Current–voltage characteristics and ethanol gas sensing properties of ZnO thin film/Si heterojunction at room temperature

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Abstract

The polycrystalline zinc oxide film/p-Si (ZnO/Si) heterojunctions were fabricated by radio-frequency (RF) magnetron sputtering. The current–voltage (IV) characteristics and ethanol gas sensing properties of the heterojunctions were investigated at room temperature (RT). It is found that the simple ZnO/Si heterojunctions can be used to detect ethanol gas at RT. Upon exposure to 200 ppm ethanol at RT, the heterojunction showed the maximum sensitivity of 67.6% at +7 V bias voltage, which is much better than the results reported before. The phenomena should be attributed to the change of carrier concentration of ZnO film and the change of potential barrier width of the junction, which is caused by the adsorption of ethanol gas molecules. The study shows that the ZnO/Si junctions have potential application as RT ethanol sensors.

Introduction

Zinc oxide (ZnO) is a promising material for gas sensor applications because of its high chemical stability, suitability to doping, non-toxicity, abundance in nature and low cost. This material has been exploited for gas sensors in forms of single crystals, sintered pellets, thick films, thin films, etc. [1], [2], [3], [4], [5]. As we know, thin films are especially suitable for such sensors, since they can be fabricated in small dimensions on a large scale with low cost and are widely compatible to microelectronic technology and circuits. Besides, in comparison with conventional sintered bulk or thick film gas sensors, thin film gas sensing materials have good sensitivity [6], [7].

As for gas sensor application, undoped and doped ZnO thin films have been investigated by many researchers for detection of toxic pollutant gases, combustible gases and organic vapors [8], [9], [10], [11], [12], [13]. As reported, the gas sensing properties of metal oxide films depend naturally on their physical or morphological properties such as grain size, porosity, thickness as well as their catalytic or surface chemical properties [14], [15], [16]. However, most of these thin film gas sensors at present are operated in the resistive mode at the temperature of over 200 °C, which makes it necessary to implement heating and thermometer structure in sensor device. In addition, operating the sensor repeatedly at high temperature will decrease its life and long-term stability [17]. Until now, there is little report on the study of gas sensing properties of ZnO thin film/Si heterojunction. In this paper, we reported the IV characteristics and ethanol gas sensing properties of ZnO/Si heterojunction. Interestingly, it is found that these ZnO/Si heterojunctions have good sensitivity to ethanol gas at room temperature (RT). The phenomena should be attributed to the change of the potential barrier width of the junction, which is caused by the adsorption of ethanol gas molecules.

Section snippets

Experimental

The ZnO/Si heterojunctions were fabricated by depositing n-ZnO thin films on p-Si (1 0 0) substrates (resistivity: 10–20 Ω cm, thickness: 480 μm) using RF magnetron sputtering. The target is an n-type ZnO ceramic disk and its purity of ZnO is better than 99.99%. Before deposition, the Si substrates were ultrasonically cleaned in ethanol and acetone, and then etched in HF solution for 3 min. Before each deposition, the base gas press inside the chamber was pumped down to less than 1.6×10−4 Pa and then

Results and discussion

Fig. 1 shows the XRD patterns (using 2θ values in the range of 10–60°) taken on the top surface of S1, S2 and S3 heterojunction. The diffraction peaks could be indexed to hexagonal wurtzite ZnO (JCPDS card No. 79-2205, a=0.3249 nm and c=0.5205 nm) [18]. With increasing film thickness the dominant ZnO (0 0 2) peak becomes sharper, indicating the well-established c-axis orientation of ZnO film. Three kinds of ZnO films with about 41, 65 and 89 nm thickness (measured from cross-section SEM images) were

Conclusion

In this study, the polycrystalline ZnO/Si heterojunctions were fabricated by RF magnetron sputtering. The I–V characteristics and ethanol gas sensing properties of the heterojunctions were investigated at RT. It is found that the simple ZnO/Si heterojunctions can be used to detect ethanol gas at RT. The phenomena should be attributed to the change of carrier concentration of ZnO film and the change of potential barrier width of the junction, which is caused by the adsorption of ethanol gas

Acknowledgements

This work is supported by the Natural Science Foundation of China (10974258), Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708061), and Program for New Century Excellent Talents in University (NCET- 08-0844).

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