本研究主要探討晶界能障散射效應對Bi0.5Sb1.5Te3薄膜材料熱電性質的影響。Bi0.5Sb1.5Te3合金材料為目前在室溫下最佳的p型熱電材料，本實驗利用磁控濺鍍法在不同製程溫度製備不同晶粒尺寸的Bi0.5Sb1.5Te3薄膜試片。實驗結果顯示隨著製程溫度由室溫上昇到200ºC，晶粒大小從25nm增加到100nm，而小晶粒尺寸的Bi0.5Sb1.5Te3薄膜試片其Seebeck係數高達280μV/K，較大晶粒的薄膜試片高約50%。藉由薄膜載子濃度、遷移率與電阻率的量測結果與載子傳輸理論模型相比較，推測此Seebeck係數的提升應來自於小晶粒尺寸薄膜試片具有較大比例的晶界能障散射效應所致。此小晶粒尺寸薄膜試片雖具有較高的Seebeck係數，但電阻率亦偏高。因此後續透過高溫短時間和低溫長時間兩種退火方式來改善室溫下鍍製的Bi0.5Sb1.5Te3薄膜試片電阻率。實驗結果顯示室溫鍍製的薄膜試片經225℃退火5分鐘後，電阻率大幅降為6.3 mΩ-cm，Seebeck係數微幅下降至210μV/K，退火後薄膜試片的最佳功率因子為7 10-4W/m-K2。

The effect of grain boundary potential scattering on thermoelectric properties of Bi0.5Sb1.5Te3 thin films has been investigated. Bi0.5Sb1.5Te3 is known to be the best p-type thermoelectric material at the room temperature regime. In this study, the Bi0.5Sb1.5Te3 thin films of various grain sizes were deposited at different substrate temperatures by the magnetron sputter deposition method. As the deposition temperature increases from room temperature to 200 ºC, the grain size of the sputtered Bi0.5Sb1.5Te3 thin films increase from 25nm to 100nm. The sputtered thin film with the smallest grain size has a very high Seebeck coefficient of 280 μV/K, which is 50% higher than that of the film of 100 nm in grain size. By comparing the measured carrier concentration, mobility, resistivity with the theoretical predictions from the carrier transport model, it is suggested that the enhancement of Seebeck coefficient for the sputtered Bi0.5Sb1.5Te3 thin films is mainly attributed to the effect grain boundary potential scattering. Since the as-deposited Bi0.5Sb1.5Te3 thin films have high electrical resistivity, two different thermal treatments, high temperature/short duration and low temperature/long duration, have been employed to reduce the resistivity of the sputtered Bi0.5Sb1.5Te3 thin films. It is found that the Bi0.5Sb1.5Te3 thin films has the lowest resistivity of 6.3mΩ-cm and a moderately reduced Seebeck coefficient of 210μV/K, leading to the highest power factor of 7 10-4W/m-K2 after annealing at 225 ºC for 5 minutes.

[1] K. Kishimoto, K. Yamamoto and T. Koyanagi, “Influences of potential barrier scattering on the thermoelectric properties of sintered n-type PbTe with a small grain size”, Japanese Journal of Applied Physics, 42, 501 (2003)

[2] K. Kishimoto, Y. Nagamoto and T. Koyanagi, “Thermoelectric properties of SiGe sintered alloys with modified grain boundaries”, in Pproceeding of the 17th International Conference on Thermoelectrics, p.257, Dresden, Germany, (1998)

[3] G. S. Nolas, J. Sharp and H. J. Goldsmid, “Thermoelectrics”, Chap. 2, p.41

[4] G. S. Nolas, J. Sharp and H. J. Goldsmid, “Thermoelectrics”, Chap. 2, p.40

[5]朱旭山, “熱電材料與元件之發展與應用”, 工業材料雜誌, 220, 93, 2005

[6] G. S. Nolas, J. Sharp and H. J. Goldsmid, “Thermoelectrics”, Chap. 5 p.118

[7] L. D. Hicks and M. S. Dresselhaus, “Effect of quantum-well structures on the thermoelectric figure of merit”, Physical Review B, 47, 12727 (1993)

[8] A. Balandin and K. L. Wang, “Effect of phonon confinement on the thermoelectric figure of merit of quantum wells”, Journal of Applied Physics, 84, 6149 (1998)

