Strained single-crystal Al2 O3 grown layer by layer on Nb (110) thin films

Article abstract of DOI:10.1063/1.2747675

The authors report on the growth of single-crystal Al2 O3 thin films on Nb (110) surfaces. Niobium is grown on α- Al2 O3 (11 2- 0), followed by the evaporation of Al in an O2 background. Initially, Al2 O3 grows layer by layer with hexagonal symmetry indicating either α- Al2 O3 (0001) or γ- Al2 O3 (111). Diffraction measurements show that the Al2 O3 initially grows clamped to the Nb with tensile strain near 10%. This strain relaxes with further deposition and beyond about 50 A, the authors observe island growth. Despite the asymmetric misfit between Al2 O3 and Nb, the strain is surprisingly isotropic. Josephson junctions employing epitaxial Al2 O3 show low effective tunnel barriers and high leakage currents.

Full text of DOI:10.1063/1.2747675

Strained single-crystal Al 2 O 3 grown layer by layer on Nb (110) thin films
Paul B. Welander and James N. Eckstein
Citation: Applied Physics Letters 90, 243510 (2007); doi: 10.1063/1.2747675
View online: http://dx.doi.org/10.1063/1.2747675
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APPLIED PHYSICS LETTERS 90, 243510 ͑2007͒
Strained single-crystal Al O grown layer by layer on Nb „110… thin films
2
3
a͒
Paul B. Welander and James N. Eckstein
Department of Physics and Frederick Seitz Materials Research Laboratory,
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
͑
Received 28 March 2007; accepted 16 May 2007; published online 15 June 2007͒
The authors report on the growth of single-crystal Al O thin films on Nb ͑110͒ surfaces. Niobium
2
3
¯
is grown on ␣-Al O ͑1120͒, followed by the evaporation of Al in an O background. Initially,
2
3
2
Al O grows layer by layer with hexagonal symmetry indicating either ␣-Al O ͑0001͒ or ␥-Al O
2
3
2
3
2
3
͑
111͒. Diffraction measurements show that the Al O initially grows clamped to the Nb with tensile
2 3
strain near 10%. This strain relaxes with further deposition and beyond about 50 Å, the authors
observe island growth. Despite the asymmetric misfit between Al O3 and Nb, the strain is
2
surprisingly isotropic. Josephson junctions employing epitaxial Al O show low effective tunnel
2
3
barriers and high leakage currents. © 2007 American Institute of Physics.
DOI: 10.1063/1.2747675͔
͓
The present challenge of constructing solid-state quan-
tum bits with long coherence times has ignited new interest
and ex situ atomic force microscopy ͑AFM͒ and x-ray dif-
fraction ͑XRD͒.
1
in Josephson junctions fabricated from single-crystal materi-
als. It has been found that critical-current 1/ f noise cannot
fully account for the observed decoherence times in junction-
The process for growing high-quality single-crystal Nb
11
films on sapphire is well understood. Our samples start
with a thick Nb base layer ͑2000 Å͒ grown on A-plane
2
¯
based qubits. However, amorphous tunnel-barrier defects
sapphire—␣-Al O ͑1120͒—with a nominal miscut of 0.1°.
2
3
can give rise to two-level charge fluctuations that destroy
Nb ͑99.99%͒ is evaporated via e-beam bombardment at a
3
,4
quantum coherence across the junction. Oh et al. have re-
cently found that tunnel junctions from epitaxial
Re/Al O /Al trilayers have a significantly reduced density
rate of about 0.3 Å/s onto a substrate held near 800 °C. The
−
11
base pressure of our chamber is about 10 torr, with the
growth pressure around 10− 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 ͑110͒
¯
9
2
3
5
of two-level fluctuators.
The pairing of Re and Al O is advantageous because of
2
3
the very small misfit between the basal planes and because
Re is less likely to oxidize compared with other supercon-
ducting refractory metals. However, epitaxial Re films de-
velop domains due to basal-plane twinning, causing the sur-
face to be rough on the length scales of a typical tunnel
orientation with Nb ͓111͔ ␣-Al O ͓0001͔, in accordance
2 3
11,12
ʈ
with the well-established three-dimensional relationship.
Nb RHEED patterns ͑Fig. 1͒ reveal a two-dimensional, re-
1
3
constructed film surface that takes one form after growth,
6
14
junction. An alternative to Al O heteroepitaxy on a close-
and a second one upon annealing. Annealed films also
show a sharp specular spot indicating long-range film flat-
ness, which is confirmed by AFM measurements. Scans
show large terraces about 2000 Å wide and monolayer step
edges that align themselves according to the substrate mis-
2
3
packed metal surface is to grow on bcc ͑110͒, where such
twinning is absent. To date, single-crystal Al O films have
2
3
7
8
9
been grown on a number of such metals: Ta, Mo, W, and
1
0
more recently Nb.
In a recent letter, Dietrich et al. reported on their inves-
tigations of ultrathin epitaxial ␣-Al O ͑0001͒ films on Nb
2
3
1
0
using tunneling microscopy and spectroscopy. Their films
were grown on Nb ͑110͒ by evaporating Al in an O back-
2
ground near room temperature. Crystallization was achieved
by annealing the sample up to 1000 °C. Subsequent micros-
copy showed the film to be atomically smooth, but spectro-
scopic scans found localized defect states around ±1 eV,
well below the 9 eV sapphire band gap.
We report here on our findings concerning the heteroepi-
taxy of Al O on Nb ͑110͒ films. Unlike the previous study,
2
3
our Al O films are grown layer by layer with codeposition
2
3
of Al and O at elevated substrate temperatures. Epitaxial bi-
layers ͑Nb/Al O ͒ and trilayers ͑Nb/Al O /Nb͒ are grown
2
3
2
3
by molecular beam epitaxy ͑MBE͒. Characterization tech-
niques include in situ reflection high-energy electron diffrac-
tion ͑RHEED͒ and x-ray photoelectron spectroscopy ͑XPS͒,
FIG. 1. Nb ͑110͒ on A-plane sapphire. Top: RHEED images along the ͑a͒
¯
¯
͓
001͔ and ͑b͒ ͓111͔ azimuths after growth at 800 °C, and ͑c͒ ͓111͔ after
2
annealing above 1300 °C. Left: 5ϫ5 ␮m AFM image of annealed Nb,
10 Å height scale. Right: XRD radial scan of the Nb ͑110͒ Bragg peak.
a͒Electronic mail: eckstein@uiuc.edu
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003-6951/2007/90͑24͒/243510/3/$23.00 90, 243510-1 © 2007 American Institute of Physics
31.252.200.218 On: Sat, 29 Nov 2014 20:20:06
0
1

