Joint improvement of conductivity and Seebeck coefficient in the ZnO:Al thermoelectric films by tuning the diffusion of Au layer

Highlights: • Joint improvement of Seebeck coefficient and conductivity is achieved in ZnO films. • Power factor is two orders of magnitude larger than film without joint improvement. • Resistivity is at the same magnitude of transparent conducting oxide due to Au layer. Joint improvement of the Seebeck coefficient S and conductivity σ is a pressing issue to overcome the limit of the figure of merit (ZT) enhancement of thermoelectric materials. This study presents a joint improvement in the ZnO:Al films by tuning the diffusion of Au layer via substrate temperature, using a radio frequency-assisted molecular beam vapor deposition method. The phenomenon occurs in the ZnO:Al films at the room temperature (RT) substrate deposited on a 10 nm Au layer (RT-ZnO:AuAl). The films have a hexagonal wurtzite structure with an in-plane (002) preferred orientation and smooth surface. The resistivity values of the films are at the same magnitude of the transparent conducting oxide (~10⁻⁶ Ω·m). The carrier concentration reaches 2.60 × 10²¹ cm⁻³ for the RT-ZnO:AuAl film. The S is about 29.5 μV/K at 240 °C. Joint improvement of conductivity and Seebeck coefficient makes the PF of the RT-ZnO:AuAl film reaches 3.58 × 10⁻⁵ Wm⁻¹ K⁻², which is two orders of magnitude larger than that of the RT-ZnO:Au film without joint improvement. The results indicate that the higher conductivity originates from the Au layer, in-plane (002) preferred orientation, and the easily excited carriers in the ZnO:Al due to the low binding energy of the Zn 2p electrons. The large S in the film is related to the high effective masses due to the impact of the diffusion of the Au layer on electronic interactions.