Electrical and thermal transport properties of atomic layer deposited Al-doped ZnO films

Abstract

As the demand of energy is growing with the industrial development in the world, thermoelectrics has gathered significant interest due to its availability to convert energy from wasted heat to electric energy. The efficiency of thermoelectric materials is determined by their figure of merit, ZT=S2σT/k, which points to the need to increase the seebeck coefficient S and electrical conductivity σ while depressing the thermal conductivity. In order to achieve high ZT values(∼1.5), tellurium-, antimony- and germanium- based compounds have been widely investigated as thermoelectric materials while their toxicity and stability to oxidation at ambient conditions have been pointed out as critical issues. Recently, as an alternative to the conventional materials, ZnO- and In2O3- based transparent conducting oxides (TCOs) have been considered as thermoelectric materials due to its thermal stability and additional merit from the transparency to the visible rays. In order to integrate the TCO materials into the thermoelectrics, it is important to control both electrical and thermal transport properties of those. In this regard, atomic layer deposition (ALD) can give a great benefit to elucidate the purpose of thermoelectrics since this novel method guarantees excellent reproducibility and adjustability of film composition. In this work, ALD is utilized as a means for preparing Al-doped ZnO (AZO) films with controllability on its thermal and electrical transport properties. Contrary to the conventional homogeneous AZO composite, ALD-AZO films exhibit a unique heterolayer structure consisting of a ZnO matrix and AlOx dopant layers embedded within the matrix. These AlOx layers not just provide additional free electrons to the system but also additional interfaces that impede the thermal transport. We show that, by widely controlling the linear density of these dopant layers, the electrical conductivity can be enhanced by more than three-fold while the thermal conductivity can be suppressed to about 22% compared to that of the undoped ZnO film. This tunable feature of ALD-AZO films suggests the feasibility of using ALD-TCOs for thermoelectric purposes, which is a significant expansion to its range of applications.