Fabrication of bismuth nanowires with a silver nanocrystal shadowmask

Abstract

We fabricated bismuth (Bi) nanowires with low energy electron beam lithography using silver (Ag) nanocrystal shadowmasks and a subsequent chlorine reactive ion etching. Submicron-size metal contacts on the single Bi nanowire were successfully prepared by in situ focused ion beam metal deposition for transport measurements. The temperature dependent resistance measurements on the 50 nm wide Bi nanowires showed that the resistance increased with decreasing temperature, which is characteristic of semiconductors and insulators.

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Fabrication of bismuth nanowires with a silver nanocrystal shadowmask
S. H. Choia)and K. L. Wang
Electrical Engineering Department, University of California at Los Angeles, Los Angeles,
California 9095-1594
M. S. Leung, G. W. Stupian, N. Presser, B. A. Morgan, R. E. Robertson, M. Abraham,
E. E. King, and M. B. Tueling
Electronics Technology Center, The Aerospace Corporation, El Segundo, California 90245-4691
S. W. Chung and J. R. Heath
Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles,
California 90095-1569
S. L. Cho and J. B. Ketterson
Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208
?Received 1 October 1999; accepted 3 January 2000?
We fabricated bismuth ?Bi? nanowires with low energy electron beam lithography using silver ?Ag?
nanocrystal shadowmasks and a subsequent chlorine reactive ion etching. Submicron-size metal
contacts on the single Bi nanowire were successfully prepared by in situ focused ion beam metal
deposition for transport measurements. The temperature dependent resistance measurements on the
50 nm wide Bi nanowires showed that the resistance increased with decreasing temperature, which
is characteristic of semiconductors and insulators. © 2000 American Vacuum Society.
?S0734-2101?00?03704-0?
I. INTRODUCTION
Nanometer-size structures such as wires and dots have
aroused considerable interests as ideal systems for testing
predictions about quantum confinement and reduced dimen-
sionality, and as building blocks for nanostructured materi-
als. A more practical reason for the study of nanostructures is
the ever present drive towards smaller sizes in the electronics
industry.1In particular, semimetallic bismuth ?Bi? with very
small effective mass and high carrier mobilities is reported to
be a good candidate to study quantum-confinement effects in
one-dimensional systems and a very promising material for
thermoelectric applications.2
Recently, a number of fabrication techniques have been
utilized to produce Bi nanowires. For example, filling porous
anodic alumina with Bi from the liquid phase resulting in
single-crystal nanowire arrays having the same crystal struc-
ture and lattice parameters as bulk.3However, it would be
difficult to make contacts to a single Bi wire to study its
one-dimensional properties.
We previously demonstrated a new fabrication technique
for the fabrication of nanometer-size polymethyl methacry-
late ?PMMA? patterns using Ag nanocrystal shadowmasks.4
In this work, we used same technique and a subsequent re-
active ion etching to fabricate Bi nanowires. 100 nm wide
platinum ?Pt? lines were written to contact a Bi single
nanowire by the FIB to permit transport measurement.
II. EXPERIMENTAL RESULTS
First, we prepared 40 nm thick Bi single-crystal films by
molecular beam epitaxy ?MBE? system. The substrates for
the MBE growth were indium doped semi-insulating
CdTe?111?B pieces, 1 cm2in size. X-ray diffraction showed
only sharp ?000l? peaks, which implied c-axis growth of Bi
perpendicular to the substrates as shown in Fig. 1. Self-
assembled high-aspect ratio Ag wire structures were trans-
ferred as a Langmuir–Schaeffer film to 40 nm thick 1%
PMMA coated MBE-grown Bi/CdTe substrates. Substrates
with the transferred Ag wires were exposed by a JEOL
6401F fieldemission scanning
?FESEM? at 700 V to provide samples with an electron dose
of 50 ?C/cm2. At 700 V, 40 nm thick PMMA was thought to
be exposed all the way to surface of Bi film. The penetration
depth of electron in silver was found to be 4 nm at 700 V by
a previous Monte Carlo simulation.4That penetration depth
is smaller than the thickness of Ag nanocrystal shadowmask.
Following the electron beam exposure, a sample was devel-
oped for 1 min in a mixture of methyl isobutyl ketone and
isopropanol in the ratio 1:3. A subsequent anisotropic reac-
tive ion etching ?RIE? process was carried out by a Plasma-
Master Model PME 1200 chlorine etcher. With a BCl3to
Ar2mixture at 20 mTorr, and a plasma sustaining power of
200 W, RIE process transferred Ag nanowire patterns to Bi/
CdTe substrates. The mask material is believed to consist of
30 nm thick Ag and 40 nm thick PMMA layers.
Its electronic orbital configuration of Bi implies that Bi
prefers to have two ionization states. The following simple
model of the Bi etching mechanism for the case of Bi?3
ionization state is proposed:
electronmicroscope
Bi→Bi?3?3e?
3
2Cl2?3e?→3Cl?
a?Electronic mail: sungchoi@seas.ucla.edu
13261326J. Vac. Sci. Technol. A 18„4…, JulÕAug 20000734-2101Õ2000Õ18„4…Õ1326Õ3Õ$17.00 ©2000 American Vacuum Society

