JOURNAL OF APPLIED PHYSICS 106, 073305 共2009兲
Atomic hydrogen interactions with amorphous carbon thin films
Bhavin N. Jariwala,1 Cristian V. Ciobanu,2,a兲 and Sumit Agarwal1,a兲
1
Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401, USA
Division of Engineering, Colorado School of Mines, Golden, Colorado 80401, USA
2
共Received 7 May 2009; accepted 6 September 2009; published online 9 October 2009兲
The atomic-scale interactions of H atoms with hydrogenated amorphous carbon 共a-C:H兲 films were
identified using molecular dynamics 共MD兲 simulations and experiments based on surface
characterization tools. Realistic a-C:H films developed using MD simulations were impinged with
H atoms with a kinetic energy corresponding to a temperature of 700 K. The specific chemical
reactions of the H atoms with the a-C:H surface were identified through a detailed analysis of the
MD trajectories. The MD simulations showed that hydrogenation occurs primarily at the sp2 sites
and converts them to sp3-hybridized C atoms. Depending on the hybridization of the next-nearest
neighbor, a dangling bond may or may not be created. The hydrogenation reaction is highly
exothermic, ⬎2.5 eV, and proceeds with a negligible activation energy barrier via a mechanism
similar to Eley–Rideal. In certain cases hydrogenation may also cleave a C–C bond. The reaction
events observed through MD simulations are consistent with the surface characterization of
D-exposed a-C:H films using Raman spectroscopy, spectroscopic ellipsometry, and in situ
attenuated total reflection Fourier-transform infrared spectroscopy. © 2009 American Institute of
Physics. 关doi:10.1063/1.3238305兴
I. INTRODUCTION
Hydrogenated amorphous carbon 共a-C:H兲 is a versatile
material with a vast number of applications, such as a wide
band gap semiconductor for high-performance electronic
devices,1 protective coatings for infrared 共IR兲 optical elements and magnetic storage disks,2–4 inert biocompatible
coatings,5,6 and as cold cathode emitters for flat-panel
displays.7–10 a-C:H films are generally deposited over large
areas at relatively low substrate temperatures by plasmaenhanced chemical vapor deposition 共PECVD兲 using hydrocarbon feed gases such as CH4 or C2H2 diluted in H2 or an
inert gas, such as Ar.11–14 The properties of these films depend on the sp3-to-sp2 hybridization ratio and the H
content.15–17 Depending on the composition, these films
demonstrate different characteristics, such as hardness, electrical resistivity, optical transparency, and chemical inertness.
Films with hardness and resistivity similar to that of diamond
are referred to as diamondlike carbon 共DLC兲.18 On the other
hand, films with high hardness, high sp2 content, and a low
optical band gap are referred to as hard graphitic a-C:H.17
During PECVD, a variety of radicals are generated in the
plasma, which impinge onto the growing film’s surface, and
react, leading to film growth. The interaction of H atoms
with the a-C:H surface is particularly important in determining both the sp3-to-sp2 hybridization ratio and the H content
in the film.18,19 The elementary steps identified during H interaction with carbon surfaces are hydrogenation, H abstraction, and H-induced chemical erosion.20–22 Hydrogenation of
a-C:H due to atomic hydrogen impingement is considered an
important step in the chemical erosion of a-C:H films.23 This
reaction is also proposed to be a primary step in the etching
a兲
Authors to whom correspondence should be addressed. Electronic addresses: cciobanu@mines.edu and sagarwal@mines.edu.
0021-8979/2009/106共7兲/073305/9/$25.00
of graphitic nuclei in the low-pressure synthesis of diamond,
as well as DLC.20,24 The interaction of H atoms with carboncased films has been studied both during the postexposure of
the films to H atoms and during film growth from hydrocarbon plasmas with the feed gas heavily diluted with H2.
Briefly, in previous studies, using a combination of Raman
spectroscopy, x-ray photoelectron spectroscopy, and atomic
force microscopy, Mikami et al.25 observed that an increase
in the H2 flow rate increased the sp3 fraction and the disorder
during the deposition of DLC films by radio-frequency 共rf兲
magnetron sputtering. Vietzke et al. studied the H-induced
chemical erosion of C atoms from a-C:H films using mass
spectrometry. These authors reported the formation of CH3
radicals along with a wide range of higher hydrocarbons due
to the reaction of thermal H atoms with a-C:H films.26 Kuppers and co-workers19,27–29 used high-resolution electron energy loss spectroscopy to study the above interactions occurring on model C:H surfaces. These authors showed that
hydrogenation of sp- and sp2-hybridized CH groups led to
the formation of sp3-hybridized CHx 共x = 2 , 3兲 groups19,23 and
suggested that hydrogenation eventually leads to erosion of
surface C atoms by desorption of volatile CxHy
molecules.23,28 The three primary interactions of H with
a-C:H—hydrogenation, H abstraction, and chemical
erosion—were proposed to synergistically lead to preferential graphite erosion, which is assumed to be the controlling
step in the low-pressure synthesis of diamond or DLC
films.20 Biener and co-workers prepared model a-C:H films
using ethane ions at 160 eV and impinged these films with H
atoms. The authors hypothesized a certain activation energy
barrier for the hydrogenation reaction and proposed that it
proceeds via an Eley–Rideal mechanism.19,21 Two different
cross sections—1.3and 4.5 Å2—were reported for the H
atom interaction with C:H films deposited in two different
106, 073305-1
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Jariwala, Ciobanu, and Agarwal
reactors.22 The reason for the difference in the cross sections
was attributed to the uncertainty in estimating the H flux
from the H atom sources.22
Interactions of H atoms with graphite and diamond surfaces have also been reported in literature. Dementjev and
Petukhov30 used Auger electron spectroscopy to demonstrate
that interaction of H atoms with a graphite surface at 300 K
changes the hybridization of the C atoms from sp2 to sp3.
