Melting and defect generation in chemical vapor deposited diamond due to
irradiation with 100 MeV Au+ and Ag+ ions
D.S. Misra a,*, Umesh Palnitkar a, Pawan K. Tyagi a, M.K. Singh a, E. Titus a,
D.K. Avasthi b, P. Vasa c, P. Ayyub c
a
Department of Physics, Indian Institute of Technology, Mumbai 400 076, India
b
Nuclear Science Center, New Delhi 100 067, India
c
Tata Institute of Fundamental Research, Mumbai 400 005, India
Abstract
Diamond windows prepared using hot filament chemical vapor deposition (CVD) were irradiated with Au and Ag ions of energy 100 and
130 MeV, respectively with fluence in the range 1010 to 3 1013 ions/cm2. Scanning electron microscopy (SEM) images showed substantial
damage at the surface of the windows irradiated with Au+ of energy 100 MeV and fluence of 3 1013 ions/cm2. At some locations on the diamond
windows deposited at 16 kPa, the surface layer appeared to have melted and flowed like a liquid exposing small crystallites underneath. Raman
spectra of windows after irradiation showed i) a decrease in the intensity of the one-phonon line at 1332 cm 1 along with an increase in its half
width; ii) substantial reduction in the intensity of the graphite G band; and, iii) the appearance of a new band at 667 cm 1. The band at 667 cm 1
did not have a corresponding Stokes line and was therefore identified as a photoluminescence (PL) band. The ion induced damage and localized
melting of CVD diamond windows are discussed in terms of the thermal spike model.
Keywords: Chemical vapor deposition (CVD); Ion bombardment; Diamond
1. Introduction
High energy and low energy ion beam irradiation effects on
the structure and optical properties of natural as well as
chemical vapor deposited (CVD) diamond have been studied
by several groups [1 – 10]. Such experiments are expected to
show a variety of interesting phenomena. For instance, one
may observe a phase transformation from diamond to graphite
or amorphous carbon, or new defect levels may arise in the
band gap of diamond. Furthermore, the recent attempt to
fabricate particle detectors [11 –15] using CVD diamond has
made it pertinent to study the influence of high-energy heavy
ion irradiation of CVD diamond films. The transfer of ion
energy to a lattice proceeds via electronic energy loss (S e) and
nuclear energy loss (S n). The ratio of energy (E) of the ions to
its atomic mass unit (amu) plays a significant role in deciding
the contributions of S e and S n to the damage produced in the
material upon irradiation [16]. In the present case, the ratio E/
amu is very close to unity (MeV/amu) and in this regime it is
expected that S e will play a dominant role and the contribution
of S n would be insignificant.
A number of groups have reported the effects of highenergy heavy ion irradiation in diamond. Hunn et al. [17]
reported the appearance of new Raman lines in the range
1400 – 1700 cm 1 and a photoluminescence (PL) band in 600–
800 cm 1 region upon irradiating natural diamond with 4 MeV
carbon ions. These were attributed to the effect of the nuclear
energy loss of the ion beam. Prawer et al. [18] reported a
dramatic increase in the conductivity by as much as seven
orders of magnitude in CVD diamond as well as natural type
IIa diamond when irradiated with 300 kV Xe and 100 kV C
ions. These results were explained in terms of graphitic
channels created in the samples due to irradiation. The
production of nanodiamond by high energy irradiation of
graphite with a beam of 350 MeV Kr ions was reported by
Daulton et al.[19], while a structural transformation of CVD
122
diamond and amorphous carbon (a-C) upon irradiation with
5 kV Ar ions was reported by Reinke et al. [20]. A few groups
[21,22] have already fabricated high energy particle detectors
using CVD diamond films.
One of the best methods for the structural characterization of
diamond and related phases is Raman spectroscopy. The
Raman spectrum of crystalline cubic (C) diamond consists of
a sharp and intense line at 1332 cm 1 representing a triply
degenerate vibrational mode at the Brillouin zone center. The
Raman spectra of graphite show a Raman line at 1580 cm 1 (G
band). In tetrahedrally bonded amorphous carbon (a-C) and
diamond like carbon the phonon selection rules are relaxed due
to the structural disorder and one observes a broad band in the
1300 –1700 cm 1 region. Raman spectra in all above cases
correspond to the phonon density of states. However, the peak
positions and the line widths of the Raman bands show
considerable variation depending on the deposition parameters
and the sp3/sp2 ratio. Raman spectra are also modified due to
the phonon confinement effects in nanocrystalline carbon and a
few studies indicate a shift in the position of the one-phonon
Raman line towards lower wave number [23].
