Amorphous InGaZnO4 films: Gas sensor response and stability

The response characteristics of amorphous-InGaZnO4 (a-IGZO(4)) thin films toward reducing/oxidizing gases (H-2/NO2), at sensor operating temperatures, are reported for the first time. The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline semiconducting metal oxides (SMOs), rendering a-IGZO(4) a highly promising model sensor system. Gas sensor tests were carried out in the temperature range of 200-400 degrees C by monitoring changes in DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H-2 or NO2 in dry air. The response (S) to H-2 was found to go through a temperature maximum (e.g. S similar to 0.7 at 350 degrees C for pH(2) = 12.5 ppm) that value being a function of pH(2). The response to NO2, on the other hand, decreased with increasing temperatures with the highest recorded values at 200 degrees C (e.g. S similar to 33 at 200 degrees C for pNO(2) = 5 ppm). The response followed an approximate power law dependence on gas partial pressure (p), S = Ap(beta), with beta taking on values of similar to 0.5-1.0 as temperature increased from 200 to 400 degrees C. Response times were found to range from 10 s to greater than 1000 s as temperature decreased. The hysteretic behavior exhibited by a-IGZO films between 150 and 400 degrees C, under temperature sweep conditions, is attributed to kinetically limited adsorption/desorption and reaction rates. (C) 2012 Elsevier B.V. All rights reserved.