Electrodeposition of bismuth thin films on n-GaAs (110)

Article abstract of DOI:10.1063/1.1886248

Bismuth thin films are formed electrochemically on n-GaAs (110). Bismuth films up to a few hundred nanometers in thickness exhibit a strong (018) texture, while thicker films are polycrystalline. The barrier height of the n-GaAsBi Schottky contacts is 0.62 eV, about 0.2 eV lower than for electrodeposited bismuth films on GaAs (100).

Full text of DOI:10.1063/1.1886248

Electrodeposition of bismuth thin films on n-GaAs (110)
Philippe M. Vereecken, Kenneth Rodbell, Chunxin Ji, and Peter C. Searson
Citation: Appl. Phys. Lett. 86, 121916 (2005); doi: 10.1063/1.1886248
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APPLIED PHYSICS LETTERS 86, 121916 ͑2005͒
Electrodeposition of bismuth thin films on n-GaAs „110…
Philippe M. Vereecken and Kenneth Rodbell
IBM T. J. Watson Research Center, Yorktown Heights, New York 10591
Chunxin Ji and Peter C. Searson
Johns Hopkins University, Department of Materials Science and Engineering, Baltimore, Maryland 21218
͑Received 4 January 2005; accepted 14 February 2005; published online 17 March 2005͒
Bismuth thin films are formed electrochemically on n-GaAs ͑110͒. Bismuth films up to a few
hundred nanometers in thickness exhibit a strong ͑018͒ texture, while thicker films are
polycrystalline. The barrier height of the n-GaAs/Bi Schottky contacts is 0.62 eV, about 0.2 eV
lower than for electrodeposited bismuth films on GaAs ͑100͒. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.1886248͔
Bismuth is a semimetal with unusual electronic proper-
ties related to its small carrier effective masses, low carrier
concentration, and long carrier mean free path.1 Bulk single
crystals can be prepared by solidification from the melt.2–4
However, thin film deposition is complicated by its low melt-
ing point and low vapor pressure. Sputter deposition results
in polycrystalline thin films with very small grain sizes,
whereas large grained polycrystalline films can be obtained
by electrodeposition. Epitaxial Bi thin films can be grown by
evaporation ͑e.g., Refs. 5 and 6͒ and by molecular beam
epitaxy ͑MBE͒ ͑e.g., Refs. 7–9͒.
In previous work we have shown that high quality, ͑012͒
textured bismuth films can be obtained on GaAs ͑100͒ by
electrodeposition and subsequent thermal annealing.10 In this
paper, we report on the direct electrodeposition of highly
oriented bismuth films on GaAs ͑110͒.
Bismuth films were electrodeposited from a solution
containing 0.02 M BiO+ in 2 M HClO4 on Te doped n-type
GaAs ͑110͒ wafers ͑ND=1.2ϫ1018 cm−3, Wafer World,
Inc.͒. Ohmic contacts were formed by thermal evaporation of
In on the back side of the wafers followed by annealing at
400 °C in a N2 atmosphere for 10 min. Solutions were pre-
pared from 0.01 M bismuth oxide ͑Bi2O3 Puratronic
99.9999%, Alfa Aesar͒ dissolved in 2 M perchloric acid
͑HClO4, 70% doubly distilled, Alfa Aesar͒. Deposition ex-
periments were performed in a custom-built Kel-F cell with
inlets and outlets for gas purging and solutions. The cell
included a platinum wire ͑Puratronic 99.9999%, Alfa Aesar͒
counter electrode and a Ag/AgCl ͑3 M NaCl͒ reference elec-
trode located in a separate compartment in contact with the
solution in the cell through a Luggin capillary. All potentials
are reported with respect to the Ag/AgCl reference ͑0.21 V
vs SHE͒. The GaAs wafers were assembled in the cell with
an exposed surface area 1 cm2.
Immediately before deposition, Ar flow through the solution
was stopped while an Ar blanket was maintained above the
solution. After deposition, the bismuth solution was replaced
by 1.2 M HCl. Finally the films were rinsed with distilled,
deionized water and dried in a nitrogen stream. All experi-
ments were performed at room temperature ͑297 K͒. Rotat-
ing disk experiments were performed in a standard three-
electrode cell with a mountable wafer chip electrode holder.
