碲化鉍系化合物為室溫範圍具有優異表現之熱電材料。優良的熱電材料必須具備高Seebeck係數、高電傳導係數以及低熱傳導係數，而以上性質可透過調控材料中之載子濃度達成。本研究計畫於軟性聚亞醯胺基板上濺鍍P型及N型兩種碲化鉍薄膜，再以電子束蒸鍍法於熱電薄膜上沉積一階梯狀銀擴散疊層，最後經由高溫熱退火處理在碲化鉍薄膜中建立一非均勻擴散區域。研究目的在於探討銀摻雜效應對P型及N型碲化鉍薄膜熱電傳輸性質之影響。研究結果顯示銀原子於熱電薄膜中呈現水平以及垂直兩個方向的不均勻擴散。銀摻雜使P型熱電薄膜載子濃度提升約4.7倍，推測為錯位型缺陷AgSb提供大量電洞所造成。而N型熱電薄膜於銀摻雜後載子濃度則僅小幅提升約1.75倍，推測為系統中過多的Te形成TeBi供體缺陷，抑制AgBi受體缺陷的產生。此外具非均勻銀摻雜之P型熱電薄膜經過掃描Seebeck量測發現Seebeck係數不對稱現象，當熱流方向為銀摻雜端至非銀摻雜端，且探針針距為5.3 mm時，偵測最大Seebeck係數285 µV/k、最大輸出電流5.41 µA以及最大輸出功率密度(power density)為10.5 nW/cm2。而N型熱電薄膜則無明顯差異，推測需於摻雜端以及非摻雜端建立足夠之載子濃度差異，才可產生Seebeck係數不對稱現象。

Bismuth telluride has been considered as a promising candidate for thin-film thermoelectric (TE) devices due to its superior thermoelectric properties at room temperature regime. Generally, a good thermoelectrics requires a large Seebeck coefficient, a high electrical conductivity and a low thermal conductivity, which can be achieved by optimization of carrier concentration. In this study, both P-type Bi-Sb-Te and N-type Bi-Se-Te thin films were deposited on polyimide substrates by RF magnetron sputtering. A step-like Ag overlayer was evaporated partially on top of the TE films using e-gun deposition. Silver atoms were driven into the TE films by thermal annealing to form a non-uniform doping profile. The research goal is to investigate the effect of Ag doping on thermoelectric transport properties of P- and N-type Bi-Te thin films. The results show that Ag elements have non-uniform distributions in both in-plane and out-of-plane directions of TE thin films. The Ag doping results in 4.7 times increase of carrier concentration in P-type films and 1.75 times increase in N-type films. It suggests that AgSb acceptor-like defects provide large amount of hole carriers in P-type films, while the excessive Te atoms occupy Bi site by forming TeBi donor-like defects counterbalances the contribution of AgBi defects in N-type films. The scanning Seebeck results reveal an asymmetric Seebeck effect in the P-type films. The P-type films with Ag doping achieve a maximum Seebeck coefficient of 285 µV/k, an output current of 5.41 µA and a power density of 10.5 nW/cm2 when heat flows in the direction from high to low doping region of P-type films with a distance between two probes of 5.3 mm. The N-type films with Ag doping do not show obviously asymmetric effect, which is likely attributed to small difference in carrier concentration between doped and non-doped region.

[1] C. B. Vining, Semiconductors are cool. Nature, 413, 577–578 (2001).
[2] L. Francioso, C. De Pascali, I. Farella, C. Martucci, P. Cretì, P. Siciliano, A. Perrone, Flexible thermoelectric generator for ambient assisted living wearable biometric sensors. Journal of Power Sources, 196, 3239–3243 (2011)
[3] G. J. Snyder, E. S. Toberer, Complex thermoelectric materials. Nature Materials, 7, 105–114 (2008).
[4] S. O. Kasap, Principles of electronic materials and devices. §4.8.2. (McGraw-Hill, 2006)
[5] D. L. Zhao, G. Tan, A review of thermoelectric cooling: Materials, modeling and applications. Applied Thermal Engineering, 66, 15-24 (2014)
[6] M. J. Huang, T. M. Chang, W. Y. Chong, C. K. Liu, C. K. Yu, A new lattice thermal conductivity model of a thin-film semiconductor. International Journal of Heat and Mass Transfer, 50, 67–74 (2007)
[7] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit. Nature, 413, (2001)
[8] H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono, K. Koumoto, Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3. Nature Materials, 6, 129 (2007)
[9] J. F. Li, W. S. Liu, L. D. Zhao, M. Zhou, High-performance nanostructured thermoelectric materials. NPG Asia Materials, 2(4), 152–158 (2010)
[10] T. Caillat, M. Carle, P. Pierrat, H. Scherrer and S, Scherrer, Thermoelectric properties of (BixSb1−x)2Te3 single crystal solid solutions grown by the T.H.M. method. Journal of Physics and Chemistry of Solids, 53, 1121–1129 (1992).
[11] J. R. Drabble, R. Wolfe, Anisotropic galvanomagnetic effects in semiconductors. Proceedings of the Physical Society Section B, 69, 1101 (1956).
[12] J. R. Drabble, R. D. Groves, R. Wolfe, Galvanomagnetic effects in n-type bismuth telluride. Proceedings of the Physical Society, 71, 430 (1958).
[13] J. R. Drabble, Galvanomagnetic effects in p-type bismuth telluride. Proceedings of the Physical Society, 72, 380 (1958).
[14] G. R. Miller, C. Y. Li, Evidence for the existence of antistructure defects in bismuth telluride by density measurements. Journal of Physics and Chemistry of Solids, 26, 173–177 (1965).
[15] Z. Starý, J. Horák, M. Stordeur, M. Stölzer, Antisite defects in Sb2−xBixTe3 mixed crystals. Journal of Physics and Chemistry of Solids, 49, 29–34 (1988).
[16] A. Kadhim, A. Hmood, H. A. Hassan, Preparation of Bi0.4Sb1.6Se3xTe3（1−x） hexagonal rods and effect of Se on structure and electrical property. Solid State Sciences, 21, 110–115 (2013).
[17] J. Navra ́til, I. Klichoa ́v, S. Karamazov, J. S ̌ra ́mkova ́, J. Hora ́k, Behavior of Ag admixtures in Sb2Te3 and Bi2Te3 single crystals. Journal of Solid State Chemistry, 140, 29-37 (1998)
[18] J.L. Cui. Transport properties of quaternary Ag–Bi–Sb–Te alloys prepared by pressureless sintering. Materials Letters, 59, 3205–3208 (2005)
[19] J.L. Cui, X. B. Xu, Microstructures and thermoelectric properties of p-type pseudo-binary AgxBi0.5Sb1.5-xTe3 (x=0.05–0.4) alloys prepared by cold pressing. Materials Letters, 60, 3669–3672 (2006)
[20] J. K. Lee, S. D. Park, B. S. Kim, M. W. Oh, S. H. Cho, B. K. Min, H. W. Lee, M. H. Kim, Control of Thermoelectric properties through the addition of Ag in the Bi0.5Sb1.5Te3 alloy. Electronic Materials Letters, 6, 4, 201-207 (2010)
[21] J. Yang, R. Chen, X. Fan, S. Bao, W. Zhu, Thermoelectric properties of silver-doped n-type Bi2Te3-based material prepared by mechanical alloying and subsequent hot pressing. Journal of Alloys and Compounds, 407, 330–333 (2006)
[22] X. Zhang, X.Y. Ma, Q.M. Lu, F.P. Zhang, Y. Q. Liu, J. X. Zhang, L. Wang, Thermoelectric properties of Ag-doped n-type (Bi2-xAgxTe3)0.96-(Bi2Se3)0.04 pseudobinary alloys. Journal of Electronic Materials, 40, 5, (2011)
[23] X. Duan, J. Yang, C. Xiao, W. Zhu, Structural and thermoelectric properties of Ag-doped Bi2(Te0.95 Se0.05)3 thin films prepared by flash evaporation. Journal of Physics D: Applied Physics, 40, 5971–5974 (2007)
[24] P. Mondal, B. Ghosh, P. Bal, Planar junctionless transistor with non-uniform channel doping. Applied Physics Letters, 102, 133505 (2013)
[25] D. A. Neman, An introduction to semiconductor devices, p.149, p.506, (McGraw-Hill, New York 2006)
[26] H. H. Huang, M. P. Lu, C. H. Chiu, L. C. Su, C. N. Liao, J. Y. Huang, H. L. Hsieh, Enhanced Seebeck coefficient of bismuth telluride compounds with graded doping profiles. Applied Physics Letters, 103, 163903 (2013)
[27] M. P. Seah, A review of the analysis of surfaces and thin films by AES and XPS. Vacuum, 34, 3-4, 463-478 (1964)
[28] F. H. Chung, D. K. Smith, Industrial Applications of X-ray Diffraction, p.847 (Marcel Dekker, New York 2000)
[29] C. H. Chiu and C. N. Liao, “A study on the diffusion behavior of Bi-Sb-Te thermoelectric materials”, p.38, p.54, 國立清華大學碩士論文2010年
[30] B. Chayasombat, S. Henpraserttae, C. Boothroyd, C. Thanachayanont, Mechanically alloyed β-Ag2Te in thermoelectric Bi2Se0.01Te2.99. Materials Letters, 116, 243–246 (2014)
[31] S. Cho, Y. Kim, A. D. Venere, G. K. Wong, J. B. Kettersonb, Antisite defects of Bi2Te3 thin films. Applied Physics Letters, 75, 1401 (1999).
[32] J. D. Keys, H. M. Dutton, Diffusion and solid solubility of silver in single-crystal bismuth telluride. Journal of Physics and Chemistry of Solids, 24, 563-571 (1963)
[33] C. Klein, C. S. Hurlburt, Manual of Mineralogy 20th edition, pp. 160-161,
(John Wiley & Sons, Inc., New York 1985)
[34] N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, (Pergamon, 1984)