Annealing Behavior of Bi 2 Te 3 Thermoelectric Semiconductor Electrodeposited for Nanowire Applications

Transcription

1 Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007, pp Annealing Behavior of Bi 2 Te 3 Thermoelectric Semiconductor Electrodeposited for Nanowire Applications Min-Young Kim and Tae-Sung Oh Department of Materials Science and Engineering, Hongik University, Seoul Jin-Sang Kim Division of Thin Film Processing, Korea Institute of Science and Technology, Seoul (Received 5 September 2006) Bi 2Te 3 films were electrodeposited for nanowire applications and the effects of annealing in a H 2 atmosphere and an Ar ambient on the thermoelectric properties of the electrodeposited Bi 2Te 3 films were investigated. The Bi 2Te 3 films electrodeposited in solutions with Bi/(Bi+Te) mole ratios of 50 % and 60 % exhibited the strong (110) preferred orientation. The behaviors of the thermoelectric properties were almost identical for the annealing treatments in a H 2 atmosphere and in an Ar ambient. The power factor of the Bi 2Te 3 film without a (110) preferred orientation, as electrodeposited in a solution with a Bi content of 40 %, was remarkably improved from W/K 2 -m to W/K 2 -m by annealing in a H 2 atmosphere and in an Ar ambient. However, no improvement of the power factor could be achieved by annealing the Bi 2Te 3 films with (110) preferred orientations. The interaction between the electrodeposited Bi 2Te 3 film and the Ti seed layer also affected the annealing behavior of the electrodeposited Bi 2Te 3 film. PACS numbers: Lw Keywords: Thermoelectrics, Bismuth telluride, Thin film, Nanowire, Electrodeposition, Annealing I. INTRODUCTION Thermoelectric devices utilizing the Seebeck effect and the Peltier effect have been widely investigated for applications to thermoelectric power generation and thermoelectric cooling [1 3]. The energy conversion efficiency of thermoelectric devices depends on the non-dimensional figure-of-merit (ZT ) of the thermoelectric semiconductors. Numerous works have been done to improve the ZT values of thermoelectric semiconductors, which depend on the Seebeck coefficient α, the electrical conductivity σ, and the thermal conductivity κ(z = α 2 σ/κ) [4 8]. However, the ZT values of thermoelectric semiconductors have been stagnant at about 1 for the last several decades, and the unavailability of thermoelectric semiconductors with values of ZT much larger than unity limits the wide spread of thermoelectric applications [9]. Recently, it has been predicted that the ZT values for low-dimensional structures, such as superlattices (2D) and nanowires (1D), can be improved significantly, compared with those of bulk thermoelectric semiconductors, due to quantum-confinement effects [10, 11]. Among low-dimensional thermoelectric devices, nanowires are more attractive because a higher figure-of-merit is pre- ohts@hongik.ac.kr; Fax: dicted and processing can be made easier by adapting a template for growth and a structural support for the nanowire array [12, 13]. Among bulk thermoelectric semiconductors, Bi 2 Te 3 -based semiconductors exhibit the highest ZT values near room temperature and can be good candidates for thermoelectric nanowires [1 3,12]. While various processing techniques, such as electrodeposition, vapor-liquid-solid epitaxy, and melt injection, can be used for nanowire fabrication, electrodeposition is attractive because it is a rapid and inexpensive process. Also, the composition and the microstructure of the electrodeposited film can be easily controlled by changing the electrodeposition parameters [12,14,15]. For bulk and sputtered Bi 2 Te 3 -based thermoelectric semiconductors, the figure-of-merit has been reported to be improved by hydrogen annealing [16, 17]. In Te-rich Bi 2 Te 3 thermoelectric semiconductors, anti-site defects, Te Bi, are produced by Te substitution into Bi sites and act as donors [18]. Also, it has been reported that heavy deformation and incorporation of oxygen in the Bi 2 Te 3 lattice generate lattice defects acting as donors [18, 19]. Hydrogen annealing causes a reduction of the dislocation density, the oxygen content, and the Te Bi anti-site defect density, resulting in an improved figure-of-merit with optimization of the carrier concentration. Contrary to bulk Bi 2 Te 3 -based semiconductors for which oxygen can be incorporated into the Bi 2 Te 3 lattice during the fabrica

