Nitrogen and hydrogen related infrared absorption in
CVD diamond films
E. Titus a,*, N. Ali a, G. Cabral a, J.C. Madaleno a, V.F. Neto a, J. Gracio a, P Ramesh Babu b,
A.K. Sikder c, T.I. Okpalugo d, D.S. Misra c
a
Department of Mechanical Engineering, University of Aveiro, 3810-193, Portugal
Materials Ireland, Polymer research Centre, School of Physics, Dublin, Ireland
c
Department of Physics, Indian Institute of Technology (IIT), Bombay, India
d
Northern Ireland Bio-Engineering Centre, NIBEC, University of Ulster, UK
b
Abstract
In this paper, we investigate on the presence of hydrogen and nitrogen related infrared absorptions in chemical vapour deposited (CVD)
diamond films. Investigations were carried out in cross sections of diamond windows, deposited using hot filament CVD (HFCVD). The results of
Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) and Raman spectroscopy carried out in a cross section of self-standing
diamond sheets are presented. The FTIR spectra showed several features that have not been reported before. In order to confirm the frequency of
nitrogen related vibrations, ab-initio calculations were carried out using GAMESS program. The investigations showed the presence of several C –
N related peaks in one-phonon (1000 – 1333 cm 1). The deconvolution of the spectra in the three-phonon region (2700 – 3150 cm 1) also showed
a number of vibration modes corresponding to spm CHn phase of carbon. Elastic recoil detection analysis (ERDA) was employed to compare the H
content measured using FTIR technique. Using these measurements we point out that the oscillator strength of the different IR modes varies
depending upon the structure and H content of CVD diamond sheets.
1. Introduction
The extraordinary behaviour of nitrogen in CVD diamond is
continuing to attract considerable interest from many researchers working on the CVD of diamond films and coatings.
Indeed, the failure to n-doped diamond and a number of
experimental results [1 – 3] showing that the single substitutional nitrogen in CVD diamond behaves rather differently than
in natural diamond, have intensified this interest. It is widely
accepted that nitrogen acts as an impurity in CVD diamond,
which strongly affects its optical, electrical and mechanical
properties. Nitrogen is one of the major impurities in natural
diamond as well and it has been used as a characteristic for the
classification of the same. A number of nitrogen related centers
in natural as well as CVD diamond have been identified using
FTIR technique [4].
In CVD diamond, hydrogen is another impurity that plays a
major role in controlling various characteristics of the material.
The source of hydrogen in CVD diamond lattice is mainly the
high concentration ( 99% by vol.) of H2 gas present in the gas
mixture. Such large concentration of H2 is required to generate
high concentration of atomic hydrogen (H) during growth that
is essential for the synthesis of diamond via CVD routes [5].
The study of IR absorption in C –H stretch region is important
not only for identifying various possible modes of vibration,
but also for the quantitative estimation of hydrogen bonded to
the diamond lattice. As the absorption is due to the
superposition of vibrations from different modes where carbon
is in different configuration, the quantitative calculation of H
content using a unique value of the oscillator strength often
proves erroneous. Jacob and Unger [6] pointed out that the
constant (A n ) proportional to the oscillator strength does not
have a unique value and depends upon the structure of the C:H
films. A n for polymer-like films with H / C ratio around 1
increases by a factor of 4 with decreasing of H / C ratio. Hence
202
order to remove the passive oxide layer of silicon. Methane
(CH4) and Hydrogen (H2) were used as precursor gases. The
flow of methane (1.6 sccm) and hydrogen (200 sccm) was
maintained constant during sample preparation. In few cases,
N2 was added in controlled manner (100 – 200 ppm) to the
gas mixture in order to ascertain the origin of certain IR
bands. The deposition pressure in the chamber was varied
from 20 to 120 Torr in order to grow the samples with desired
quality. The temperature of the substrates was maintained at
880 -C and measured using a K-type thermocouple. The
thickness of the films was in the range: 30– 32 Am. A window of
diameter 6 –7 mm was cut in the center of the diamond sheets
deposited on silicon substrates by chemical etching [8]. This
facilitates to record the IR spectra on self-standing sheets and
the unwanted interference from the substrates could be easily
avoided. Several sets of sheets were deposited and subjected to
FTIR spectroscopy in order to check the reproducibility of the
data and methodology. The IR spectra were recorded in the
range 400 –4000 cm 1 with a resolution of 4 cm 1 on a Nicolet
Fourier transform spectrometer in transmission mode. The
Raman spectra were recorded at room-temperature using a
Renishaw micro-Raman system with an excitation wavelength
of 514.5 nm from an Ar+ ion laser. The diamond films were
irradiated with 2.4 MeV He2+ for ERDA analysis. Depth
profiles were obtained by deconvoluting the energy lost as the
ions travel into and exit from the sample.
for the determination of A n , the exact concentration of H atoms
bonded to carbon using an independent method is required.
