Reliable anatase-titania nanoclusters functionalized GaN sensor devices for UV assisted NO₂ gas-sensing in ppb level

Internet of Things applications require ultra-low power, integrable into electronic circuits and mini-sized chemical sensors for automated remote air quality monitoring system. In this work, a highly sensitive and selective detection of nitrogen dioxide (NO₂) has been demonstrated by functionalizing gallium nitride (GaN) submicron wire with titania (TiO₂) nanoclusters. The two-terminal GaN/TiO₂ sensor device was fabricated by top-down approach. The photo-enabled sensing makes it possible to operate this sensor at room-temperature, resulting in a significant reduction in operating power. The GaN/TiO₂ sensor was able to detect NO₂ concentrations as low as 10 ppb in air at room temperature (20 °C) with a quick response-recovery process. The sensor was found highly selective toward NO₂ against other interfering gases, such as ethanol (C₂H₅OH), ammonia (NH₃), sulfur dioxide (SO₂), methane (CH₄) and carbon dioxide (CO₂). Furthermore, principal component analysis has been performed to address the cross-sensitive nature of TiO₂. The sensor device exhibited excellent long-term stability at room temperature and humidity and was quite stable and reliable at various environmental conditions. Continuous exposure of the device to siloxane for a one-month period has shown a very small degradation in sensor response to NO₂. Finally, interaction of NO₂ gas molecules with the GaN/TiO₂ sensor has been modeled and explained under the light of energy band diagram. The photoinduced oxygen desorption and subsequent charge transfer between TiO₂ nanoclusters and NO₂ molecules modulate the depletion region width within the GaN, thus contributing to a high performance NO₂ gas sensing.