Metal-semiconductor ohmic contact of SnO2-based ceramic gas sensors

SE@RS ACFIJA~ORS B

CHEMICAL

Sensors and Actuators B 41 (1997) 163-167

Metal-semiconductor

ohmic contact of SnO,-based ceramic gas sensors

Xiaohua Zhou, Yulong Xu *, Quanxi Cao, Suyan Niu Department

of Technical

Physics,

Xidian

Umkersity,

Xi’an

710071,

China

Received 5 August 1996; received in revised form 3 February 1997; accepted 11 February 1997

Abstract The realisation of an ohmic contact between metal electrodes and SnO, semiconducting ceramics by a metal-n + -n structure can cancel harmful effects in SnO, gas sensors that are caused by the unmodified contact. This contact structure does not only increase the sensitivity of SnO, gas sensors, but also decreases the scatter in gas-response properties of these sensors. Especially, it is necessary to adopt the metal-n+ -n electrode structure when ceramic SnO, gas sensors operate at lower temperatures. 0 1997 Elsevier Science S.A. Kevw~ords:

Gas sensors; Metal-semiconductor

contact; Schottky barrier; SnO, ceramics

1. Introduction

2. Experimental

Ceramic gas sensors based on tin dioxide are becoming increasingly important in various fields of applications to measure partial pressures of a large variety of species. The resistance of tin dioxide ceramic gas sensor consists of three parts, i.e. bulk resistance, surface resistance and contact resistance between metal electrodes and semiconducting ceramics. The reduction of the contact resistance is useful for improving the properties of ceramic gas sensor. It is well known that tin dioxide is an n-type semiconductor, and an ohmic contact of lower resistance can be obtained by metal-n +-II structure, which is an important method of decreasing the metal-semiconductor contact resistance. Because of the structure of SnO, ceramics itself, which is polycrystalline and has a twoelement composition, its ohmic contact to metal is much more complex than that of an elemental semiconductor to metal. Therefore, in this work we have only made an initial investigation on the metal-semiconductor ohmic contact between tin dioxide gas sensing ceramics and metal electrodes.

2.1. Preparation

* Corresponding author. + 86 29 7426049; fax: + 86 29 5262281; e-mail: [email protected] 0925-4005/97,‘$17.00 0 1997 Elsevier Science S.A. All rights reserved, SO925-4005(97)00109-3

PII

of samples

In order to compare the difference between two gas sensor types, i.e. ceramic SnO, gas sensors with and without the metal-n +-n contact structure, three different sensor constructions were made and measured, respectively. Tin dioxide powder was made from SnCl, raw material through the liquid-phase co-precipitation method [I] and mixed with the catalyst and organic binder according to a certain ratio. The resulting fresh paste was printed to an alumina tube substrate equipped with two gold electrodes and Pt wires. After drying, the paste was sintered at 680°C for 50 min in air. This construction is here called sample 1. The difference of sample 2 construction from sample 1 is in the composition of paste. The paste composition of sample 2 was obtained by the addition of 6 wt.% Sb,O, powder to the paste composition of sample 1. Sample 3 construction was prepared as follows: the paste of sample 2 was first printed to cover the two electrodes of alumina tube. After drying, the paste of sample 1 was printed to the surface of the whole tube substrate-(see sample 3 in Fig. 1). The other processes

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Sample 1

B 41 (1997)

163-167

Sample 3

Sample 2

Fig. 1. Structure sections of the three samples constructions: (.I) Pt-lead wire; (2) heater: (3) alumina tube; (4) gold electrode; (5) SnO, ceramic; (6) Sb,O,-doped SnO, layer.

were the same as for samples 1 and 2. The structures of the three sample constructions are shown in Fig. 1. 2.2. Results of measured samples

2.2.1. Resistance of tke three samples in ail Fig. 2 shows the resistance of the three samples in air as a function of temperature. When the heating voltage is zero, the temperature is 20°C. 2.2.2. Sensitivity of the three samples to alcohol vapour and other gases In the case of the heating voltage V, = 3.5 V (corresponding to 190°C)? the sensitivities (R,,/R,,,) of the samples to a few different concentrations of C*H,OH vapour in air are shown in Fig. 3. It is quite evident from the results in Fig. 3 that the sensitivity of the sensors is improved by the metal-n’-n semiconductor contact structure. When samples 1 and 3 were exposed to an atmosphere containing 200 ppm C,H,OH vapour in air and the heating voltage was changed, the obtained sensitivities are shown in Fig. 4. From this figure it is seen that the maximums of the two curves appear at the same heating voltage, which means that the optimum operating temperatures of both gas sensor samples are the same. The sensitivities of the three samples to some other gases are plotted in Fig. 5. 2.2.3. Comparison of scatter in sensitivity between samples 1 and 3 Three sensors were randomly taken from both two sample constructions 1 and 3, respectively, and their sensitivities were measured at exposure to different concentrations of alcohol vapour in air. The obtained results are shown in Fig. 6. From the results in Fig. 6, it is evident that the sensors with the sample 3 construction have a smaller scatter in sensitivity values than sensors with the sample 1 construction.