In the present study, we have irradiated self-supporting
windows (thickness å 14 Am) of CVD diamond with 100 MeV
Au+ and 130 MeV Ag+ beams. The ion fluence in the high
energy beam is two orders of magnitude smaller than in the low
energy beams and the dominant damage in the case of high
energy beams is expected to arise from electronic energy loss
as discussed later. The damage to the lattice due to S n occurs
near the stopping range of the beam and is mostly localized.
Detailed Raman studies of the irradiated samples reveal an
interesting phase transformation leading to the origin of a
defect level in the band gap, which is also confirmed by
photoluminescence measurements.
2. Experimental details
The diamond films were deposited with 0.8% CH4 and
balance H2 on Si substrates maintained at 890 -C using hot
filament chemical vapor deposition (HFCVD) apparatus
whose details are described elsewhere [24]. In an earlier
work [24] we have demonstrated the reproducible nature of
the process and deposited diamond films under different
deposition pressure to produce the films with varying nondiamond content characterized using Raman spectroscopy.
The films deposited in the pressure range 2.6 –7.9 kPa contain
dominantly sp3 bonded carbon having cubic diamond
structure with grainsize ranging from 5 to 10 Am. Raman
spectra of these films show very little or no non-diamond
peak. The films deposited in the pressure range 10.6 –16 kPa,
on the other hand, are dominated by cauliflower morphology
with grainsizes in the range 2– 5 Am. The Raman spectra of
these films show a dominant Raman band at 1500 cm 1
indicating that the non-diamond impurities are substantially
high [25]. We now report a study of the influence of highenergy heavy ion irradiation on both types of specimen. The
irradiation of the self-supported diamond windows (thickness
å 14 Am) was carried out using 100 MeV Au+ and 130 MeV
Ag+ beams under a vacuum 1.33 10 4 Pa at 300 K. The ion
beam irradiation time was varied to generate the fluences of
1012, 3 1012, 1013 and 3 1013 ions/cm2. The ion beam was
scanned over an area of 10 mm 10 mm aperture to give a
uniform irradiation profile. Raman spectra were recorded on
the virgin as well as irradiated samples in the back-scattering
geometry using a Spex 1403 monochromator and conventional photon counting system. The excitation source was the
488 nm line from an Ar+ laser. Photoluminescence (PL)
emission spectra were obtained at room temperature using a
Spex 1681 spectrometer with excitation wavelengths of
300 nm and 370 nm from a Xe arc lamp.
3. Results and discussion
The electronic energy loss and the nuclear energy loss for
100 MeV Au+ and 130 MeV Ag+ beams were calculated using
the Transport of Ions in Matter (TRIM) program (figure not
shown here) and the density of CVD diamond was taken to be
the same as natural diamond. At the surface of the diamond
sample, the calculated electronic energy loss S e for the Au+ and
Ag+ beams are, respectively, 1.9 104 and 1.6 104 eV/nm.
As the ions penetrate deeper the value decreases to zero near
the end of the range. The values of S n, on the other hand are
very small at the surface and increase to a maximum near the
stopping range. The TRIM calculations showed that the
electronic energy loss is more than the nuclear energy loss
on the surface of the sheet.
The Raman spectra of the virgin and irradiated samples are
shown in Fig. 1(a) – (b). Fig. 1(a) shows the Raman spectra of
the virgin diamond window deposited at 2.7 kPa, as well as
irradiated with 130 MeV Ag+ ions and fluence 1013 ions/cm2.
Expectedly, Raman spectra of virgin sample deposited at
2.7 kPa contain only the Raman line at 1332 cm 1
corresponding to the one-phonon mode in crystalline diamond,
and lines from non-diamond phases are absent. Upon
irradiation, the intensity of the one-phonon line decreases,
but in contrast to the results reported [26] with low energy ion
irradiation, there is no indication of the appearance of lines
from non-diamond phases. Fig. 1(b) shows the Raman spectra
of the virgin and Au+ irradiated diamond window, deposited at
16 kPa. The energy of the beam and fluence were 100 MeV and
3 1013 ions/cm2 respectively. As mentioned earlier, these
samples contain high concentration of non-diamond impurities
in their virgin state. Raman spectra of the virgin samples show
a weak Raman line at 1332 cm 1 followed by a band in the
region 1500 – 1700 cm 1 which corresponds to the nondiamond impurities. The irradiation with Au+ diminishes the
one-phonon Raman line at 1332 cm 1 almost entirely in these
samples but the notable result is the disappearance of the nondiamond impurity band above 1500 cm 1 in both the samples.
