# Surface and Interface Properties of 10–12 Unit Cells Thick Sputter Deposited

Authors: L. V. Saraf; C. M. Wang; M. H. Engelhard; P. Nachimuthu
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

Ultrathin and continuous epitaxial films with relaxed lattice strain can potentially maintain more of its bulk physical and chemical properties and are useful as buffer layers. We study surface, interface, and microstructural properties of ultrathin
(∼10–12 unit cells thick) epitaxial ceria films grown on single crystal YSZ substrates. The out-of -plane and in-plane lattice parameters indicate relaxation in the continuous film due to misfit dislocations seen by high-resolution transmission electron microscopy (HRTEM) and substrate roughness
of ∼1-2 unit cells, confirmed by atomic force microscopy and HRTEM. A combination of secondary sputtering, lattice mismatch, substrate roughness, and surface reduction creating secondary phase
was likely the cause of surface roughness which should be reduced to a minimum level for effective use of it as buffer layers.


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## Body

## 1. Introduction

Cerium
oxide is a widely studied material
across several disciplines due to its use in oxygen storage, catalysis, and solid
oxide fuel cells [1–3]. Epitaxial
cerium oxide layers also act as buffer layers for metal oxide semiconductors
(MOS-s), silicon on insulator (SOI) for
ferroelectrics, and superconducting thin films. It has a cubic fluorite
structure that is very stable in a broad temperature and pressure range. The
reversible oxygen storage, oxygen diffusion, surface desorption, electrical
conductivity of ceria are widely studied by us and others in the catalysis
community [2, 4–7]. Doping ceria with
+3 valence elements creates oxygen vacancy which can be an electrolyte for
solid oxide fuel cells with oxygen ion conductivity in the intermediate
temperature range [3]. It has been shown that growth of epitaxial ceria on
silicon can create metal oxide emiconductor (MOS) layers useful in electronics [8].
Due to good lattice and thermal expansion coefficient matching with c-oriented
epitaxial yttrium barium copper oxide (YBCO) superconductors, ceria had been
consistently studied as a buffer layer to grow superconductors [9, 10]. The
buffer layers have also been shown to create proximity effect in
superconductors [11]. Because strained films can undergo structural phase
transition modifying its physical and chemical properties, it may become
essential to minimize the changes in physical and chemical properties of ultrathin
ceria buffer layers while maintaining its overall continuous and epitaxial/oriented
nature.Due to nearly matching lattice parameters,
similar cubic structures and thermal expansion coefficients for ceria and
yttrium stabilized zirconia (YSZ), epitaxial growth of ceria on YSZ was demonstrated
by us [12, 13]. In general, techniques like molecular-beam epitaxy or atomic
layer deposition have been utilized to grow epitaxial films [14]. Especially,
when ultrathin layer depositions are critical, these techniques have good control
over the film growth parameters. Reactive oxide DC sputtering on the other hand
has other advantages like compatibility with semiconductor processes, higher
growth rates, a high degree of ionization of evaporated species, and the ability
to grow on relatively large areas [14]. There have been some reports discussing
ultrathin ceria films less than 5 nm in thickness grown by sputtering [15, 16].
One of the major challenges of this technique is to avoid secondary sputtering
from the surface of already deposited films. Since the binding energies of most
of the solids are on the order of few electron volts, for films with few unit
cell in thickness, minor changes in incident ion energy can lead to variation
in secondary sputtering rate from the film surfaces which can cause morphology
changes resulting in increased roughness. Different types of thin film growth
modes are modeled and reviewed by Gilmer et al. [17]. In this report, we
discuss surface and interface properties of 10–12 unit cell
thick epitaxial ceria film with the film surface roughness in the range of 1-2 unit cells.
Defects in the form of misfit dislocations cause relaxation in ceria films. A
combination of secondary sputtering, lattice mismatch, substrate roughness, and
secondary phase at the surface were most likely responsible for the film
surface roughness.


