove the passive oxide
layer of silicon. Few depositions were done on the
Fig. 1. (a)–(c): The SEM of the diamond films; deposited at 120 torr and temperature gradients; (a) at 50 8C cmy1; (b) at 100 8C cmy1; (c)
at 150 8C cmy1; and (d) at 120 torr, without gradient.
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substrates without treatment with diamond powder for
comparison. Chromium thin film (of 1 mm=10
˚ was deposited as heater by thermal evapmm=200 A)
oration on the backside of the substrates for the generation of the gradient The gradient was created across
the sample by applying DC power to both ends of the
heater using chromel wires. The gradient was measured
by measuring the temperature at both ends of the sample
using chromel-alumel thermocouples attached to the
substrate holder. The reactant gases were methane and
hydrogen at the flow rates of 1.6 and 200 sccm,
respectively. The substrate temperature was measured at
the center of the substrate and was held constant for all
depositions at 880 8C. The substrates were treated with
hydrogen plasma for 0.5 h prior to the deposition.
Self-standing diamond sheets were used for IR analysis by making a window of diameter 6–7 mm in the
center. In order to make the window, the diamond films
grown on silicon substrates were mounted on a glass
plate with the diamond facing the glass. Subsequently
the whole assembly was covered by wax. Wax was then
removed from the center and the assembly was dipped
in a mixture of HF and HNO3 (1:3) in the fourth step.
The above procedure resulted in silicon etching and the
window was then removed from the glass plate and
cleaned thoroughly using de-ionized water, trichloroethelene and acetone. Since there is no interaction
between diamond surface and the etchants, the chemistry
of this surface should be unperturbed. The infrared
spectra were recorded by a Nicolet Fourier Transform
Infrared spectrometer in the transmission mode. Spectra
were recorded on all the samples in the range of 400–
4000 cmy1.
3. Results and discussions
Fig. 1(a)–(c) shows the SEM micrographs of the
diamond films deposited at 120 torr applying different
temperature gradients. A typical SEM micrograph showing the morphology of the sample deposited without
temperature gradient at 120 torr is shown in Fig. 1(d).
It is evident from these micrographs that the films
contain dominantly (100) textured grains. Moreover, the
size of the grains increases significantly with the
increase in temperature gradient. At lower gradients the
crystal size was less than 1 mm. As the gradient
increases further the crystal size exceeds 1 mm subsequently increasing to approximately 3.6 mm for a gradient of 150 8C cmy1. The density of the grains,
however, decreases as a function of the gradient. Any
further increase in the gradient resulted in reduction of
the density of the textured grains. The sample deposited
at 120 torr without gradient shows a surface morphology
typically observed in very poor quality diamond films
(Fig. 1d). The quality of the film can be gauged from
the color of the self-standing diamond window as well
Fig. 2. SEM of diamond film deposited at pressures (a) 20 torr with
temperature gradient 1508 cmy1; and (b) 20 torr without temperature
gradient.
as from the IR results. The good quality films will be
white in color and transparent in IR region. The grains
in poor quality films typically have a cauliflower structure evidently containing a large concentration of nondiamond carbon impurities. We could grow continuous
film at pressure 20 torr and at a temperature gradient
150 8C but the crystals were not (100) textured. Fig.
2(a),(b) shows the SEM micrographs of the films
deposited at 20 torr with and without temperature
gradient. It is evident that the crystals grown are of very
high quality but (100) texture is absent.
In order to understand the mechanism responsible for
the formation of (100) textured grains, we deposited the
films on the silicon substrates for various deposition
times with and without the gradient. Bare untreated
silicon (100) substrates were subjected to hydrogen
plasma etching at 880 8C with and without the gradient
with an objective to reveal the structural transformation
on the surface of the substrate that might lead to the
1406
Fig. 3. SEM of (a) Si substrate subjected to hydrogen plasma etching without temperature gradient across the substrate; and (b) in the presence
of the temperature gradient 150 8C; (c) and (d) show respectively the (100) oriented diamond grains deposited for 1 and 2 h.
orientation of the grains. The results of the SEM studies
on the above samples are shown in Fig. 3a–d. It is
interesting to note that the silicon surface develops
micro-pores when subjected to hydrogen plasma etching
in presence of the temperature gradient (150 8C cmy1)
(Fig. 3a). The size of the pores on the surface ranges
from submicron to 1–2 micron. In contrast, no pores
can be seen on the silicon surface subjected to hydrogen
plasma etching without the gradient as shown in Fig.
3b. Fig. 3c,d shows the micrographs of the (100)
textured diamond grains deposited for 1 and 2 h,
respectively.