Figure 1 shows typical current–voltage curves for
n-GaAs ͑110͒ in 0.02 M BiO+ in 2 M HClO4. The onset
potential for bismuth deposition is about −0.14 V. For the
stationary electrode, a characteristic reduction peak is seen at
−0.3 V due to the nucleation and diffusion-limited growth of
bismuth. On the reverse scan, deposition continues to a po-
tential of about 0 V. The total deposition charge during the
scan ͑0.1 C cm−2͒ corresponds to a Bi film of about 70 nm or
340 equivalent monolayers. The deposition of Bi results in
the formation of a Schottky junction so that for UϾ0 V the
n-GaAs/Bi junction is under reverse bias and no anodic cur-
rent is seen until about 0.4 V when the band bending is suf-
ficiently large that tunneling becomes possible and bismuth
stripping is observed.12,13 The stripping charge was 0.90–
0.93 of the deposition charge indicating a small contribution
of hydrogen evolution at the more negative deposition poten-
tials.
At a rotating disk electrode the deposition current be-
comes diffusion controlled at about −0.3 V depending on the
Prior to each experiment, the wafer surfaces were pre-
treated by rinsing first in 1 M NH4OH for about 15 min and
then in 6 M HCl for 10–15 min. The GaAs surface was then
etched in
a mixture of concentrated H2SO4, H2O2
͑30 vol %͒ and water ͑3:1:1 by volume͒ for 10 s with agita-
tion, rinsed with distilled, deionized water, and finally rinsed
in 6 M HCl to remove the formed oxide.11 After removing
the HCl, deaerated 2 M HClO4 was introduced into the cell
while maintaining the flow of Ar. After about 15 min the
supporting electrolyte solution was removed and the deaer-
ated 20 mM BiO+ in 2 M HClO4 was introduced to the cell.
FIG. 1. Current–voltage curves for n-GaAs ͑110͒ in solution containing
20 mM BiO+ and 2 M HClO4 at a scan rate of 20 mV s−1: ͑a͒ stationary
conditions and ͑b͒ rotating disk electrode at a rotation rate of 5 s−1. The inset
shows the diffusion limited deposition current at −0.5 V versus the square
root of the rotation rate.
0003-6951/2005/86͑12͒/121916/3/$22.50
86, 121916-1
© 2005 American Institute of Physics
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121916-2
Vereecken et al.
Appl. Phys. Lett. 86, 121916 ͑2005͒
FIG. 3. 2␪ vs ␹ x-ray diffraction patterns for ͑a͒ 100 nm and ͑b͒ 1 ␮m thick
bismuth films deposited on GaAs ͑110͒.
FIG. 2. Atomic force microscope images of ͑a͒ 3.5 nm, ͑b͒ 16 nm, and ͑c͒
100 nm electrodeposited bismuth films on n-GaAs ͑110͒.
been reported using MBE ͑Refs. 15 and 16͒ and PVD at low
substrate temperatures.17
rotation rate. The inset of Fig. 1 shows that the diffusion
limited deposition current is proportional to the square root
of the rotation rate in accordance with convective-diffusion
mass transport. The diffusion coefficient for BiO+, deter-
mined from the slope, is 0.6ϫ10−5 cm2 s−1.
Bismuth thin films up to 1 ␮m in thickness were depos-
ited at constant potential using a two step process, without
agitation. A short ͑0.1–0.25 s͒ nucleation pulse at −0.275 V
was followed by growth at −0.02 or −0.025 V to the final
film thickness. During the nucleation pulse the current den-
sity was between 5 and 40 mA cm−2, corresponding to
1–5 nm of bismuth. The as-deposited films were bright and
silver colored although they became gold colored after rins-
ing and drying.
Figure 2 shows AFM images of bismuth films for depo-
sition charges of 5, 22, and 135 mC cm−2. For a deposition
charge of 5 mC cm−2 ͑17 ML͒, the surface is covered by
isolated 20–50 nm diameter bismuth islands up to 10 nm in
height. The relatively flat islands are in contrast to the hemi-
spherical islands typically associated with diffusion-limited
growth. For a deposition charge of 22 mC cm−2 ͑74 ML͒ the
islands have coalesced resulting in a relatively smooth 16 nm
thick film. For a deposition charge of 135 mC cm−2 ͑454
ML͒, the films are polycrystalline with grain sizes of
50–100 nm and an average thickness of 100 nm.
