Diamond & Related Materials 14 (2005) 1348 – 1352
www.elsevier.com/locate/diamond
Synthesis and characterization of superhard
aluminum carbonitride thin films
A.L. Ji, L.B. Ma, C. Liu, C.R. Li, Z.X. CaoT
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, P.O. Box 603, Beijing 100080, China
Received 18 June 2004; received in revised form 11 January 2005; accepted 15 January 2005
Available online 9 March 2005
Abstract
Oxygen-free aluminum carbonitride thin films were grown on Si (100) substrates by reactive magnetron sputtering of Al target with the
gas mixture of Ar, CH4 and N2 as precursor. A complementary set of techniques including X-ray photoelectron spectroscopy, energydispersive X-ray spectrometry, X-ray diffraction, transmission electron microscopy and atomic force microscopy was employed for the
characterization of the deposit chemistry, structure as well as morphology. Film growth proceeds along the preferred [0001] direction with the
basal planes twisted because of the frustration in arranging the building blocks for aluminum carbonitrides. Under given conditions, the
deposits show a declining tendency of crystallization with increasing carbon content. Strong covalent bonding and structural disorder give the
film’s extreme mechanical rigidity: Berkovich hardness is over 27.0 GPa for all the deposits, and an extreme value of 53.4 GPa was measured
in Al47C20N33.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Aluminum carbonitride; Magnetron sputtering; Thin film; Hardness
1. Introduction
Covalent compounds comprising light elements have
many excellent properties such as wide band gap, superior
chemical and mechanical stability, due to the small atomic
radius and strong interatomic bonding. Among them,
carbides, nitrides and borides of silicon and aluminum have
attracted considerable interest from multidisciplinary
researchers and have found extensive applications in various
industry branches besides microelectronics and optoelectronics [1–3]. In recent years, the research work has been
shifting gradually, but steadily, from the binary materials to
ternary ones, since there is a much larger space of property
tuning in the latter. Generally, the atomic and/or electronic
structures of a ternary system can be tailored by composition modification, for instance, to obtain a particular quality.
Various deposition techniques have been exploited in the
T Corresponding author.
E-mail address: [email protected] (Z.X. Cao).
0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.diamond.2005.01.036
synthesis of ternary compounds of light elements such as B–
C–N and Si–C–N, and a wealth of excellent materials
properties have been manifested in such systems [4–8].
It can be anticipated that the ternary Al–C–N compounds
will show some individual characteristics as they contain
metal atoms. In fact, the complementary combination of
properties such as extreme electrical insulation, high
dielectric constant, readiness of metallization and so on
promises multifaceted applications of aluminum carbonitrides in electronics. The ternary Al–C–N system provides a
host of wide-gap materials as illustrated in Fig. 1, yet more
novel structures are waiting to be thoroughly explored.
Recently, there is an increasing effort targeted at Al–C–N
and also quaternary compounds containing these three
elements—novel structures and high-quality thin films [9].
In 2001, Kouvetakis’s group first succeeded in synthesizing
the cyanide crystal Al(CN)3 using a wet chemistry method
[10]. According to our first principles calculation, this new
aluminum carbonitride has a direct band gap of about 7.0
eV, as calibrated against the wurtzite AlN. Jiang et al.
prepared the Al–C–N thin films with reactive sputtering in
A.L. Ji et al. / Diamond & Related Materials 14 (2005) 1348–1352
1349
scopy (TEM, Tecnai F20) and X-ray diffraction (XRD,
Rigaku, D/max-2500 PC) using the Cu Ka irradiation were
employed for structural characterization. Surface morphology of the films was assessed by an atomic force microscope (AFM, Digital Instruments) operated in contact mode.
In doing the hardness measurement with a Berkovich
indenter (CSEM), the maximum load was carefully manipulated to restrict the indent depth within 80 nm, so that the
influence of the soft substrate is negligible.
3. Results and discussion
the hope that the incorporation of aluminum may stabilize
the hypothetical h-C3N4 structure [11,12]. In the present
work, we report the synthesis of oxygen-free, polycrystalline aluminum carbonitride films with reactive magnetron
sputtering. A complementary set of techniques has been
employed for the chemical and structural characterization of
the deposits. Nanoindentation reveals a maximum hardness
nearly at the lower limit for natural diamond.
2. Experimental
Films were grown on Si (100) substrates by reactive
magnetron sputtering of aluminum target (4 N purity) with
the gas mixture of argon, nitrogen and methane. Before
being transferred into the preparation chamber of a customdesigned magnetron film growth facility, which was preevacuated to a pressure ~10 4 Pa, the substrates had been
ultrasonically cleaned successively in alcohol and acetone
and rinsed with de-ionized water following 3-min etching in
1.0% HF solution to remove the native oxide. As a matter of
routine, the target would be presputtered under proposed
growth conditions for 1 h in order to obtain a required
metal-atom supply. In the course of film growth, the
substrate temperature was held at 300 8C, the pressure
maintained at 1.6 Pa, and the total gas flow rate kept at 4.0
sccm, of which 2.4 sccm reserved for argon. Composition
regulation for the deposits was realized through varying the
flow rates of nitrogen and methane—for the ternary deposits
concerned here, the flow ratio CH4/(CH4+N2) was varied
from 5% to 25% at a 5% interval.