[9] Y. H. Shing, Y. Chang, A. Mirshafii, L. Hayashi, S.S. Roberts, J, Y. Josefowicz and N. Tean, “Sputtered Bi2Te3 and PbTe thin films”, Journal of Vacuum Science and Technology A, 1, 503 (1983)

[10] E. Chales, E. Groubert and A. Boyer, “Structural and electrical properties of bismuth telluride films grown by the molecular beam technique”, Journal of Materials Science Letters, 7, 575 (1988)

[11] A. Boulouz, A. Giani, F. Pascal-Delannoy, M. Boulouz, A. Foucaran and A. Boyer, “Preparation and characterization of MOCVD bismuth telluride thin films”, Journal of Crystal Growth, 194, 336 (1998)

[12] H. Zou, D. M. Rowe and G. Min, “Preparation and characterization of p-type Sb2Te3 and n-type Bi2Te3 thin film grown by co-evaporation”, Journal of Vacuum Science and Technology A, 19, 899 (2001)

[13]郭世偉碩士論文，”Bi/Te複合薄膜濺鍍製程暨其熱電性質研究”，國立清華大學材料所(2004)

[14]廖柏瑞碩士論文，”濺鍍製程參數對Bi/Te複合熱電薄膜影響之研究”，國立清華大學材料所(2005)

[15]佘東和碩士論文，” Bi/Te多層複合濺鍍薄膜製程與熱電性質之
研究”，國立清華大學材料所(2006)

[16] D.M. Rowe, “CRC Handbook of Thermoelectric”, (Boca Raton, 1994) Chap.7

[17] K. Kishimoto, M. Tsukamoto, and T. Koyanagi, “Temperature dependence of the Seebeck coefficient and the potential barrier scattering of n-type PbTe films prepared on heated glass substrates by rf sputtering” Journal of Applied Physics, 92, 5331 (2002)

[18] G. S. Nolas, J. Sharp and H. J. Goldsmid, “Thermoelectrics” Chap. 2, p. 39

[19] K. Kishimoto, and T. Koyanagi, “Preparation of sintered degenerate n-type PbTe with a small grain size and its thermoelectric properties” Journal of Applied Physics, 92, 2544 (2002)

[20] Y. G. Kim, K. H. Kang and K. S. Gam, “Measurement of the Seebeck coefficients of binary Cu–Ni alloys”, Measurement Science and Technology, 15, 1266 (2004)

[21] L. J. van der Pauw, “A Method of Measuring the Resistivity and Hall coefficient on Lamellae of Arbitrary Shape”, Philips Technical Review, 20, 220 (1958)

[22] H. Noro, K. Sato and H. Kagechika, “The thermoelectric properties and crystallography of Bi-Sb-Te-Se thin films grown by ion beam sputtering” Journal of Applied Physics, 73, 1252 (1993)

[23] G. S. Nolas, J. Sharp and H. J. Goldsmid, “Thermoelectrics”, Chap. 2, p. 38

[24] M. Stordeur, M. Stolzer, H. Sobotta and V. Riede “Investigation of the valence band structure of thermoelectric (Bi1-xSbx)2Te3 single crystals”, Physica Status Solidi B, 150, 165 (1988)

[25] J. Seo, D. Lee and C. Lee, “Microstructure, mechanical properties and thermoelectric properties of p-type Te-doped Bi0.5Sb1.5Te3 compounds fabricated by hot extrusion” Journal of Materials Science Letters, 16, 1153 (1997)

[26] J. Seo, C. Lee and K. Park, “Effect of extrusion temperature and dopant on thermoelectric properties for hot-extruded p-type Te-doped Bi0.5Sb1.5Te3 and n-type SbI3-doped Bi2Te2.85Se0.15” Materials Science and Engineering B, 54, 135 (1998)

[27] H. C. Kima, T. S. Oha and D. B. Hyun, “Thermoelectric properties of the p-type Bi2Te3–Sb2Te3–Sb2Se3 alloys fabricated by mechanical alloying and hot pressing” Journal of Physics and Chemistry of Solids, 61, 743 (2000)

[28] Y. Horio and T. Hoshi, “Amorphous thermoelectric alloys and thermoelectric couple using “, United States Patent 5726381