APPLIED PHYSICS LETTERS 88, 022102 ͑2006͒
Zhi Liang Bao and Karen L. Kavanagh
Department of Physics, Simon Fraser University, 8888 University Drive, Burnaby,
British Columbia V5A 1S6, Canada
͑
Received 3 October 2005; accepted 10 November 2005; published online 9 January 2006͒
Bismuth films formed by electrodeposition on n-GaAs ͑111͒ at 70 °C are found to be single
crystalline, ͑0001͒ oriented, with trigonal surface morphologies typical of high quality single
crystals. Diode current-voltage characteristics display low reverse-bias leakage currents and average
barrier heights of 0.77±0.02 eV ͑n=1.07͒. A necessary requirement for single crystalline growth is
͓
Bismuth is a semimetal that has many extraordinary
transport properties due to its highly anisotropic Fermi sur-
face and small effective carrier mass. It is the most diamag-
High-resolution x-ray measurements ͑monochromated
beam, width 12 arcsec͒ comparing -2 scans to tilt scans
showed that the Bi ͑006͒ peak width was 0.14° comparable
to the theoretical width expected for a perfect 70 nm thick Bi
1
2
–4
netic of metals, has the largest magnetoresistence,
and
4
,5
12
quantum size effects were also first reported in Bi. Using
vacuum deposition techniques, epitaxial Bi has been ob-
film ͑0.12°͒. Thus, a very small broadening occurred due to
defects. X-ray pole figures using the Bi ͕02 2គ 4͖ reflections
showed the expected threefold symmetry ͓Fig. 1͑b͔͒ of a
single Bi crystal. The spots at higher tilt angles are due to the
GaAs ͕311͖ planar reflections which are tilted 3° away from
the Bi ͕02 2គ 4͖ reflections. They confirm the epitaxial in-plane
arrangement between the two materials. The weak extra
spots visible in between at the hexagonal symmetry positions
5
6
7
8
tained on cleaved mica, BaF , GaSb, GaAs͑110͒, and
2
9
CdTe using molecular-beam-epitaxy ͑MBE͒ techniques, or
on Au/Si͑100͒ obtained by annealing electrodeposited poly-
4
crystalline Bi films. Highly textured films have also been
reported for ͑001͒ and ͑011͒ GaAs orientations using
1
0
electrochemistry. However, the direct epitaxial growth of
single crystalline Bi films using electrochemistry has not
been reported.
1
3
͑
circled͒ are likely due to twinning defects. We see further
evidence of these from secondary electron images of the sur-
face obtained by field emission, scanning electron micros-
copy ͑SEM͒. Figure 2 is a typical example showing triangle-
shaped surface facets visible, consistent with the trigonal Bi
crystal symmetry. The dark lines are likely surface steps at
twinning boundaries inferred from the rotation in surface
facet direction inside these regions.
Recently, we have shown that the addition of ammonium
sulfate, ͑NH ͒ SO , into iron sulfate ͑FeSO ͒ aqueous solu-
4
2
4
4
tions can lead to the electrodeposition of high quality epitax-
11
ial Fe films on GaAs ͑111͒, ͑110͒, and ͑001͒ substrates. In
this paper, we report on a similar effect of ͑NH ͒ SO on the
4
2
4
electrodeposition of single crystalline Bi films epitaxially on
GaAs ͑111͒ substrates.
Finally, single crystalline structure was also confirmed
from TEM. A plan-view electron diffraction pattern is shown
in Fig. 1͑c͒. The corresponding images reflect single crystal-
line material, consistent with the diffraction pattern, which
clearly shows the expected hexagonal symmetry. The extra
spots around each hexagonal spot are due to double
The substrates were oriented ͑±0.5°͒, n-type ͑Si-doped,
1
8
17
1
–2ϫ10 or 2–3ϫ10 ͒ GaAs ͑111͒-B ͑As-terminated͒
2
epiready wafers cleaved into 5ϫ6 mm pieces. The electro-
lyte was composed of a saturated solution of reagent-grade,
bismuth ͑III͒ nitrate pentahydrate, Bi͑NO ͒ ·5 H O,
14
3
3
2
diffraction in the Bi and GaAs. Indexing of this pattern
͑
NH ͒ SO , and deionized water. Photoresist was used to
4 2 4
confirms that the Bi out-of-plane trigonal axis ͑0001͒ is par-
allel to the GaAs ͑111͒, and that the Bi in-plane hexagonal
mask the substrate backside and to define the deposition win-
dows exposed to the electrolyte. Prior to the electrodeposi-
tion, the substrates were dipped into an aqueous ammonium
hydroxide solution ͑10%͒ ͑10 s͒ to remove the native oxide,
rinsed in distilled water ͑10 s͒, and then immediately trans-
ferred into the electrolyte. An InGa ohmic contact was used
to connect the GaAs substrate to the constant current power
͕
͗
10 1គ 0͖ directions are parallel to the GaAs in-plane hexagonal
110͘ directions, consistent with the x-ray diffraction results.
Film crystalline quality declined for growths at room
temperature, as indicated by broader x-ray peak widths and
lower peak intensities, however, a preferred ͑0001͒ texture
remained. The Bi͑NO ͒ ·5 H O electrolyte by itself is
3
3
2
2
supply. Galvanostatic electrodeposition ͑10 mA/cm ͒ was
strongly acidic and the addition of ammonium sulfate inhib-
its the formation of metal oxide ions in the electrolyte and at
the film surface. Electrodeposition without the addition of
͑NH ͒ SO , resulted in textured or polycrystalline films at all
carried out at 70 °C without electrolyte agitation. Growth
rates were approximately 70 nm/min based on cross-
sectional transmission electron microscopy ͑TEM͒.
4
2
4
Low-resolution x-ray diffraction -2 scans for
growth temperatures, typical of Bi deposited in acidic
9
,10,12
7
0±5 nm thick Bi/GaAs films are shown in Fig. 1͑a͒. Bis-
muth ͑003͒, ͑006͒, and ͑009͒ diffraction peaks and the GaAs
111͒ and ͑333͒ substrate peaks are detected, indicating a
electrolytes.
No evidence of oxygen in the films or an
interfacial oxide layer was detected in our films using cross-
sectional TEM investigations.
͑
rhombohedral Bi structure with its trigonal axis normal to the
substrate surface. The peak positions indicate that the Bi film
is relaxed in the out-of-plane direction.
A typical current-voltage characteristic for a Bi/GaAs
͑111͒ diode grown on a lower doped substrate at 70 °C is
shown in Fig. 3. All diodes displayed close to ideal thermi-
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
003-6951/2006/88͑2͒/022102/3/$23.00 88, 022102-1 © 2006 American Institute of Physics
31.211.208.19 On: Wed, 03 Dec 2014 06:41:57
0
1
Bao, Zhi Liang
