Changes in Carrier Concentration and Debye Length: Experimental Evidence from van der Pauw Hall Measurements on NO_xSensing of In₂O₃

Abstract

Changes in conductivity, carrier concentration and mobility of In 2 O 3 thin film on exposure to NO x at different temperatures were measured using van der Pauw technique. The species responsible for these changes were probed by laser Raman spectroscopic technique. The direct correlation between the adsorbed species and the observed changes in the electrical properties of In 2 O 3 thin film toward NO x as a function of temperature had been elucidated.Among various metal oxides, SnO 2 , WO 3 and In 2 O 3 are well known semi-conductors for gas sensing applications. 1-9 Their gas sensing action and signal transduction mechanism are still the subjects of interest to many research teams. 10-14 As the gas sensing action is being a surface phenomenon, probing the changes in conductivity and carrier concentration in presence of trace levels of the pollutant gas is expected to provide a meaningful insight into the mechanism of the gas sensing. Hall measurements have been employed to probe the surface chemical interactions induced changes in the physical quantities under in-situ sensing conditions. [15][16][17][18][19] Sanjines et al used Hall measurement studies on SnO 2 thick film toward hydrogen and measured the changes in conductivity, carrier concentration and carrier mobility of the film. Ogawa et al also carried out Hall voltage measurements on ultra fine SnO 2 thin film in the presence of ethanol and discussed the variation of the carrier mobility with increase in concentration of ethanol. In the present investigations, In 2 O 3 thin films with granular morphology are employed and the changes in the physical quantities upon exposure to NO x (mixture of NO and NO 2 ) are probed by Hall voltage measurements using van der Pauw technique. The details of these experiments and the results obtained are presented in this paper. ExperimentalThin films of In 2 O 3 were deposited on 10 × 10 mm polished alumina substrates by pulsed laser deposition. A KrF laser (M/s Lumonics, Canada) operating at a wavelength of 248 nm with repetition rate of 10 Hz and a fluence of ∼5 J/cm 2 was used for this purpose. In 2 O 3 targets for deposition were made from the high purity powders of In 2 O 3 (99.9%, M/s Aldrich, USA). The powders were pelletized into disks of 15 mm diameter and ∼ 3 mm thickness and then sintered at 1273 K for about 48 h in air. The temperature of the substrate was maintained at 773 K during film deposition under an oxygen partial pressure of about 2 × 10 −1 mbar. Prior to the film deposition on the substrate, gold electrical pads were screen printed on four corners of the substrate for four probe measurements. The substrate to target distance was maintained at 4.5 cm and the deposition was carried out for 20 min. The deposited thin films were analyzed by X-ray diffraction using model D 500, M/s Siemens, Germany. The thickness of In 2 O 3 thin films was measured using profilometer Dektak 3 of M/s Veeco, USA. The morphology of the films was studied using Atomic Force Microscope model Solver Pro, M/s NT -MDT, Russia.Hall ...

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