In this paper, we report on results obtained from sample
characterizations using scanning electron microscopy (SEM),
elastic recoil detection analysis (ERDA), Raman and Fourier
Transform Infrared (FTIR) spectroscopic techniques. The
samples were cross sections of self-standing diamond sheets
deposited at various pressures (20 – 120 Torr). The FTIR
spectra showed several features that have not been reported
earlier. In order to confirm the frequency of nitrogen related
vibrations, ab-initio calculations were carried out using
GAMESS program. The quality of the films was studied
by the determination of its H content in the sheets. Using
different hydrogen measurement technique, we point out that
the oscillator strength of the different IR modes varies
depending upon the structure and H content of CVD
diamond sheets.
2. Experimental
Diamond sheets of the thickness varying from 10 –50 Am
were deposited using a HFCVD [7]. Boron doped silicon
wafers with (100) orientation were used as substrates for the
deposition diamond films. The substrates were polished using
2 Am diamond powder and subsequently ultrasonically
cleaned in acetone. Prior to the deposition, the substrates
were dipped in HF followed by de-ionized water cleaning in
3. Results and discussion
Fig. 1 shows the SEM micrographs of diamond films
deposited in the absence (a) and presence (b) of nitrogen. It is
evident from these micrographs that both the films contain
dominantly (111) oriented diamond grains. However, the
nitrogen added film exhibited uniform and smaller grains
below micron scale.
Fig. 2 shows the baseline corrected FTIR spectra of the
diamond sheets (with 200 ppm nitrogen doping at 20 Torr
chamber pressure) in one phonon, two phonons and three phonon
Intensity (a.u.)
Fig. 1. SEM micrographs of the diamond films deposited in the absence (a) and
presence (b) of nitrogen.
1000
2000
3000
-1
Wavenumbers (cm )
Fig. 2. Baseline corrected FTIR spectra of the diamond sheets (with 200 ppm
nitrogen doping and 20 Torr chamber pressure) in one phonon, two phonons
and three phonons regions (below 1333, 1333 – 2666 and 2667 – 3100 cm 1,
respectively).
203
Table 1
Characteristic vibration frequencies observed in our films
Wave number (cm 1)
Mode of vibration
1109
1332
1339
2027
2161
2820
2832
2850
2880
2920
2960
3025
Nitrogen related
Isolated nitrogen atoms
C_C
C_C
C_C
N – CH3
O – CH3
Sym. SP3CH2
Sym. SP3CH3
Asym. SP3CH2
Asym. SP3CH2
SP2 CH
regions (below 1333, 1333 – 2666 and 2667 – 3100 cm 1,
respectively). The summary of the various modes of vibrations and their frequencies in CVD diamond sheets is
presented in Table 1. The IR absorption in the region below
1333 cm 1 is forbidden by symmetry in pure diamond
crystal. However, due to the presence of nitrogen and hydrogen
related defects in natural and CVD diamond, the IR absorption
bands are visible in this region as well. In our samples one
phonon region was typically masked due to the presence of the
intense interference fringes. Therefore, the comparison of the
sheets with different nitrogen concentration was a difficult task.
However, in few of the samples where the fringes were not so
strong, certain unique features in one phonon have been
observed and discussed in some detail below.
The expanded FTIR spectra of nitrogen un-doped (a) and
doped (b) in one phonon region are shown in Fig. 3. In Fig.
3(a), it shows a dominant peak at 1332 cm 1 and a shoulder at
1109 cm 1. The intensity of the shoulder at 1109 cm 1
increased significantly (Fig. 3b) in the sheets doped with
nitrogen. It appears that these two bands are related with the N
impurity centers in the sheets. In natural diamond, the 1332
cm 1 band has been assigned to isolated nitrogen [9]. Some
aliphatic chemical compounds [10] also show the C –N stretch
mode at 1332 cm 1. However, the band at 1109 cm 1 has not
been identified in diamond (CVD or natural). As the peak
becomes stronger with nitrogen doping, it is likely that this
Intensity (a.u.)