For the 1 ␮m thick film, the appearance of rings shows
the gradual loss of texture and the emergence of a polycrys-
talline film. Perpendicular to the surface, streaks correspond-
ing to Bi ͑006͒, ͑024͒, ͑205͒, and ͑207͒ are evident. We note
that strong ͑012͒ texture has been previously reported for
electrodeposited Bi films on GaAs ͑100͒.10
Figure 4 shows a typical semilogarithmic current–
voltage curve for a n-GaAs͑110͒/Bi junction with a 200 nm
bismuth film. The n-GaAs/Bi junctions showed rectifying
behavior with an exponential forward current at negative po-
tentials and a reverse current of about 10−4 A cm−2 at posi-
tive potentials. The barrier height ⌽B determined from the
forward bias current region according to thermionic emission
theory18 was about 0.62 eV with an ideality factor, n=1.19.
Previously, we have measured barrier heights of about
0.80 eV for bismuth films electrodeposited on
n-GaAs ͑100͒ ͑ND=3ϫ1016 cm−3͒.12 Hence, the measured
barrier height for n-type GaAs/Bi Schottky junctions is
about 0.2 eV lower for ͑110͒ than for ͑100͒ single crystal
GaAs. The relatively high value for the ideality factor n is
due to the relatively high donor density ͑ND=1.2
ϫ1018 cm−3͒.
Figure 3 shows the 2D plane ͑2␪−␹͒ x-ray diffraction
pattern for 100 nm and 1 ␮m bismuth films.14 The spots in
the diffraction pattern for the 100 nm films, characteristic of
films up to several hundred nanometers in thickness, indicate
that the films have very strong in-plane texture. Perpendicu-
lar to the film plane ͑␹=0͒, only the spots for GaAs ͑220͒
and Bi ͑018͒ are seen. The unit cell for the GaAs ͑110͒ plane
is 4.0 Åϫ5.65 Å with four atoms in the corners. The Bi
͑018͒ plane has two layers, one slightly above the other, with
a rectangular unit cell. The unit cells, with four atoms in the
corners, are 4.546 Åϫ5.486 Å and 4.546 Åϫ5.755 Å, cor-
responding to misfits of 13% in the short direction and
−2.9% or 1.8% in the long direction. These results suggest
that the Bi ͑018͒ plane can form an epitaxial 1ϫ1 overlayer
on GaAs ͑110͒. Formation of epitaxial ͑1ϫ1͒ bismuth over-
layers up to a few monolayers thick on GaAs ͑110͒ have
FIG. 4. Current–voltage curves for a n-GaAs͑110͒/Bi junction with a
200 nm bismuth film. The series resistance was about 3 ⍀ cm2. The inset
shows the time dependence of the barrier height after deposition.
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121916-3
Vereecken et al.
Appl. Phys. Lett. 86, 121916 ͑2005͒
Technol. A 17, 3473 ͑1997͒.
The time dependence of the barrier height of a
n-GaAs͑110͒/Bi contact is shown in the inset of Fig. 4.
The small 10 meV increase from an initial value of
0.616 eV to 0.626 eV after one month suggests that the bis-
muth film is dense and continuous completely covering the
GaAs surface. The 10 meV change is probably due to slow
oxidation at the edges of the sample.12
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14X-ray measurements were obtained using a Bruker 3-axis goniometer
͑Bruker AXS, Inc., Madison, WI͒ General Area Diffraction Detector Sys-
tem ͑GADDS͒ with a Rigaku RU 300 ͑The Woodlands, TX͒ rotating an-
ode generator ͑Cu K␣ rotating anode source͒ operating at 40 kV and
100 mA. The Bruker GADDS system uses a stationary areal detector
͑Bruker HI-STAR͒ to acquire x-ray intensities as the sample is rotated
about the substrate normal.
This work was supported by the JHU MRSEC ͑NSF
Grant No. DMR00-80031͒.
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