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Sensors and Materials, Vol. 32, No. 7 (2020) 2433–2441
MYU Tokyo
S & M 2272

Fabrication and Thermoelectric Propertiesof Chromium Silicide Thin Films
Takao Mori,1,2* Takashi Aizawa,1 S. N. Vijayaraghavan,1 and Naoki Sato1
1
2

WPI-MANA and CFSN, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8671, Japan(Received March 30, 2020; accepted May 15, 2020)

Keywords: thermoelectric, thin film, magnetic semiconductor, power factor, thermal conductivity

Thermoelectric thin films are candidate materials that can be used to supply the energy
harvested to autonomously power Internet of Things (IoT) sensors and devices. This work
deals with the fabrication of chromium silicide thin films and the characterization of their
thermoelectric properties. The films were grown by utilizing a high-temperature molecularbeam epitaxy (MBE) apparatus under different conditions. The highest power factor of morethan 0.6 mW/m K 2 was obtained for the chromium silicide film deposited at a temperature of
900 °C. The thermal conductivity of the thin film was observed to be approximately one-thirdthat of bulk CrSi2.

1.

Introduction

The application of thermoelectric power generation has still not been realized in a large
market. In addition to the performance of power generation, cost has also been a big problem.
One promising application that has gained interest is thermal energy harvesting to dynamicallypower innumerable Internet of Things (IoT) sensors and devices.(1,2) In this case, themaintenance-free possibility of thermoelectrics is attractive, because changing batteries for ahuge number of sensors and devices is not feasible. It is therefore important for IoT applications
to find methods appropriate for processing and fabricating modules for high-performancematerials. The use of inorganic thermoelectric thin films is one possibility.
The performance of thermoelectric materials is determined by their figure of merit:
ZT = α2σκ−1T,(1)where α2σ is the power factor, and the conversion efficiency is a function of ZT. To
achieve high ZT, a high electrical conductivity σ, but a low thermal conductivity κ, and alarge Seebeck coefficient α, but a high σ, are required. Therefore, there are some obvious
paradoxes and trade-offs in the requirements of the physical properties of materials to achieve
high thermoelectric performance. There have been several good overall reviews of recent
advancements in enhancing the ZT of thermoelectric materials.(3,4)*

Corresponding author: e-mail: MORI.Takao@nims.go.jp
https://doi.org/10.18494/SAM.2020.2889
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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We have proposed the use of magnetism to enhance the properties of thermoelectricmaterials. Magnetic ion doping has been demonstrated for various systems, such as CuGaTe2,(5)Bi2Te3,(6) and SnSe,(7) to be a possible route to increase the power factor. Such an increase is
not automatic and occurs in cases where electrical carriers have strong coupling with doped
magnetic moments, effectively modifying transport properties. Namely, this interaction“drags” the carriers, leading to an increase in effective mass that in turn increases theSeebeck coefficient. This will be detrimental to mobility, but the overall increase in power
factor has been realized.(5–7) Some good-performance thermoelectric compounds have also
been recently discovered amongst magnetic semiconductors, where magnetic elements are
the main constituents.(8–13) Carrier-doped CuFeS2 exhibits a large power factor at room
temperature (RT), speculated to originate from strong magnetic interactions.(8) Theoretical
calculations have also shown that CuFeS2 has an antiferromagnetic configuration, resulting
in the largest Seebeck coefficient and power factor.(9) At low temperatures, the contributionof carrier–magnon interactions to the Seebeck coefficient was also indicated.(10) Sulfides
with a spinel structure, i.e., thiospinels, were generally observed to have poor thermoelectric
properties.(14) Good thermoelectric properties of ZT = 0.43 were revealed in CuCr2S4 by tuning
the carrier concentration by Sb doping, whereby the intrinsic properties of a magnetic systemwith a relatively large effective mass were manifested at relatively large power factors.(11) Cr2Se
is another magnetic system with a relatively good thermoelectric performance near RT, andimportantly, p, n tuning is possible without external element doping.(12) In the Heusler alloy
Fe2VAl system, where itinerant weak ferromagnetic behavior was engendered by doping, spin
fluctuation was found to increase the Seebeck coefficient, for example by 50% at 400 K.(13)Ultrahigh performance at ZT > 4 has also been reported in related Heusler films.(15) Inorganic
thermoelectric thin films have been fabricated and investigated for various material systems,(16)for example, thermoelectric thin films of oxides,(17,18) borides,(19,20) and stannides.(21)Here, we report on the fabrication and thermoelectric properties of chromium silicide thinfilms. Chromium silicide is of interest for us, because for its future outlook, as a magnetic
system(22) with high potentials as described above, and the thermoelectric properties of bulk
CrSi2 were investigated previously.(23,24) A relatively large power factor of ~1 mW/m K 2 hasbeen reported for a pure bulk material at RT.(24) Nakasawa et al. succeeded in increasing this to~1.4 mW/m K 2 at RT via Mo doping.(25) The reported high thermal conductivity of 12 W/m K
at RT for the bulk material(25) also makes it an attractive system that could benefit from being
formatted into the thin film form. These are the reasons why we have focused on the chromium
silicide system in our investigations of the fabrication of thin films and their thermoelectric
properties.