(b)
(a)
1100
1200
1300
1400
-1
Wavenumber (cm )
Fig. 3. Expanded FTIR spectra of nitrogen undoped (a) and doped (b) films in
one phonon region.
peak is nitrogen related. In order to check if the frequency of
C –N stretching mode lies around 1332 cm 1 and reduces due
to the increase in number of nitrogen atoms around carbon, abinitio calculations were carried out using GAMESS program.
The Hartree Fock calculations were carried out using 6-31G (d,
p) basis set. Each geometry was first optimized to obtain
minimum energy configuration and then all the normal modes
were obtained for the optimized geometry. The calculated
frequencies (Table 2) are not only from pure C – N stretching
vibrations but also have the contributions from other stretching
and bending vibrations. It is observed that with increase in the
number N atoms of the parent molecule N –CH3, the C – N
stretching frequency decreases appreciably. Also, the environment around C atom shifts the value of C –N frequency. Thus
the peak at 1109 cm 1 may be either due to the different
environment around C –N-species, or larger number of N
atoms bonded to C – N configuration. There is no evidence of
N –O, O – H or C – O transitions indicating that O impurities are
negligible in the sheets.
Our ab-initio calculations were comparable with literature
values and experimental data obtained in this study (Table 2).
Nitrogen related centers have been reported in natural and
CVD diamond by Colling et al. [11]. Major peaks
corresponding to N centers, in one phonon region, are at
1280 cm 1 (a pair of nearest neighbor substitutional nitrogen
atoms known as A center), 1175 cm 1 (due to an aggregate of
6– 8 N atoms known as B center), 1135 and 1344 cm 1 (due to
isolated nitrogen). A few other nitrogen related defects due to
platelets are also reported by Davies [12].
Table 2
C – N stretching frequencies for various geometries obtained from ab-initio
calculation
204
Fig. 4. Typical FTIR spectra in CH stretch region of undoped diamond sheets
grown at 120 Torr.
Two phonon absorptions are intrinsic to pure diamond. The
IR bands observed in the experimental spectrum (Fig. 2) at
1978, 2028 and 2161 cm 1 are due to C – C coupling. Similar
bands in CVD diamond have also been reported by other
workers [13].
As the absorption in the CH stretching region is very
significant, the three phonon region (2700 to 3100 cm 1) has
been studied in detail. The vibrations of the carbon –hydrogen
bonds in N –CH3 group are also expected in this region. The
spectrum in this region is superposition of the CH vibrations
from spm CHn , where m, n = 1, 2, 3. Sharp absorption peaks in
the CH region indicate strongly bonded hydrogen. The
experimental spectrum was fitted using Gaussian peaks after
background correction. Analysis of the spectra was carried out
on the basis of peak assignment available in the literature [14].
The half width and amplitude of the bands were taken as fitting
parameters. The frequency of the vibration shifted slightly for
C –H bonds in different local environments, which enabled us
to differentiate between various CHn groups. Since each
individual spm CHn configuration is characterized by a specific
IR absorption peak, one can use these spectral peaks to analyze
the relative hybridization of the carbon atoms.
The typical IR spectra in CH stretch region of un-doped
diamond sheets grown at 120 Torr are shown in Fig. 4. The
deconvolution of the CH band shows mainly seven peaks in
this region (Table 1). Closer examination of the CH stretch
region shows that diamond sheets deposited at 120 Torr also
contain hydrogen bonded to sp2 carbon (3025 cm1). This is,
however, considerably small compared to hydrogen bonded to
sp3 carbon and is not prominent in 20 Torr sample. The
dominant absorption at 2850 and 2920 cm 1 are indicative of
the symmetric and asymmetric stretch bands of sp3 CH2 group,
respectively. The bands at 2880 and 2960 cm 1, on the other
hand, are due to symmetric and asymmetric stretch modes of
sp3 bonded CH3 groups. The fitting procedure in the CH region
was repeated for spectrum of samples intentionally doped with
nitrogen. In addition to symmetric and asymmetric stretch
bands of sp3 CH2,3 groups, peaks at 2820 appeared in the
fitting procedure for nitrogen doped samples. The peak at 2820
is related to N center and there are CH stretch vibrations of the
C –H bond in the N –CH3 group. This band has also been
assigned to H terminated diamond (111) surface [15]. However,
it is observed that the intensity of the band at 2820 cm 1
increased in the samples deposited with N2 doping. Moreover,
the stretch mode of C – N bond observed at 1332 cm 1 in this
experiment also supports the above statement. A less intense
peak appears at 2832 cm 1 which is identified as oxygen
related which appears due to the oxygen impurity.