Balooch and Olander31 observed that over a temperature
range of 400–2200 K, H atoms etch graphite to form CH4
and C2H2.
It is well known that hydrocarbon gases heavily diluted
in hydrogen are used as a precursor for the growth of microcrystalline and nanocrystalline diamond at relatively high
temperatures.32–34 However unlike the formation of nanocrystalline Ge 共Ref. 35兲 and Si 共Ref. 36兲 due to H2 dilution of
GeH4 and SiH4, respectively, the mechanism for nanodiamond formation due to H2 dilution is not well understood.
This is because C can bond in sp3- as well as sp2-hybridized
states unlike Ge and Si. Angus and co-workers20,24 were the
first to propose that during chemical vapor deposition of diamond, atomic H etches the graphitic nuclei preferentially
over the diamondlike phase. Furthermore, it has been proposed that abstraction of H from a surface CHx group 共x = 1,
2, or 3兲 by atomic H is a key step that provides a bonding site
for the incoming hydrocarbon species and thus, controls diamond growth.20 However, the detailed mechanisms of the set
of reactions that lead to such structural transformations and
their energetics have not been identified.
In this article, we focus on the interactions of H atoms
with a-C:H surfaces using classical molecular dynamics
共MD兲 simulations and experiments based on surface diagnostic tools such as Raman spectroscopy, spectroscopic ellipsometry 共SE兲, and surface IR spectroscopy. Using MD, we
have created realistic a-C:H films, developed a method to
characterize the sp3-to-sp2 hybridization ratios in the films,
and visualized the reaction pathways of H with the a-C:H
surface. We further show that the reaction pathways observed
in the MD simulations are consistent with our experimental
measurements. Specifically, we observe that hydrogenation
of the a-C:H surface by thermal H atoms proceeds at the sp2
sites via a highly exothermic mechanism similar to Eley–
Rideal in that it has a negligible activation energy barrier.
This reaction generally converts the sp2 C atoms to sp3 C
atoms.
This article is organized as follows. In Sec. II, we
present the computational details for creating realistic a-C:H
films and describe the MD simulations to visualize the trajectories of H atoms impinging on these films. In Sec. III, we
describe the experimental approach used to deposit hard
a-C:H films and the in situ and ex situ surface diagnostic
techniques used to identify the hydrogenation/deuteration reaction occurring at the surface of these films. The computational and experimental results are presented and discussed
in Sec. IV. Finally, our key conclusions are summarized in
the Sec. V.
II. MOLECULAR DYNAMICS SIMULATIONS
A. Interatomic potential
The modified extended Brenner 共mXB兲 potential, developed by Sbraccia et al.,37 was employed for the MD simulations. The mXB potential is an improvement over the extended Brenner 共XB兲 potential,38 which is a modified version
of the Brenner potential for hydrocarbons39 that includes
treatment of the interactions for Si along with C and H. The
original Brenner potential for hydrocarbons39 has certain
shortcomings, such as a short cutoff distance for C–C interaction, which results in an underestimation of sp3 fractions in
a-C:H 共Refs. 40 and 41兲 and the omission of the torsional
potential term for hindered rotation around C–C bonds.42 The
XB potential also has a few limitations, such as the presence
of false minima in the potential energy curves of ethane with
respect to the C–C distance.37 The mXB potential eliminates
many of the drawbacks of other Brenner-type potentials by
modifying the parameters of the XB potential. The mXB
potential employs longer C–C interaction cutoff radii to
eliminate the underestimation of sp3 C–C bonds during
simulations.37 The range of local interactions is slightly increased by reducing the first neighbor cutoff radii and increasing the second neighbor cutoff radii for C–C, C–H, and
H–H interactions.37 The modification of the cutoff radii as
well as changes in several other parameters eliminate the
occurrence of overcoordinated C atoms and lead to the correct potential energy curves for ethane.37 Thus far, the mXB
potential has been successfully used to study the chemisorption of hydrocarbon molecules on Si surfaces.37,43–45
B. Realistic a-C:H films
Using MD, plasma-deposited thin films can be simulated
by repeated impingement of radicals onto a substrate, provided the plasma composition is well characterized. Employing this procedure, Maroudas and co-workers36,46–48 created
amorphous hydrogenated silicon 共a-Si:H兲 films by repeated
impingement of SiH3 radicals onto a Si共001兲 substrate. SiH3
