Thermoelectric properties optimization of Al-doped ZnO thin films prepared by reactive sputtering Zn–Al alloy target
Introduction
Wasted heat, for example, in cars, power plants, and smelting furnace, is a rich source of energy that could be harvested and thermoelectric generators have been considered as promising energy harvesters [1], [2]. Electrical power harvest from waste heat requires high stability thermoelectric materials with large power factor (PF) and low thermal conductivity [2], [3]. Therefore, the thermoelectric materials suitable for high-temperature thermoelectric power generation have been limited to only a few compounds such as SiGe and FeSi2 [4]. However, these compounds must be using surface protection to prevent oxidation or vaporization at high temperature. Thanks to advantages of thermoelectric oxides including high-temperature thermal stability under conditions that oxidize semiconductor alloys, sufficient source supply, and environment friendly compositions, oxide thermoelectric materials have been recognized as promising candidates for applications in high-temperature thermoelectric power generation [5]. Among n-type thermoelectric oxides, the nontoxic and low-cost ZnO based thermoelectric material has attracted much attention because its excellent charge carrier transport properties are tunable via doping [2], [6], [7]. In order to improve the thermoelectric properties of thermoelectric materials, thin film technique is used due to the stronger quantum confinement compared to that of their bulk materials [8], [9].
ZnO thin film is a kind of n-type semiconductor material which is technologically important due to its wide range of optical and electrical properties such as wide band gap of 3.3 eV, large excitonic binding energy of 60 meV at room temperature, controllable electrical conductivity via doping, etc. [10]. Due to these characteristics, ZnO thin film has been widely used in solar cells [11], surface acoustic waves [12], [13], [14], dilute magnetic semiconductors [15], [16], resistive memory [17], transparent conductors [18], etc. As for thermoelectric application, various dopants have been introduced into ZnO to improve its thermoelectric properties [5], [10], [19], [20], [21], [22], [23]. Among these works, it is found that Al-doped ZnO (AZO) is one of the best thermoelectric materials for high temperature thermoelectric application. Compared to the bulk AZO thermoelectric materials, only a few studies of the thermoelectric properties of AZO thin films have been reported, and the improved thermoelectric properties of AZO thin films are required for practical applications. Therefore, in this work, we focused on the optimization of deposition and subsequent thermal processing of AZO thermoelectric thin films.
ZnO thin films deposited by magnetron sputtering usually use ZnO ceramic target. This method has the following disadvantages: firstly, the ZnO ceramic target is more expensive; secondly, for industrial application, it is difficult to sinter a large ZnO ceramic target of high quality and large ZnO target is broken easily during the sputtering process; thirdly, the modulation of the deposited ZnO thin film component and carrier concentration is limited by the ceramic target component, and the incorporation of impurities introduces impurity level which has a negative impact on the optical and electrical properties of ZnO thin films. Therefore, in this work, AZO thermoelectric thin films were prepared by direct current (DC) reactive magnetron sputtering Zn–Al alloy target. To optimize the thermoelectric properties of AZO thin films, the influence of annealing temperature and Al-doping content on the thermoelectric properties was investigated systematically.
Section snippets
Experimental
Direct current reactive magnetron sputtering using Zn–Al alloy target was used to deposit AZO thin films on BK7 glass substrates at room temperature. Before deposition, the substrates were ultrasonically cleaned in acetone, alcohol and deionized water for 5 min, respectively. The chamber was pumped to a base pressure of 8.0 × 10−4 Pa. The working pressure was kept at 0.5 Pa with Ar of 40 sccm as the working gas and O2 of 6 sccm as the reactive gas. Prior to deposition, a 10 min pre-sputtering process
Results and analysis
The sheet resistivity and thickness of the thin films at room-temperature are shown in Table 1. The sheet resistivity changes are inconspicuous at first when the annealing temperature is below 823 K. However, when the annealing temperature is 823 K, the sheet resistivity increases a magnitude larger than that annealing below 823 K. Considering the thickness of the thin films annealing at various temperatures, the thin film annealed at 823 K is thinner than others, which may be result of the
Conclusions
AZO thin films were deposited by direct current reactive magnetron sputtering using Zn–Al alloy target at room-temperature and the deposited AZO thin films were annealed at various temperatures. The annealing temperature and Al-doping content were varied to obtain the optimized thermoelectric properties at high temperature. AZO thin film annealed at 773 K shows a relatively stronger texture and better crystallinity, which promises its high electrical conductivity. The absolute value of Seebeck
Acknowledgments
Supported by Special Project on the Integration of Industry, Education and Research of Guangdong Province (2012 B091000174), Basical Research Program of Shenzhen, China (JC201005250053A, JC201104210094A, JCYJ20120817163755062) and Natural Science Foundation of SZU (Grant No. 80100035699).
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