Ammonia sensitivity of rf sputtered tellurium oxide thin films

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Abstract

In this work we present the results of a study concerning the sensitivity properties of tellurium oxide (TeO2) thin films to ammonia gas. TeO2 films were grown on quartz substrates by means of the rf reactive sputtering method using a tellurium target in an Ar–O2 atmosphere with different O2/Ar ratio ranging from 35/65 to 50/50. The structure, surface morphology and chemical composition of the prepared films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The energy gap Eg of the films was evaluated from transmission (T) and reflection (R) spectra. To characterize the films as sensors, experiments with ammonia (NH3) at concentration levels of 100–500 ppm and at operating temperatures ranging from 130 to 220 °C were performed. The films deposited with a O2/Ar ratio of 45/55 and thermally treated at 450 °C for 60 min were polycrystalline and showed satisfactory response to NH3 at the optimum operating temperature of 170 °C. At this temperature the response and recovery times were about 3.1 and 5.6 min, respectively. The sensitivity of the TeO2 films follows a power law relationship with the increase of the gas concentration.

Introduction

In the last years, electrical conductive metal oxides have been widely investigated for the detection of toxic pollutant gases, combustible gases and organic vapours. The mechanism of gas detection with these materials is based, in large part, on reactions that occur at the sensor surface, resulting in a change in the electron concentration. When exposed to reductive gas, such as NH3, chemisorbed oxygen on the semiconductor surface reacts with the reductive gas species and the electrons trapped by the oxygen are released into the semiconductor, changing the conductance. For n-type semiconductor, the conductance increases due to the increase of electron concentration in the semiconductor. For p-type semiconductor, the conductance decreases because of the combination of holes with electrons released from surface reaction. This change in conductivity is directly related to the amount of a specific gas present in the environment, resulting in a quantitative determination of the gas concentration [1]. Ammonia detection and monitoring are required due to the importance of this compound in various applications such as food technology, agricultural activity and medical diagnosis. Many industrial processes, such as the production of fertilizers, plastics, explosives, pulp and paper, oil refinery and power generation use ammonia as a raw material. In addition, ammonia is an air pollutant. It can affect the health of plants and animals, as well as humans.

Metal oxide semiconductor-based sensors for NH3 gas are most attractive because they are compact, sensitive, of low-cost, and have low-power consumption. Unfortunately it is not easy to develop a sensor which combines all the mentioned advantages. Different metal oxide materials have been reported to be usable as NH3 sensors such as SnO2 [2], ZnO [3], WO3 [4], TiO2 [5], Fe2O3 [6] and MoO3 [7]. The main disadvantage of these sensors is their high operating temperature (>200 °C). Maintenance of sensor at high temperature increases power consumption and reduces sensor life. Moreover, it is necessary to implement heating and thermometer structures in sensor devices. In order to develop NH3 sensors operating at lower temperature with adequate parameters (sensitivity, selectivity, rapidity of response–recovery time), many efforts are currently in progress to optimize device performances by different methods by using doping, catalyst, fabrication of nanostructures, new deposition technologies, etc.

Tellurium dioxide (TeO2) is an interesting semiconductor, both in its crystalline and amorphous forms, with physical and chemical properties that make it suitable for fundamental theory and for technological applications [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, TeO2 has not been widely investigated as a material for gas sensing, and little has been published on this subject. Liu et al. [10], [11] investigated the gas properties of TeO2 nanowires synthesized by the thermal evaporation of tellurium metal in air. These materials showed good sensing performance to toxic gases such as NO2, NH3 and H2S at room temperature. Previously, the NO2 gas sensitivity at room temperature of sputtered TeO2 thin films was investigated by us [17]. The films prepared by thermal annealing showed a promising sensitivity and response towards tested gas.

To the best of our knowledge, no report about the detection of NH3 gas by sputtered TeO2 thin films is available in literature. In this study we examine TeO2 thin films obtained by reactive sputtering method for NH3 sensing applications. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to determine the structure, morphology and chemical composition of the films. The optical properties were investigated using UV–vis–NIR spectrophotometry. The NH3 sensing characteristics of the films were studied by electrical conductance measurements.

Section snippets

Experimental

In the present study, tellurium oxide thin films were deposited by rf sputtering method using a metallic Te target (purity 99.999%, diameter 152 mm) on non-intentionally heated quartz substrates. The substrates were cleaned properly and spin dried before being loaded into the sputtering system. The target to substrate distance was 8 cm. The rf power and the total pressure during deposition were 350 W and 5 mTorr, respectively. Different samples were deposited by varying the O2/Ar ratio (35/65,

Results and discussion

Energy dispersive X-ray spectroscopy showed that the films deposited with a O2/Ar ratio of 35/65 in the sputtering chamber consisted mainly of Te and then they were not analyzed in this work. Moreover, as-deposited TeO2 films and thermally treated films with a O2/Ar ratio of 50/50 did not show enough response to the presence of NH3 gas. The films deposited with a O2/Ar ratio of 45/55 showed a good response as were thermally treated at 450 °C for 60 min. Therefore, with regard to the gas-sensing

Conclusions

TeO2 films were grown on quartz substrates by means of the rf reactive sputtering method using a tellurium target in an Ar–O2 atmosphere with different O2/Ar ratio. The films deposited with a O2/Ar ratio of 35/65 in the sputtering chamber consisted mainly of Te and then they were not analyzed in this work. Moreover, as-deposited TeO2 films and thermally treated films with a O2/Ar ratio of 50/50 did not show enough response to the presence of NH3 gas. The films deposited with a O2/Ar ratio of

Acknowledgements

The authors thank A.R. De Bartolomeo, G. D’Elia and L. Monteduro for their technical assistance during the measurements.

Tiziana Siciliano received her physics degree and PhD degree both from University of Lecce, Italy in 2000 and 2005, respectively. Her main research activity concerns the characterization of materials for gas sensor application. She is now a researcher at the University of Salento.

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    Tiziana Siciliano received her physics degree and PhD degree both from University of Lecce, Italy in 2000 and 2005, respectively. Her main research activity concerns the characterization of materials for gas sensor application. She is now a researcher at the University of Salento.

    Massimo Di Giulio has made deep and various experimental researches in the main field of the physical thin film deposition, by thermal vacuum evaporation, electron beam deposition, assisted ion beam deposition, rf sputtering. He has also studied the physical, optical, photoelectronic and transport properties of several materials, either as bulk or as thin/thick films. Actually, he is an associate professor of applied physics at the University of Salento.

    Marco Tepore received his degree in engineering of materials from University of Lecce, Italy in 2004 talking his thesis concerning the preparation and characterization of thin films of organic materials prepared by Langmuir–Blodgett technique for gas sensor application. Actually, he works at the department of material science as fellow.

    Emanuela Filippo received her physics degree and PhD degree both from University of Lecce, Italy in 2000 and 2005, respectively. Since 2000, she works at the department of material science as fellow. Her research interest includes metal nanostructures synthesis and characterization and the study of thin films for applications in technology.

    Gioacchino Micocci received his physics degree from University of Lecce, Italy in 1973. Actually, he is a full professor of applied physics at the University of Salento. His research activity is focused on the growth of semiconductor materials both in the form of bulk and thin films for applications in technology.

    Antonio Tepore received his physics degree from University of Lecce, Italy in 1971. Actually, he is a full professor of applied physics at the University of Salento and director of the department of material science of this university. His research activity is mainly devoted to structural, electrical and optical properties of semiconductor materials for applications in technology.

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