In-plane thermal conductivity of disordered layered W Se2 and (W)x (W Se2) y superlattice films

Article abstract of DOI:10.1063/1.2800888

It was recently reported that misoriented layered W Se2 and (W)x (W Se2) y films possess extremely low cross-plane thermal conductivity. Here, we report that the in-plane thermal conductivity results for W Se2 and W4 (W Se2) 10 films measured by using a suspended device are about 30 times higher than the cross-plane values because of the in-plane ordered and cross-plane disordered structures and about six times lower than that of compacted single-crystal W Se2 platelets. The additional W layers in the W4 (W Se2) 10 films were found to greatly increase the in-plane electrical conductivity relative to the W Se2 films, but reduce the in-plane lattice thermal conductivity assuming the Wiedemann-Franz law.

Full text of DOI:10.1063/1.2800888

In-plane thermal conductivity of disordered layered W Se 2 and ( W ) x ( W Se 2 ) y
superlattice films
Anastassios Mavrokefalos, Ngoc T. Nguyen, Michael T. Pettes, David C. Johnson, and Li Shi
Citation: Applied Physics Letters 91, 171912 (2007); doi: 10.1063/1.2800888
View online: http://dx.doi.org/10.1063/1.2800888
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/17?ver=pdfcov
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APPLIED PHYSICS LETTERS 91, 171912 ͑2007͒
In-plane thermal conductivity of disordered layered WSe and „W… „WSe …
2
x
2 y
superlattice films
Anastassios Mavrokefalos
Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
Ngoc T. Nguyen
Department of Chemistry, Materials Science Institute, University of Oregon, Eugene, Oregon 97403, USA
Michael T. Pettes
Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
David C. Johnson
Department of Chemistry, Materials Science Institute, University of Oregon, Eugene, Oregon 97403, USA
a͒
Li Shi
Department of Mechanical Engineering and Center for Nano and Molecular Science and Technology, Texas
Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA
͑
Received 30 July 2007; accepted 1 October 2007; published online 26 October 2007͒
It was recently reported that misoriented layered WSe and ͑W͒ ͑WSe ͒ films possess extremely
2
x
2 y
low cross-plane thermal conductivity. Here, we report that the in-plane thermal conductivity results
for WSe and W ͑WSe ͒ films measured by using a suspended device are about 30 times higher
2
4
2 10
than the cross-plane values because of the in-plane ordered and cross-plane disordered structures
and about six times lower than that of compacted single-crystal WSe platelets. The additional W
2
layers in the W ͑WSe ͒ films were found to greatly increase the in-plane electrical conductivity
4
2 10
relative to the WSe films, but reduce the in-plane lattice thermal conductivity assuming the
2
Wiedemann-Franz law. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2800888͔
Thin film heterostructures have been widely used in op-
toelectronic devices and are currently under active investiga-
tion for thermoelectric energy conversion applications. It
employed a metal layer on top of the patterned film as a
11
heater and resistance thermometer. The uncertainties of the
metal layer thickness and ␬ introduce uncertainties to the
obtained ␬ of the silicon film.
1
,2
was recently found that the cross-plane thermal conductivity
3
͑
␬͒ of disordered WSe thin films is as low as 0.05 W/m K.
In this letter, we employ a suspended microdevice to
2
This result is the lowest ␬ ever reported for a dense solid to
measure the in-plane ␬ of patterned and suspended WSe and
2
this date. The disordered WSe thin films is an expansion of
͓͑W͒ ͑WSe ͒ ͔ superlattice films. The measurement results
2
x
2 y z
a previous work developed by Johnson and co-workers
where a multilayer superlattice structure is formed by inter-
show that the in-plane ␬ is about 30 times higher than the
cross-plane ␬ obtained by Cahill and co-workers using the
TDTR method. The high ␬ anisotropy is attributed to the
in-plane ordered and cross-plane disordered structures of the
films. Nevertheless, the in-plane values of the films are still
about six times lower than that of compacted single-crystal
4
,5
facial nucleation along the interfaces. The extremely low
cross-plane ␬ is attributed to phonon localization induced by
the random stacking of the two-dimensional crystalline
WSe sheets in the cross-plane direction. On the other hand,
2
it is expected that the in-plane ␬ is much higher than the
cross-plane value because phonons are much less localized in
the in-plane direction within each layer of the polycrystalline
WSe platelets. Moreover, the additional W layers in the
2
͑W͒ ͑WSe ͒ films were found to increase the electrical con-
x
2 y
ductivity to be three orders of magnitude higher than that of
6
film.