In addition, an intense band appears at 667 cm 1 in the
samples irradiated with Au+ ions. As this band is not present in
the anti-Stokes part of the Raman spectra (not shown here), it
may be due to photoluminescence from a radiation-induced
defect level in the samples. It is also interesting to note that this
band is not present in the Raman spectra of Ag+ irradiated
123
(a)
(b)
Unirradiated
Intensity (arb.units)
Intensity (arb.units)
Unirradiated
Au irradiated
(3x1013) ions/cm2
Ag irradiated
1350
1500
Raman Shift
1650
(cm-1)
500
1000
1500
Raman Shift (cm-1)
Fig. 1. Raman spectra of the virgin and irradiated diamond windows: (a) with Ag+ ions of fluence 1013 ions/cm2, (b) with Au+ ions of fluence 3 1013 ions/cm2. The
windows are deposited at different pressure as discussed in the text. (b) shows appearance of a peak at 667 cmsuper 1 assigned to a defect level due to irradiation.
Also please note the disappearance of non-diamond band (~ 1500 cm 1) in the irradiated windows (b).
470
611
496
509
Intensity (arb.units)
421
412
sample. This could be explained using the projected range of
both Ag+ ions and Au+ ions. The projected range of 100 MeV
Au+ ions and 130 MeV Ag+ ions are 11.4 Am and 14 Am in
diamond respectively. As the thickness of the diamond sheets
in the present study is 14 Am, it is likely that the Ag+ ions are
transmitted whereas the Au+ ions are stopped in the windows.
Since the number of defects produced peaks near the end of the
range, it may be expected that in such a case the Au+ ions
create much more crystal defects than the Ag ions.
To confirm these findings, photoluminescence spectra were
recorded on the windows irradiated with Ag+ and Au+ ions.
Fig. 2(a) and (b) show the PL emission spectra from the
samples irradiated with Ag+ and Au+ beams with fluence 1013
and 3 1013 ions/cm2, respectively. The spectra of the sample
Au irradiated
(3x1013)
Ag irradiated (1013)
300
450
600
Wavelength (nm)
Fig. 2. Photoluminescence spectra of Au+ and Ag+ ions irradiated diamond
windows. The fluence of Au+ and Ag+ ions was 3 1013 and 1013 ions/cm2,
respectively. In Au+ ions irradiated sample a PL peak at 509 nm corresponding
to 667 cm 1 band in Raman spectra can be seen. This is absent in Ag + ions
irradiated windows.
irradiated with Ag+ beam show higher intensities at 300 and
600 nm due to scattering of the incident wavelength. The
spectrum of the sample irradiated with Au+ shows a much
higher intensity of defect level induced PL than from that
irradiated with Ag+ beam. The Raman peak at 667 cm 1, in the
spectra of Au+ irradiated samples, corresponds to 509
nanometer in the PL spectrum. No evidence is observed for
the same defect in Ag+ irradiated sample in either the Raman or
the PL spectra. This implies that the Raman peak at 667 cm 1
is typical of the samples irradiated with Au+ ions. Other PL
peaks were observed at 412 nm, 421 nm, and 496 nm in Ag+
and Au+ irradiated sheets.
The appearance of a broad band around 667 cm1 in all
samples irradiated with Au+ ions of fluence of 3 1013 ions/cm2
is another important result. The band is not visible in the antiStokes side and therefore could be confirmed to be a PL band.
The presence of this band is confirmed in the PL spectra of the
diamond sheets irradiated with fluence 3 1013 ions/cm2 as
shown in Fig. 2(a). This corresponds to 509 nm peak in PL
spectrum. Interestingly the Raman spectra of Ag+ irradiated
sheets do not show any PL band at 667 cm 1 and the same is
confirmed in the PL spectra where no band is observed at 509 nm
(Fig. 2(b)). The appearance of this peak may be related to the
atomic displacement caused due to the ionic collisions between
the Au+ ions and C atoms of the diamond lattice. In case of Ag+
ions the displacement may be smaller due to the lower mass of
Ag+ ions. Alternately, the range of Ag+ ion beam may be higher
than the thickness of the window due to lower mass of Ag+. This
may result in complete transmission of Ag+ from window and
the end-of-range damage, where maximum displacement
occurs, might be absent in Ag+ irradiated samples.
Besides the peak at 509 nm other major peaks as observed
in the PL spectra of Au+ irradiated windows are 412 nm,
421 nm, 470 nm and 496 nm. The strongest PL peak is
observed at 470 nm which is popularly known as TR12-center
[27]. This center is observed in all types of irradiated diamond
samples including natural as well as CVD diamonds. It is
particularly strong in the samples irradiated with high-energy
heavy ions and has been ascribed to the defect involving
124
vacancies and interstitial carbon atoms [28]. The peak at 412
nm corresponds to N3-center which has three N atoms
associated with it. The configuration of the defect involves
two N atoms forming a bond and the third one bonding to
carbon atom leaving a vacancy in the center [29]. The peaks
observed at 421 nm and 496 nm are again ascribed to the effect
of irradiation in diamond [30]. The peak at 496 nm is observed
with irradiation and is popularly known as H4-center. The
possible configuration of this defect is an aggregate of N atoms
bound to a vacancy. A possible explanation for the observation
of all these centers could be the creation of vacancies near the
surface due to the electronic energy loss of the ions in the
diamond windows due to irradiation. The impurities (N, H and
O etc.) present in CVD diamond then could be activated due to
irradiation producing the above centers.
Scanning electron microscopy (SEM) micrographs of the
diamond windows before irradiation are shown in Fig. 3(a) –
(b). SEM studies show that the sheets grown at lower
deposition pressure contain sharp faceted crystallites, whereas
blunt spherical crystallites are seen in the sheets grown at
higher deposition pressure. The non-diamond impurities
increase in the windows grown at higher pressure, resulting
in loss of sharpness of the facets. A comprehensive set of SEM
images of the Au+ and Ag+ irradiated diamond windows is
shown in Fig. 4(a) –(h). These images show the morphology of
the grains after irradiation. Fig. 4(a) and (b) show the images of
2.7 kPa deposited sheets irradiated with Ag+ of 130 MeV and
fluence 1013 ions/cm2. Fig. 4(c) and (d) are the images of
diamond windows deposited at 16 kPa irradiated with Au+ ions
of 100 MeV and fluence 1013 ions/cm2. The morphology of the
grains is dominantly (110) oriented in Fig. 4(a) and (b) which
are at different magnifications. The damage due to irradiation
with the ion beam is clearly visible on the facets of the grains
seen in Fig. 4(b) where smooth facets developed roughness. We
know that the electronic energy loss is highest at the surface
and may result into the local damage of the sp3 bonded carbon
network. The effect of irradiation with Au+ ions is similar as
shown in Fig. 4(c) and (d). The most striking damage is
obtained at the surface of diamond windows irradiated with
Au+ of fluence of 3 1013 ions/cm2, as is evident in Fig. 4 (e) –
(g). The damage is substantial and it can be clearly noted that at
some locations on the sheets the surface layer of the diamond
sheet has melted and flowed like a liquid exposing small
crystallites underneath the surface (refer to Fig. 4(g)). To the
best of our knowledge this extent of damage has not been
reported elsewhere. In the 8.0 kPa deposited sheets, however
the damage caused due to irradiation of Au+ beam is not as
high as shown in Fig. 4(h).
The high-energy heavy ions, such as used in the present
work, lose their energy dominantly due to electronic energy loss.
With the impact of the ion with the material a large
concentration of secondary electrons in the energy range few
eV to 1 keV are generated. Initially these are confined in a small
volume of about 1 nm3. The fast electrons move away quickly
but the slower ones transfer their energy via the electron phonon
coupling to the lattice. The energy transfer takes place over a
time scale of 10 12 s. The value of the coupling parameter plays
a significant role in deciding the energy transfer to the lattice. In
several metallic and semi-conducting materials [31,32] this
parameter has been used as a guide for the track generation due
to heavy ion irradiation. Due to exceptionally high thermal
conductivity of diamond films we do not see any track formation
with heavy ion irradiation. But the melting and recrystallization
of the diamond windows are evident in the films deposited at
16.0 kPa with Au+ ions of fluence 3 1013 ions/cm2 (Fig. 4(g)).
The melting temperature of diamond is in excess of 4300 K
as shown by the groups working at Oak Ridge and Brookhaven
National laboratories recently. As the ion beam heating may not
be very significant, the melting of diamond lattice may
therefore be a result of the thermal spike generated due to the
secondary electrons produced in the heavy ion’s passage in the
sample. The temperature profile in the case of a spike model
assuming the linear heating source confined at the axis of the
beam can be obtained by solving the following equations [33]:
dTe
d
dTe
Ke ðTe Þ dTe
qCe ðTe Þ
Ke ðTe Þ
¼
þ
dr
r
dt
dr
dr
gðTe T Þ þ AðrÞ
qC ðT Þ
dT
d
dT
K ðT Þ dT
¼
K ðT Þ
gðTe T Þ
þ
dt
dr
dr
r dr
ð1Þ
ð2Þ
where C e, C and K e, K are the specific heat and thermal
conductivity for the electronic system and lattice, respectively,q is
the material density, g is the electron–atom coupling, A(r) is the
energy brought on the electronic system in a time considerably
Fig. 3. SEM images of unirradiated diamond windows deposited at (a) 2.7 kPa and (b) 16.0 kPa.