was previously experimentally identified as the primary radical for the deposition of device-quality a-Si:H films from a
SiH4 plasma.49 Stuart et al.50 used a Brenner-type empirical
potential to simulate the structure of amorphous carbon films
and characterized the model films in terms of ring size distributions, concluding that the sp2 content was generally
larger than in experiments. Krstic et al., Reinhold et al., and
Stuart et al. also studied the interactions of H and D with
amorphous carbon films,51 analyzing in detail the energy and
angle spectra of sputtered particles,52,53 hydrogen
reflection,54 and methane production55 at the impact of deuterium and or hydrogen with amorphous carbon. Neyts et
al.56,57 performed MD simulations of a-C:H films deposited
in a C2H2 / Ar expanding thermal plasma by impinging the
substrate with the hydrocarbon radicals experimentally detected by Benedikt et al.58,59 This procedure is, however, not
feasible for simulating a-C:H films deposited using other
plasma sources due to an insufficient knowledge of the relative flux of the large number of radicals that are generated in
these hydrocarbon discharges. Therefore, in this study, we
have developed a stepwise procedure for creating realistic
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073305-3
J. Appl. Phys. 106, 073305 共2009兲
Jariwala, Ciobanu, and Agarwal
(a)
(b)
(c)
(d)
FIG. 1. 共Color online兲 Side view of the simulation cell showing the transformation from a diamond crystal to an a-C:H film. 共a兲 The diamond crystal with
periodic boundary conditions in the x and y directions. 共b兲 The diamond crystal after removing a certain fraction of C atoms while keeping the bottom five
layers fixed to simulate the underlying bulk substrate. 共c兲 The amorphized film obtained after thermalization at 1000 K for 5 ps followed by relaxation using
a conjugate-gradient energy-minimization algorithm. 共d兲 The final a-C:H film after hydrogenation of the amorphized structure and thermalization at 700 K for
25 ps.
a-C:H films from crystalline diamond. The procedure involves tuning of the bulk film density, randomization of the
atomic coordinates, and hydrogenation of the film such that
the resulting structure and composition are similar to plasmadeposited a-C:H films. The stepwise procedure is as follows.
共a兲
共b兲
共c兲
共d兲
We begin with a crystalline, unreconstructed diamond
共100兲 slab with periodic boundary conditions in the
共100兲 plane, as shown in Fig. 1共a兲. This slab initially
has 1176 C atoms with in-plane dimensions of 17.6
⫻ 17.6 Å2. The bottom five layers of this slab are fixed
to simulate the underlying bulk substrate.
The bulk density of diamond 共3.5 g / cm3兲 is much
higher than the experimentally reported range of densities for a-C:H films 共1.6– 2.2 g / cm3兲. Therefore, a
certain number of C atoms, calculated based on the
desired density of the a-C:H film, are randomly removed from the slab, as shown in Fig. 1共b兲. The remaining atoms, excluding the bottom five layers, are
then given random displacements of at most one-third
of a C–C bond length in diamond.
The slab is thermalized at 1000 K for 5 ps to allow the
atoms to rearrange and relax. The resulting film is then
relaxed to its local minimum energy configuration using a conjugate-gradient algorithm. The film obtained
through this procedure is shown in Fig. 1共c兲.
During this procedure, several carbon atoms change
their hybridization from sp3 to sp2, but the film also
contains a large number of dangling bonds. To determine the location of these dangling bonds, the neighbors of each C atom are first identified based on a
maximum bond length of 1.7 Å, which is ⬃10%
greater than the sp3-sp3 bond length of 1.54 Å. The
dangling bonds are then identified based on the number
of nearest neighbors; a bond length cutoff of 1.50 Å is
used to differentiate between sp2- and sp3-hybridized C
atoms, which would have different number of neighbors for complete coordination. These dangling bonds
are passivated by H atoms and the film structure is
again relaxed. The somewhat arbitrary cutoff of 1.50
Å, determined by numerical experimentation, leads to a
sufficient number of H atoms in the film that is representative of hard a-C:H. This relaxed, hydrogenated,
and amorphized film is again thermalized at 700 K for
25 ps. Some overhydrogenation of the film is observed
due to the choice made for the bond length cutoff, but
the extra H atoms desorb as stable hydrogen molecules
during the thermalization process. The total energy of
the film becomes almost constant at 700 K after 25 ps.
The film obtained using the above procedure is shown
in Fig. 1共d兲.