## 2. Experiment

DC magnetron sputtering was used to grow
cerium oxide thin films on YSZ. A cerium metal target was sputtered in a (1 : 1) mixture
of argon and oxygen gases. The (001) oriented single-crystal YSZ substrates
were sequentially cleaned in acetone, methanol, and water using an ultrasonic
bath. In the growth chamber, the cerium target and YSZ substrate were kept
approximately 8 inches apart. The total sputter power was kept at 100 Watts to
avoid formation of particulates on the substrates. A 10–15-minute presputtering
was carried out to clean the target surface. The films were grown on YSZ
substrates at750∘C and 12 mbar total
Ar+O2 pressure.
After deposition, the substrates were cooled at a rate of 5∘C/min.
More importantly, sputtering time with a growth blocking shutter
was used to control film
thickness. In the present case, 10–12 unit cells thick
ceria films were achieved after sputtering for 1 minute.High-resolution X-ray
diffraction (XRD) and measurements were carried out on a Phillips X’pert θ-2θ diffractometer. Jade 6.0 software from
Materials Data Inc. was used to facilitate the fitting of the X-ray profile
scans. X-ray reflectivity (XRR) measurements
were carried out using a Philips X’Pert multipurpose diffractometer. The thickness,
roughness, and density of the films were determined from the fitting of XRR
data by BEDE REFS V4.00 code. The a-axis parameter of the YSZ substrate
was measured at 0.5143 nm. For the calculation purposes, bulk lattice parameter
of ceria was used 0.541 nm [5–7, 12, 13]. A Digital
Instruments Nanoscope IIIa Multimode microscope was utilized for atomic force
microscopy (AFM) analysis using a tapping mode. In
the case of low- and high-resolution transmission electron microscopy
(HRTEM) measurements, a standard tripod wedge
polishing method followed by Ar-ion-beam thinning was used to obtain an electron
transparent sample. HRTEM analysis was carried out on a JEOL 2010 microscope with
a lateral resolution of 0.194 nm. The digital recording of the images
was captured using a CCD
camera (1024 × 1024 pixels) followed by processing with digital micrograph. XPS
measurements were carried out using a physical electronics quantum 2000 scanning
ESCA microprobe. A focused
monochromatic Al Kα X-rays (1486.7 eV) source with 16-element
multichannel detection was used in the experiment. The X-ray beam was a 101 W,
107 μm diameter beam which was scanned over an area of 1.4 mm × 0.2 mm. The
high-energy resolution data was collected using pass energy of 23.5 eV. The
binding energy scale was calibrated by setting Cu 2p3/2 position at
932.6 eV and Au 4f7/2 position at 83.9 eV.


