Electrical anisotropy of W-doped ReSe2 crystals

Article abstract of DOI:10.1149/1.2209589

Single crystals of W-doped Re Se2 have been grown by chemical vapor transport process with bromine as the transporting agent. Single crystalline platelets up to 3×3 mm surface area and 100 μm in thickness were obtained. From the X-ray diffraction patterns

Full text of DOI:10.1149/1.2209589

J100
Journal of The Electrochemical Society, 153 ͑8͒ J100-J102 ͑2006͒
0
013-4651/2006/153͑8͒/J100/3/$20.00 © The Electrochemical Society
Electrical Anisotropy of W-Doped ReSe Crystals
S. Y. Hu, * C. H. Liang, K. K. Tiong, Y. S. Huang, and Y. C. Lee
2
a,
b
b,z
c
d
a
Department of Electronic Engineering, Research Center for Micro/Nano Technology,
Tung Nan Institute of Technology, Taipei, 22202, Taiwan
b
Department of Electrical Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan
Department of Electronic Engineering, National Taiwan University of Science and Technology,
c
Taipei 10607, Taiwan
d
Department of Computer Science and Information Engineering, Research Center
for Micro/Nano Technology, Tung Nan Institute of Technology, Taipei 22202, Taiwan
Single crystals of W-doped ReSe have been grown by chemical vapor transport process with bromine as the transporting agent.
2
Single crystalline platelets up to 3 ϫ 3 mm surface area and 100 ␮m in thickness were obtained. From the X-ray diffraction
patterns, the doped crystals are found to crystallize in the triclinic-layered structure. The electrical anisotropy has been investigated
along and perpendicular to the b-axis on the van der Waals plane by temperature-dependent conductivity and Hall effect mea-
surements. The influence of the dopant will be compared and discussed.
©
2006 The Electrochemical Society. ͓DOI: 10.1149/1.2209589͔ All rights reserved.
Manuscript submitted November 14, 2005; revised manuscript received April 14, 2006. Available electronically June 13, 2006.
Rhenium diselenide ͑ReSe ͒ is a diamagnetic indirect semicon-
was allowed to cool down slowly ͑40°C/h͒ to ϳ 200°C. The am-
poule was then removed and wet tissues applied rapidly to the end
2
ductor belonging to the family of transition-metal dichalcogenides
crystallized in a distorted layered structure of triclinic symmetry
away from the crystals to condense the Br vapor. When the am-
2
¯
1,2
poule reached room temperature, it was opened and the crystals
removed. The crystals were then rinsed with acetone and deionized
water. Figure 1 shows the scanning electron microscopy ͑SEM͒ pho-
tograph of the as-grown W-doped crystal with the b-axis as indi-
cated. Single crystalline platelets up to 3 ϫ 3 mm surface area and
͑
space group P1͒. It is a subject of considerable interest because
of its extremely anisotropic electrical, optical and mechanical
3
properties, as a promising solar-cell material in electrochemical
4
,5
cells. Owing to the potential technological applications of the ma-
terial, a variety of efforts have been devoted to the theoretical and
6
-8
1
00 ␮m in thickness were obtained. The tungsten content x was
experimental understanding of the solid-state properties of ReSe2.
estimated by energy dispersive X-ray analysis ͑EDX͒. A consider-
able discrepancy exists between the nominal doping ratio and that
determined by EDX. The nominal concentration is much larger than
the actual one because no W could be detected in EDX even though
It is known that doping of semiconductor material can lead to a
change of the electrical properties. To date, few related works con-
cerning the effects of tungsten ͑W͒-doped on the electrical aniso-
9
tropy of ReSe have been reported.
2
1
0
this method is sensitive to concentrations of x Ͼ = 0.1%. The Re
and W metals are most likely to be chemically transported at differ-
ent rates and most of the doping material must remain in the un-
transported residual charge.
