Amorphous InGaZnO4 films: Gas sensor response and stability

Abstract The response characteristics of amorphous-InGaZnO 4 (a-IGZO 4 ) thin films toward reducing/oxidizing gases (H 2 /NO 2 ), 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 °C by monitoring changes in DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H 2 or NO 2 in dry air. The response ( S ) to H 2 was found to go through a temperature maximum (e.g. S  ∼ 0.7 at 350 °C for pH 2  = 12.5 ppm) that value being a function of pH 2 . The response to NO 2 , on the other hand, decreased with increasing temperatures with the highest recorded values at 200 °C (e.g. S  ∼ 33 at 200 °C for pNO 2  = 5 ppm). The response followed an approximate power law dependence on gas partial pressure ( p ), S  =  Ap β , with β taking on values of ∼0.5–1.0 as temperature increased from 200 to 400 °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 °C, under temperature sweep conditions, is attributed to kinetically limited adsorption/desorption and reaction rates.