The chemical composition of the deposits was determined by energy-dispersive X-ray spectrometry (EDX)
attached to a scanning electron microscope (Sirion, Fei)
excited with a primary electron energy of 10 KeV, together
with X-ray photoelectron spectroscopy (ESCA-lab) using
the Mg Ka line (hv=1253.6 eV), which was also exploited
to study the bonding states. Transmission electron micro-
140
Al
120
Intensity (arb. units)
Fig. 1. Ternary Al–C–N composition triangle showing the currently
available stoichiometric compounds.
Under given conditions, films of a thickness varying
about 1.3 Am, as determined from the cross-sectional
scanning electron micrographs, were obtained after a 2-h
growth. Oxygen-free aluminum carbonitride films were
obtained, as confirmed by the EDX spectrum shown in
Fig. 2. For the comparison study presented in this work, the
samples are AlN, Al49C13N38, Al50C15N35, Al47C20N33,
Al48C25N27 and Al4C3, for which the composition was
calculated from both the XPS and EDX spectral data. It is
noteworthy that, under given conditions, the content of Al in
the resulting aluminum carbonitride films remains at about
50%, while that of C and N competes. To infer the chemical
binding in the deposits from XPS spectral lines, we make a
close examination of the sample Al47C20N33 as an example.
In Fig. 3, the Al 2p line centered at 74.1 eV has a welldeveloped, featureless spectral profile; the full-width-athalf-maximum is 1.97 eV, which is a typical value for Al in
AlN and sapphire [13]. The C 1s line is much broader than
the N 1s line. It can be decomposed into three distinct peaks
with a center separation of 4.0 eV; for the N 1s line this is
however less than 2.5 eV. Moreover, the main peak in C 1s
line also has a lower weight in the whole profile than in the
case of N 1s line. Considering as well the fact that only one
Raman spectral line at about 660 cm 1, corresponding to the
Al–N E 22 mode [14], is detectable (not shown) in the
100
80
60
N
40
20
0
0
C
500
1000
1500
2000
Energy (eV)
Fig. 2. Typical EDX spectrum for the aluminum carbonitride deposit. No
trace of oxygen contamination can be recognized.
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A.L. Ji et al. / Diamond & Related Materials 14 (2005) 1348–1352
4000
Intensity (arb. units)
(a)
Al 2p
3500
3000
2500
2000
1500
1000
500
85
80
75
70
65
Binding Energy (eV)
4000
Intensity (arb. units)
(b)
3500
Fig. 4. High-resolution TEM micrograph taken on the sample Al50C15N35.
Ribbons of faults around the grains can be recognized.
C 1s
3000
2500
2000
1500
295
290
285
280
275
Binding Energy (eV)
7000
(c)
Intensity (arb. units)
6000
N 1s
5000
4000
may carry certain ambiguity due to the lack of established
reference data on this material.
Fig. 4 represents a typical TEM micrograph taken on the
sample Al50C15N35. The selected-area-electron-diffraction
pattern can be indexed for a hexagonal or rhombohedral
structure, given the electron beam impinging along a
slightly tilted [0001] direction. Fringes in the micrograph
originate in the (112̄0) prism planes with a spacing of
roughly 2.666 2. This corresponds to a lattice constant of
a=3.482, a plausible value for the homologous series of
aluminum carbonitrides [15,16]. More information about the
structure can be inferred from the XRD pattern (Fig. 5), in
which only one peak at 2hc368 can be assigned to the
(000l) reflection from the deposits, l denotes the number of
basal planes in a unit cell. It varies remarkably for different
allotropes of aluminum carbonitrides—for the rhombohedral Al10C3N6 it is as large as 30 [15]. With increasing C
3000
2000
1000
404
400
396
392
Binding Energy (eV)
Fig. 3. Profile of XPS spectral lines (a) Al 2p, (b) C 1s and (c) N 1s
measured in the sample Al47C20N33.
deposits of smaller C content, and this peak vanishes in
samples containing more than 15% carbon, one may
contemplate that the incorporated C atoms assume a rather
disordered arrangement, while the network of AlN is well
preserved. This point will become clearer from the
subsequent structural analysis. We will not go to the next
step further to designate the component peaks as the socalled sp2C–N or sp3C–N bonds, since such an assignment
Fig. 5. XRD pattern for the series of aluminum carbonitride films, with AlN
and Al4C3 at the two extremes. Only the (000l) reflection from basal planes
of the deposits exclusive of Al4C3 is evidenced.
