Microstructure and Thermoelectric Properties of p-Type Bi 0.5 Sb 1.5 Te 3 and n-Type Bi 2 Te 2.7 Se 0.3 Films Deposited
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Microstructure and Thermoelectric Properties of p-Type Bi0.5Sb1.5Te3 and n-Type Bi2Te2.7Se0.3 Films Deposited by Pulsed Laser Ablation Raghuveer S. Makala, K. Jagannadham, B.C. Sales1, Hsin Wang2 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC-27695-7907 1 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 2 High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Thin films of p-type Bi0.5Sb1.5Te3, n-type Bi2Te2.7Se0.3 and n-type with SbI3 doping were deposited on mica substrates using Nd-YAG pulsed laser ablation at temperatures ranging from 3000C to 5000C. These films were characterized using X-ray diffraction, SEM and TEM. X-ray mapping and EDS were used to determine the composition. The films showed uniform thickness and high crystalline quality with a preferred (00n) alignment with the substrates. The film quality in terms of composition and crystal perfection is studied as a function of growth temperature. It was found that films deposited at 3500C gave improved crystallinity and thermoelectric characteristics. The Seebeck coefficient, electrical resistivity and Hall mobility were measured as a function of temperature and compared with the measurements on the bulk. Correlation of thermoelectric properties with microstructure is discussed. INTRODUCTION Bi2Te3-based materials are well known to be good materials for thermoelectric devices near room temperature. High efficiency thin film thermoelectric devices are widely foreseen to have applications in microelectronics such as heat dissipators for various devices. The crystal structure of these materials is composed of atomic layers in the order of Te/Bi/Te/Te/Bi /Te/Bi/Te/Te/….. oriented along the c-axis and the Te/Te layers are held together by weak Van der Waal forces [1]. Ternary compounds: Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3 have better thermoelectric characteristics for p-type and n-type respectively. Numerous deposition techniques have been attempted including evaporation, flash evaporation [2], molecular beam epitaxy [3], chemical vapor deposition, sputtering [4], and laser ablation [5]. A primary difficulty in the deposition of thermoelectric films is maintaining stoichiometry. The growth structure is very important since grain boundaries and crystalline defects effect the resistivity by scattering charge carriers. In the present effort, we have used pulsed laser deposition [5]. Pulsed laser deposition (PLD) has the advantage of maintaining stoichiometry of the target composition in the thin films. Moreover, PLD is a non-equilibrium process. In the present effort, we deposited the p-type thermoelectric material Bi0.5Sb1.5Te3 and the n-type Bi2Te2.7Se0.3 on mica substrates and characterized the microstructure and thermoelectric properties.
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EXPERIMENTAL DETAILS We have used Nd-YAG laser (λ = 266nm) pulses, operating at 10Hz frequency to ablate the thermoelectric target material. The target materials were made from a stoichiometric mixture of
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