Micro thermoelectric devices can directly convert heat energy into electric energy even with small heat source ^[1]. To prepare thermoelectric materials for the micro devices, electrodeposition have numbers of advantages, for example patterned formation of micro structures by selective deposition. Among the thermoelectric materials, Bi-Te has high performance at room temperature, and we have fabricated micro thermoelectric devices using electrodeposited Bi-Te ^[2]. We have attempted to improve their performance, and one of the approaches was annealing. However, when the device was annealed, the problems such as increase of electric resistance ^[2] and crack formation were observed. Although the effect of annealing on Bi-Te films has been widely investigated ^{[3, 4]}, its application to the devices has rarely been reported. In addition, the effect of Cu addition on the thermoelectric properties of Bi-Te has been reported for several preparation methods such as spark plasma sintering ^{[5, 6]}, pressureless sintering ^[7] and gas-induced-reduction method ^[8], yet not in electrodeposition. Hence, in this study, effect of Cu addition to the Bi-Te films using electrodeposition and also the effect of the annealing on the films were investigated, in order to improve the thermoelectric performance. Then, the application to the device was attempted.

Electrodeposition of Bi-Te was carried out using a three-electrode system with a stirring paddle at room temperature. A glass substrate coated with 100 nm Au/10 nm Cr lavers was used as working electrode, Pt as counter electrode, and Ag/AgCl as reference electrode, respectively. Electrolyte composition is shown in Table 1. Cu-nitrite was added to the electrolytes, and additive amount was changed. Deposition potential was changed from -50 mV to -160 mV for p-type, and was +50 mV for n-type. The deposited films were annealed at 230 ^oC in Ar+H₂ atmosphere. The thermoelectric performance of the films was evaluated by electric resistivity and Seebeck coefficient. The size of p-n legs in the device is 200 μm in diameter and 20 μm in height. Patterned substrates for the selective deposition were formed using photolithography, and the details of fabrication procedure were described in our previous study ^[2].

First, both n-type Bi-Te and p-type Bi-Sb-Te films were prepared with various conditions of Cu additive amount and deposition potential, followed by annealing. Subsequently, their thermoelectric performance before and after the annealing was evaluated. As a result, it turned out that increase in resistivity after the annealing was suppressed for the p-type Bi-Sb-Te films, and higher performance was obtained at the optimized conditions, with additive amount of 0.5 mM and deposition potential of -120 mV. This condition was applied to prepare the p-type material for the device. On the other hand, the performance of n-type Bi-Te films decreased by adding Cu, so Cu free condition was applied to form the device. Using these conditions, the device was fabricated, and power curves were measured. The device with Cu added p-type Bi-Sb-Te, the maximum power increased after the annealing from 1.36 μW to 3.50 μW.

From these results, it was confirmed that addition of Cu to Bi-Sb-Te films effectively prevented the increase in resistivity after the annealing, resulting in high thermoelectric performance of the devices.

This research was partially supported by a Grant-in-Aid for Scientific Research from MEXT, Japan.

References

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