Thermoelectric properties of nanostructured bismuth–telluride thin films grown using pulsed laser deposition Phuoc Huu Le a , Chien-Neng Liao b , Chih Wei Luo c,1 , Jihperng Leu a,⇑ a Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan, ROC b Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan, ROC c Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC article info Article history: Received 4 February 2014 Received in revised form 21 May 2014 Accepted 2 July 2014 Available online 10 July 2014 Keywords: Bi 2 Te 3 Thermoelectric properties Nanostructures Pulsed laser deposition (PLD) abstract Nanostructured n-type bismuth telluride (Bi 2 Te 3 ) thin films were grown on SiO 2 /Si (1 0 0) substrates at argon ambient pressure (P Ar ) of 80 Pa by pulsed laser deposition (PLD). The effects of film morphologies, structures, and compositions on the thermoelectric properties were investigated. At a substrate temper- ature (T s ) of 220–340 °C, stoichiometric films with highly (0 0 l)-oriented and layered structures showed the best properties, with a carrier mobility l of 83.9–122.3 cm 2 /Vs, an absolute Seebeck coefficient |a| of 172.8–189.7 lV/K, and a remarkably high power factor (PF) of 18.2–24.3 lW cm 1 K 2 . By contrast, the Te-rich films deposited at T s 6 120 °C with (0 1 5)-preferred orientations and columnar-small grain struc- tures or the Te-deficient film deposited at 380 °C with Bi 4 Te 5 polyhedron structure possessed poor prop- erties, with l < 10.0 cm 2 /Vs, |a| < 54 lV/K, and PFs 6 0.44 lW cm 1 K 2 . The morphology of highly (0 0 l) oriented-layered structures and the stoichiometry predominantly contribute to the substantial enhance- ment of l and |a|, respectively, resulting in remarkable enhancement in PF. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Thermoelectric (TE) materials are of interests for applications as heat pumps and power generators [1–4]. The performance of TE materials is evaluated in terms of a dimensionless figure of merit, ZT = a 2 rT/j, in which a, r, j, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and absolute temperature, respectively. To achieve a high ZT value, a TE material must exhibit a high power factor (PF), a 2 r, and low thermal con- ductivity, j. However, increasing the ZT value is challenging because of the coupling among the TE parameters [3]: the relation- ship between a and the carrier concentration n (expressed by |a|  n 2/3 [3]) limits the enhancement of the PF (=a 2 r), whereas the proportional relationship between electrical conductivity and electronic thermal conductivity (the Wiedmann–Franz law) restricts the improvement of the r/j ratio. Bismuth telluride (Bi 2 Te 3 )-based materials have been widely exploited for Peltier-coolers and thermoelectric generators at low temperature regime [5–8]. Nanocrystalline and nanostructured Bi 2 Te 3 -based films conduct heat poorly because of extensive phonon scattering at grain boundaries [9–12], but the electrical transport properties of the films are impaired because of lattice imperfections and grain-boundary defects [9], indicating that fur- ther investigation is required to determine how to improve PF or the electronic part of ZT. Currently, enhancing the PF of Bi 2 Te 3 - based thin films is challenging. Besides the coupling among TE material properties [3], the control of film stoichiometry is a key factor for obtaining better TE properties [13–16]. Yet, it is a chal- lenge to grow stoichiometric films because of the tendency for re-evaporation of volatile elements (i.e. Te, Se) at elevated T s [15,16], and the low sticking coefficient Te (<0.6) at T s beyond 300 °C [17,18]. Numerous charge carriers arising from vacancy defects of volatile elements can constrain the enhancement of |a|; however, low carrier concentrations can suppress electrical conductivity if carrier mobility (l) is poor. Substantial effort has been devoted to enhancing the PF and ZT values of Bi 2 Te 3 thin films grown using various vapor-deposition techniques. Moreover, nanocrystalline and nanostructured Bi 2 Te 3 thin films have recently attracted considerable attention because they exhibit superior TE performance [9,10,13,19–22]. The layered-hexagonal Bi 2 Te 3 films fabricated using radio-frequency magnetron sputtering possessed a PF of 8.8 lW cm 1 K 2 for an (0 1 5)-oriented film, and a PF of 33.7 lW cm 1 K 2 for a highly (0 0 l)-oriented layered film [19,21]. Furthermore, PFs of 27 lW cm 1 K 2 and 39.9 lW cm 1 K 2 were measured for http://dx.doi.org/10.1016/j.jallcom.2014.07.018 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +886 35131420. E-mail addresses: cwluo@mail.nctu.edu.tw (C.W. Luo), jimleu@mail.nctu.edu.tw (J. Leu). 1 Tel.: +886 35712121x56196. Journal of Alloys and Compounds 615 (2014) 546–552 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom