Thermoelectric Devices Fabricated Using Amorphous Indium Gallium Zinc Oxide

Abstract

Thermoelectric (TE) devices are promising candidates for renewable energy resources. Superior TE materials are achieved by obtaining high power factors (PF = S 2 s), where S is the Seebeck coefficient, s is the electrical conductivity. Recently, it is necessary to investigate flexible TE devices for harvesting distributed energy at room temperature. On the other hands, amorphous InGaZnO(a-IGZO) has been already in practical use as a TFT device. The a-IGZO has enormous potential such as transparent, a low temperature process, controllability of the carrier concentration and so on. However, a few studies have been conducted on the TE properties of IGZO. Therefore, I consider a-IGZO can be applied to a transparent flexible TE element by optimizing the TE properties. There is a trade-off between the S and sunder variation of the carrier concentration. Optimization of the carrier concentration is important for a high performance TE material. In this study, I optimized the carrier concentration in a-IGZO thin film and evaluated the effects of oxygen ratio on the TE properties. In addition, the obtained TE properties were theoretically analyzed. At first, a-IGZO films (200 nm) were deposited on a glass substrate at 300 K using RF magnetron sputtering. During the sputtering process, the oxygen flow ratio (O2/(Ar + O2)) was set at values of 0 ~ 6 %. After the sputtering process, the samples were annealed at 300 °C for 2 h in a N2 atmosphere. After annealing, Mo electrodes were formed. TE properties such as S and s were measured in the range of 100 K ~ 400 K by physical property measurement system (PPMS). Carrier concentration (n) was measured by Hall effect method at room temperature. To analyze the measured data, I utilized percolation model that the potential barrier hinders conduction for electron transmission when the Fermi level (E f ) is located in the tail-states. E f , and other physical parameters were obtained by fitting theoretical equations to the measured S and s. The measured s and S with theoretical results are compared. The s increased and S decreased with decreasing oxygen flow ratio. The fitting was performed in the temperature range from 150 to 350 K. The theoretical results of s fit well with the experimental results. The fitting results of S show good agreement around low temperature and under the higher oxygen flow ratios, the measured S was slightly different from the theoretical result because of nondegenerated behavior. Carrier concentration with dependence of S and s shows the relationship of the trade off between S and s . The carrier concentration increased with decreasing oxygen flow ratio. This result can be adequately explained by the internal defects resulted from the increment of oxygen vacancies. PF showed a gradual increase along with the carrier concentration up to ~ 7.7 × 1019 cm−3 and then decreased. Therefore, the PF has the maximum value of 0.082 × 10-3 W / mK2. Figure 3 indicates a trend for E f to increase with increasing carrier concentration. It was confirmed that the obtained value of E f by fitting at which PF was maximal was equivalent to the potential barrier height. These finding leads to a conclusion that the TE properties o a-IGZO were controlled by the relationship between E f and the potential barriers. The TE properties of a-IGZO thin films were evaluated with controlled carrier concentrations to obtain better TE properties. By optimizing the carrier concentration, I found that the PF had a maximal value of 0.082 × 10−3 W/mK2 at 300 K. This optimized PF was larger than the value of the previous report. Theoretical analysis revealed the relationship of E f with PF.