The H contents in CVD diamond sheets were measured
using comparison and integration method. These methods has
been established for the calculation of H content in the
diamond sheets and the details of this techniques are reported
elsewhere [16]. The H contents were also calculated using
ERDA in few of the samples, which is a primary technique for
quantifying and depth profiling of hydrogen in thin films. It has
been used to depth profile, bonded and unbonded hydrogen in
diamond thin films. The results of the H content in the CVD
diamond sheets obtained using different methods are presented
in Table 3. The significant observation is that there is a
substantial difference in the value of H content obtained using
comparison method and ERDA. The difference widens in the
sheets grown at higher pressures. As discussed earlier, ERDA
measures the total (bonded and unbonded) H content whereas
the IR absorption gives only the bonded H content. This
implies that there is significantly high concentration of
unbonded H in diamond sheets. It mainly lies at interstitials,
grain boundaries, internal voids, vacancy clusters etc. and can
be easily dislocated from the diamond samples upon heating.
The unbonded H has been recognized as a possible cause for
the change in electrical conductivity of the CVD diamond
sheets upon annealing [17].
The integrated absorbance of each band can be used to
estimate the hydrogen concentration in a particular mode as the
Table 3
Hydrogen content in the sheets calculated from different methods
Sample
Pressure (Torr)
Comparison method (at.%)
Integration method (at.%)
ERDA (at.%)
1
2
3
4
5
6
20
40
60
80
120
140
0.051
0.032
0.055
0.056
0.064
0.077
0.072
0.025
0.188
0.258
0.329
0.595
0.18
0.24
0.36
0.64
–
–
205
Table 4
Variation of proportionality constant with different modes
Intensity (a.u.)
1. 20 Torr
2. 40 Torr
3. 60 Torr
4. 80 Torr
5. 120 Torr
Frequency (cm 1)
A n 1019
1
2
3
4
5
1300
1400
1500
1600
1700
1800
Raman shift (cm-1)
Fig. 5. Raman spectra recorded in the full range of one set of diamond sheets
grown with different deposition pressures ( P d).
concentration of the oscillating species is proportional to the
integrated intensity of the absorption band. The total hydrogen
concentration (Hc) is given by the following equation:
Z
aðxÞd ðxÞ
Hc ¼ An
dx
where A n is the proportionality factor and a(x) is the
absorption coefficient at frequency x. A n is proportional to
the inverse of oscillator strength. The difference in the value of
hydrogen concentration therefore signifies the importance of
the uncertainty in the value of A n. According to this
measurement by the integration method, the oscillator strength
should decrease as the H content in the sheets increases. The
details of the above equation and calculations are discussed in
detail and published elsewhere [16].
Raman spectra were recorded on all the sheets which were
deposited at various conditions in order to estimate the nondiamond content in the sheets. Spectra were recorded with the
use of Ar+ laser (k = 514.5 nm and power 50 mW ) in the range
1200 –1700 cm 1 with a step size of 2 cm 1. However, to
resolve the fine structure of Raman diamond line, in the range
of 1280 –1380 cm 1, a step size of 0.5 cm 1 was used. The
spectra recorded in the full range of one set of sheets that were
Fig. 6. FTIR spectra in CH stretch region of nitrogen doped diamond sheets
grown at 20 Torr.