125
Fig. 4. SEM images of irradiated diamond windows (a) and (b) 2.7 kPa deposited window irradiated with Ag+ ions of fluence 1013 ions/cm2; (c) and (d) 16.0 kPa
deposited windows irradiated with Au+ ions of fluence 1013 ions/cm2; (e) – (g) are images of 16.0 kPa deposited windows irradiated with Au+ ions of fluence 3 1013
ions/cm2 and (h) is 8.0 kPa deposited windows irradiated with Au+ ions of fluence 3 1013 ions/cm2.
less than the electronic thermalization time and r is the radius in
cylindrical geometry with the heavy ion path as the axis.
The above equations can be simplified to estimate the
temperature in the path of ion beam as T = (dE / dx)e / kR2i C i q i ;
where T is temperature, (dE / dx)e is electronic energy loss, R i is
the radius of the cylindrical spike region assumed to be 1.5 nm,
C i is the specific heat of diamond and q i is the density. Our
calculations using this simple formula predict a temperature of
6200 K using Au+ ions. For Xe and Kr ions the same
expression results in the temperatures in excess of 7557 K and
6343 K as shown by Didyk et al. [34]. The temperature during
the thermal spike may therefore reach in excess of melting
temperature of diamond and results in the melting inside the
spike region. This would occur only for a very short time
(10 11 to 10 12 s) and the material would recrystallize within
the spike before new ion arrives. However, as the diamond
structure is metastable it may not recrystallize as diamond and
other forms of carbon might be produced.
126
We must emphasize however that we have clear evidence of
melting only in the sheets deposited at 16.0 kPa. These sheets
contain large concentration of non-diamond impurities at the
grain boundaries as well as on the surface which may act as
catalyst for the melting [35]. The radius of the cylindrical spike
region for non-diamond (amorphous) carbon is estimated to be
about 1.5 nm for the Au+ ions [35]. The track radius depends
upon E/amu ratio and may be lower than this value. As the
thermal conductivity of non-diamond impurities may not be as
high as diamond and non-diamond impurities also contain free
volume, voids etc., the effective size of spike region may
increase and the material remains in molten state for sometime
to cause the macroscopic flow. The energy deposited by beam
in the samples may also be significant and add to the above.
The above also implies that the diamond sheets deposited at
2.7 kPa will not melt during heavy ion irradiation as these do not
contain non-diamond impurities. We have some preliminary
evidence for the same in Fig. 4(h) showing an SEM image of a
diamond sheet deposited at 8.0 kPa. As this sheet contains lesser
concentration of non-diamond impurities the melting of the sheet
is not seen under Au+ irradiation. It is an interesting proposition
to work with and we propose to conduct experiments in this
direction. It is also evident from Fig. 4(b), which depicts the
image of the sheet irradiated with Ag+ ions that the irradiation
results in local generation of defects. The facets of cubic
diamond can be seen developing rough features on the surface.
4. Conclusion
The diamond windows grown by HFCVD technique were
irradiated with Au+ and Ag+ ions of energy 100 and 130 MeV,
respectively. The fluence of the ions was varied in the range
1010 to 3 1013 ions/cm2. The main conclusions of the above
study are as follows:
The SEM images show the substantial damage at the surface
of the windows when these were irradiated with a fluence of
3 1013 ions/cm2 Au+ ions of energy 100 MeV. At some
locations on the diamond windows, the surface layer has
melted and flowed like a liquid exposing small crystallites
underneath. For the fluence below 3 1013 ions/cm2, the
damage is not as extensive. On the good quality facets the ion
irradiation results in the development of roughness typical of
non-diamond impurities. The Raman spectra show three
remarkable changes in the sheets after irradiation: 1) a decrease
in the intensity of the one-phonon line at 1332 cm 1 along
with an increase in its half width; 2) substantial reduction in the
intensity of the graphite G band; and 3) the appearance of a
new band at 667 cm 1. The band at 667 cm 1 does not have
corresponding Stokes line and is therefore identified as a PL
band. The PL spectra of the irradiated windows clearly show
the emergence of a new PL band at 509 nm corresponding to
667 cm 1 band observed in the Raman spectra.
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