2.

Materials and Methods

Chromium silicide thin films were deposited on sapphire (0001) substrates (Shinkosha Co.,
Ltd.) using a molecular beam epitaxy (MBE; EV-500, Eiko Co., Ltd.) apparatus. Each sapphire
substrate was degas-heated at 1000 ℃ for 1 h in vacuum before film deposition.  Si and Cr were
evaporated using special high-temperature K-cells, namely, MB-3000Si (Eiko Co., Ltd.) with aSensors and Materials, Vol. 32, No. 7 (2020)

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Ta crucible for Si and TUBO-e (CreaTec Fischer & Co., GmbH) with a vitreous carbon crucible
for Cr, respectively. The deposition conditions are listed in Table 1. As common conditions,the growth time was 90 min, and the heating temperature of the Cr cell was 1150 °C. The film
thickness was measured using a stylus profilometer (Dektak 6M, Bruker). The crystal structure
was evaluated by X-ray diffraction (XRD), using a powder diffractometer (SmartLab 3, Rigaku
Corp.).
The thermoelectric properties of the deposited thin films were measured using ZEM-3(ULVAC). The cross-plane thermal conductivity was evaluated using a customized focused
picosecond time-domain thermoreflectance (TD-TR) instrument (PicoTR, Picotherm Corp.) ina front-heating/front-detection configuration.(26–28) A 100-nm-thick Pt thin film was depositedon chromium silicide thin film surfaces using a DC sputtering system to detect transient
temperature changes. A 1550 nm infrared pulsed laser with a repetition frequency of 20 MHz
and a pulse duration of 0.5 ps was used as a heat source. A 780 nm probe laser was used todetect thermoreflectance signals. Here, we assumed the specific heat of all films as 3R fromthe Dulong–Petit law (0.575 J g−1 K−1) for the calculation of the thermal conductivity.

3.

Results and Discussion

Figures 1 and 2 show the XRD patterns of the MBE-grown chromium silicide thin films.
As summarized in Table 1, various thin films were obtained under different thin film growth
conditions.
The substrate temperatures of RT and 400 °C were apparently too low to achieve chromium
silicide film growth, and phases were obtained from 600 °C and above. For substrate heating
temperatures of 600 and 800 °C, the majority phase was the CrSi metallic phase with significant
CrSi2 growth. For 900 and 1000 °C substrate heating, CrSi2 was obtained as the majority phase,with CrSi and Cr5Si3 as minority phases.
In an attempt to reduce the amount of the metallic CrSi phase and relatively increase theamount of the main CrSi2 phase, the Si cell was heated up to 1525 °C. As can be observed inTable 1Chromium silicide film growth parameters and phases.
Substrate
Si cell
Sample ID
Thickness (nm)temp. (℃)
temp. (℃)
RT
RT
1500
—
400
400
1500
—
600 deg
600
1500
98.3
800 deg
800
1500
134.9
900 deg
900
1500
134.0
1000 deg
1000
1500
106.6
600/1525 deg
600
1525
107.1
800/1525 deg
800
1525
—
900/1525 deg
900
1525
132.0
1000/1525 deg
1000
1525
115.0

Majority
phase
—
—
CrSi
CrSi
CrSi2
CrSi2
CrSi2
CrSi2
CrSi2
CrSi2

Minority
phases——
CrSi2
CrSi2, Cr5Si3
CrSi, Cr5Si3
CrSi, Cr5Si3
CrSi
CrSi, Cr5Si3
CrSi, Cr5Si3
CrSi, Cr5Si3

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Fig. 1. (Color online) XRD patterns of chromium silicide thin films with 1500 °C Si cell heating. The indicatedtemperatures are the substrate heating temperatures.

Fig. 2. (Color online) XRD patterns of chromium silicide thin films with 1525 °C Si cell heating. The indicatedtemperatures are the substrate heating temperatures.

Fig. 2, this did not result in a relative reduction in the amount of the CrSi phase. Apparently,
the opposite behavior was observed and the thermoelectric properties subsequently degraded.
Namely, the power factor of the best sample in this series was around half that of Si heatedSensors and Materials, Vol. 32, No. 7 (2020)

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at 1500 °C. The tantalum material of our Si cell might have reacted with Si at higher
temperatures, and it was not easy to tune the conditions further to optimize the CrSi2 MBE thinfilm growth on our apparatus. This should be examined in future attempts together with doping
to tune the properties.
The thermoelectric properties of the chromium silicide thin films, namely, the Seebeckcoefficient, resistivity, and power factor, are shown in Fig. 3. The Seebeck coefficient of
the 900 deg film with substrate heating is larger than 140 µV/K near RT, slightly smaller
but similar in magnitude to what was previously reported for bulk CrSi2.(24) This can beconsidered to be the effect of the metallic CrSi minority phase. Reductions in the electricalconductivities and power factors of thin films of various thermoelectric materials compared

(a)

(b)

(c)

Fig. 3.

(Color online) Thermoelectric properties: (a) Seebeck coefficient, (b) resistivity, and (c) power factor.

