APPLIED PHYSICS LETTERS 86, 073101 共2005兲

Fabrication of nanoscale charge density wave systems
Katsuhiko Inagaki,a兲 Takeshi Toshima, Satoshi Tanda, and Kazuhiko Yamaya
Department of Applied Physics, Graduate School of Engineering, Hokkaido University,
Kita 13 Nishi 8 Kita-ku, Sapporo 060-8628, Japan

Shinya Uji
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

共Received 1 June 2004; accepted 14 December 2004; published online 7 February 2005兲
Nanoscale charge density wave systems of quasi-one-dimensional o-TaS3 crystals were fabricated.
Gold electrodes 400 nm wide were made by standard lift-off technique on o-TaS3 nanocrystals
prepared by deposition on silicon substrates. Interface resistance was higher than 100 G⍀ just after
evaporation, and were significantly reduced by electron-beam irradiation. The electrodes were tested
down to 80 mK, and were found quite durable for cryogenic measurement. The temperaturedependence of the resistance of the nanocrystal was represented as the variable-range-hopping-typeconduction with one dimension over the wide range of temperature, from 4.2 to 100 K. Thisbehavior was different from that of conventional bulk samples.
© 2005 American Institute of Physics. 关DOI: 10.1063/1.1862782兴
The charge density wave 共CDW兲 is an emergence ofmacroscopic quantum coherence of length greater than
1 ␮m, accompanied with lattice deformation of the wave
vector 2kF.1 Much attention has been given to nanoscale
metal trichalcogenides 共MX3: M = Nb, Ta; X = Se, S兲 crystals
due to their quasi-one-dimensional 共1D兲 transport
properties.2–4 Reduction of the system size is expected tolead new macroscopic quantum phenomena because a wholenanocrystal becomes a single-domain CDW system, in contrast to a bulk crystal consisting of many CDW domains.
Investigation of the nanoscale CDW system will be a key
issue to realize prospecting applications, which exploit quantum collective motion of CDWs; for instance, field-effecttransistors,5 electron pumps,6 and ultrafast memory.7 Moreover, the recent discovery of MX3 topological 共ring, Möbius,and figure-of-eight兲 crystals8 has opened a field of possible
CDW devices by exploiting interference of long-range CDW
order through their nontrivial topology. Since the system sizemust be smaller than the dephasing length of the CDW, thenanoscale CDW system is also crucial to observe the topological effect.
Studies on the nanoscale CDW were, however, often
hindered by unexpectedly high interface resistance between
the crystal and the electrodes.2–5 To reduce interface resistance, various techniques were exploited; for example, fixing
a sample to gold electrodes with glue,2 and making use ofsoft metal 共e.g., indium兲 as electrodes.5 Therefore, a reproducible method for fabrication of electrodes is crucial forfurther study of the nanoscale MX3 system. In additions, it is
known that the electrodes themselves play an important rolein the sliding dynamics of CDWs.9 Hence, geometry of theelectrodes should be determined rather than simply using
rounded shapes of silver paint, which has been used to attachlead wires to samples.
We report here the fabrication of nanoscale CDW systems of o-TaS3. The electrodes were fabricated by the standard electron-beam lithography technique. The contact resistance was too large to measure electrical properties, but ita兲Electronic mail: kina@eng.hokudai.ac.jp

