Thermoelectric properties of Bi 2 Te 3 based thin films fabricated by pulsed laser deposition
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1044-U09-08
Thermoelectric properties of Bi2Te3 based thin films fabricated by pulsed laser deposition Shun Higomo1,2, Takashi Yagi3, Haruhiko Obara1, Atsushi Yamamoto1, Kazuo Ueno1, Tsutomu Iida2, Naoyuki Taketoshi3, and Tetsuya Baba3 1 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 2 Department of Material Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8501, Japan 3 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 3, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan ABSTRACT Bi2Te3-based thin films were fabricated on glass substrates by the pulsed laser deposition (PLD) method. The vapor pressures of Bi and Te are significantly different, so controlling the stoichiometric composition is difficult when using conventional physical vapor deposition techniques, and the thermoelectric properties of Bi2Te3 films are sensitive to the film composition. PLD is a promising technique for the fabrication of telluride-based films such as Bi2Te3 due to its superior capability for controlling the film composition. Another advantage of PLD is the flexibility that it allows in terms of atmosphere in the reaction chamber; high concentrations of gases such as oxygen or argon can be introduced. We have measured various compositions of Bi2Te3-based films, and have identified the optimal compositions for both ntype and p-type material. The thermal conductivities of these Bi2Te3 films were evaluated by an exact measuring system, and the results were twice as low as those of conventional bulk materials. These results suggest that PLD has significant advantages for the deposition of inplane Bi2Te3-based thin films. INTRODUCTION Thermoelectric materials can be used to convert between heat and electrical energy. They not only enable precise temperature control, but they are also promising materials for dealing with some of the problems related to energy resources and global warming. Their performance can be quantified using the dimensionless figure-of-merit, ZT: ZT =
S2 T ρ(κ c + κ L )
(1)
where S is the Seebeck coefficient, ρ is the electrical resistivity, κc is the carrier conductivity, κL is the lattice thermal conductivity, and T is the absolute temperature, respectively. Bi2Te3-based compounds are the materials that exhibit the highest values of ZT at ‘low’ temperatures, typically from room temperature to 450K. They feature a rhombohedric structure for the spacing group (R3m), and it is easier to represent the structure by imagining a hexagonal unit cell. The hexagonal cell is formed by stacking the layers as shown in Figure 11. The parameters of the lattice constant are a = 4.3835 Å, c = 30.360 Å2. In the direction perpendicular to the a-planes, the van der Waals bonds between Te(1)-Te(1) have a significant influence which
contributes to the st
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