The a-C:H films generated by the procedure above were
characterized for the sp2-to-sp3 hybridization ratio and the H
content by developing a more accurate set of criteria than the
one discussed above. Since C atoms can be sp3-, sp2-, or
sp-hybridized, and can also have some dangling bonds left
unpassivated after the procedure described above, it would
be incorrect to identify the type of hybridization of a C atom
based solely on its number of nearest neighbors. Moreover,
the bond lengths 关see Fig. 2共a兲兴 and bond angles have a wide
distribution in amorphous materials, making it difficult to
identify the type of bonding configuration. We have therefore
used bond energy cutoffs to differentiate between a C–C
double bond and a single bond. Figure 2共b兲 shows the bond
energy distribution for the film shown in Fig. 1共d兲, which
gives two sharp peaks centered at ⫺4.91 and ⫺3.64 eV with
minimal overlap because the difference in the bond energy of
a C–C double bond 共C = C in ethylene is ⫺6.37 eV兲 and a
single bond 共C–C in ethane is ⫺3.56 eV兲 is a few eV. The
cutoff energy to differentiate a single bond from a double
bond was chosen to be ⫺4.15 eV, which is the energy that
corresponds to the minimum in the bond energy distribution
in Fig. 2共b兲. If the C–C bond energy is below ⫺4.15 eV, the
bond is characterized as a double bond; otherwise it is a
single bond. Since no bond energies below ⫺8 eV are obtained for the final relaxed and thermalized film, there are no
sp-hybridized C atoms in the film 共for reference, the C ⬅ C
bond energy in acetylene is ⫺8.4 eV兲. We also did not observe any C atoms with more than four neighbors, indicating
no over coordination in the film. To determine the hybridization state of a given C atom, the number of neighbors and the
bond energies of the C–C neighbors are used. If the number
of nearest neighbors is 4, the C atom is sp3-hybridized. If the
number of neighbors for the central C atom is 3, it may have
1 C and 2 H, or 2 C and 1 H, or 3 C atoms as neighbors. In
any case, if one of the carbon neighbors has a double bond
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073305-4
J. Appl. Phys. 106, 073305 共2009兲
Jariwala, Ciobanu, and Agarwal
120
(a)
100
1.43 Å
Counts
80
60
1.57 Å
40
20
0
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Bond length (Å)
120
(b) -4.91 eV
100
FIG. 3. Schematic of the capacitively coupled plasma reactor with the in
situ ATR-FTIR spectroscopy setup.
Counts
80
60
graphitic a-C:H film deposited at high bias voltages using
PECVD.18,61
Figure 2共c兲 shows the total C–C radial distribution function 共RDF兲 calculated for the a-C:H film created using the
above mentioned procedure. The RDF clearly indicates the
amorphous nature of the film, as evidenced by the absence of
peaks beyond ⬃2.65 Å. The first peak at 1.43 Å shows a
large number of conjugated sp2-hybridized C atoms in the
film, indicating that a hard graphitic film was created using
this procedure. The shoulder at 1.54 Å shows that there are
also sp3-hybridized C atoms present in the film, but their
fraction is smaller than that of sp2-hybridized C atoms.
-3.64 eV
40
20
0
-6
5
-5
-4
-3
-2
Bond energy (eV)
(c)
4
g(r)
3
C. H interactions with a-C:H
2
1
0
-1
0
1
2
3
4
r (Å)
5
6
7
8
FIG. 2. 共a兲 Bond length distribution for the a-C:H film generated using MD
simulations. The length distribution was fitted with two Gaussians with a
significant overlap. 共b兲 Bond energy distribution for the a-C:H film in Fig.
1共d兲 fitted using two Gaussians centered at ⫺4.91 and ⫺3.64 eV, representing the sp2 and sp3 fractions, respectively. The dotted line represents the
bond energy cutoff at ⫺4.15 eV for the sp2- and sp3-hybridized states. 共c兲
Total carbon-carbon RDF, g共r兲, for the a-C:H film in Fig. 1共d兲. The g共r兲 was
calculated for the amorphous region and all the H atoms along with the
bottom five fixed layers were excluded.
with the central C atom 共indicated by the bond energy兲, the
central C atom is sp2-hybridized with no dangling bond. If
all C–C neighbors have single bonds, the central atom is
sp3-hybridized with a dangling bond. Similar logic using
bond energies of C–C neighbors is implemented to determine
the hybridization of C atom with a different number of
neighbors 关for details, refer to the chart in supplementary
information, Fig. 1-SI 共Ref. 60兲兴. The density of the final film
is 2.2 g / cm3 with 33% atomic H, which is representative of
experimentally deposited, hard a-C:H films.18 The sp3 fraction in the film is 20%, which is characteristic of a hard
H atoms were repeatedly impinged at random locations
on to the computationally generated a-C:H film from a distance of 2 Å above the topmost atom of the surface with
initial velocities directed normal to the surface and corresponding to a temperature of 700 K. The entire system was
equilibrated for 4 ps between any two consecutive H atom
impingement events. The time step used for the MD simulations was 0.1 fs and the atomic trajectories of the H atoms
were visualized by taking snapshots of the atomic configurations at 1 fs intervals. To calculate the energetics of the
reactions observed, the MD configurations before and after
the reaction were relaxed using a conjugate-gradient algorithm.
III. EXPERIMENTAL
The a-C:H films used for the experiments were deposited in a PECVD reactor. The reactor consists of a stainlesssteel vacuum chamber with a base pressure of ⬃10−7 Torr
共see Fig. 3兲. The plasma is generated between two asymmetric parallel plates: The plate spacing was set at 4 cm using a
linear-shift manipulator. Radio frequency power at 13.56
MHz was applied to the upper electrode via an in-house-built
L-type matching network. Hard a-C:H films were deposited
at room temperature on a Si substrate on the powered electrode using a 1:1 mixture of CH4 and Ar 关50 SCCM 共SCCM
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073305-5
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denotes cubic centimeter per minute at STP兲 each兴 as the
precursor gases, a chamber pressure of 50 mTorr, and 100 W
power to the plasma source.