3. Analysis of the results and discussion By comparison with sample 1, sample 3 has the following advantages: (1) Because of the reduction of the metal-semiconductor contact resistance, the resistance of sample 3 in air decreases, especially its air resistance at room temperature. From Fig. 2 it is clear that SnO, ceramics modified with Sb,O, (sample 2) can be used as an II+ layer between the metal electrode and sensitive SnOz layer because the room-temperature resistance of sample 2 in air is much lower than that of sample 1. The 100 ,

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Heating Voltage ( V ) Temperature (“C ) Fig. 2. Resistance of samples I, 2 and 3 in air as a function of heating voltage V, (temperature).

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Alcohol Vapor Concentration (ppm) Fig. 3. Sensitivity of samples 1,2 and 3 as a function of alcohol vapour concentration in air at IJH= 3.5 V.

electronegativities of Sb and Sn are similar, being 1.8 and 1.9, respectively. The coordination numbers of Sb5+ and Sn4+ in SnO, are the same and they have ionic radius of 61 and 69 pm, respectively [2]. Therefore, when Sb,O, is added into SnO,, Sb5 + ions behave as donors decreasing the resistivity of SnO,. Because gold as an electrode in SnO, sensor has work function of about 5.10 eV [3] and SnO, ceramics is a n-type oxide semiconductor, a contact barrier is formed between them when they are in a close contact, as shown in Fig. 7(a). The width of the contact barrier, the depletion layer or space charge region, is inversely

8o 70

200 ppm CzHsOH Vapour

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Heating Voltage ( V ) Temperature (“C ) Fig. 4. Sensitivity of samples 1 and 3 as a function of heating voltage in the case of exposure to 200 ppm alcohol vapour in air.

Actuators

B 41 (1997)

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proportional to a constant donor concentration in SnO, ceramics and can be reduced by the metal-n +-, structure [4], as shown in Fig. 7(b). For that reason the tunnelling of conduction electrons through the contact barrier increases so markedly that the contact resistance of the components decreases. With the increase of temperature, the Fermi level of SnO, semiconductor falls and its work function increases, which means that the height of the contact barrier decreases and thermoelectronic emission of electrons increases. Thus, when temperature is above 220°C (V, = 4 V), there is very little difference between the resistance of samples 1 and 3. (2) The sensitivity of the sensor to alcohol vapour is improved. From results in Fig. 3 it is seen that sample 2 is insensitive to alcohol vapour and the sensitivity of sample 3 to alcohol vapour is higher than that of sample 1. On the other hand, from results in Fig. 5, it is seen that both samples 1 and 3 are insensitive to other gases, such as H,, CO,CH, and petrol vapour. As a result, the use of the metal-n + -y1 contact not only improves the sensitivity of the sensor to alcohol vapour, but also does not change the sensitivity of the sensor to other gases, i.e. the selectivity of the sensor does not change with n’ layer addition. (3) The scatter in sensitivity has decreased. From results in Fig. 6 it is clear that the scatter in sensitivity values of samples 3 is smaller than that of samples 1. Because sample 3 has the metal-n +-n contact, the harmful effects caused by the unmodified contact can be decreased and the sensing properties of the sensors improved. (4) The difference between the forward and reverse resistance of a sensor (at a polarity change) is reduced. If a measured resistance value of the sensor is called the forward resistance RI,+,, the resistance measured with changed polarity is known as the reverse resistance R,,. Table 1 illustrates typical measured results for R,, and R,, from samples I,2 and 3 in the case of the heating voltages of 0 and 3.5 V. There are two reasons for the difference between the forward and reverse resistance. The first reason is an asymmetry between two contact barriers of a sensor, i.e. there are some differences in the height and width between the two contact barriers. The second reason is an asymmetry in grain boundary barriers, as shown in Fig. 8. When the heating voltage is above 3.5 V (T= 190°C), the influence caused by the asymmetry of the barriers decreases because the capability of the carriers to cross the barriers increases. A comparison of results between samples 1 and 3 in Table 1 shows that the metal-n + -n contact is useful to eliminate the polarity of the sensor.