A.L. Ji et al. / Diamond & Related Materials 14 (2005) 1348–1352
content, the position of the reflection shows slight variation
among them, but the intensity decreases steadily along with
an ever broadened profile: the full-width-at-half-maximum
changes from 0.138 for AlN to 0.438 for Al48C25N27. In the
case of aluminum carbide, no reflection from the deposit can
be detected. These results indicate that film growth proceeds
with a preferred [0001] orientation, and the packing of the
basal planes becomes more frustrated when more C atoms
are incorporated.
The crystallinity of the deposits also has its manifestation
in the morphology. In Fig. 6, AFM height images of the
aluminum carbonitrides are represented. Under given
growth conditions, the surface becomes smoother with
increasing C content, following a severe amorphization
tendency. In the sample Al49C 13N38, the surface is
composed of distinct grains with averaged dimension of
200 nm, root-mean-squared roughness amounts to 6.30 nm
(Fig. 6a). When the C carbon reaches 25% as in the sample
Al48C25N27 (Fig. 6d), the surface has an amorphous
appearance and roughness measures only 3.11 nm. The
sample in Fig. 6c has a very compact morphology, and a
minimum roughness 2.81 nm is measured.
Aluminum carbonitrides, together with AlN can be
categorized as superhard, as determined from the hardness
1351
Fig. 7. Berkovich hardness and elastic modulus as function of the film
composition.
measurement. Fig. 7 displays the Berkovich hardness as a
function of film composition, values given here are average
over 10 test points for each sample. All the measured values
are over 27.0 GPa, and the maximum is 53.4 GPa,
approaching the lower limit for natural diamond. For AlN
film, the measured Berkovich hardness is thus larger by
roughly a factor of 1.8 as measured on the (0002) plane of
the AlN crystal. From the curve, one sees that the hardness
maximizes at a C content of 20%. With more carbon
Fig. 6. AFM height images in a range of 2.02.0 Am, from (a) to (d), for samples Al49C13N38, Al50C15N35, Al47C20N33, and Al48C25N27.
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A.L. Ji et al. / Diamond & Related Materials 14 (2005) 1348–1352
incorporated the deposits then turn softer. By assuming a
Poisson ratio of 0.15, this is a reasonable value for materials
exhibiting a hardness over 27 GPa. The corresponding
Young’s modulus is also calculated. The maximum Young’s
modulus amounts to 328 GPa.
Growth of aluminum carbonitride films under given
conditions seems proceeding with the incorporation of
carbon into the network of Al–N blocks, which contains
only Al–N–Al–N–Al zigzagged chain of bond lengths
alternating between 1.89 and 1.92 2 [15], resulting in a
declining crystallinity with increasing C content. The
carbide films exhibit simply no reflection in the XRD
pattern. Carbon atoms are bonded to Al atoms in Al2C2 and
Al2C blocks; thus, the film preserves either hexagonal or
rhombohedral symmetry of the bulk, and the spacing of the
basal planes remains within the range for the homologous
aluminum carbonitride series [15,16]. Geometrical frustration in the arrangement of the blocks leads to a defect-rich
structure. Grains with the [0001] direction registration
preserved extend only a range of a few nanometers; they
are thus surrounded by ribbons of stacking faults (Fig. 4).
Sometimes, strain contrast of a characteristic length as large
as a few nanometers due to stress relaxation can be observed
in TEM micrographs (not shown). The inclusion of C atoms
severely distorts the primary AlN network. Consequently,
with more carbon incorporated, the deposits gradually lose
their crystallinity as concluded from both AFM and XRD
results. Another direct evidence of lost order is the absence
of the characteristic Raman peak at 660 cm 1 in samples of
large C content. The extreme hardness at Al47C20N33 can be
explained in terms of enhanced disorder in microstructure
vs. a diminishing crystallinity, thus an increasing fraction of
softer amorphous phase, in the deposits, as the C content is
increased. The hardness for aluminum carbide film is quite
small (1.7 GPa) due to its amorphism, as confirmed by the
complete absence of reflection in the XRD pattern (Fig. 5).
More effort should be dedicated to obtaining crystalline
Al4C3 samples for a systematic comparison study of
aluminum carbonitrides.
4. Summary
Aluminum carbonitride thin films of varied C contents
were deposited. No oxygen impurity in the deposits can be
resolved by the EDX spectrum. TEM and electron
diffraction revealed a polycrystalline nature of the deposits,
with the grains growing along the preferable [0001]
direction. For a C content up to 25%, spacing of the basal
planes in the aluminum carbonitride films suffers only a
slight variation, and it falls within the values for the bulk
homologous series. With the incorporation of more C atoms,
the deposits show a declining crystallinity under given
conditions. A maximum Berkovich hardness of 53.4 GPa
was measured in the sample Al47C20N33.
Acknowledgments
This work was supported by the National Science
Foundation of China Grant Numbers 60306009 and
10404034, the China State Key Projects of Basic Research
Grant Number 2002CB613500.
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