## 3. Results and Discussion

Figures1(a) and 1(b) represent HRTEM and TEM micrographs of ceria thin film grown on YSZ
single crystal. A good cube-on-cube epitaxy can be seen in the HRTEM image
(Figure 1(a)). Good film coverage and the overall uniformity of the layer can be seen
in Figure 1(b). A diffraction pattern is shown as inset of
Figure 1(b), where
film and substrate reflections related to (h 0 0) family of planes are evident
indicating epitaxial growth. The Fourier transformation
of HRTEM image (Figure 1(a)) is shown in
Figure 2. The misfit dislocations,
indicated by arrows in film and substrate and ellipses (in the interface
region), can be seen in the image. XRR and AFM measurements on ceria thin film are
shown in Figures 3(a) and
3(b), respectively. Fitting of the XRR profile indicates
that the thickness of the ceria films is 6 nm which is in good agreement with TEM images shown in
Figure 1. The inset of Figure 3(a) is the XRD scan, where no other orientations than (200) and (400) are
observed. The AFM image in Figure 3(b) shows the surface morphology of 6 nm
thick ceria film. The root-mean-square roughness was calculated to be 0.3 nm
for the 500 nm ×  500 nm scan area using a silicon cantilever with a nominal tip radius
of curvature of 10 nm. Since this represents average roughness, HRTEM image
indicates surface
roughness variation around 1-2 unit cells (up to ~1 nm). The inset of Figure
3(b) shows an AFM image of commercially as-received YSZ single-crystal substrate
used in the present study, after a typical cleaning cycle mentioned above. Step-like features ~100 nm wide and several nanodots were
observed on the substrates. A significant size distribution of nanodots indicates
that these features may have been resulted from the crystal polishing process.
In one of the previous studies, these nanodots on as-received YSZ single
crystal substrates were also connected to the atomic scale irregularities
resulting from point defects. [18] That study also observes that after
annealing at 1250∘C resulted in removal of much of these nanodots
with an ordering in step-like features.HRTEM and TEM
micrographs of 10–12 unit cell thick epitaxial ceria film grown on single
crystal YSZ.
(a)(b)Figure 2
The Fourier
transformation of 10–12 unit cell thick ceria film. The misfit dislocations,
indicated by arrows and ellipses.(a) XRR and
(b) AFM measurements on 10–12 unit cell thick epitaxial ceria film. Insets
of (a) and (b) represent an XRD scan and AFM image of the as-received YSZ
substrate, respectively.
(a)(b)To
determine out-of-plane and in-plane lattice constants of ceria thin films, slow
HRXRD measurements were carried out for (200) and (420) planes. The data are shown
in Figures4(a) and
4(b). We have calculated that c- (out-of-plane) and a-axis
(in-plane) lattice constants were 0.539 nm and 0.54 nm, respectively, indicating
negligible strain. The visualization of (200) and (420) planes in ceria lattice
are shown as the insets to Figures 4(a) and
4(b). The difference in the
broadness of ceria and YSZ peaks in this figure clearly highlights the difference
between the bulk and the ultrathin nature of these layers.
Figure 5 shows XPS
spectrum collected at 20∘ emission angle from a typical epitaxial
ceria film grown on YSZ single crystal [19]. At this emission angle, the data
is mostly independent of the film thickness more than 5 nm. Low emission angle
XPS provides information about ceria surface states due to enhanced signal from
roughly first ~5 nm. We observed that even though the bulk film indicates
epitaxial ceria (Ce4+) film, as shown in inset XRD spectrum of
Figure 5, surface has the presence of Ce3+ valence state. The
results point toward reduction in ceria with a possibility of phase separation
at the surface. Since surface roughness and phase separation are mutually
dependent factors, phase separation is likely a major contributing factor for
ceria surface roughness [20].(a) Out-of-plane (200) and (b) in-plane (420)
XRD scans of 10–12 unit cell
thick ceria film, necessary to calculate the respective lattice
parameters. The insets indicate visualization of (200) and (420) planes in
a ceria lattice.
(a)(b)Figure 5
XPS Ce 3d
photoemission spectrum of a typical epitaxial CeO2/YSZ film
when the data is collected at emission angles of 20∘. Inset: XRD scan of
the same film.Ideally, a critical thickness can be
achieved in every epitaxial film, where
lattice mismatch gets accommodated in
the form of a uniform strain [21]. Above such thicknesses, a misfit dislocation
is an energetically favored process. In the case of ceria/YSZ pair, we have
observed a critical thickness ~64 nm [12]. Several extrinsic experimental
factors such as gas flow rates, their mixing ratios and substrate temperatures
can also affect this parameter. A combination of XRD peak broadening, peak
position shifts, and analysis of HRTEM images by Fourier transformation can
give some information about the nature of strains and misfit dislocations. Minor
c and a axis strains indicate relaxation of ceria films due to misfit
dislocations observed in Fourier transformed HRTEM image. In a recent report,
similar lattice parameter values were obtained at these thicknesses when the
films were grown on hexagonal r-plane Al2O3, where lattice
mismatch is even greater [15]. In that case, no full film coverage was obtained
until ceria thickness reaches 8 nm. A “lattice constant”-“oxygen incorporation”
relationship was also discussed suggesting that at extremely low-thicknesses
ceria may not be fully oxidized. This phenomenon may enhance the lattice
parameters by as much as 3.7% in ultrathin epitaxial ceria films [15]. This argument
combined with XPS result indicates
that there is some oxygen deficiency at the surface if we consider Ce3+ state from a
CeO2 oxygen deficient form and near complete oxidation
of the state if it is from Ce2O3 like features.Ceria-YSZ combination has low lattice mismatch
of ~5% [YSZ (a-0.514 nm) and ceria (a-0.541 nm)] and nearly matching thermal
expansion coefficients from room temperature to around the present growth
temperature range [22]. Even though misfit dislocations, as seen in Fourier
transformed HRTEM image in Figure 2 relax majority of strain, effects of substrate
lattice mismatch on film surface roughness can not be completely ruled out. Generally,
the substrate coverage during the growth of ultrathin epitaxial films rapidly
changes during initial growth stages. High surface mobility of the deposited
atoms can be achieved with the help of relatively high growth temperatures. Several
types of thin film growth modes such as layer by layer and nucleation followed
by islands formations are discussed in the literature [17]. The techniques like
XRR and AFM can be effectively utilized to measure surface roughness. In our
case, an rms roughness
of 0.3 nm roughly represents an average thickness variation less than c-axis of
a ceria unit cell. When we compare it to the overall thickness, it is about 5%
of the total thickness. By taking into account the lack of atomic layer manipulation capability in
sputter process, given roughness is in an acceptable range. Additionally, when
we consider as-received substrate roughness, surface-phase separation, and
secondary sputtering, these roughness parameters can be further justified.


## 4. Conclusion

We have studied surface and interface properties
of 10–12 unit cell thick epitaxial ceria films grown on single crystal YSZ
substrates by DC magnetron sputtering. An average roughness of ~0.3 nm or
about 5% of the total film thickness with roughness variation in the range of
1-2 unit cells was observed. The relaxation in epitaxial ceria is caused by lattice
misfit dislocations. Because the surface of typical epitaxial ceria has Ce3+ valence state, major factors contributing the surface roughness were single-crystal
substrate roughness, lattice mismatch, the secondary sputtering caused by the
sputter process, and phase separation at the ceria surface. Minimizing all of
these parameters can more help to establish better control over the process and
effective use of ceria as buffer layers.



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*Source: 206019-2008-05-21.xml*