The SE 共Woollam M-44兲 data were analyzed by regression analysis using a three-layer optical model with a surface
roughness/a-C:H film/native SiO2 structure on a 1 mm thick
c-Si substrate. The a-C:H film layer was fitted with the
Tauc–Lorentz 共TL兲 model for amorphous semiconductors,
proposed by Jellison and Modine.62 The surface roughness
layer was fitted using the effective medium approximation
with 50% void fraction and 50% of the underlying a-C:H
film layer.63 The Tauc band gap, film thickness, surface
roughness, and optical constants for the a-C:H films were
obtained using this model. Micro-Raman spectroscopy 共LabRamHR兲 was performed using a 532 nm diode laser in a
backscattering geometry. To observe the hydrogenation/
deuteration reaction, the as-deposited film was placed on the
grounded electrode and exposed to D atoms from a D2
plasma for 1 h at a substrate temperature of 150 ° C, a pressure of 100 mTorr, and 10 W rf power to the plasma. The
plasma was operated at a very low power density to minimize the bombardment of the surface with energetic ions.
After the D2 plasma exposure, the a-C:H film was again
characterized using ex situ SE and micro-Raman spectroscopy.
For in situ characterization of H共D兲-atom interactions
with a-C:H, an isotope-exchange experiment was conducted
using real-time, in situ attenuated total reflection Fouriertransform infrared 共ATR-FTIR兲 spectroscopy, as shown in
Fig. 3. Briefly, in this setup, an IR beam from a FTIR spectrometer was directed using a series of mirrors and lenses
into the flat face of a 1 ⫻ 10⫻ 50 mm3 trapezoidal internal
reflection crystal 共IRC兲 with its short faces beveled at 45°.
The IR beam was internally reflected on the bottom and top
faces since the angle of incidence was greater than the critical angle for total internal reflection. In this geometry, there
were 25 reflections on each flat face. The IR beam exiting
from the opposite beveled edge of the IRC was collimated
using a KBr lens and was directed by a series of mirrors onto
a mercury cadmium telluride A 共MCT-A兲 detector. We used
KBr windows on both sides of the beam path, which are
transparent in the IR up to 400 cm−1. However, the spectral
range was limited by the ZnSe IRC, which is transparent in
the IR up to ⬃700 cm−1. Therefore, the change in absorbance due to the stretching vibrations of CHx and CDx 共x
= 1 , 2 , 3兲 groups at ⬃3000 and ⬃2000 cm−1, respectively,
were observed simultaneously. We used ZnSe as the IRC
because its refractive index, ⬃2.2, is closely matched to that
of hard a-C:H. For the range of refractive indices typical of
dense a-C:H films 共1.7–2.3兲,64 the reflectivity at the films’
interface with ZnSe is very low and therefore, total internal
reflection occurs at the vacuum-film interface and not the
crystal-film interface. Thus, the IR beam passes through the
film twice for each reflection, greatly enhancing the signalto-noise ratio. A hard a-C:H film was deposited at room temperature on the powered electrode on a ZnSe IRC as well as
on a Si wafer using a CH4 plasma operated at a flow rate of
25 SCCM for CH4, a chamber pressure of 100 mTorr, and 25
W rf power to the plasma source. Compared to the condi-
tions used for the deposition of films for ex situ characterization, a relatively high pressure and low power were used to
reduce ion bombardment of the surface. Films deposited at
conditions identical to the ones used for the ex situ studies
described above led to highly conductive a-C:H films, which
rendered the ZnSe IRC opaque in the IR due to free electron
absorption.65 To confirm that the film was indeed hard
a-C:H, the one deposited on the Si wafer was characterized
using SE 共see Sec. IV B兲. The ZnSe crystal with the asdeposited film was then placed on the grounded electrode
and a background IR spectrum was collected. The film was
exposed to a D2 plasma at a substrate temperature of 150 ° C,
a pressure of 100 mTorr, and 10 W rf power to the plasma
source. We used D2 instead of H2 to clearly distinguish between H atoms already in the film from the D atoms impinging from the gas phase since the stretching vibrations of CHx
and CDx are well separated in the IR. After every 20 s of D2
plasma exposure, an IR difference spectrum was collected,
which represents changes in the film composition due to interactions with D atoms. It should be pointed out that a small
fraction of the D atoms and D2 molecules impinge onto the
surface with energies much higher than in the MD simulations as the neutral species undergo elastic and inelastic collisions with high-energy ions in the plasma sheath at the
grounded electrode. However, this small fraction of highenergy neutrals and ions should not drastically alter the comparison with the MD simulations where the H atoms are
incident at thermal energies.
IV. RESULTS AND DISCUSSION
A. MD simulations
A large fraction of the H atoms that were impinged on
the a-C:H film reflected back into the gas phase. Most of the
remaining atoms were adsorbed on sp2 sites on the surface,
hydrogenating the film. Three such hydrogenation reactions
are shown in Fig. 4, where the sp2-hybridized C atoms are
shown as dark gray spheres 共red, in the online version兲, the
sp3-hybridized C atoms are light gray 共blue兲, and the hydrogen atoms are shown as smaller spheres. The haloed small
sphere represents the incoming H atom. Reactions 共a兲 and 共b兲
in Fig. 4 resulted in a change in the hybridization state of the
adsorption site from sp2 to sp3 with complete passivation of
the carbon atom, whereas in reaction 共c兲 such conversion was
accompanied by the formation of a dangling bond.