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4. Conclusions For the reduction of the contact resistance and scatter in the gas-response properties and for the increase

of sensitivity at low operation temperatures of ceramic SnO, gas sensors, metal-n + -II semiconductor contacts may be a good choice.

Acknowledgements 90 80 - -n-Sample

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The authors thank the Foundation of Ministry of Electronic Industry of China for the support of this work. X.H. Zhou and Yulong Xu would like to thank Professor 0. Toft Sorensen of Riso National Laboratory, Denmark, for his support and kind help.

-

SnOz Ceramic -

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VH = 3.5 v (T=19O"C) 01

I 0

200

400

600

800

1000

1200

Alcohol Vapour Concentration (ppm) Fig. 6. Comparison of scatter in the sensitivity values between samples 1 and 3 at exposure to different concentrations of alcohol vapour in air at Vn = 3.5 V.

(a)

Sample 1

02)

Sample 3

Fig. 7. Simple energy-band diagram for samples 1 and 3: EC, the bottom of conduction band; E,, Fermi level; Ev, the top of valence band; 6, the width of depletion layer.

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Table 1 Forward and reverse resistance, R, and R,,, measured from samples 1, 2 and 3 at heating voltage V, = 0 and 3.5 V. Relative errors between the two resistance values are also given in the table

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Sample 1 Sample 2 Sample 3

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Ev

Fig. 8. Schematic description of asymmetries in the contact barriers and grain boundary barriers.

References [l] Y.L. Xu, X.H. Zhou, Research of Gas Sensors for Ceramic Semiconductor, West Electrons l(1) 10 (1990) 20 (in Chinese). [2] Y.L. Xu, Elements of oxide and compound semiconductors, Xidian University Press, Xi’an, 1991, pp. 48 (in Chinese). [3] E.H. Rhoderick, Metal-semiconductor contact, Clarendon Press, Oxford, Ch. 1, 1978. [4] S.M. SZE, Physics of Semiconductor Devices, Wiley, New York, 1969, pp. 370.

Biographies Xinoha Z/IOZ~ has been an associate professor at Xidian University, China, since 1994. He graduated from Department of Technical Physics, Xidian University in 1977 and completed his studies of theory physics postgraduate class, Northwest University of china, in 1985. Since 1986, he has been engaged in teaching and research of electronic materials and devices as a lecturer, an associate professor in Xidian University. His research interests are in the area of gas sensors, va-ristors and advanced electroceramics. He has published two textbooks and about forty papers. He has once worked at Materials Department, Rise National Laboratory, Denmark as a visiting scholar and guest scientist for 17 months.

Yulorzg Xu is a professor at the Department of Technical Physics, Xidian University. He graduated from the Department of Physics, Ji-Lin University of China, in 1961. Since that time, he has been engaged in teaching and research of solid-state electronic materials and device in Xidian University. His research interests are in the area of metal oxide gas sensors, semiconductor heterojunction device and electronic ceramics. His research activities include more than 70 journal and conference papers and sixbooks. Professor Xu is a fellow of CIE (Chinese Institute of Electronics) and an old senior member of IEEE. He has been in abroad for many times. From 1981- 1983 and in 1990 he studied and worked two times in the German university, Karlsruhe University and Technical University of Aachen, as a visiting scientist. In the summer of 1996 he stayed in the Materials Department, Rise National Laboratory of Denmark, as a guest professor in the company of his wife, associate professor Yucheng Xu. Quark Cao has been an associate professor at Xidian University since 1993. He graduated from Department of Technical physics, Xidian University in 1975 and completed his studies of theory physics postgraduate class, University of Science and Technology of China (USTC), in 1985. Since that time, he has been engaged in teaching and research of electronic materials and components as a lecturer, an associate professor in Xidian University. His research interests are in the area of gas sensors, varistors and physical and chemical analysis of sensitive materials. He has published one textbook and about forty papers.

Suynlz Nizr has been a engi’neer at Xidian University since 1989. She graduated from Beijing Electronic and Machine School in 1966. Since that time, she has been engaged in experiments and research of electronic materials and components. Her research interests are in the area of ceramics and thin film. She has published five papers.