was found to be significantly improved after electron-beamirradiation. We found that the nanoscale o-TaS3 crystal
showed a variable-range-hopping 共VRH兲 conduction over a
wider range of temperatures than those observed in previousstudies.
To obtain nanoscale o-TaS3 samples, we synthesized
single crystals by the chemical vapor transport method. A
pure tantalum sheet and sulfur powder were put in a quartz
tube. The quartz tube was evacuated to 1 ⫻ 10−6 Torr andheated in a furnace at 530 °C for 5 h. The reaction duration
was chosen much shorter than the typical condition since weneeded rather smaller crystals. The crystals were sonicated intoluene for 15 min and kept from perturbation for several
hours to sediment unwanted larger crystals. The dispersion
was then deposited on a silicon substrate with thermal oxide
layer of 1 ␮m. After blow-drying, the crystals were inspected with an optical microscope. The obtained crystals
were typically 0.2 ␮m in width and 10–100 ␮m in length.
Electrodes were fabricated by standard electron-beamlithography with a scanning electron microscope 共SEM;
JSM-5200, JEOL兲 equipped with a homemade writing system. On the substrate, 50-nm-thick gold markers and contact
pads were written prior to deposition. Polymethylmethacrylate 共PMMA兲 resist was spun to 500 nm thick, followed by
prebaking at 170 °C for an hour. From the position of thecrystal relative to the markers, a mask pattern was designedand loaded to the electron-beam writer. After the developing
process by methyl isobutyl keton diluted by isopropylalcohol
共1:2兲, 50-nm-thick gold thin film was evaporated. An adhesion layer of titanium was optionally evaporated between thegold and the substrate to investigate whether the adhesion
layer affects the contact resistance. Lift-off was done withacetone at room temperature. We needed a further process toobtain an Ohmic contact. Each electrode was heated by irradiation of an electron beam with a SEM 共JSM-820, JEOL兲.
The electron current was monitored at the sample stage andby the probe current detector inserted in the beam line.
Figure 1 is a scanning electron micrograph of a TaS3
crystal on a silicon substrate with gold electrodes. It is shownthat the gold electrodes were well defined and located pre-0003-6951/2005/86共7兲/073101/3/$22.50
86, 073101-1© 2005 American Institute of Physics
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Inagaki et al.

FIG. 1. Scanning electron micrograph of an o-TaS3 nanocrystal attachedwith gold electrodes.

cisely on the nanocrystal. The width of the electrode was 400
nm with a separation of 1 ␮m. Each electrode connects to abonding pad 共100⫻ 100 ␮m2兲. We fabricated gold electrodes
for several nanocrystals; however, roughly a half of themfailed. In most cases of failure, the crystal was washed away
at the lift-off stage. This was probably because of strong
affinity between the TaS3 surface and the PMMA. Anotherchoice of electron beam resist rather than PMMA may solve
the problem. The use of a Ti adhesive layer increased theyield on lift-off, although it affected contact resistance. Wewill discuss this problem shortly.
Figure 2共a兲 shows room-temperature I-V characteristicsof the sample both before 共the broken line兲 and after 共thesolid line兲 electron-beam treatment. The resistance was originally larger than 100 G⍀, while it decreased markedly to 21k⍀ after a 30 s irradiation of electron beam at an acceleration
voltage of 25 kV and a beam current of typically 1⫻ 10−6 A. Since this was taken by two-terminal measurement, the obtained resistance consists of sample resistance,
contact resistance, and the current-limiting series resistor of
2 k⍀. By taking into account the sample dimension 2 ⫻ 0.2⫻ 0.2 ␮m3, the resistivity is calculated as 3 ⫻ 10−4 ⍀ m. This

Appl. Phys. Lett. 86, 073101 共2005兲

is larger than the bulk resistivity by two orders. The crosssection that actually effects electric conduction is 10−2 of thatwhich appears in the micrograph, if the resistivity is same as
the bulk sample. Previous studies of o-TaS3 thin wires haveshown a similar tendency.3
Ohmic conduction was not obtained for the electrodewith a Ti adhesive layer. Figure 2共b兲 shows the I-V curves ofbefore 共the broken line兲 and after 共the solid line兲 the electron
irradiation. The curve did not change after the electron beam
was irradiated for a period longer than 30 s. Titanium ischemically active and easily forms a bond to other materials.
That is why it is often used as an adhesive layer between thesubstrate and the electrode. From the obtained I-V curves, webelieve a thin Ti layer was oxidized or sulfurized when it wasdeposited onto the o-TaS3 surface, and it became a tunnel
barrier.
During irradiation of an electron beam in SEM, thesample sometimes gets dirty due to contamination with carbonate. We believe this plays a small role in the transportmeasurements for the following reasons. 共1兲 The typicalsample size is 10–100 ␮m in length, and we use the optical
microscope for locating of the sample. 共2兲 Since the gold
electrodes are evaporated after deposition of the sample, themetal layer covers and protects the contact area during irradiation. 共3兲 The electron beam is irradiated only on themetal-sample contact, which is to be heated locally, althougha weak electron beam is scattered when we look at thesample. The difference between the I-V characteristics
shown in Figs. 2共a兲 and 2共b兲 supports our observation.
The fabricated electrodes were found to be very durable.
We cooled the electrodes to 80 mK several times, and observed no degrading of the electrode. Even after storage forsix months, electric properties showed only little change.
This is very astonishing because conventional methods toattach the electrodes on MX3 crystals are fragile and can only
be used for a couple of measurements. Hence, our method
will be important for future applications of MX3 nanocrystals
to electronics.
Let us discuss the temperature dependence of resistanceof the nanoscale o-TaS3 crystal. It is known that the temperature dependence of the resistance of conventional bulk
samples is complex and consists of several regimes. At room
temperature, a bulk o-TaS3 crystal is metallic. Below thePeierls temperature 共T P ⬃ 220 K兲, the resistance rises exponentially 关exp共⌬ / kBT兲兴, where ⌬ ⬃ 800 K. At lower temperatures than ⬃100 K, the potential of exponential rise changes⌬ ⬃ 200 K. This change in the potential is described in termsof “soliton transport”.10 Below 20 K, the formula of the temperature dependence changes from the Arrhenius form toVRH conduction, represented as␴共T兲 = ␴0 exp关− 共T0/T兲1/共d+1兲兴,