In the reaction shown in Fig. 4共a兲, the incoming H atom
bonded to the sp2-hybridized C atom is labeled 1 共C1兲. Before the attachment of H to C1, the central, sp2-hybridized
atom C2 was bonded with three sp2-hybridized neighbors
C1, C3, and C4, similar to a C atom in graphene. The binding of H to C1 changed its hybridization from sp2 to sp3, as
reflected by the change in the C1–C2 bond energy from
⫺5.03 to ⫺3.69 eV, and by an increase in the C1–C2 bond
length from 1.42 to 1.54 Å. The hybridization state of C2 did
not change, and no dangling bond was created on this atom.
This was inferred from the energies 共⫺4.94 and ⫺4.92 eV兲
and the lengths 共1.40 and 1.39 Å兲 of the C2–C3 and C2–C4
bonds, which were consistent with a central sp2-hybridized
atom 共C2兲 bonded with two sp2 neighbors, similar to a C
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073305-6
J. Appl. Phys. 106, 073305 共2009兲
Jariwala, Ciobanu, and Agarwal
attached to C1, it changed its hybridization from sp2 to sp3.
The bond energy of the C1–C2 and C1–C3 bonds changed
from ⫺5.28 to ⫺3.88 eV and from ⫺5.02 to ⫺3.69 eV,
respectively; the bond lengths increased from 1.40 to 1.54 Å
for C1–C2, and from 1.42 to 1.54 Å for C1–C3. Thus, we
concluded that C1–C2 and C2–C3 have become single bonds
upon the adsorption of H. However, no dangling bond was
created on C3 as the double-bond character of C3–C4 and
C3–C6 increased from 33% to 50%. Similarly, the doublebond character for the C2–C5 bond increased, eliminating a
possible dangling bond on C2. The exothermic energy of this
reaction was 3.06 eV.
In the reaction shown in Fig. 4共c兲, the incoming H atom
attaches to the sp2-hybridized atom C1. After the H atom
attaches to C1, the bond between C1 and C2 transforms into
a single bond, as shown by the bond energy change from
⫺6.27 to ⫺3.87 eV and the bond length increase from 1.32
to 1.54 Å. Since the C2–C3 bond is also a single bond, a
dangling bond is created at C2. After the reaction, C2 becomes sp3-hybridized with three neighbors 共2 C and 1 H兲
and a dangling bond; this was concluded from analyzing the
variations in bond energies and bond lengths. The exothermicity of the reaction obtained was 2.49 eV. In all of the
above reactions, the H atom attaches to the sp2 site instantaneously 共within 1 fs兲 without being thermally equilibrated
with the surface before attachment. Hence, the reaction proceeds via an Eley–Rideal type of mechanism. For all reactions in Fig. 4, the activation energy barrier from the MD
analysis was negligible. The formation of a dangling bond on
a nearest neighbor of the adsorption site depends on the hybridization state of the first- and second-order neighbors of
that site. In all the above reactions, the sp2 site converts to a
sp3 upon hydrogenation which leads to an increase in the sp3
fraction in the film; this is consistent with our experimental
observations of increased sp3 fraction after D2 plasma exposure of an as-deposited a-C:H film.
Another interesting reaction observed is shown in Fig. 5,
in which the attachment of a H atom leads to the splitting of
a surface cluster. The incoming H atom attaches to C1, which
has a 共first-order兲 neighbor sp3-hybridized 共C2兲 and another
(a)
1
1
∆E= 2.28 eV
2
2
4
3
4
3
6
(b)
1
6
3
2
3
1
4 ∆E= 3.06 eV
4
2
5
5
(c)
1
1
2
∆E= 2.49 eV
2
3
3
FIG. 4. 共Color online兲 Relaxed atomic configurations for three hydrogenation reactions at different sp2 sites on the a-C:H film’s surface. For clarity,
only atoms in the vicinity of the reaction event are shown. The cyan spheres
indicate sp3-hybridized C atoms, the red spheres indicate sp2-hybridized C
atoms, and the relatively small blue spheres indicate H atoms. The incoming
H atom is haloed. The C atom, to which the incoming H atom was attached,
and its first and second neighbors are labeled numerically. The exothermic
energy of each reaction event is also indicated. No dangling bonds were
created in reactions 共a兲 and 共b兲 due to a change in the bond energies of the
first and second neighbors. In reaction 共c兲 a dangling bond was created.
atom in benzene. In other words, the double-bond character
of the C2–C3 and C2–C4 bonds increased from 33% to 50%
resulting in the elimination of the possible dangling bond.66
The exothermic energy of the hydrogenation reaction 共a兲 was
2.28 eV.
In the reaction shown in Fig. 4共b兲, the incoming H atom
also attached to the sp2-hybridized atom C1 with two
nearest-neighbor sp2 atoms, C2 and C3. The next-nearest
neighbors of C1 were also sp2-hybridized. After the H atom
6
1 3
4
7
2
6
5
1
4
3
7
2
5
1
4.26 eV
2.94 eV
4
3
6
7
2
5
FIG. 5. 共Color online兲 Potential energy surface illustrating H atom attachment to a sp2 C with two C and one H neighbors. The color coding and numbering
is the same as in Fig. 4. The exothermic energy for both steps also indicated. In the first step, the H atom attaches to a sp2-hybridized atom C1 and a dangling
bond is created on its first neighboring sp2-hybridized atom C3 and both C atoms are converted to sp3 C atoms. In the second step, a neighboring C–C bond
between C1 to which H atom gets attached and its first neighbor C2 is broken and C atoms C1 and C3 get converted back to sp2 C atoms.