2 Annealing Behavior of Bi 2Te 3 Thermoelectric Semiconductor Min-Young Kim et al tion process [16], an electrodeposited Bi 2 Te 3 is rather free from incorporated oxygen due to the nature of electrodeposition during which metal ions and hydroxyl ions move to opposite electrodes. Thus, there would be no noticeable difference between the effect of an annealing treatment in a reducing atmosphere and the effect of one in an inert ambient on the thermoelectric characteristics of electrodeposited Bi 2 Te 3. In this study, Bi 2 Te 3 films were electrodeposited in solutions with different Bi/(Bi+Te) mole ratios for nanowire applications, and the annealing effects on the thermoelectric properties of electrodeposited Bi 2 Te 3 films were investigated. Annealing was accomplished in a 50 % H % Ar atmosphere as a reducing ambient and also in a 100 % Ar atmosphere as an inert ambient to compare the effects of the annealing atmosphere on the thermoelectric properties of the electrodeposited Bi 2 Te 3. II. EXPERIMENTS Bismuth-telluride films were electrodeposited from nitric-acid aqueous solutions containing 50-mM Bi-Te in 1-M HNO 3 at a constant potential of 50 mv. Bi 2 O 3 and TeO 2 were successively dissolved in HNO 3 with varying the Bi/(Bi+Te) mole ratio, i.e. the Bi content, in the electrodeposition solution from 30 % to 60 %. To form bismuth-telluride films for thermoelectric property measurements, we sputter-deposited a 1-µm-thick Ti layer onto a SiO 2 /Si substrate as a conducting layer for electrodeposition. A three-electrode electrochemical cell system was employed for electrodeposition of 50-µmthick bismuth-telluride films. A Ti/SiO 2 /Si substrate was used as a cathode, a Pt mesh electrode was used as an anode, and a Ag/AgCl electrode was used as a reference electrode. Bismuth-telluride films were electrodeposited without stirring at a constant potential of 50 mv for 4 h at room temperature by using a potentiostat/galvanostat (Princeton Applied Research model 263A). An electrodeposited bismuth-telluride film was separated from a Si substrate by etching a Ti seed layer to eliminate any effects of the conducting Ti seed layer on the thermoelectric measurements and to eliminate any interaction between the bismuth-telluride film and the Ti seed layer during annealing. The electrodeposited bismuth-telluride films were annealed at 450 C for 4 h in a 50 % H % Ar atmosphere and in a 100 % Ar ambient. The crystalline phases of the electrodeposited bismuth-telluride films were characterized by using X-ray diffraction (XRD). The microstructures of the electrodeposited and the annealed Bi 2 Te 3 films were observed using scanning electron microscopy (SEM). The Seebeck coefficients (α) of the electrodeposited and the annealed bismuth-telluride films were measured at room temperature by applying a temperature difference of 20 C at Fig. 1. X-ray diffraction patterns of the Bi 2Te 3 films electrodeposited in 50-mM Bi-Te solutions with Bi contents, i.e. (Bi+Te) mole ratios, of (a) 30 %, (b) 40 %, (c) 50 %, and (d) 60 %. both ends of the films. The electrical resistivities (ρ) were measured using a 4-point probe. The carrier concentrations and the mobilities of the electrodeposited and the annealed Bi 2 Te 3 films were characterized by using Hall measurements with AC magnetic fields. The power factors (P ) of the electrodeposited and the annealed Bi 2 Te 3 films were evaluated using the relation P = α 2 /ρ. III. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffraction patterns of the films electrodeposited in 50-mM Bi-Te solutions with various Bi/(Bi+Te) mole ratios. For films electrodeposited in solutions with a Bi content between 30 % and 60 %, Bi 2 Te 3 diffraction peaks were mainly observed, indicating that the films electrodeposited in such solutions were Bi 2 Te 3 films. This result confirms that the electrodeposition rate of Te was faster than that of Bi [20,21]. The films electrodeposited in solutions with a Bi content of 30 % exhibited the (015) peak as the largest diffraction peak. However, the (110) intensity increased for the films electrodeposited in the solution containing more Bi 3+ ions, and the films processed in solutions with Bi contents of 50 % and 60 % exhibited strong (110) preferred orientations. It has been also reported that the orientation of an electrodeposited Bi 2 Te 3 film varies from a random orientation to a (110) preferred orientation with increasing deposition potential [20]. The Seebeck coefficient and the electrical resistivity

3 -672- Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007 Fig. 2. Seebeck coefficient of the as-electrodeposited and the annealed Bi 2Te 3 films as functions of the Bi content, i.e. the Bi/(Bi+Te) mole ratio, of the electrodeposition solution. Fig. 3. Electrical resistivity of the as-electrodeposited and the annealed Bi 2Te 3 films as functions of the Bi content of the electrodeposition solution. of the electrodeposited Bi 2 Te 3 films are shown in Figs. 2 and 3, respectively. For later comparison, the Seebeck coefficient and the electrical resistivity of the Bi 2 Te 3 films annealed in a H 2 atmosphere and an Ar ambient are also illustrated in Figs. 2 and 3. The aselectrodeposited Bi 2 Te 3 films possess Seebeck coefficients with minus signs, indicating that the electrodeposited Bi 2 Te 3 films have n-type conduction. As Te-rich Bi 2 Te 3 has been reported to be n-type and Bi-rich Bi 2 Te 3 p-type [20], all the films electrodeposited in solutions with a Bi content between 30 % and 60 % in this work could be considered as Te-rich Bi 2 Te 3. The Seebeck coefficient of the as-electrodeposited Bi 2 Te 3 films in this study was in the range of µv/k, which was equal to the values reported by others for eletrodeposited Bi 2 Te 3 films of stoichiometric composition [20]. However, the electrical resistivities of the electrodeposited Bi 2 Te 3 films, shown in Fig. 3, were much lower than the value of about 10 mω-cm reported for stoichiometric Bi 2 Te 3 films [20], which was attributed to the higher carrier concentrations of our films. The electrical resistivity of the electrodeposited Bi 2 Te 3 film became lower with increasing Bi content of the electrodeposition solution, which was consistent with the report that Te incorporation into the electrodeposited Bi 2 Te 3 film caused the resistivity to increase [22]. As Fig. 4 shows, the Seebeck coefficients of the Bi 2 Te 3 films, electrodeposited not only by us but also by others [20], were much lower than the 200 µv/k reported for bulk Bi 2 Te 3 [7], which was attributed to the high carrier concentration. In Fig. 4, the carrier concentrations of the Bi 2 Te 3 films annealed in a H 2 ambient and an Ar atmosphere are also illustrated for later comparison. The Fig. 4. Carrier concentration of the as-electrodeposited and the annealed Bi 2Te 3 films as functions of the Bi content of the electrodeposition solution. Seebeck coefficient of an n-type thermoelectric semiconductor can be expressed as α = k B e (γ + C ln n c), (1) where k B is the Boltzmann constant, e is the electric charge of an electron, γ is the scattering factor, n is the carrier concentration, and C is a constant [4 7]. The carrier concentrations, /cm 3, of the

4 Annealing Behavior of Bi 2Te 3 Thermoelectric Semiconductor Min-Young Kim et al Fig. 5. Power factor of the as-electrodeposited and the annealed Bi 2Te 3 films as functions of the Bi content of the electrodeposition solution. Fig. 6. X-ray diffraction patterns of the Bi 2Te 3 films annealed in a 50 % H % Ar atmosphere. The Bi contents of the electrodeposition solution were (a) 30 %, (b) 40 %, (c) 50 %, and (d) 60 %. as-electrodeposited Bi 2 Te 3 films were much higher than the carrier concentrations of /cm 3 required for optimization of the thermoelectric properties. Fig. 5 shows the power factor (P = α 2 /ρ) of the as-electrodeposited Bi 2 T 3 film as a function of the Bi content in the electrodeposition solution. For later comparison, the power factors of the Bi 2 Te 3 films annealed in a H 2 ambient and an Ar atmosphere are also illustrated in the Figure. Because of the low Seebeck coefficient, the power factor of the as-electrodeposited Bi 2 Te 3 films is quite low compared to the values reported for Bi 2 Te 3 -based bulk thermoelectric materials [4 7]. The power factor of the as-electrodeposited Bi 2 Te 3 film was maximized to W/K 2 -m for a film electrodeposited in a solution with a Bi content of 60 %. The power factor of the as-electrodeposited Bi 2 Te 3 films was the same magnitude as that of the sputtered or evaporated (Bi,Sb) 2 Te 3 films, but was one order lower than the values obtained for bulk (Bi,Sb) 2 (Te,Se) 3 alloys due to the low Seebeck coefficient [4 7,17]. Because the Seebeck coefficient of the aselectrodeposited Bi 2 Te 3 films were too low due to high electron concentration, to optimize the thermoelectric properties, the electrodeposited films were annealed in a 50 % H % Ar atmosphere as a reducing ambient or a 100 % Ar atmosphere as an inert ambient at 450 C for 4 h. Figs. 6 and 7 illustrate the XRD patterns of the Bi 2 Te 3 films after annealing in a H 2 atmosphere and an Ar ambient, respectively. The orientation of the electrodeposited Bi 2 Te 3 films was preserved even after annealing treatments in a H 2 atmosphere and an Ar ambient. The Bi 2 Te 3 films electrodeposited in solutions with Bi contents of 50 % and 60 % preserved the strong Fig. 7. X-ray diffraction patterns of the Bi 2Te 3 films annealed in an Ar ambient. The Bi contents of the electrodeposition solution were (a) 30 %, (b) 40 %, (c) 50 %, and (d) 60 %. (110) preferred orientation after annealing at 450 C for 4 h. In Figs. 2 to 5, the Seebeck coefficient, the electrical resistivity, the carrier concentration, and the power factor of the Bi 2 Te 3 films annealed in a H 2 atmosphere and an Ar atmosphere were compared with the values of the as-electrodeposited films. The variations in the behav-

5 -674- Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007 Fig. 8. Mobility of the as-electrodeposited and the annealed Bi 2Te 3 films as functions of the Bi content of the electrodeposition solution. iors of the thermoelectric properties, shown in Figs. 2 to 5, were almost identical for both the H 2 annealing process and the Ar annealing process. These results confirm that oxygen atoms were not incorporated in the Bi 2 Te 3 film during the electrodeposition process. With annealing treatments both in a H 2 atmosphere and an Ar atmosphere, as shown in Fig. 4, the electron concentration of the Bi 2 Te 3 film became lower. Such a reduction in the electron concentration could be attributed to a decrease in the Te Bi anti-site defect density and the dislocation density during the annealing process. As illustrated in Fig. 2, the Seebeck coefficients of the Bi 2 Te 3 films without a (110) preferred orientation, as electrodeposited in solutions with Bi contents of %, became substantially larger due to a reduction in the electron concentration. Especially, the Bi 2 Te 3 film electrodeposited in a solution with a Bi content of 40 % exhibited a Seebeck coefficient of V/K after annealing, which is about half the value reported for bulk Bi 2 Te 3 single crystals [7]. However, the Seebeck coefficients of the Bi 2 Te 3 films with (110) preferred orientations, as electroplated in solutions with Bi contents more than 50 %, decreased with annealing treatment, even though the electron concentration was lowered with the annealing process. While Te-rich Bi 2 Te 3 films were n-type, Bi-rich ones exhibited p-type conduction [20]. With Te vaporization during the annealing process [23], the compositions of the Bi 2 Te 3 films electrodeposited in solutions with % Bi contents became closer to the composition for the p-n transition, causing a decrease in the Seebeck coefficient. The interaction between an electrodeposited Bi 2 Te 3 film and a Ti seed layer can also affect the Seebeck co- Fig. 9. Scanning electron micrographs of the Bi 2Te 3 films (a) electrodeposited in a solution with a Bi content of 40 %, and (b) annealed in a H 2 atmosphere, or (c) annealed in an Ar ambient. efficient of an annealed Bi 2 Te 3 film. In this study, the interaction between the Bi 2 Te 3 film and the Ti seed layer was eliminated by etching off the Ti seed layer before annealing. In our previous work where the Ti seed layer was not removed [21], the Seebeck coefficient was improved to 197 µv/k by annealing in a 50 % H % Ar atmosphere for the Bi 2 Te 3 film electrodeposited in a solution with a Bi content of 40 %. The discrepancy between the result of this work and that of previous work [21] was caused by the interaction between the electrodeposited Bi 2 Te 3 film and the Ti seed layer during the annealing process. In Fig. 3, the electrical resistivities of the electrodeposited Bi 2 Te 3 films were reduced by the annealing treatments both in a H 2 atmosphere and an Ar ambient. The resistivity ρ is given as ρ = 1/(qnµ e ), where q is the electron charge, n is the electron concentration, and µ e is the electron mobility. With the lowering of the electron concentration by the annealing treatment, the decrease in the resistivity with annealing process can be attributed to improved mobility as shown in Fig. 8. The mobility of the electrodeposited Bi 2 Te 3 films was substantially improved by more than 3 times by annealing both in a H 2 atmosphere and in an Ar ambient, which was partly due