The growth mechanism of (100) and (111) oriented
diamond facets has been debated in the literature considerably. Early studies showed that selective hydrogen
etching resulted in diamond orientation on the substrates
w17,18x. Further b-SiC was considered as a transfer
layer growing diamond epitaxially on Si substrates
w19,20x. Jiang and Klages showed that (001) oriented
diamond could grow epitaxially w3,21–23x on (001)
silicon by applying negative bias. It was demonstrated
w22,23x using high-resolution transmission electron
microscopy that (001) diamond grew directly on (001)
Si substrate. In our case, the texture changes from (111)
to (100) by applying temperature gradient. Hence, it
may be possible that micropores created on substrates
remove the native oxide layer and expose the underline
(100) Si plane for nucleation of (100) diamond which
results in parallel growth of (100) faces on (100)
nucleated substrates.
Considerable work exists in the literature on the
growth mechanism of (100) and (111) oriented crystal
faces. It is well known that for a kinetically controlled
growth process, the crystal morphology is determined
by the appearance of facets that have the slowest growth
rate in normal direction. As (110) surface is a stepped
face and encounters no steric repulsion between neighboring H atoms, its growth rate is highest w24x normal
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spectra of (100) textured sheets contain two dominant
peaks at 2860 cmy1 and 2930 cmy1 that correspond to
symmetric and asymmetric stretch bands of CH2 group.
A weak band corresponding to sp3 CH3 symmetric
stretch band at 2778 cmy1 is also visible. Similar FTIR
spectroscopy results in (100) textured diamond windows
are reported by other groups as well w28,29x. In contrast,
the spectra of the non-textured samples contain a broad
band containing various peaks corresponding to multiple
CHn modes.
The integrated absorbance of each band can be used
to estimate the hydrogen concentration in a particular
mode. As discussed by McNamara et al. w30x, the
concentration of the oscillating species is proportional
to the integrated intensity of the absorption band. The
total hydrogen content is:
NHsAn
Fig. 4. FTIR spectra of (a) (100); and (b) (111) textured diamond
samples grown at 120 torr.
to the substrate. In most cases, therefore, either (100)
or (111) texture is present in films. According to the
established growth model of Franklach w25x, the growth
rate of (100) facets depends on the concentration of
CH2 or CH3 species while that of (111) facets depends
on both CH3 and C2H2 concentration. As in the present
case, the deposition of (100) facets is dominant, we
shall concentrate on growth mechanism of the same.
High-resolution electron energy loss spectroscopy
(HREELS) on (100) diamond surfaces prepared at 800
8C reveal w26x that the observed loss peaks are consistent
with CH2 radicals. Low Energy Electron Diffraction
(LEED) studies by Hamza et al. w24x showed that
warming CVD diamond surface from 500 to 750 K
showed 1=1 reconstruction pattern that was assigned
to dehydrogenated surface. Annealing to 1300 K resulted
in 2=1 pattern ascribed to monohydride surface. It
would appear, therefore, that (100) growth mainly
occurs via CH2 species. This is difficult to comprehend
because the distance between two H atom on –CH2
˚ and will
bonded on diamond (100) surface is only 74 A
result in strong steric repulsion. It has been shown w27x
that the dimerization of –CH2 to C-H dimer with
possible vacancies in structure might be responsible for
(100) growth via-CH2 species.
Another strong indication that (100) diamond grows
via-CH2 bonded species arises from FTIR spectroscopy.
In FTIR spectra, various peaks in the range 2700–3100
cmy1 corresponding to spmCHn species that are incorporated during the growth, are observed. The spectra of
the self-standing diamond windows fabricated with
(100) textured sheets and the sheets with poor morphology are shown in Fig. 4. It should be noted that
both sheets are deposited at 120 torr and 880 8C. The
|
aŽv.dv
v
(1)
where An is the proportionality factor and a(v) is the
absorption coefficient at frequency v. An is proportional
to the inverse of oscillator strength. We have chosen
An to be 1=1020 and assumed it to be constant for all
the modes and samples w31x. a(v), the frequencydependent absorption coefficient, is taken to be a Gaussian function. The concentration of hydrogen is higher
(0.064 at.%) in poorly textured samples as compared to
(100) textured sample (0.048 at.%).
4. Conclusions
The (100) textured diamond films can be grown on
Si substrate by creating a temperature gradient across
the substrate during growth. The size of the crystals
increases with the temperature gradient. Good quality
films were formed at pressure 120 torr and temperature
880 8C. The (100) texture was developed only on
pretreated substrate. The films grown on untreated substrate consists of very minute crystals. The FTIR results
of (100) textured sample shows prominent CH2-related
peaks whereas the film without texture comprises multiple CH, CH2 and CH3 peaks. This may be due to the
fact that the (100) growth of diamond proceeds via
CH2 species.
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