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
J. Appl. Phys. 106, 073305 共2009兲
Jariwala, Ciobanu, and Agarwal
160
180
(a)
160
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
400
600
800
1000
1200
D (°)
180
y (°)
0
Wavelength (nm)
180
160
180
(b)
160
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
400
600
800
1000
1200
D (°)
sp2-hybridized 共C3兲. C1–C2 is a single bond and C1–C3 is a
pure double bond. The second neighbors of C1 and first
neighbors of C2 are the sp2-hybridized atoms C4 and C5.
The second neighbors of C1 and first neighbors of C3 are the
sp3-hybridized atoms C6 and C7. The hybridization state of
C1 transforms from sp2 to sp3 after the H attachment, leaving a dangling bond on C3 as now all the three C atoms
neighboring C3 are sp3-hybridized. The dangling bond on C3
is subsequently eliminated by the breaking of C1–C2 and the
ensuing conversion of C1, C2, and C3 to sp2. After the
breaking of the C1–C2 bond, both C2–C4 and C2–C5 become double bonds with energies of ⫺5.03 and ⫺5.26 eV,
respectively; the C1–C3 bond transforms into a pure double
bond 共bond energy of −6.23 eV兲 as the remaining two
neighbors of C3 共C6 and C7兲 are sp3-hybridized. The attachment of the H atom and the breaking of C1–C2 occur spontaneously, before the incoming H atom reaches thermal equilibrium with the surface. Hence, the entire reaction proceeds
via an Eley–Rideal type of mechanism. The activation barriers for the hydrogenation reaction and for the subsequent
breaking of the C1–C2 bond are negligible, and both of these
reactions are highly exothermic with exothermicities of 1.32
and 2.94 eV, respectively 共Fig. 5兲. While it appears that reactions such as those shown in Fig. 5 lead to an increase in
the number of sp2 sites, further attack by incoming H atoms
eventually converts such sites to sp3. Therefore, the overall
result of hydrogenation is again an increase in the sp3 fraction of the film due to H atoms attaching to sp2 C atoms and
converting them to sp3.
y (°)
073305-7
0
Wavelength (nm)
FIG. 6. Ellipsometry data and the corresponding fits for a hard a-C:H film
共a兲 before and 共b兲 after 1 h of D2 plasma exposure. The fits were obtained
using a three-layer model with the a-C:H layer described by the TL model
for amorphous semiconductors.
B. Surface diagnostics
G band 1540 cm
-1
Before
D band 1382 cm
Raman intensity
Figure 6 shows the SE data for the a-C:H film, collected
over the spectral range of 300–1300 nm, before and after D2
plasma exposure. A refractive index of 2.01 at 600 nm was
obtained for the as-deposited film, typical of hard a-C:H.67
After D2 plasma exposure, the Tauc band gap of the film
increased from 0.72 to 0.81 eV indicating that the sp3 fraction in the a-C:H film increased.68 This is consistent with our
MD simulations, which show hydrogenation of sp2 sites by
H atoms increases the overall sp3 content. Further hydrogenation leads to eventual etching of the film through desorption of stable hydrocarbon molecules. Furthermore, the surface roughness of the film increased from 1.2 to 3.5 nm,
while the film thickness decreased from 339 to 302 nm indicating H/D-induced etching of a-C:H films, which previously has been reported in literature.22
The micro-Raman spectra of the a-C:H film before and
after D2 exposure are shown in Fig. 7. The visible Raman
spectra of a-C:H films consist of the G band at ⬃1550 cm−1
and the D band at ⬃1350 cm−1.69 The G band corresponds
to the bond stretching vibrations of all pairs of sp2 atoms in
both rings and chains, and the D band corresponds to the
breathing mode of sp2 atoms in rings. Therefore, if the D
band is not observed, there are no sp2 ring structures in the
film. Ferrari and Robertson70 proposed a model that discusses in detail the relationship between the a-C:H film’s
band gap, the position of the G band, and the ratio of intensities of the D band, I共D兲, to that of the G band, I共G兲. Ac-
-1
G band 1535 cm
-1
1
After
D band 1325 cm
1000
1200
-1
1400
1600
1800
Raman Shift (cm-1)
FIG. 7. Raman spectra at 532 nm excitation for an a-C:H film before and
after exposure to a D2 plasma. The spectra were baseline corrected and
deconvoluted using Gaussian line shapes.
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073305-8
J. Appl. Phys. 106, 073305 共2009兲
Jariwala, Ciobanu, and Agarwal
C-H
bending
C-D
stretching
film, would be much greater than the increase due the
abstraction-passivation reaction.