In this article we report the electrical anisotropy of the W-doped
ReSe2 single crystals. ReSe2 crystallizes in a distorted 1T-MX2
structure with clustering of Re4 diamond units forming a one-
dimensional chain within the van der Waals ͑VdW͒ plane. The dia-
For anisotropic electrical conductivity, a selected sample was ori-
ented and cut into a rectangular shape. Electrical connections to the
crystal were made by means of four parallel gold wires ͑parallel or
perpendicular to b-axis͒ with spiral shape laid across the basal sur-
face of the thin crystal and attached to the crystal surface by means
of highly conducting silver epoxy. The wires near each end of the
rectangular crystal acted as current leads, while the two contact
wires on either side of the central line were used to measure the
potential drop across the crystal. The voltage drop measured by a
sensitive potentiometer is taken to be the average value obtained on
reversing the current through the sample. The data was checked for
ohmic contact quality. For Hall effect measurements, the Hall volt-
age was measured between side arms of the sample which are op-
posite to each other. The measurements were made by taking data
mond chain clusters in the metal sheet of ReSe resulted in a lattice
2
distortion from the ideal octahedral layered structure resulting in an
electrical biaxial character of the compound. Therefore, in-plane an-
isotropic response is expected for linearly electrical field along and
1,2
perpendicular to the b-axis. The effects of dopant ͑W͒ on the
anisotropic electrical properties were studied and discussed.
Experimental
Single crystals of W-doped and undoped ReSe layered crystals
2
were grown by the chemical vapor transport process, respectively
with Br as the transporting agent. The total charge used in each
2
growth experiment was ϳ10 g. The stoichiometrically determined
weight of the doping material ͑ϳ0.5% nominal concentration͒ was
added. Prior to the crystal growth, a quartz ampoule containing Br
2
3
͑
ϳ5 mg/cm ͒ and the elements ͑W, 99.99% pure; Re, 99.99% pure;
Se, 99.999%͒ was cooled with liquid nitrogen, evacuated to
−
6
10
Torr and sealed. It was shaken well for uniform mixing of the
powder. The ampoule was placed in a three-zone furnace and the
charge pre-reacted for 24 h at 800°C with the growth zone at
1
000°C, preventing the transport of the product. The furnace was
then equilibrated to give a constant temperature across the reaction
tube, and was programmed over 24 h to give the temperature gradi-
ent at which single crystal growth took place. A temperature gradi-
ent of about 2°C/cm with the temperature range from
1050 to 1000°C over a reaction length of 25 cm gives optimal con-
dition for the crystallization of the samples. After 360 h, the furnace
*
Electrochemical Society Student Member.
E-mail: b0114@mail.ntou.edu.tw
Figure 1. The SEM of as-grown W-doped ReSe2 sample with the b-axis as
indicated and the experimental polarization schemes for electrical anisotropy.
z
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Journal of The Electrochemical Society, 153 ͑8͒ J100-J102 ͑2006͒
J101
Figure 2. XRD patterns of W-doped ReSe and undoped ReSe single crys-
2
2
tals.
for both directions of electrical and magnetic fields and averaging
the results, thus eliminating errors caused by misalignment of Hall
probes, thermal emfs, and all the thermomagnetic effects which are
dependent on the direction of the electric or magnetic fields. A RMC
model 22 closed-cycle cryogenic refrigerator equipped with a model
4
025 digital thermometer controller was used for temperature-
dependent measurements. The measurements were done between 20
and 300 K with a temperature stability of 0.5 K or better.
Results and Discussion
X-ray diffraction.— For X-ray diffraction ͑XRD͒ studies, several
small crystals from batches of as-grown W-doped ReSe were finely
2
ground with a mixture of glass powder and the X-ray powder pat-
terns were taken and recorded by means of a slow-moving radiation
detector. The Cu K␣ radiation ͑␭ = 1.542 Å͒ was employed and a
silicon standard was used for the experimental calibration. The
structural parameters with reasonable standard errors ͑±0.01%͒
were determined by the least-squares fit of SPSS SigmaPlot math-
ematical software. Figure 2 shows the XRD pattern of the undoped
ReSe2 together with W-doped ReSe2 and confirmed the triclinic
symmetry, while the lattice parameters a, b and c of Re-doped
samples were carefully determined to be 6.715 ± 0.005 Å,
Figure 3. ͑a͒ Temperature dependence conductivity of W-doped ReSe and
undoped ReSe2 single crystals. ͑b͒ Temperature dependence conductivity
anisotropy of W-doped ReSe2 and undoped ReSe2 single crystals.