the WSe film, but reduce the in-plane lattice thermal con-
2
7
While the 3␻ method and the time domain thermal re-
ductivity ͑␬ ͒ according to an estimation based on the
l
8
flectance ͑TDTR͒ technique have been widely used to mea-
Wiedemann-Franz law.
sure the cross-plane ␬ of thin films, measuring the in-plane ␬
has been difficult because of parasitic heat conduction in the
substrate. Ju et al. extended the 3␻ method to obtain the
in-plane ␬ of a thin film by using microfabricated heater
The WSe films and the ͑W͒ ͑WSe ͒ superlattice films
2
x
2 y
were synthesized using modulated elemental reactant
5
technique. The elements were sequentially deposited in a
high vacuum chamber ͑1ϫ10− Torr background pressure͒
7
9
bridges of varying widths. The ␬ of the film was extracted
onto unheated prepolished Si wafer ͑roughness of ±3 Å͒. W
by fitting a two-dimensional heat conduction model to the
measurement results. Zink et al. obtained the in-plane ␬ of a
thin film evaporated on the back side of a silicon nitride
membrane device by comparing the measured thermal con-
͑
99.95% purity͒ was deposited at 0.2 Å/s using 3 kW elec-
tron beam guns, while Se ͑99.995% purity͒ was delivered
using a Knudsen effusion cell depositing at 0.5 Å/s. A com-
puter program controlled the thicknesses of the elements de-
posited, opening a shutter for either a specified period of
time or responding to the integrated thickness signal of a
quartz crystal monitor. For the superlattice sample measured
in this work, approximately four-unit-cell thick W and ten-
1
0
ductances before and after the evaporation. Liu and
Asheghi patterned a thin Si film into a suspended beam and
a͒Author to whom correspondences should be addressed. Electronic mail:
lishi@mail.utexas.edu
unit-cell thick WSe were deposited 24 times to produce a
2
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003-6951/2007/91͑17͒/171912/3/$23.00 91, 171912-1 © 2007 American Institute of Physics
16.165.95.79 On: Wed, 10 Dec 2014 17:28:00
0
2

171912-2
Mavrokefalos et al.
Appl. Phys. Lett. 91, 171912 ͑2007͒
FIG. 1. ͑Color online͒ ͓͑a͒–͑d͔͒ SEMs
of the transfer process of a patterned
thin film to the microdevice using a
nanomanipulator. Prior to film transfer,
electron beam lithography was used to
pattern arrays of ZEP-520 resist lines
on the film, which were used as an
etching mask to etch through the film
using reactive ion etching with CHF3
chemistry. The etched film pattern in
͑
a͒ was obtained after the resist lines
were removed using oxygen plasma.
e͒ SEM of a patterned ͑W͒ ͑WSe ͒
2 10
͑
4
thin film assembled on the microde-
vice. The arrow indicates one of the
four Pt lines deposited on top of the
film.
superlattice film. Electron probe microanalysis and Ruther-
ford backscattering measurements show that the repeat thick-
ness of the W ͑WSe ͒ layer is 83.6±0.1 Å. After annealing
the as deposited amorphous reactants in an open nitrogen
atmosphere at 600 °C, the W:Se ratio was found to be 0.93–
using the microdevice consisting of two membrane thermal
1
2–15
reservoirs.
The data reduction procedure is based on a
4
2 10
straightforward one-dimensional heat conduction analysis
that does not involve any fitting parameters. In the two-probe
thermal measurement procedure based on the suspended
1
3,14
0
.95:2 for the WSe samples.
device,
the obtained ␬ consists of an error due to the
2
Individual thin films were patterned using electron beam
contact thermal resistance between the two membranes and
the nanostructure, which is reduced by depositing Pt locally
at the two contacts. In a four-probe thermal measurement
lithography and reactive ion etching. The patterned thin film
was transferred to the suspended microdevice using a sharp
tungsten tip actuated by a Zyvex S100 Nanomanipulator
System inside a scanning electron microscope ͑SEM͒. Be-
cause the adhesion between the film and the silicon substrate
is very poor, the film could be peeled off by the tip and
transferred directly to the suspended microdevice, as illus-
trated in Figs. 1͑a͒–1͑d͒. To enhance the electrical and ther-
mal contacts between the film and the microdevice, we used
electron beam induced metal deposition to deposit a 250 nm
wide, 150 nm thick, 2 ␮m long Pt line on top of the thin film
and each of the four underlying Pt electrodes prepatterned on
the suspended device, as illustrated in Fig. 1͑e͒.
