Interfacial structure of V2AlC thin films deposited on -sapphire

348 D. P. Sigumonrong et al. / Scripta Materialia 64 (2011) 347–350
described in detail elsewhere [16]. During deposition, the
substrate was kept at floating potential and heated to
750 °C. The film was grown to a thickness <strong>of</strong> approximately
1.5 lm (±5%), as determined by scanning electron
microscopy. The chemical composition <strong>of</strong> the film
was quantified using energy-dispersive X-ray analysis
with an EDAX Genesis 2000 system. The atomic concentration
<strong>of</strong> each element deviates by less than 4% from
the stoichiometric value. Hence, the composition <strong>of</strong> the
film reported here is close to stoichiometric. Phase analysis
was carried out by X-ray diffraction (XRD) with a
Bruker D5000 diffractometer in Bragg–Brentano geometry
and a Cu X-ray source. Figure 1 shows the X-ray
diffractogram <strong>of</strong> the <strong>V2AlC</strong> film grown in this work.
All major XRD peaks can be assigned to <strong>V2AlC</strong> except
for the peak indicated by the arrow with an integrated
intensity (area under the peak) <strong>of</strong> 1% <strong>of</strong> the total diffracted
intensity. Based on XRD, the <strong>V2AlC</strong> film studied
in this work is 99% phase pure.
Structural investigation <strong>of</strong> the interface was carried
out by TEM using a 300 kV Tecnai G 2 F30. Fast Fourier
transformation (FFT) was carried out using the
Digital Micrograph package. A low-magnification
cross-sectional image <strong>of</strong> the film is depicted in Figure 2a,
which reveals that the <strong>V2AlC</strong> film is polycrystalline. At
the interface, the orientation relationship between the
film and the substrate is determined by the corresponding
selected area diffraction (SAED) pattern (shown in
Fig. 2b) as follows:
– <strong>V2AlC</strong> (0 0 0 1)//a-Al2O3 ð1120Þ,
<strong>V2AlC</strong> ð2020Þ//a-Al2O3 ð3300Þ,
and <strong>V2AlC</strong> ½1210Š//a-Al2O3 [0001]
The small deviation <strong>of</strong> the two-zone axis is most likely
due to the finite size effect <strong>of</strong> the SAED aperture used.
The lattice mismatch between the film and substrate
can be calculated by f =(as af)/af; in which as and af
are the d-spacings <strong>of</strong> the Al2O3 and <strong>V2AlC</strong> planes which
are perpendicular to the interface [18]. The d-spacings <strong>of</strong>
Al2O3 ð3300Þ and <strong>V2AlC</strong> ð2020Þ employed are the
stress-free lattice parameters <strong>of</strong> <strong>V2AlC</strong> and Al2O3. The
interfacial mismatch is calculated to be 8.16%.
Figure 2(c) shows an HRTEM image obtained from
the interface, indicating (local) epitaxial growth <strong>of</strong>
<strong>V2AlC</strong> <strong>thin</strong> film on ð1120Þ a-Al2O3. In Figure 2(c),
the ð2110Þ plane <strong>of</strong> Al2O3 transfers continously into
Figure 1. X-ray diffractogram <strong>of</strong> the V 2AlC film grown on ð1120Þ
a-Al2O3 substrate at 750 °C.
the ð1013Þ plane <strong>of</strong> <strong>V2AlC</strong>, both <strong>of</strong> which are inclined
to the interface. This feature <strong>of</strong> the interfacial micro<strong>structure</strong>
is more clearly revealed by Figure 2d, in which
the processed image obtained by calculating the FFT
after masking both the Al2O3 {2110} and <strong>V2AlC</strong>
{1013} reflections on the opposite side <strong>of</strong> the unscattered
beam, as shown in the inset image in Figure 2d.
For the <strong>V2AlC</strong> film grown on a-Al2O3 substrates, a
semicoherent interface forms which is characterized by
coherent regions separated by misfit dislocations. Thus
the lattice mismatch at the interface is accommodated
by the misfit dislocations. In Figure 2d, the misfit dislocations
are evenly distributed in V 2AlC near the interface.
This is expected since <strong>V2AlC</strong> is less stiff than
Al2O3 [19]. Hence, epitaxial growth <strong>of</strong> <strong>V2AlC</strong> <strong>films</strong> on
ð1120Þ Al 2O 3 substrates may at least in part be caused
by small lattice mismatch, compensated by misfit dislocations.
It is important to note that the thickness <strong>of</strong> the
interfacial region where local epitaxy observed is on the
order <strong>of</strong> 100 nm. It may be speculated that the relatively
high deposition rate employed here hinders global epitaxy.
The growth rate, at 37.5 nm min 1 , was one order
<strong>of</strong> magnitude larger than the rates reported in the literature
resulting in the formation <strong>of</strong> epitaxial Ti2AlC <strong>thin</strong>
<strong>films</strong> [12,13].
In order to investigate the origin <strong>of</strong> the experimentally
observed local epitaxy, we applied ab initio calculations
to probe the atomistic and electronic nature <strong>of</strong> the
V 2AlC//a-Al 2O 3 interface. These ab initio calculations
were performed using the OpenMX code [20], based
on DFT [21] and basis functions in the form <strong>of</strong> linear
combinations <strong>of</strong> localized pseudoatomic orbitals [22].
The electronic potentials were fully relativistic pseudopotentials
with partial core correction [23,24], and a local
density approximation was applied [25]. The basis
functions used were generated based on a confinement
scheme [26] and specified as follows: Al6.0-s2p2d1,
O5.0-s2p2, V7.5-s2p2d2 and C4.5-s2p2. The energy cut<strong>of</strong>f
(150 Ryd) and k-point grid (1 1 1) wi<strong>thin</strong> the real
space grid technique [27] were adjusted to reach an accuracy
<strong>of</strong> 10 6 H atom 1 . Spin polarization was not considered
since the total energy differences between spin
polarized and non-polarized <strong>V2AlC</strong> configurations are
negligible [28]. The interface was described using a slab
model where periodic boundary conditions are broken
in one crystallographic direction by inserting a vacuum
layer (here thickness = 10 A ˚ ).
Two interface configurations have been studied in this
work. V 2AlC (0001)//a-Al 2O 3 ð1120Þ and V 2AlC
½1210Š//a-Al2O3 [0 0 0 1], which is identical to the one
observed by TEM; as well as <strong>V2AlC</strong> (0 0 0 1)//a-Al2O3
ð1120Þ and V 2AlC ½1100Š//a-Al 2O 3 [0 0 0 1] each consist
<strong>of</strong> 60 atoms corresponding to the substrate and 128 and 94
atoms, respectively, corresponding to the film. The most
stable surface termination <strong>of</strong> ð1120Þ a-Al 2O 3 reported
in the literature is oxygen [29,30]; consequently, this termination
was adopted here. In both configurations, the
substrate is stoichiometric a-Al2O3 with six atomic layers
in thickness and the film is stoichiometric V 2AlC with
eight atomic layers in thickness. Two layers at the bottom
<strong>of</strong> the interface belonging to a-Al2O3 were fixed to represent
the (infinite) bulk. For each <strong>of</strong> these interfacial
configurations, the following stacking sequences were