Strained single-crystal Al2O3 grown layer-by-layer on Nb (110) thin
  films
Strained single-crystal Al2O3 grown layer-by-layer on Nb (110) thin films
Paul B. Welander and James N. Eckstein
Department of Physics and Frederick Seitz Materials Research Laboratory,
University of Illinois at Urbana-Champaign, Urbana, IL 61801
(Dated: April 1, 2007)
We report on the layer-by-layer growth of single-crystal Al2O3 thin-films on Nb (110). Single-
crystal Nb films are first prepared on A-plane sapphire, followed by the evaporation of Al in an O2
background. The first stages of Al2O3 growth are layer-by-layer with hexagonal symmetry. Electron
and x-ray diffraction measurements indicate the Al2O3 initially grows clamped to the Nb lattice with
a tensile strain near 10%. This strain relaxes with further deposition, and beyond about 50 Å we
observe the onset of island growth. Despite the asymmetric misfit between the Al2O3 film and the
Nb under-layer, the observed strain is surprisingly isotropic.
The present challenge of constructing solid-state quan-
tum bits with long coherence times [1] has ignited new
interest in Josephson junctions fabricated from single-
crystal materials. It has been found that critical-current
1/f noise cannot fully account for the observed deco-
herence times in junctions-based qubits [2]. However,
amorphous tunnel-barrier defects can give rise to two-
level charge fluctuations that destroy quantum coher-
ence across the junction [3, 4]. Oh et al have recently
found that tunnel-junctions from epitaxial Re/Al2O3/Al
tri-layers have a significantly reduced density of two-level
fluctuators [5].
The pairing of Re and Al2O3 is advantageous because
of the very small misfit between the basal planes and
because Re is less likely to oxidize compared with other
superconducting refractory metals. However, epitaxial
Re films develop domains due to basal-plane twinning,
causing the surface to be rough on the length scales of
a typical tunnel-junction [6]. An alternative to Al2O3
hetero-epitaxy on a close-packed metal surface is to grow
on bcc (110), where such twinning is absent. To date
single-crystal Al2O3 films have been grown on a number
of such metals: Ta [7], Mo [8], W [9], and more recently
Nb [10].
In a recent paper, Dietrich et al reported on their in-
vestigations of ultra-thin epitaxial α-Al2O3 (0001) films
on Nb using tunneling microscopy and spectroscopy [10].
Their films were grown on Nb (110) by evaporating Al in
an O2 background near room temperature. Crystalliza-
tion was achieved by annealing the sample up to 1000 ◦C.
Subsequent microscopy showed the film to be atomically
smooth, but spectroscopic scans found localized defect
states around ±1 eV, well below the 9 eV sapphire band
We report here on our findings concerning the hetero-
epitaxy of Al2O3 on Nb (110) films. Unlike the previ-
ous study, our Al2O3 films are grown layer-by-layer with
co-deposition of Al and O at elevated substrate tem-
peratures. Epitaxial bi-layers (Nb/Al2O3) and tri-layers
(Nb/Al2O3/Nb) are grown by molecular beam epitaxy
(MBE). Characterization techniques include in situ re-
∗This article has been submitted to Applied Physics Letters.
flection high-energy electron diffraction (RHEED) and
x-ray photo-electron spectroscopy (XPS), and ex situ
atomic force microscopy (AFM) and x-ray diffraction
(XRD).
The process for growing high-quality single-crystal Nb
films on sapphire is well understood [11]. Our samples
start with a thick Nb base layer (2000 Å) grown on A-
plane sapphire – α-Al2O3 (112̄0) – with a nominal miscut
of 0.1◦. Nb (99.99%) is evaporated via e-beam bombard-
ment at a rate of about 0.3 Å/s onto a substrate held
near 800 ◦C. The base pressure of our chamber is about
10−11 torr, with the growth pressure around 10−9 torr.
After deposition, the film is annealed above 1300 ◦C for
30 min. During growth and annealing the film surface is
monitored with RHEED.
Epitaxial Nb on A-plane sapphire grows in the (110)
orientation with Nb [11̄1] ‖ α-Al2O3 [0001], in accordance
with the well-established three-dimensional relationship
[11, 12]. Nb RHEED patterns (Figure 1) reveal a two-
dimensional, reconstructed film surface that takes one
form after growth [13], and a second one upon annealing
[14]. Annealed films also show a sharp specular spot in-
FIG. 1: Nb (110) on A-plane sapphire. Top: RHEED images
along the (a) [001] and (b) [11̄1] azimuths after growth at 800
◦C, and (c) [11̄1] after annealing above 1300 ◦C. Left: 5 × 5
µm2 AFM image of annealed Nb, 10 Å height scale. Right:
XRD radial scan of the Nb (110) Bragg peak.
http://arxiv.org/abs/0704.0118v1
FIG. 2: Top: RHEED images from epitaxial Al2O3 on Nb
(110), taken along the [11̄00] azimuth after deposition of (a)
4 Å, (b) 25 Å, and (c) 125 Å. Bottom: 5× 5 µm2 AFM scans
on Al2O3 films that are 20 Å (left, 10 Å height-scale) and 100
Å (right, 50 Å) thick.
dicating long-range film flatness, which is confirmed by
AFM measurements. Scans show large terraces about
2000 Å wide and monolayer step-edges that align them-
selves according to the substrate miscut (Figure 1). An-
nealed Nb films typically have an rms surface roughness
less than 2 Å.