2820
2.045
2832
4.075
2850
1.085
2880
1.263
2920
6.899
2960
1.215
3020
4.645
grown with different deposition pressures ( P d), are shown in
Fig. 5. A sharp Raman line at 1332.5 cm 1 was observed in all
the sheets, which implies that all the sheets contain good
crystalline diamond [18]. A broad band corresponding to
the non-diamond impurities [19] also appeared at around
1500 cm 1 in the sheets. It was observed that the nondiamond carbon components in the sheets increased systematically with the increase of P d. It can be explained with the
rate of etching and growth of sp2 and sp3 bonded carbon in
the CVD environment. Atomic H is known to etch H from
the growing surface and to stabilize sp3 precursors for further
growth. Therefore, a continuous and sufficient supply of
impingement flux density of atomic hydrogen (IFDH) on the
growing surface is required in CVD process for depositing
diamond. Insufficient IFDH will leave a few C – H bonds
intact and hence hydrogen may get incorporated in the
diamond lattice. Inside the diamond lattice, the termination of
sp3 carbon bond with H may give rise to the sp2 bonding in
the surrounding environment. This implies that the bonded H
in the diamond films, which is a result of insufficient IFDH,
will give rise to more hydrogenated carbon impurities. In
HFCVD process, the dissociation of H2 into H atoms takes
place in the vicinity of the hot filament. The recombination of
H atoms occurs during its movement towards the substrate.
The recombination rate will increase because the mean free
path of H atoms decreases with increase in P d. This will
result in lower IFDH at higher P d and higher non-diamond
carbon as well as higher H concentration in the sheets. This
hypothesis, although a simplified picture of a complex
situation, can be used to explain the well observed correlation
between the H content and the non-diamond carbon
impurities reported by Windischmann et al. [20]. However,
we have observed higher graphitic content in the intentionally
nitrogen doped samples (20 Torr) despite of its sharp and
well defined diamond peak (Fig. 6). The poor quality of the
Fig. 7. Variation of the values of A n for different diamond sheets grown at
different deposition pressures.
206
nitrogen doped CVD diamond films is also reported by
Adhikari et al. [21].
It is known that Raman scattering coefficient is significantly
higher for graphite than diamond [19]. Thus very small
concentration of sp2 phase could be easily detected using
Raman spectroscopy. This also implies that the sheets deposited
without nitrogen at low pressure (< 60 Torr) where sp2 bonded
phase is detected to be very weak are of high purity. On the
other hand, sheets grown at high pressure show substantial
concentration of sp2 phase of carbon. At the same time a strong
diamond line is also evident in high pressure grown sheets.
These sheets can, therefore, be treated as a composite material;
a mixture of sp2 and sp3 phase of carbon [22].
As mentioned earlier, the calculation of the value of the
proportionality constant A n for different modes of vibrations is
quite difficult unless there is an independent technique which
can calculate the exact number of H bonded in different modes.
Other workers have also emphasized this difficulty in CVD
diamond [22]. Using the value of H content calculated for
different modes using comparison method, we have estimated
the value of A n for various C – H modes in CVD diamond.
These values are listed in Table 4. It is observed that A n
decreased with increase of H content in the sheets. Fig. 7 shows
the variation of the values of A n for different sheets which were
grown at different deposition pressures. Inset shows the change
in intensity ratio of nondiamond (I nd) and diamond (I d)
Raman band with deposition pressure. It can be seen that there
is an order of magnitude difference in the value of A n for the
sheets with high and low values of I nd / I d. This implies that by
knowing the quality of the films in terms of non-diamond/
diamond intensity ratio from Raman spectra, one can accordingly choose a suitable value of A n to calculate the bonded
hydrogen in the film. The variation in the values of A n is
expected by considering the fact that the environment
surrounding each C – H mode varies significantly with deposition pressure and the structure of the sheets.
4. Conclusion
Nitrogen doped diamond films exhibited a significant
change in grain size and film morphology. FTIR spectroscopy
has proven to be a useful tool for the analysis of nitrogen and
hydrogen in CVD diamond films. Nitrogen induced films
showed several additional peaks which were confirmed as
nitrogen related using ab-initio calculations. Proportionality
constant (A n ) was calculated for different modes of C – H
vibrations and a correlation was made between the hydrogen
content and A n. Hydrogen concentration (IR active) of the
samples deposited at various conditions has been calculated
with comparison and integration method and a comparison of
these values with ERDA measurements confirmed the presence
of large number of unbound hydrogen present in the CVD
diamond sample.
Acknowledgements
FCT, Portugal and DST, India is highly acknowledged for
funding this work. Authors would like to thank Dr. S.
Kanagaraj for proof reading the paper.
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