| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 102 === 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.
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