Materials Chemistry and Physics 119 (2010) 145–148 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Composition and thermoelectric power factor variation of (Bi 2 Te 3 ) 0.96 (Bi 2 Se 3 ) 0.04 crystal in growth direction M. Allahkarami a,∗ , L. Seyed Faraji a,1 , G. Kavei b , Y. Zare b a School of Mechanical and Aerospace Engineering, Helmerich Advanced Technology Research Center, Oklahoma State University, 700N Greenwood Ave, Tulsa, OK 74106, USA b Thermoelectric Lab, Semiconductor Device Fabrication Division, Material and Energy Research Center (MERC), Iran article info Article history: Received 13 March 2009 Received in revised form 22 June 2009 Accepted 21 August 2009 Keywords: Semiconductors Crystal growth Thermoelectric effects Solidification abstract (Bi 2 Te 3 ) 0.96 (Bi 2 Se 3 ) 0.04 crystal, which is an n-type thermoelectric semiconductor, has many applications in thermoelectric cooling systems. Single crystal of this composition was grown by Traveling Heater Method. A sensible gradient in thermoelectric power factor was observed in the first quarter length of the prepared crystalline ingot. Characterizing the crystallization procedure and ingot composition, the gradient was attributed to the variation of the Bi 2 Se 3 concentration of Bi 2 Te 3 –Bi 2 Se 3 quasi-binary solid solution system. The structural properties were characterized by means of XRD analyses. Results of composition variation (Bi 2 Se 3 distribution function) were in good correlation with experimental thermoelectric power factor measured along the grown rod. Published by Elsevier B.V. 1. Introduction Bismuth telluride-based compositions are widely used in ther- moelectric coolers applications and recently there is optimistic effort to improve material performance using nano-technology developments. Such enhancement is the result of a significant reduction in thermal conductivity caused by strong phonon scat- tering by interfaces in the nano-structures [1–4]. In thermoelectric systems, electrical current is conducted through series of semi- conductor pairs of p- and n-type. Thermoelectric power factor of this configuration is defined as ˛ 2  where ˛ and  are Seebeck coefficient and electrical conductivity, respectively. p-Type semi- conductors are prepared from optimal state of Bi 2 Te 3 –Sb 2 Te 3 solid solutions and n-type semiconductors are prepared from optimal state of Bi 2 Te 3 –Se 2 Te 3 solid solution. Non-homogeneity in Se con- tent is observed in Bi 2 Te 3 –Se 2 Te 3 single crystal when prepared by Traveling Heater Method [5,6]. Controlling Se content is not con- venient. Also, variation of crystal structure due to substitution of Se for Te (leading to variation of Se content in composition) is not homogeneous. Various techniques are applied to make the composition homogeneous, mainly based on powder metallurgy, mechanical alloying and hot pressing [7–12]. Compositions created by means of these techniques have good mechanical properties, but low figure-of-merit. Bismuth telluride thin film has been stud- ied for its possible thermoelectric applications [13–16], it is known ∗ Corresponding author. Tel.: +1 918 327 1582; fax: +1 270 897 1179. E-mail address: masoud.allahkarami@okstate.edu (M. Allahkarami). 1 Tel.: +1 918 327 1582; fax: +1 270 897 1179. that film’s electron transport properties depend on its chemical composition which is directly depends on accuracy of chemical composition of evaporated material. In the present work, (Bi 2 Te 3 ) 0.96 (Bi 2 Se 3 ) 0.04 crystals grown by Traveling Heater Method and variation of Se content along the crys- tal due to this method is investigated. A model is represented for calculating Bi 2 Se 3 content along the crystal which is fairly justi- fied by experimental results of thermoelectric power factor (˛ 2 ) measurement. 2. Experimental The solid solution was prepared in a quartz capsule containing raw material purified up to 5N purity, in rocking furnace. To improve properties of devices, Trav- eling Heater Method was applied. In this method, a heater travels from bottom of capsule to the top very slowly, melting content of capsule just near it. As heater moves upward, lower regions of materials in capsule are solidified according to phase diagram [7], as illustrated in Fig. 1 and crystal is formed under the melted part. Each crystalline plane is crystallized with respect to the direction of its lower planes. The heater was set to travel at a rate of 8 mm h -1 . The morphology of crystal was evaluated with a Cambridge Scanning Electron Microscope (SEM) operating at 25 kV. Crystal structural and phase evolutions during deposition have been monitored by X-ray diffraction analysis using a Philips diffrac- tometer with Cu K radiation ( = 1.5405 Å). All X-ray analyses were performed with steps of 0.02 ◦ and duration of 1 s per step. Electrical conductivity was measured by the bar method using a laboratory made device with four-point probe to ensure high accuracy. To measure Seebeck coeffi- cient, two Ni–Al(-) and Ni–Cr(+) thermocouples were fixed by two screws at the distance of 6 mm along the crystallized bar. One end of ingot was heated by a tiny wounded heater to generate temperature gradients about 5–10 ◦ C. Several mea- surements were carried out along the bar-shaped sample. Seebeck coefficient was obtained from the slope of thermo-electromotive force to the temperature differ- ence and then ˛ 2  equation is used to calculation the power factor. 0254-0584/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.matchemphys.2009.08.047