243510-2
P. B. Welander and J. N. Eckstein
Appl. Phys. Lett. 90, 243510 ͑2007͒
FIG. 3. Strain vs film thickness for epitaxial Al O on Nb ͑110͒. ͑b͒ Strain
2
3
of a 100 Å film measured during deposition. ͑छ͒ 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.
FIG. 2. Top: RHEED images from epitaxial Al O on Nb ͑110͒, taken along
2
3
¯
the ͓1100͔ azimuth after deposition of ͑a͒ 4 Å, ͑b͒ 25 Å, and ͑c͒ 125 Å.
2
Bottom: 5ϫ5 ␮m AFM scans on Al O3 films that are 20 Å ͑left, 10 Å
2
height scale͒ and 100 Å ͑right, 50 Å͒ thick.
the Al O growth is layer by layer ͑Frank–van der Merwe
2
3
mode͒. Beyond this thickness, the two-dimensional ͑2D͒
streaks evolve into three-dimensional ͑3D͒ spots, indicating
the growth of islands ͑Stranski-Krastanov mode͒.
cut. ͑Fig. 1͒ Annealed Nb films typically have a rms surface
roughness less than 2 Å.
As the transformation from 2D to 3D growth occurs, the
measured spacing between RHEED streaks/spots increases,
indicating a shrinking of the Al O surface lattice. Using the
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 ͑2␪-␻͒ of the
Nb ͑110͒ Bragg peak from a 2000 Å thick film, with inten-
sity fringes indicating a structural coherence that extends
over the entire film thickness. Rocking curves typically have
a full width at half maximum of about 0.03°. In addition,
measurements of specular and off-axis Bragg peaks demon-
strate that a 2000 Å thick annealed Nb film is strained 0.1%
or less with respect to bulk.
2
3
RHEED from the base layer Nb as a ruler, we find that the
Al O film experiences a tensile strain that relaxes with in-
2
3
creasing thickness, as shown in Fig. 3. The strain-thickness
¯
curve is determined from RHEED along the ͓1100͔ azimuth
during Al O deposition near 750 °C. With respect to
2
3
C-plane sapphire ͑a=4.759 Å͒, the tensile strain is nearly
1
1
0% initially and by 20 Å has fallen to about 8%. After
00 Å of deposition, the Al O exhibits a tensile strain of
Al O is deposited in situ onto similar Nb films at a
2
3
2
3
around 3%.
substrate temperature of around 750 °C. Using a standard
After deposition and cooling in O , Al O , films of vari-
effusion cell, Al ͑99.9995%͒ is evaporated at about 0.1 Å/s
2
2
3
−
6
ous thicknesses show further lattice relaxation ͑Fig. 3͒. On
average, RHEED measurements near room temperature
show a strain reduction of about 1% when compared to mea-
surements just after Al deposition. Thermal contraction ac-
counts for a significant portion of the strain change during
in an O ͑99.995%͒ background up to 5ϫ10 torr. Under
2
these growth conditions, we estimate that the O flux is about
2
1
5
1
000 times greater than that of Al. After deposition, the
sample is cooled before turning the O off. Al O films in-
2
2
3
cluded in this report range in thickness from 15 to 125 Å.
Chemical analysis of the Al O is carried out in a XPS
cooling. ͓Both Nb and Al O have expansion coefficients in
2
3
2
3
−
6
−1
this temperature range around ͑7–8͒ϫ10 K .͔ However,
due to the limited precision of our measurements, the pres-
ence of other strain-relief mechanisms cannot be determined.
Nevertheless, the measured tensile strain in epitaxial
Al2O3 films on Nb ͑110͒ is significant. What’s more, the
system adjacent to the growth chamber. Measurements of the
Al 2p, O 1s, and Nb 3d levels indicate that the Al is com-
pletely oxidized with no measurable oxidation of the under-
lying Nb. The observed energy difference between the O 1s
and Al 2p levels is 457.1 eV, in good agreement with what
1
6
has been reported for sapphire ͑456.6 eV͒. The Nb 3d level
¯
strain is fairly isotropic—RHEED patterns along the ͕1100͖
shows no side bands which would indicate oxide formation.
azimuths reveal relatively small variations. The strain for
each azimuth is determined by averaging opposite directions,
RHEED of the Al O3 thin film reveals a hexagonal
2
C-plane-like surface in the Nishiyama-Wasserman orienta-
¯
¯
e.g., ͓1100͔ and ͓1100͔, to reduce systematic errors. The
mean and range of the measured tensile strain for the three
azimuths are shown in Fig. 3. The strain isotropy is surpris-
1
7
¯
tion: ␣-Al O ͑0001͒ ͓1100͔
ʈ
Nb ͑110͒ ͓001͔. ͓Because
2
3
both ␣-Al O ͑Ref. 18͒ and ␥-Al O ͑Ref. 19͒ have close-
2
3
2
3
packed planes, no definitive crystal structure can be inferred.
Hexagonal Miller indices will be employed for defining crys-
tallographic orientations by convention only.͔ Diffraction im-
ages from various stages of growth are shown in Fig. 2.
Immediately after the oxide deposition begins, the Nb dif-
fraction pattern and specular spot disappear. After about 2
ML ͑4 Å͒, the Al O diffraction pattern becomes visible. At
¯
¯
ing since the misfit along the Nb ͓001͔ or ␣-Al O ͓1100͔
2
3
direction is rather large ͑20%͒, while along the Nb ͓110͔ or
¯
¯
␣-Al2O3 ͓1120͔ it is much smaller ͑−1.7%͒. Despite such an
anisotropic misfit, the Al O films exhibit isotropic strain.
Thin Al O films are also very flat. AFM imaging of a
20 Å thick film shows an atomically flat surface with mono-
layer steps ͑c/6=2.165 Å͒ and a rms roughness of about 2 Å
͑Fig. 2͒. On the other hand, the surface of a 100 Å thick film
2
3
2
3
2
3
a thickness of 25 Å, RHEED shows an elongated specular
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spot and well-defined first-order streaks. Up to about 50 Å,
131.252.200.218 On: Sat, 29 Nov 2014 20:20:06

243510-3
P. B. Welander and J. N. Eckstein
Appl. Phys. Lett. 90, 243510 ͑2007͒
with our findings could be expected since the lattice con-
stants of Ta and Nb are nearly identical. One difference
though is that Chen et al. observed a Kurdjumov-Sachs re-
7
¯
¯
lationship, ␣-Al O ͑0001͒ ͓1100͔
ʈ
Nb ͑110͒ ͓111͔, instead
2
3
of the Nishiyama-Wasserman orientation that we observe.
In summary, single-crystal Nb/Al O and Nb/Al O /Nb
2
3
2
3
multilayers were grown by MBE. Various methods of mate-
rial analysis suggest that these layers are all high quality. Our
principal finding is that epitaxial Al O on Nb ͑110͒ grows
2
3
under uniform tensile strain despite the anisotropic misfit. As
the Al O film thickness is increased, the strain relaxes and
2
3
the surface roughens. The overlayer Nb grows with a ͑110͒
surface orientation under growth conditions that would yield
Nb ͑111͒ on C-plane sapphire. Josephson junctions fabri-
cated from these epitaxial multilayers show low effective
barrier heights and high leakage currents.
FIG. 4. ͑Color online͒ XRD pole figure for an epitaxial Nb/Al O /Nb
2
3
trilayer grown on A-plane sapphire. Both Nb layers have a ͑110͒ surface
orientation. This scan shows the off-axis ͗110͘ Bragg peaks. The four peaks
connected by the dashed rectangle are approximately four times stronger
than the others.
AFM and XRD analyses were carried out in the Center
for Microanalysis of Materials, University of Illinois at
Urbana-Champaign, which is partially supported by the U.S.
Department of Energy under Grant No. DEFG02-
is comprised of islands about 1000 Å wide and 50 Å in
height. This agrees well with our interpretation of Al O
RHEED—evidence for islands in the diffraction images ap-
peared after about 50 Å of deposition.
For those samples where an epitaxial Nb overlayer is
deposited in situ, the substrate is warmed back up above
2
3
9
1ER45439. This project was funded by the National Sci-
ence Foundation through Grant No. EIA 01-21568.
7
00 °C. Under these conditions, growth on C-plane sapphire
1
11,12
M. A. Nielson and I. L. Chang, Quantum Computation and Quantum
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2
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1