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with the bulk material, have typically been observed.(16) The difference in polycrystalline
quality between bulk samples and thin films can be a typical origin of this behavior, but the
Seebeck coefficient is not affected. The 600 and 800 deg films with relatively high contentsof metallic CrSi exhibit lower resistivities and smaller Seebeck coefficients. Metallic minority
phases were previously observed in some cases to markedly increase the electrical conductivity
and power factor;(29) however, in this case, even with this possible benefit, the 900 deg thinfilm exhibits a higher resistivity than the bulk, which is likely due to the detrimental thin-film
effect mentioned previously. Although there is an apparatus limitation at present as discussed
above, if we can finely tune the amount of the metallic CrSi phase in CrSi2 films, this can leadto the enhancement of the overall power factor. Such a metallic minority phase can generate anonpercolating metallic partial network in the material, which will not markedly degrade theSeebeck coefficient, while enhancing the electrical conductivity significantly.(29)The best power factor of the presently obtained chromium silicide films is ~0.6 mW/m K 2
near RT obtained for the 900 deg thin film. This is smaller than ~1 mW/m K 2 reported for bulk
CrSi2, but for thermoelectric thin films with abundant elements, it is a relatively large value atRT.(17–21)
A significant increase in power factor to ~1.4 mW/m K 2 at RT was previously reported
for Mo-doped bulk CrSi2, and doping experiments should be carried out in future studies.
Furthermore, as mentioned in the introduction, the increase in the power factor has beenachieved by considering magnetism,(5–9) and as a future outlook, this possibility should also be
investigated for chromium silicide thin films.
The behavior of the series of films with 1525 °C Si cell heating is as expected, withlower Seebeck coefficients due to the relatively higher content of metallic CrSi (Fig. 4). The
maximum power factor obtained for the 1000/1525 deg film is only around half that of the1500 °C Si cell heating series.
The determined thermal conductivities of the selected thin film samples are listed in Table 2.
The electronic thermal conductivity was determined using the Wiedemann–Franz law,where κE = LσT. 2.44 × 10−8 V2/K 2 was used as the Lorenz number L here. κE is relatively
small, and it can be observed that the major part of thermal conductivity is the lattice thermal
conductivity.
The thermal conductivity of bulk CrSi2 has been reported as 12 W/m K at 300 K.(25)Compared with the highest power factor film 900 deg sample, there is an approximately
one-third reduction in thermal conductivity. The metallic CrSi minority phase is difficult
to qualitatively gauge, since it can have opposite effects, namely, the thermal conductivity
decreases via interfaces, but it also increases via highly conductive channels. In any case, all
the chromium silicide films synthesized under standard conditions exhibited a lower thermal
conductivity than bulk CrSi2, so the thin-film format is effective in reducing the thermal
conductivity.
We consider other possible effects of the differences in the thickness between the thin films,
from 98.3 nm to 134.9 nm, on the thermoelectric properties measured. The measurements
themselves should not have any effect. Regarding the intrinsic properties, even for the thinnest
film, in this range we do not expect any specific electronic effects that are not observed inSensors and Materials, Vol. 32, No. 7 (2020)

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(a)

(b)

(c)

Fig. 4.

(Color online) Thermoelectric properties: (a) Seebeck coefficient, (b) resistivity, and (c) power factor.

Table 2
Thermal conductivities of chromium silicide films.
Thermal diffusivity
Thermal conductivity κSample ID
at 300 K (10 −6 m 2/s)
at 300 K (W/mK)
600 deg
2.85
7.20
800 deg
4.05
10.23
900 deg
3.33
8.40
1000 deg
3.01
7.61
600/1525 deg
2.73
6.90
900/1525 deg
3.65
9.22
1000/1525 deg
3.54
8.95

Electronic thermal conductivity κE
at 323 K (W/mK)
0.79
1.19
0.23
0.26
—
—
—

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the bulk form, such as quantum confinement or electronic structure modulation. However,according to a previous report on the theoretical evaluation of the thermal conductivity of
CrSi2,(30) phonons with a mean free path of about 100 nm do not have significant contributions
to thermal transport. Therefore, the differences in the thickness between our samples are notexpected to have any effect on the thermoelectric properties.

4.

Conclusions

We fabricated chromium silicide thin films by MBE. The film with the highest
thermoelectrical performance had CrSi2 as the main phase. The highest power factor of
more than 0.6 mW/m K 2 at RT was obtained for the chromium silicide film with a deposition
temperature of 900 °C and a Si cell heating temperature of 1500 °C. For thermoelectric thinfilms with abundant elements, the obtained power factor was relatively large, which made thethin films promising systems to be investigated further. These thin films exhibited a thermal
conductivity that was approximately one-third that of bulk CrSi2. Bulk CrSi2 exhibited
significant increases in power factor via doping; this should also be further examined in the
fabricated thin films, together with an investigation on any correlation with magnetic properties,
and their tuning.

Acknowledgments
This work was supported by Japan Science and Technology Agency (JST), Mirai Program
Grant Number JPMJMI19A1, CREST JPMJCR19Q4, and JSPS KAKENHI Grant Number
JP17H02749 and JP16H06441.

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