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Bi?3
2Cl2→Bi?3?3Cl?→3BiCl3?volatile?.
Following formation of Bi?3and Cl?3ions by plasma, vola-
tile products of BCl3were formed and washed away. Ar in
the etch gas mixture might contribute to the reduction of the
undercut profile. We have found a mixture of BCl3and Ar2
to be an excellent choice of gases to be used in RIE of Bi,
producing vertical profiles and etching rates at about 100
nm/min. The RIE chemical mechanism for Bi is found to be
similar to that of GaAs etching by chlorine gases. Through
the RIE process, 50 nm wide and 40 nm high Bi nanowires
were fabricated on CdTe substrates.
Submicron-size platinum ?Pt? contacts on a Bi single
nanowire were prepared by in situ FIB metal deposition at 25
kV, 6 pA to allow temperature dependent resistance mea-
surements. Figure 2 shows the FESEM micrograph of a 50
nm wide, 40 nm high Bi nanowire to which Pt contacts were
made by the FIB. The electrical resistance of the wire mea-
sured was typically on the order of 1–20 ?. The temperature
dependence of the resistance of 50 nm wide Bi nanowire is
shown in Fig. 3. The data show that resistance increases with
decreasing temperature, which is characteristic of semicon-
ductors and insulators. As quantum confinement is intro-
duced into the Bi nanowire system, the external conduction
subband and valence subband edges move in opposite direc-
tions to eventually form a positive energy band gap (Eg)
between the lowest L-point conduction subband edge and the
highest T-point valence band edge, thereby leading to a
semimetal-semiconductor transition (Eg?0? as the wire size
is decreased below the critical wire width of Bi. Dresselhaus
et al. found that Bi makes a transition at critical wire radius
of 52 nm.5In the Bi nanowires, the carrier mobility is sup-
pressed by carrier confinement along the direction of wire
and by surface imperfection.
III. CONCLUSIONS
In conclusion, 50 nm wide Bi nanowires were fabricated
by low energy electron beam lithography using Ag nanoc-
rystals as a shadowmask and a subsequent chlorine reactive
ion etching process. Temperature dependent resistance mea-
surements show that the Bi nanowire fabricated has semicon-
ductor properties rather than metallic properties. Our method
may be a good candidate of fabrication technique for study-
ing Bi nanowires.
ACKNOWLEDGMENTS
The authors gratefully acknowledge support by the Aug-
mentation Awards for Science and Engineering Research
Training ?AASERT? Fellowship Grant No. N00014-96-1-
1258, the Office of Naval Research MURI, and ARO MURI
and the U.S. Air Force under Contract No. F04701-93-C-
0094. One of the authors ?J.R.H.? acknowledges the Office of
Naval Research Contract No. N00014-981-0422, and the
Packard Foundation.
FIG. 1. X-ray diffraction pattern of MBE-grown Bi film on CdTe ?111?B
substrate.
FIG. 2. FESEM micrograph of a 50 nm wide Bi nanowire with metal con-
tacts. Pt contacts to the wire were prepared by in situ FIB metal deposition.
FIG. 3. Temperature dependence of the resistance of the 50 nm wide Bi
nanowire.
1327 Choi et al.: Fabrication of bismuth nanowires with a silver
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