1
2,15
procedure reported recently,
the temperature drops at the
two contacts are measured using the nanostructure sample as
a thermocouple element so that the error due to contact ther-
After the film was assembled on the microdevice, the
etched edge of one suspended W ͑WSe ͒ film was ana-
4
2 10
lyzed using high resolution transition electron microscopy
HRTEM͒, which showed, in Fig. 2, moiré fringes indicative
͑
of slightly misoriented crystal layers throughout the sample.
Tungsten ͑110͒ lattice fringes were prominent in the bright
field images and it is most likely that these thick W layers are
responsible for the linear moiré patterns. Selected area dif-
fraction ͑SAD͒ revealed the polycrystalline nature of the su-
perlattice showing a discrete number of comparatively large
W grains within the illuminated area. The interplanar spac-
ings of both W and WSe were within 0.5% of those reported
2
for bulk W and WSe by x-ray powder diffraction. Addition-
2
ally, SAD showed only ͕hk0͖ reflections from the WSe2
layer, indicating the ͓0001͔ axis perpendicular to the trans-
port direction for the in-plane measurement as expected.
The procedure for measuring the in-plane ␬ of the film is
FIG. 2. Top-view HRTEM of the etched edge of a suspended ͑W͒ ͑WSe ͒
2 10
4
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the same as that for measuring the ␬ of individual nanowire
film assembled on the microdevice. The inset is the SAD pattern.
216.165.95.79 On: Wed, 10 Dec 2014 17:28:00

171912-3
Mavrokefalos et al.
Appl. Phys. Lett. 91, 171912 ͑2007͒
For sample 4, the electron contribution ͑␬ ͒ to ␬ can be
e
estimated from the measured electrical conductivity accord-
ing to the Wiedemann-Franz law. As shown in Fig. 3, ␬ is
e
about one-third of the measured total ␬ for sample 4 at room
temperature. Hence, ␬ contributes to about two-thirds of the
l
total ␬ of sample 4. Not shown in Fig. 3, the estimated ␬ is
e
about five orders of magnitude lower than the total ␬ of
sample 2 so that ␬ of the WSe sample is essentially the
l
2
same as the measured total ␬. Comparing the deduced ␬l
results of the WSe film and the W ͑WSe ͒ film, one can
2
4
2 10
note that the addition of W layers in the W ͑WSe ͒ film
4
2 10
FIG. 3. Measured thermal conductivity of the four thin film samples.
reduces its in-plane ␬ by about 30% compared to the WSe
l
2
Samples 1 and 2 are 162 nm thick WSe film. Samples 3 and 4 are 140 nm
2
film.
thick ͑W͒ ͑WSe ͒ films. Also plotted is the electron contribution ͑␬ ͒ to
4
2
10
e
The measurement results show that the in-plane ␬ of the
misoriented layered WSe2 and ͑W͒ ͑WSe ͒ superlattice
the thermal conductivity of sample 4 estimated from the measured electrical
conductivity using the Wiedemann-Franz law.
x
2 y
films is about 30 times higher than the cross-plane value. The
highly anisotropic ␬ can be attributed to the in-plane ordered
and cross-plane disordered structures of the rotationally dis-
ordered films. Nevertheless, the in-plane values of the disor-
dered films are still about six times lower than that of com-
pacted single-crystal platelets. Moreover, it is intriguing to
observe that adding W layers in the film could reduce the
in-plane ␬l. The lower in-plane values in the film than in the
single crystal could be caused by smaller grain size in the
films. The further suppression with the addition of W layers
in the superlattice films could also be caused by further de-
crease in the grain size because of strains induced by the
mal resistance can be eliminated from the obtained ␬ and
Seebeck coefficient of the sample. The four-probe electrical
conductivity can also be obtained using the measurement
device. However, this four-probe thermal measurement
method cannot be applied with great accuracy to the thin film
samples measured in this work because the samples have
either low Seebeck coefficient or low electrical conductivity
and cannot be used to measure the contact temperature drops
accurately. The results reported here are thus the two-probe
1
2
␬
. From our previous measurements of other nanofilms and
1
4
nanofibers, the contact thermal resistance can be about
10%–30% of the total thermal resistance of the sample.
additional W layers or by smaller ␬ in the W layers than in
l
Four thin film samples have been measured. As shown in
the WSe layers. Meanwhile, it is also possible that misori-
2
Fig. 3, the obtained in-plane ␬ results at room temperature
entation of adjacent layers or addition of W layers in the film
increases scattering of the in-plane phonon modes by the
boundaries between adjacent layers.
