Defect structure and sensing mechanism of SnO2 gas sensors: Comparative electrical and spectroscopic studies

Sohd State lonics 28-30 (1988) 1631-1636 North-Holland, Amsterdam

DEFECT STRUCTURE AND SENSING MECHANISM OF SnO2 GAS SENSORS: COMPARATIVE ELECTRICAL AND SPECTROSCOPIC STUDIES K.D. SCHIERBAUM, H.D. WIEMH()FER and W. GOPEL Institut J~r Physikalische und Theoretische Chemie, Auf der Morgenstelle 8, D- 7400 Tiibingen. FederalRepublic of Germany Re~:eived 13 August 1987

We investigated the interaction ofundoped SnO, single crystals and films with O,, H:O, NO, and NO,. Equilibrium condu~,tivities were determined at high temperatures (T> 1000 K), and for oxygen partial pressures l 03 Pa_<'Po, ~ l0 s Pa. Changes of electrical conductivities and of work functions were measured as a function of time during adsorption-desorption cycles at temperatures between 300 and 800 K. Large effects were observed with O~ and NO2, whereas no detectable changes of sheet conductances occurred during NO exposure. The results are interpreted in terms of a bulk defect model determining the high temperature response at T> 1000 K, and in terms of an acceptor type of chemisorption of O, and NO2 determining the low temperature response at T< 500 K. The latter is usually followed by reactions involving subsurface defects, i.e. oxygen vacancies.

1. Introduction SnO: single crystals and films are of imerest in the fields of chemical sensors and catalysis. We have studied the interaction of well defined samples, both under ultra high vacuum, UHV, and under atmospheric pressure conditions in order to determine experimental conditions of reversible SnOe/gas interactions and in order to investigate the elementary steps of gas sensing mechanisms.

2. Experimental Undoped SnO2 thir) films with thicknesses between 13 and 400 nm were prepared by RF-sputtering, electron beam evaporation of S n Q , and by vacuum sublimation of Sn and subsequent oxidation in N2/O: mixtures. Experimental techniques used to investigate the samples under UHV and atmospheric pressure conditions are shown schematically in fig. t. ConductiviIies were de~.ermined by using the four point Van-der-Pauw method. Changes of work functions were measured with a Nezoelectrically driven Kelvin probe. Purities and elemental composition of" SnO2 thin films were determined by X-ray photoelectron spectroscopy, XPS. Film struc-

tures including grain boundaries were investigated by scanning Auger microscopy, SAM, scanning electron microscopy, SEM, and energy dispersive X-ray emission, EDX. Valence band sm~c~ures were investigated by ultraviolet photoelectron spectroscopy. UPS. For experimental details, see [ I 1.

3. ][ute~'ac~ion wi~h O2 At temperatures above 1000 K, reversible changes of conductivities were observed as a function of oxygen partial pressure. The results (fig. 2) lead to a power low a.,. [Po: ]-1/6, which confirms existing point defect models for pure crystalline SnO2_, with double positively charged oxygen vacancies acccrding to the equilibrium -~ +2e' ~O~ . 2 O~+Vo _ The standard enthatDy of the formation of oxygen vacancies Vo AH '~ ~ 17 eV was ob~amea from an analysis of the temperature dependence of equilibrium constants of the defect formation. It is in line with results from measurements of complex impedances of polycrystalline samples [ 2 ]. Diffusion coefficients of vficancy diffusion at high temperatures as estimated from time-dependen~ conductivity mea-

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K.D. Schierbaum et aL/SnO: gas sensors

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surements on thin SnO2 films after stepwise change of 02 partial pressure were in the order of 10-,4 cm: s-i at T = 1000 K. Stoichiometric surfaces could be prepared by low temperature oxygen exposure, and subsequent thermal desorption to Tmax=470 K. UHV-treatment at higher temperatures and longer times leads to the formation of (sub-)surface defects which are detected by characteristic increases in the surface conductivity Atr (fig. 3) because of the complete ionisation of these defects. The surface conductivity Aa is defined by the deviation of the sheet conductance a m = [ln(2)/~t]R -! from the value abd= [ln(2)/~t]Rg' in flat-situation [3]: Aft-ri o - trb'd

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1634

K.D. Schierbaum et aL/Sn02 gas sensors

face and bulk contributions of results obtained in gas exposure experiments can be done by measuring simultaneously surface conductivity Aa and work function changes A@ with typical results shown ~n fig. 5 for characteristic temperatures. At low temperatures, chemisorption and corresponding charge transfer lead to drastic effects in 6@ and relatively small effects in A¢, whereas at higher temperatures the charge transfer in chemisorbed sta'ms is less important and subsurface annealing of defects leads to drastic effects in A¢. For a quantitative evaluation, see, e.g., our earlier work on ZnO and TiO2 [ 5 ]. No change of valence band structures, of binding energies, and of relative intensities of Sn- and O-core levels was observed after thermal formation of point defects in UHV.

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ductivity, and d= thickness of the sample. From Aa, an excess concentration of (sub-)surface defects in the order of l0 ~ cm-= can be estimated by using mobility data from the literature [4]. Fig. 4 shows changes in surface conducfivifies as a function of~.im_e upon oxygen exposure t o SnlD 2 thin films with different ccncentrations of (sub-)surface defects prepared uhder UHV-conditions at different temperatures. These measurements lead to an estimarion of excess defects, i.e. oxygen vacancies acting as donors, in (sub-)surface reg~ei,,; ~.... temperature treatment [5 ]. The separation of sur-

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K.D. Schierbaum et al./Sn 02 gas sensors

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the sample. The observed drifts in the baseline result from pa.,'fia! annealing of subsurface defects, i.e. oxygen vacancies, after decompostion of N O , at the surface. 5. Influence of water

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1636

K.D. Sch ferbaurn et al./Sn02 gas sensors

st~-uc'mres of SnO2 films and single crystals before and after acceptor-type chemisorption [ 6]. The bulk defec'~ levels ED~ and ED, are determined by singly Vo ~nd doubly ionized oxygen Vo vacancies, the concentration of which is adjusted by high tempex'ature treatment at T> 1000 K. This concentr~tion determines ( E c - E~ )bDuring chemisorption at low temperatures work function changes A q ~ = A x ' e A V are observed. Under these conditions the concentration of physisorbed species Xphys iS below detection limit. Annealing of (sub-)surface defects can be done by 02 exposure at elevated temeperatures, a n d / o r by NO2 exposure, which even at room temperature leads ~to stoichiometric passivation layers. In the presence of air an annealing of subsurface defects (which leads to drifts in conductivities) by NO2 is negligible and makes possible the use of conductivity measure.ments for SnO2 sensor applications under these conditions.

Acknowledgements Technical assistance and helpful discussions with Dr. D. Schmeisser and W. Net', are gratefully acknowledged. The work is supported by the BMFT and Land Baden-Wiirttemberg.

References [ 1] W. G6pel. Techn. Messen2 ( 1985)/47; 3 (1985) 92; 5 (1985) 175. [2 ] J. Maier and W. Gtipel, J. Solid State Chem. 72 (1988) 293. [3] W. GiSpel and U. Lampe, Phys. Rev. B 22 (1980) 6447.

[4] Z.M. Jarzcbski and J.P. Marton, J. Electrochem. Soc. 123 (1976) 199C. [5 ] W. G6pel, G. Rockerand R Feierabend, Phys. Rev.28 (1983) 3427. [6] W. G6pel, Prog. Surface Sci. 20 (1985) 1.