6 Annealing Behavior of Bi 2Te 3 Thermoelectric Semiconductor Min-Young Kim et al to the grain growth during the annealing process. Fig. 9 shows the SEM micrographs of the Bi 2 Te 3 films electrodeposited in a solution with a Bi content of 40 % and the films annealed in a H 2 atmosphere and an Ar ambient. Substantial grain growth to change the grain morphology occurred during the annealing processes both in a H 2 atmosphere and an Ar ambient. As Fig. 5 shows, the power factor of the Bi 2 Te 3 film electrodeposited in a solution with a Bi content of 40 % was remarkably improved to W/K 2 - m by annealing in a H 2 atmosphere and an Ar ambient at 450 C for 4 h. Such power factors were much higher than the values reported for both sputtered and evaporated (Bi,Sb) 2 Te 3 films [7,17,24]. However, no improvement in the power factor could be achieved by annealing Bi 2 Te 3 films with (110) preferred orientations, as electrodeposited in a solution with a Bi content of 50 % or 60 %. The interaction between the electrodeposited Bi 2 Te 3 film and the Ti seed layer also affected the power factor of the annealed film. In our previous work where the Ti seed layer was not removed [21], a power factor of W/K 2 -m was obtained for the Bi 2 Te 3 film electrodeposited in a solution with the Bi content of 40 % by annealing in a H 2 atmosphere. centration. After annealing, the Bi 2 Te 3 film electrodeposited in a solution with a Bi content of 40 % exhibited a Seebeck coefficient of µv/k. The power factor of the Bi 2 Te 3 film without a (110) preferred orientation, as electrodeposited in a solution with a Bi content of 40 %, was remarkably improved from W/K 2 -m to W/K 2 -m by annealing in a H 2 atmosphere and an Ar ambient at 450 C for 4 h. Such power factors were much higher than the values reported for both sputtered and evaporated (Bi,Sb) 2 Te 3 films. However, no improvement in the power factor could be achieved with annealing treatment for the Bi 2 Te 3 films with (110) preferred orientations. The interaction between the electrodeposited Bi 2 Te 3 film and the Ti seed layer was confirmed to affect the annealing behavior of the electrodeposited Bi 2 Te 3 film. ACKNOWLEDGMENTS This research was supported by a grant (code : M105KO K ) from the Center for Nanostructured Materials Technology under the 21st Century Frontier R & D Programs of the Ministry of Science and Technology, Korea. IV. CONCLUSIONS Bi 2 Te 3 films were fabricated by using electrodeposition with various Bi/(Bi+Te) mole ratios in the electrodeposition solution, and the effects of annealing in a H 2 atmosphere and an Ar ambient on the thermoelectric properties of the electrodeposited Bi 2 Te 3 films were investigated. While the Bi 2 Te 3 film electrodeposited in a solution with a Bi content of 30 % exhibited the (015) peak as the largest diffraction peak, the films processed in solutions with Bi contents of 50 % and 60 % exhibited strong (110) preferred orientations. Such preferred orientation relationship of the electrodeposited Bi 2 Te 3 films was preserved after annealing at 450 C for 4 h. The Seebeck coefficient of the as-electrodeposited Bi 2 Te 3 films, µv/k, was smaller than the 200 µv/k reported for bulk Bi 2 Te 3 due to the high electron concentration of the electrodeposited Bi 2 Te 3 films. Among the as-electrodeposited Bi 2 Te 3 films, the one processed in an electrodeposition solution with a Bi content of 60 % exhibited a maximum power factor of W/K 2 -m, which was the same magnitude as those reported for the sputtered or the evaporated (Bi,Sb) 2 Te 3 films, but one order lower than the values obtained for bulk (Bi,Sb) 2 (Te,Se) 3 thermoelectric semiconductors. 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