Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In2O3 thin films have been prepared from commercially available pure In2O3 powders by high vacuum thermal evaporation (HVTE) and from indium iso-propo xide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrod es. The as-deposited HVTE and SG films have been annealed at 500 degrees C for 24 and I h, respectively. The film morphology, crystalline phase and ch emical composition have been characterised by SEM, glancing angle XRD and X PS techniques. After annealing at 500 degrees C the films' microstructure t urns from amorphous to crystalline with the development of highly crystalli ne cubic In2O3-x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3-x (SG) after annealing. SEM characterisation has highlighted substantial mor phological differences between the SG (highly porous microstructure) and HV TE (denser) films. All the films show the highest sensitivity to NO2 gas (0 .7-7 ppm concentration range), at 250 degrees C working temperature. At thi s temperature and 0.7 ppm NO2 the calculated sensitivities (S = R-g/R-a) yi eld S = 10 and S = 7 for SG and HVTE, respectively. No cross sensitivity ha ve been found by exposing the In2O3 films to CO and CH4. Negligible H2O cro ss has resulted in the 40-80% relative humidity range, as well as to I ppm Cl-2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted to have a significant cross to the NO2 response. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.