−
1
−1
are in the range of 1.2–1.6 W m K , which is about 30
times higher than the cross plane ␬ obtained by Cahill and
co-workers using the TDTR method on WSe2 and
W ͑WSe ͒ thin film samples synthesized under the same
This work is supported by ONR ͑Program manager: Dr.
Mihal Gross͒. M.T.P. is supported by a NSF Graduate Re-
search Fellowship. Nanopatterning was performed at UT
Austin Microelectronics Research Center supported by NSF
NNIN. We thank David Cahill for suggesting this measure-
ment.
x
2 y
condition. The anisotropy ratio is much higher than that of
compacted single-crystal horizontal WSe platelets, which
2
−
1
−1
has an in-plane ␬ of 9.7 W m
K
and a cross-plane ␬ of
−
1
−1
2
.09 W m K . The increased ␬ anisotropy verifies the in-
plane ordered and cross-plane disordered natures of the rota-
tionally disordered layered structure of the films. However,
the in-plane ␬ is still about six times lower than that of the
compacted single-crystal platelets. The lower in-plane values
in the disordered films could be caused by smaller lateral
grain size ͑about 6–10 nm from both diffraction and TEM
measurements͒, which is evident in the two times lower in-
1
T. C. Harman, P. J. Taylor, M. P. Walsh, and B. E. LaForge, Science 297,
2
229 ͑2002͒.
2
R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature
3͑
London͒ 413, 597 ͑2001͒.
C. Chiritescu, D. G. Cahill, N. Nguyen, D. Johnson, A. Bodapati, P.
Keblinski, and P. Zschack, Science 315, 351 ͑2007͒.
S. Moss, M. Noh, K. H. Jeong, D. H. Kim, and D. C. Johnson, Chem.
Mater. 8, 1853 ͑1996͒.
−
1
−1
4
5
plane electrical conductivity ͑0.014 ⍀ cm ͒ in the WSe
2
thin film ͑sample 2͒ than in the compacted single-crystal
WSe platelets. However, the in-plane ␬ reduction could also
M. Noh, C. D. Johnson, M. D. Hornbostel, J. Thiel, and D. C. Johnson,
2
Chem. Mater. 8, 1625 ͑1996͒.
K. E. Goodson, Science 315, 342 ͑2007͒.
D. G. Cahill, Rev. Sci. Instrum. 61, 802 ͑1990͒.
D. G. Cahill, Rev. Sci. Instrum. 75, 5119 ͑2004͒.
Y. S. Ju, K. Kurabayashi, and K. E. Goodson, Thin Solid Films 339, 160
be caused by increased scattering of the in-plane phonon
modes by the boundaries between adjacent layers in the dis-
ordered films.
6
7
8
9
The measured in-plane ␬ results of the two WSe2
samples and one of the two W ͑WSe ͒ samples are very
10͑
1999͒.
4
2 10
B. L. Zink, B. Revaz, J. J. Cherry, and F. Hellman, Rev. Sci. Instrum. 76,
similar and slightly higher than that of the other W ͑WSe ͒
4
2 10
110
24901 ͑2005͒.
sample, i.e., sample 4. At room temperature, the in-plane
W. J. Liu and M. Asheghi, ASME Trans. J. Heat Transfer 128, 75 ͑2006͒.
A. Mavrokefalos, M. T. Pettes, F. Zhou, and L. Shi, Rev. Sci. Instrum. 78,
034901 ͑2007͒.
1
2
3
electrical conductivity of sample 4 was found to be about
−
1
−1
4
0
00 ⍀ cm , much higher than the value of
1
−
1
−1
L. Shi, D. Y. Li, C. H. Yu, W. Y. Jang, D. Kim, Z. Yao, P. Kim, and A.
Majumdar, ASME Trans. J. Heat Transfer 125, 881 ͑2003͒.
C. H. Yu, S. Saha, J. H. Zhou, L. Shi, A. M. Cassell, B. A. Cruden, Q.
Ngo, and J. Li, ASME Trans. J. Heat Transfer 128, 234 ͑2006͒.
F. Zhou, J. Szczech, M. T. Pettes, A. L. Moore, S. Jin, and L. Shi, Nano
.014 ⍀ cm for the WSe film ͑sample 2͒. Hence, the
addition of W layers in the W ͑WSe ͒ film increases the
2
4
2 10
14
electrical conductivity by four orders of magnitude, making
the sample metalliclike with a small room-temperature See-
1
5
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beck coefficient of 1.5 ␮V/K relative to the Pt electrode.
Lett. 7, 1649 ͑2007͒.
2
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