JOURNALOF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 13 (2002) 571±579 Electrical conductivity of indium sesquioxide thin ®lm T. BAK, J. NOWOTNY, M. REKAS, C. C. SORRELL Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia P. A. BANDA*, W. WLODARSKI Department of Communication and Electronic Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Vic 3001, Australia This paper reports electrical properties of In 2 O 3 thin ®lm (100 nm) at elevated temperatures (667±1118K) and under controlled oxygen activity. The present study, based on the measurements of electrical conductivity (EC) using the method proposed by van der Pauw, includes the following determinations: * Dynamics of EC changes during gas/solid equilibration of the O 2 /In 2 O 3 system; * The dependence of EC on oxygen partial pressure dependence; * The dependence of EC on temperature. # 2002 Kluwer Academic Publishers 1. Introduction Indium sesquioxide, In 2 O 3 , thin ®lms were applied as sensors for O 3 [1±3] and NO x [4±6]. So far, mainly electrical conductivity (EC) has been applied as a sensing property. Consequently, understanding of the effect of processing on electrical properties of In 2 O 3 , such as EC, is important for processing thin ®lms with the properties desired for sensing of speci®c gases. The purpose of the present study is the determination of EC of In 2 O 3 thin ®lms within a wide range of temperatures, T , and oxygen partial pressures, p(O 2 ) and aims at understanding the effect of both T and p(O 2 ) on electrical transport in In 2 O 3 thin ®lms. 2. Brief literature overview In 2 O 3 is known as an n-type semiconductor exhibiting relatively high EC. Its band gap is E g  3:1 eV [9]. This oxide material has a wide range of applications, such as an air electrode for solid oxide fuel cells [10±14], transparent electrodes for electro-optical applications [15±17], infrared re¯ectors of solar collectors [18±20], photo-electrodes for electrochemical hydrogen genera- tion from water [21] and gas sensors [22±29]. The properties of In 2 O 3 including EC, are determined by its defect structure. Therefore, EC may be applied for veri®cation of defect chemistry models that were considered in terms of point defects, such as interstitial cations [30,31], oxygen vacancies [32,33], oxygen interstitial [34] and defect complexes [35]. However, the reports on defect chemistry of In 2 O 3 are not very consistent. Therefore, further studies are required in order to derive defect models that could explain the properties of In 2 O 3 over a wide range of temperatures and oxygen partial pressures. There has been an accumulation of reports on electrical properties of In 2 O 3 and the effect of doping with aliovalent ions on these properties [15, 20, 35±58]. Unfortunately, the majority of reports [15,24,35,37± 45, 47, 48, 53±55, 58] concern lower temperatures (100± 300 K) at which In 2 O 3 remains in a quenched state. One should emphasize, however, that only EC data deter- mined in equilibrium are suitable for the considerations in terms of defect chemistry. TheabsolutevalueofECofIn 2 O 3 remains in the range 10 2 ±10 8 O  1 cm  1 . This depends on oxygen non- stoichiometry, type of additions, and their concentration. The mobility of electrons is about 10 2 cm 2 V  1 s  1 . The mobility slightly increases with temperature (the activa- tion energy of the mobility is 0.08 eV [53]. According to Weiher [57] the activation energy of EC for In 2 O 3 single crystal at elevated temperatures is E  1.55eV. Similar values were reported for a polycrystalline material [32, 51, 52]. According to Rupprecht [58] the activation energyofECofIn 2 O 3 thin®lms,determinedabove770K, depends substantially on oxygen partial pressure. The dependence between EC and p(O 2 ) may be expressed by the following relation: s  const. p(O 2 ) 1=m s 1 where the parameter m s has been frequently used for veri®cation of defect chemistry models [9]. *Present address: IMEC, Microsystems, Components and Packing Division, Biosensors Group, Kapeldreef 75, B-3001 Heverlee, Belgium. 0957±4522 # 2002 Kluwer Academic Publishers 571