Amorphous InGaZnO4 films: Gas sensor response and stability

The response characteristics of amorphous-InGaZnO4 (a-IGZO4) thin films toward reducing/oxidizing gases (H2/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-IGZO4 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 H2 or NO2 in dry air. The response (S) to H2 was found to go through a temperature maximum (e.g. S ∼ 0.7 at 350 °C for pH2 = 12.5 ppm) that value being a function of pH2. The response to NO2, on the other hand, decreased with increasing temperatures with the highest recorded values at 200 °C (e.g. S ∼ 33 at 200 °C for pNO2 = 5 ppm). The response followed an approximate power law dependence on gas partial pressure (p), S = Apβ, 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.