2
11
increase is observed by adding W dopant. The corresponding value
for undoped ReSe and W-doped ReSe at 300 K is 3.67
2
2
1
6
−3
16
−3
ϫ 10 cm and 9.60 ϫ 10 cm , respectively. This result to-
6.626 ± 0.005 Å and 6.739 ± 0.005 Å, respectively. These numbers
gether with the observation of increasing of conductivity shows that
are similar to that of the undoped ReSe and consistent with those
2
9
the incorporation of W into ReSe could cause increases of the car-
previously reported.
2
rier concentrations of doped samples and hence improves electrical
conductivity. Temperature-dependent Hall mobility along and per-
pendicular to the b-axis can be derived from the conductivity and
Temperature dependence electrical conductivity and Hall effect
measurements.— The electrical transport along ͑E
dicular ͑E Ќ b͒ to the b-axis polarization schemes are shown in Fig.
. A study of electrical anisotropy was undertaken by performing the
ʈ
b͒ and perpen-
Hall voltage by the relation ␮H = R ␴ and the results are shown in
H
1
Fig. 4. For the samples, over the temperature range between 20 and
about 160 K, the mobility increases with increasing temperature,
characteristic of domination of impurity ͑or defect͒ scattering.
Above 160 K the mobility decreases when temperature increases,
which corresponds to the region where electron-phonon scattering
effect is dominant. In general, the mobility of the doped sample is
smaller than that of the undoped one. Hall mobilities are very low
temperature-dependent conductivity measurement along different
crystal orientations. Shown in Fig. 3a is the result of the
temperature-dependent conductivity measurements along ͑␴ b͒ and
ʈ
perpendicular ͑␴Ќb͒ to the b-axis of undoped ReSe and W-doped
2
ReSe . The results show the conductivity of the doped sample is
2
found to be higher than that of the undoped one and the conductivity
along the b-axis, ␴ b
ʈ
larger than that of the perpendicular one, ␴Ќb
.
and the mobility along the b-axis, ␮ b is larger than that of the
perpendicular one, ␮Ќb. The low values of mobility indicate the
general semiconducting properties of layered crystals of undoped
ʈ
The larger value of the conductivity parallel to the b-axis is related
to the strongest bonding force of the samples, which exists along the
crystal orientation of the Re-cluster chains. From the conductivity
ReSe2 and doped ReSe due to the larger effective masses of
2
1
2
measurements, the anisotropic ratio of undoped ReSe and W-doped
carriers. Electrical transport properties n, ␴ and ␮H from conduc-
tivity and Hall effect measurements at 300 K are summarized in
Table I. In view of the low mobility, we may conclude that carrier
concentration is the dominant factor in assisting the electrical con-
2
ReSe is around 4 and 3.5, respectively and is shown in Fig. 3b. The
2
anisotropy shows a decrease with adding W dopant. The temperature
variation of carrier concentration evaluated from the Hall measure-
ment is similar to conductivity measurement in which a marked
ductivity for undoped and doped ReSe . Note also that the measured
2
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Journal of The Electrochemical Society, 153 ͑8͒ J100-J102 ͑2006͒
Conclusions
Single crystals of W-doped ReSe on the electrical anisotropy
2
properties were studied via the temperature-dependent conductivity
and Hall effect measurements. Hall effect measurements reveal that
doped and undoped samples are all n-type semiconductors. The re-
sults indicate that incorporation of small amount of tungsten into
ReSe could cause increases of the carrier concentrations of doped
2
samples and hence improves electrical conductivity. The larger
value of conductivity parallel to b-axis is most likely related to the
strong bonding force of ReSe which exists along the crystal orien-
2
tation of Re-cluster chains and the anisotropy shows a decrease with
adding W dopant.
Acknowledgments
S.Y.H., C.H.L., and K.K.T. acknowledge the support of the Na-
tional Science Council grant no. NSC94-2112-M-019-005. Y.C.L.
acknowledges the support of the National Science Council grant no.
NSC94-2112-M-236-001.
Figure 4. Temperature dependence mobility of W-doped ReSe and undoped
2
ReSe single crystals along and perpendicular to b-axis, respectively.
2
National Taiwan Ocean University assisted in meeting the publication
costs of this article.
values vary considerably among crystals even when the crystals
were taken from the same batch. Such differences are probably due
to the different uncontrollable doping concentrations or nonstoichi-
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