共1兲

where T0 is a characteristic temperature and d = 1 is dimension of the system.11,12
On the other hand, the observed temperature dependenceof the nanoscale o-TaS3 crystal was significantly different
from that of the bulk crystals. Figure 3 shows the o-TaS3
nanocrystal differs from that of the bulk samples at the following points: 共1兲 Peierls transition was not clearly observedin the nanocrystal, and 共2兲 resistance was described in termsof VRH conduction over the wide range of temperatures
共100 to 4.2 K兲 with d = 1 共inset of Fig. 3兲.

FIG. 2. I-V characteristics of TaS3 nanocrystals with 共a兲 50-nm-thick gold
electrodes only, and 共b兲 50-nm-thick gold electrodes with a 20-nm-thick Ti
adhesive layer. The broken line and the solid line in each pane show the data
before and after the electron-beam treatment, respectively. The solid curvein 共a兲 shows that the Ohmic conduction of 21.9 k⍀ is achieved for theelectrodes without an adhesive layer. On the other hand, use of the Ti adhesive layers prevents Ohmic conduction 共b兲.
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Appl. Phys. Lett. 86, 073101 共2005兲

Inagaki et al.

ties. We believe that in this regime the carrier is dislocationof the CDW, or charged soliton. The energy to excite a soliton differs in each position due to impurities. Further studies
will unveil the physical nature of the VRH conduction ino-TaS3 by careful comparison of nanoscale samples withconventional bulk ones.

FIG. 3. Temperature dependence of resistance of the o-TaS3 nanocrystal.
Inset shows the linear conductance from 4.2 to 100 K as a function of T−1/2.
The data fit to a linear line very well, suggesting VRH conduction:␴ ⬀ exp关−共T0 / T兲1/2兴.

The authors thank Prof. Pertti Hakonen, Helsinki University of Technology, Prof. Noriyuki Hatakenaka and Prof.
Munehiro Nishida, Hiroshima University, and Prof. Andrew
Cleland, University of California, Santa Barbara, for fruitful
discussions. This work was partly supported by Grant-in-Aid
for Scientific Research, Japan Society for the Promotion ofScience.
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14This is contrast to the case of NbSe3, in which not all of the Fermi surface
contributes to the Peierls transition, and there still remain electron stateseven after the transition.
1The first feature is interpreted as the result of 1D fluctuation because the cross section is small enough.3 The CDW
actually occurs in “quasi”-1D materials, consisting of abundle of conductive chains. The CDW phase is stabilized
by correlating adjacent chains through a Coulomb interaction. If the cross section, and hence the number of the chains,is reduced, the system cannot gain its free energy by themechanism of Peierls transition. We believe that the interpretation is consistent with our sample whose cross section is4 ⫻ 10−2 ␮m2.
Another feature is rather surprising. The observed resistance fell into the VRH relation 共1兲 with good agreement
over the wide range of temperatures. The physical picture ofthe VRH conduction is, however, inconsistent with the standard CDW mechanism. Assumptions for deriving Eq. 共1兲 include a finite density of states at the Fermi level.13 In theCDW state of o-TaS3, the whole Fermi surface is relevant to
the transition and no electron density remains.14 That is why
previous studies for bulk crystals showed good agreement in
the Arrhenius form.
Actually, the previous studies of o-TaS3 were unlikely toprovide acceptable interpretation of the VRH conduction, although it must be related with randomly distributed impuri-Downloaded 01 Apr 2007 to 133.87.166.22. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp