AlGaN/GaN Heterostructure Based Schottky Diode Sensors with ZnO Nanorods for Environmental Ammonia Monitoring Applications

AlGaN/GaN heterostructure Schottky diodes with ZnO nanorod functionalization are shown to be capable of detecting NH3 balanced with air at a concentration of ∼0.3 ppm at 25°C and ∼0.1ppm at 300°C. The diodes show reproducible current response to repeated cycling of the NH3 exposure at all temperatures in this range. The diode current at fixed voltage decreased upon exposure to the NH3, which is opposite to what occurs with exposure to hydrogen. This suggests the detection mechanism involves reaction of ammonia with oxygen species on the ZnO nanorods, increasing the negative charge on the interface with AlGaN. The detection sensitivity displayed an activation energy of 0.071 eV and increased monotonically with ammonia concentration at all temperatures, from 3.36% (25°C) to 12.59% (300°C) for 2 ppm at a voltage of 5 V. The diodes could detect ammonia for either polarity applied bias. The absolute current change and sensitivity upon exposure to ammonia increased with measurement temperature.