C-H
stretching
Absorbance
V. SUMMARY AND CONCLUSIONS
0.01
1580 cm-1
(aromatic sp2 C=C)
1620 cm-1
(olefinic sp2 C=C)
1000
1500
2000
2500
3000
Wavenumber (cm-1)
3500
FIG. 8. IR difference spectra showing the change in absorbance in the CHx
and CDx stretching regions due to exposure of an a-C:H film to D atoms
from a D2 plasma.
cording to this model, as the band gap decreases, both the G
peak position as well as the I共D兲 / I共G兲 ratio decrease. The G
peak position and the I共D兲 / I共G兲 values for the a-C:H film
before D2 plasma exposure were 1540 cm−1 and 1.47, respectively, which decreased to 1535 cm−1 and 0.57, respectively, after D2 plasma exposure. Since both values decreased, it was concluded that the band gap of the film and
its sp3 content increased. Thus, Raman spectroscopy data
complement the band gaps obtained through SE, and are also
consistent with the MD simulations.
The film deposited for the in situ ATR-FTIR spectroscopy study was under slightly different deposition conditions, as discussed in Sec. III. This film had a band gap of
1.65 eV and a refractive index of 1.9 共at 600 nm兲: These
values are within the range reported for hard a-C:H.18,71 The
difference IR spectrum obtained after 30 min of D2 plasma
exposure is shown in Fig. 8. The decrease in absorbance in
the C–H stretching region around ⬃2900 cm−1 and the corresponding bending region around ⬃1400 cm−1 was clearly
identified in the spectrum.72 The peaks at 1620 and
1580 cm−1 were assigned to olefinic sp2 C = C and aromatic
sp2 C = C stretching vibrations, respectively.72 In the difference spectrum in Fig. 8, the large decrease in absorbance due
to the above mentioned peaks indicated chemical erosion of
the a-C:H film due to D2 plasma exposure: The D atoms
from the plasma insert into the C = C bonds, which eventually lead to the formation of stable molecules that desorb into
the gas phase. If there was only D insertion with no chemical
erosion, only the bands at 1620 and 1580 cm−1 would show
a decrease in absorbance with a shift in frequency in the CHx
region due to the formation of higher hydrides and a change
in hybridization from sp2 to sp3. This is further confirmed by
the increase in absorbance in the ⬃2100 cm−1, which was
primarily due to sp3 CDx 共x = 1 , 2 , 3兲 species.22 This increase
in absorbance could be interpreted as either due to the deuteration of the sp2 sites, which converts them to sp3 sites 共see
Sec. IV A兲, or due to the D-atom passivation of dangling
bonds that were created by abstraction of H by D from surface sp3 CHx 共x = 1 , 2 , 3兲 sites. Since it is known that rate of
hydrogenation is an order of magnitude greater than the rate
of abstraction,19 the increase in absorbance due to sp3 CDx
共x = 1 , 2 , 3兲 species, created by hydrogenation of the a-C:H
The specific interactions of H or D atoms with hard
a-C:H films were investigated using a combination of atomistic simulations and experiments based on surface spectroscopic techniques. To study these interactions, realistic
a-C:H films were created using MD simulations, which were
subsequently impinged with H atoms at thermal energies. We
developed a procedure to characterize the sp2 and sp3 hybridization states in the a-C:H film, which was based not just
on the number of nearest neighbors, as previously reported in
literature, but also on the bond energy. This facilitated a
more accurate description of the reaction pathways of H atoms with a-C:H surfaces due to the ability to identify the
creation of dangling bonds and the corresponding change in
hybridization. The analyses of the MD trajectories revealed
that the hydrogenation of hard a-C:H films occurs primarily
at the sp2 sites via an Eley–Rideal type of mechanism. The
hydrogenation reaction was highly exothermic with a negligible activation energy barrier. This fast mechanism therefore provides a way to transform the unsaturated sp2 sites on
the surface into sp3 CHx sites, which is followed by etching
of these sites. Thus, at high fluxes of atomic hydrogen that
are used to deposit microcrystalline and nanocrystalline diamond films,32 the Eley–Rideal-based hydrogenation reaction
provides an efficient way to eliminate the sp2 carbon in the
film. We observed that hydrogenation at the sp2 sites may or
may not create a dangling bond: This depended on the first
and second neighbors of the C atom to which H atom was
attached. The experimental data were consistent with the MD
simulations. Both the SE and Raman data indirectly showed
that the sp3 content of the a-C:H film increased upon exposure to D atoms. The ATR-FTIR data confirmed D addition
to the a-C:H film and subsequent etching. While the set of
reactions discussed herein show that interaction with H or D
atoms led to an overall change from sp2 to sp3 hybridization
in these films, the films remained amorphous and did not
evolve to diamondlike structures. Thus the interaction of H
atoms with a-C:H films is more complex than with amorphous films of other group IV elements such as Si and Ge,
which show increased crystalline order upon H exposure.35,73
We speculate that other reaction pathways must also exist
that eventually lead to ordering of these films at temperatures
much lower than those required for thermal annealing.
ACKNOWLEDGMENTS
This research was supported by the ACS-PRF 共Grant
No. 44934-G5兲, the NSF under Grant Nos. CMMI-0825592
and CBET-0846923, and the Renewable Energy MRSEC
program at the Colorado School of Mines 共NSF Grant No.
DMR-0820518兲. We also acknowledge the use of supercomputing resources provided by the Golden Energy Computing
Organization at Colorado School of Mines acquired with financial assistance from the NSF 共Grant No. CNS-0722415兲
and the National Renewable Energy Laboratory.
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