Effect of self-grown seed layer on thermoelectric properties of ZnO thin films

• Thermoelectric properties of 2% Al doped ZnO (AZO) thin films are reported.
• A self-grown seed layer develops during the nucleation process of the thin film growth.
• Samples with thicker self-grown seed layer show the best thermoelectric performance.
• The seed layer plays a crucial role for the electrical and thermal properties.
• Best sample figure of merit is about 5 times larger than that of bulk ZnO.

The influence of self-grown seed has been studied on thermoelectric performance of 2% Al-doped ZnO (AZO) thin films. The thickness and orientation of c-axis domains in seed layer change on different substrates while other deposition conditions were kept unchanged for a comparable study. The changes occur because of the different nucleation process of thin film growth on substrate interface and the different lattice mismatch between AZO and substrate. Thin films are grown by pulsed laser deposition on single crystals (SrTiO3 (STO) and Al2O3) and cheap amorphous fused silica (FS) substrates at 400 °C. All thin films are c-axis oriented. The grains are highly connected and elongated in shape which leads to high thermoelectric properties. The thicker self-grown seed layer is found in thin film deposited on FS substrates which shows best performance: electrical conductivity σ = 93 S/cm and Seebeck coefficient S = − 203 μV/K, which estimate power factor (S2.σ) about 0.37 × 10− 3 Wm− 1K− 2 at 600 K. The value of thermal conductivity (κ) was found lowest (4.89 Wm− 1K− 1) for thin film deposited on FS than the other thin films (6.9 Wm− 1K− 1 on Al2O3 and 6.55 Wm− 1K− 1 on STO) at 300 K. The figure of merit, ZT = S2.σ.T/κ, is calculated 0.046 at 600 K, 5 times larger than the ZT of our previous reported bulk AZO, which is promising for practical applications of thermoelectric oxide thin films.