XRD measurements on these Nb films show sharp
Bragg peaks and narrow rocking curves, both indicative
of single-crystal growth. Figure 1 shows a radial scan
(2θ-ω) of the Nb (110) Bragg peak from a 2000 Å-thick
film, with intensity fringes indicating a structural coher-
ence that extends over the entire film thickness. Rock-
ing curves typically have a FWHM of about 0.03◦. In
addition, measurements of specular and off-axis Bragg
peaks demonstrate that a 2000 Å-thick annealed Nb film
is strained 0.1% or less with respect to bulk.
Al2O3 is deposited in situ onto similar Nb films at a
substrate temperature of around 750 ◦C. Using a stan-
dard effusion cell, Al (99.9995%) is evaporated at about
0.1 Å/s in an O2 (99.995%) background up to 5 × 10
torr. Under these growth conditions we estimate that
the O2 flux is about 1000 times greater than that of Al
[15]. After deposition the sample is cooled before turning
the O2 off. Al2O3 films included in this report range in
thickness from 15 to 125 Å.
Chemical analysis of the Al2O3 is carried out in an
XPS system adjacent to the growth chamber. Measure-
ments of the Al 2p, O 1s and Nb 3d levels indicate that
the Al is completely oxidized with no measurable oxida-
tion of the underlying Nb. The observed energy differ-
ence between the O 1s and Al 2p levels is 457.1 eV, in
good agreement with what has been reported for sap-
phire (456.6 eV) [16]. The Nb 3d level shows no side
bands which would indicate oxide formation.
RHEED of the Al2O3 thin film reveals a hexagonal
FIG. 3: Strain vs. film thickness for epitaxial Al2O3 on Nb
(110). (•) Strain of a 100 Å film measured during deposition.
(⋄) Strain observed for a number of samples after deposition
and cooling, with error bars indicating the range of strain
values measured along different RHEED azimuths.
C-plane-like surface in the Nishiyama-Wasserman orien-
tation: α-Al2O3 (0001) [1̄100] ‖ Nb (110) [001] [17]. (Be-
cause both α-Al2O3 [18] and γ-Al2O3 [19] have close-
packed planes, no definitive crystal structure can be
inferred. Hexagonal Miller indices will be employed
for defining crystallographic orientations by convention
only.) Diffraction images from various stages of growth
are shown in Figure 2. Immediately after the oxide depo-
sition begins the Nb diffraction pattern and specular spot
disappear. After about 2 ML (4 Å) the Al2O3 diffraction
pattern becomes visible. At a thickness of 25 Å, RHEED
shows an elongated specular spot and well-defined first-
order streaks. Up to about 50 Å the Al2O3 growth is
layer-by-layer (Frank-van der Merwe mode). Beyond this
thickness the 2D streaks evolve into 3D spots, indicating
the growth of islands (Stranski-Krastanov mode).
As the transformation from 2D to 3D growth is
occurring, the measured spacing between RHEED
streaks/spots increases, indicating a shrinking of the
Al2O3 surface lattice. Using the RHEED from the base-
layer Nb as a ruler, we find that the Al2O3 film experi-
ences a tensile strain that relaxes with increasing thick-
ness, as shown in Figure 3. The strain-thickness curve is
determined from RHEED along the [1̄100] azimuth dur-
ing Al2O3 deposition near 750
◦C. With respect to C-
plane sapphire (a = 4.759 Å), the tensile strain is nearly
10% initially and by 20 Å has fallen to about 8%. After
100 Å of deposition, the Al2O3 exhibits a tensile strain
of around 3%.
After deposition and cooling in O2, Al2O3 films of var-
ious thicknesses show further lattice relaxation (Figure
3). On average, RHEED measurements near room tem-
perature show a strain reduction of about 1% when com-
pared to measurements just after Al deposition. Ther-
mal contraction accounts for a significant portion of the
strain change during cooling. (Both Nb and Al2O3 have
expansion coefficients in this temperature range around
7-8×10−6 K−1.) However, due to the limited precision
FIG. 4: XRD pole figure for an epitaxial Nb/Al2O3/Nb tri-
layer grown on A-plane sapphire. Both Nb layers have a (110)
surface-orientation. This scan shows the off-axis 〈110〉 Bragg
peaks. The four peaks connected by the dashed rectangle are
approximately four times stronger than the others.
of our measurements, the presence of other strain-relief
mechanisms cannot be determined.
