Investigation of the oxygen gas sensing performance of Ga2O3 thin films with different dopants
Introduction
The development of new and more efficient materials for gas sensing is a challenge of the near future, as the market for these devices continues to grow. Metal oxides, such as TiO2, WO3 and Ga2O3 have gained a great deal of interest in the scientific and technology communities. In particular, transition metal oxides are promising for electrochromic devices due to their reversible thermal coloration and for gas sensing as their optical and electrical properties change in presence of oxidizing or reducing gas species [1], [2]. Metal oxide thin films have been traditionally used as gas sensing materials. They offer the possibility of “tailoring” the sensitivity and selectivity towards specific gas species. These samples can be obtained via the sol–gel technique, which represents a reliable, low-cost chemical route, widely used for the deposition of these materials.
Oxygen sensors have practical use in monitoring and controlling systems of combustion engines, waste gases and chemical processes, etc. with the current focus of research being novel materials for fast, stable, sensitive and selective gas sensing [3]. Ga2O3 has emerged as a viable gas sensing material as a result of the pioneering work of Fleischer et al. [4]. Pure Ga2O3 thin films have strong sensitivity to oxygen gas at operating temperatures above 700 °C [5], [6]. By doping these films, sensors with higher responses and lower operating temperatures may be obtained.
Gas measuring systems incorporating metal oxide semiconductor (MOS) thin films exploit the films change in resistance to the introduced gas. The resistance change depends on the semiconductor and gas type, whether it be reducing or oxidizing. These sensors operate on the principle that the surface conduction of the sensors varies in relation to the adsorption of the ambient gas. Such changes have been noted since the earliest studies of semiconducting materials [7], [8]. Many reactions on the surface of metal oxides are possible which can be acceptable as the gas sensing mechanism. However, the most dominant reaction in semiconductor gas sensing is a reversible gas adsorption mechanism that occurs on the sensor’s surface. The adsorbed gas atoms inject electrons into or extract electrons from an n-type semiconducting material depending on whether they are reducing or oxidizing respectively [9].
Doping may dramatically change the electrical properties of metal oxide films, hence we present semiconducting Ga2O3 thin films with Ce, Sb, W and Zn as dopants in this paper. The resistivity of the Ga2O3 thin films can be tailored with different dopant and the concentrations. The sol–gel process was employed to prepare these films taking the advantages of atomic level mixing and low temperature synthesis. Their electrical response has been measured in the operating temperature range of 300–600 °C.
Section snippets
Thin film preparation
The sol–gel process was employed to realize the semiconducting metal oxide thin films. Precursor solutions of gallium isopropoxide, cerium isopropoxide, tungsten ethoxide, antimony butoxide, (Chemat Technology, Inc., USA), and zinc acetyleacetonate hydrate ([CH3COCHC (O)CH3]2Zn·xH2O, Sigma–Aldrich, USA) were mixed to achieve solutions of the doped gallium oxide thin films. The precursor solutions with an analytic purity were used. Since the metal alkoxides are very sensitive to moisture, the
Results and discussion
The ionic size of Ce4+, Sb5+, W6+, Zn2+ and Ga3+ are very close to each other, i.e. 0.101, 0.062, 0.072, 0.074 and 0.062 nm, respectively (from Pauling’s ionic radii data). Therefore, Ce4+, Sb5+, W6+ and Zn2+ can go into the lattice as substitutional metal dopants. Zn2+ may acts as an acceptor, while Ce4+, Sb5+ and W6+ may act as donors. The incorporation of Zn and Sb in Ga2O3 can be described by the following reactions:
An additional advantage of using donor
Conclusions
Gas sensors based on Ce, Sb, W and Zn doped Ga2O3 semiconducting thin films have been fabricated. The sol–gel process was employed to prepare the gas selective layers. The sensors were exposed to various concentrations of oxygen gas in an ambient of nitrogen and the gas sensing performance has been examined. It was observed that sensors doped with Zn showed preferred gas sensing at operating temperatures below 450 °C. Sensors doped with Ce had lower response than that of Zn films and operated at
Acknowledgements
The work was partially supported by the Cooperative Research Centre (CRC) for Microtechnology, Australia, the project title “High performance gas sensing films”.
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