Regardless, the measured tensile strain in epitaxial
Al2O3 films on Nb (110) is significant. What’s more,
the strain is fairly isotropic – RHEED patterns along the
{1̄100} azimuths reveal relatively small variations. The
strain for each azimuth is determined by averaging oppo-
site directions – eg. [1̄100] and [11̄00] – to reduce system-
atic errors. The mean and range of the measured tensile
strain for the three azimuths is shown in Figure 3. The
strain-isotropy is surprising since the misfit along the Nb
[001] or α-Al2O3 [1̄100] direction is rather large (20%),
while along the Nb [11̄0] or α-Al2O3 [1̄1̄20] it is much
smaller (−1.7%). Despite such an anisotropic misfit, the
Al2O3 films exhibit isotropic strain.
Thin Al2O3 films are also very flat. AFM imaging of
a 20 Å-thick film shows an atomically flat surface with
monolayer steps (c/6 = 2.165 Å) and an rms roughness
of about 2 Å (Figure 2). On the other hand, the surface
of a 100 Å-thick film is comprised of islands about 1000
Å wide and 50 Å in height. This agrees well with our
interpretation of Al2O3 RHEED - evidence for islands
in the diffraction images appeared after about 50 Å of
deposition.
For those samples where an epitaxial Nb over-layer
is deposited in situ, the substrate is warmed back up
above 700 ◦C. Under these conditions growth on C-plane
sapphire would yield (111)-oriented films [11, 12]. How-
ever, XRD analysis indicates that the top Nb layer is
(110)-oriented with Nb [001] ‖ α-Al2O3 [1̄100], [01̄10]
and [101̄0]. A pole scan of off-axis 〈110〉 Bragg peaks
is shown in Figure 4, and despite the surface orientation,
the Nb over-layer reproduces the hexagonal symmetry of
the Al2O3 film. The top Nb film grows in three domains
of roughly equal weight rotated with respect to one an-
other by 120◦, with one domain aligned to the base Nb
layer. This type of film structure has been observed for
Nb growth on C-plane sapphire, but only under the fol-
lowing conditions: evaporation above 1000 ◦C [20], post-
growth annealing up to 1500 ◦C [21], and niobium sput-
tering near 850 ◦C [22]. That we observe this growth
structure for evaporation near 700 ◦C suggests that the
surface lattice of the Al2O3 film, while hexagonal, is not
identical to that of C-plane sapphire.
Tunnel-junctions were fabricated from several of these
epitaxial tri-layers. The I-V characteristics showed
a large conductance shunting the Josephson junction.
While an inhomogeneous morphology may cause such
a conductance, no metallurgical pinholes were ever ob-
served in our Al2O3 films. Devices with 20 Å Al2O3 lay-
ers had critical current densities around 104 A/cm2 and
normal state conductances near 109 S/cm2. Assuming a
homogeneous barrier, the latter value gives an effective
barrier height of about 1.3 eV. This is similar to the en-
ergy of sub-gap states found spectroscopically by Dietrich
et al [10] in epitaxial Al2O3 on Nb.
Among the previous studies of Al2O3 epitaxy on bcc
(110) metals, only Chen et al reported any measure of
tensile strain [7]. For Al2O3 films 5-40 Å thick on Ta
(110) they measured a lattice enlargement of about 9%.
The agreement with our findings could be expected since
the lattice constants of Ta and Nb are nearly identi-
cal. One difference though is that Chen et al observed a
Kurdjumov-Sachs relationship, α-Al2O3 (0001) [1̄100] ‖
Nb (110) [11̄1] [7], instead of the Nishiyama-Wasserman
orientation we observe.
In summary, single-crystal Nb/Al2O3 and
Nb/Al2O3/Nb multi-layers were grown by MBE.
Various methods of materials analysis suggest these
layers are all high-quality. Our principal finding is that
epitaxial Al2O3 on Nb (110) grows under uniform tensile
strain, despite the anisotropic misfit. As the Al2O3 film
thickness is increased the strain relaxes and the surface
roughens. The over-layer Nb grows with a (110) surface
orientation under growth conditions that would yield
Nb (111) on C-plane sapphire.
AFM and XRD analysis was carried out in the Cen-
ter for Microanalysis of Materials, University of Illinois
at Urbana-Champaign, which is partially supported by
the U.S. Department of Energy under grant DEFG02-
91ER45439. This project was funded by the National
Science Foundation through grant EIA 01-21568.
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