We report and discuss our results on the performance of our photoactivated In₃O₂/GaN NW sensors, for monitoring NO₂ at various levels of relative humidity and operating power. A 1ppm sensitivity as high as 29% was demonstrated at operating power of 5 mW and relative humidity 40%.

Nanostructure-based semiconductor gas sensors such as nanowires, nanotubes or nanorods have seen significant progress in recent years, owning to their large surface-to-volume ratio proprieties, which lead to considerably increased sensitivity and improve the time response to analyte gases. In particular, Gallium Nitride (GaN) nanostructure-based gas sensors have generated a lot of interest due to its unique combination of sensor “friendly” properties such as, direct bandgap, excellent carrier mobility, high heat capacity and high breakdown voltage. These properties combined with suitable surface engineering make GaN and GaN-based nanostructures an excellent candidate for portable gas sensors.

In this work, Gallium Nitride (GaN) nanowire (NW)-based NO₂ sensors, functionalized with either pure In₂O₃ or In₂O₃ covered with an evaporated Au nanolayer, were developed for low operating power and high sensitivity. An AlGaN buffer layer was first deposited on a Si substrate to minimize lattice mismatch and improve adhesion between the Si substrate and the GaN NWs. Then the GaN NWs were patterned by stepper lithography-assisted dry-etching, followed by Induced Coupled Plasma etching using a metal hard mask to protect the GaN NW. The nanowire width target ranged between 200-600nm. Subsequently, electrodes composed of Ti/Al/Ti/Au metal stacks were deposited on top of the GaN to form Ohmic contacts. A thin In₂O₃ layer metal oxide receptor was deposited on all devices, using RF magnetron sputtering on top of the exposed GaN NW and this was followed for a subgroup of devices by coating the In₂O₃ with an evaporated Au nanolayer. Finally, rapid thermal annealing (RTA) at 700 ⁰C was performed to crystalize the receptor layers and improve the ohmic contact. The surface quality of the resulting sensors was inspected with the help of Scanning Electron Microscope (SEM) micrographs.

Several devices of each group (In₂O₃ and In₂O₃/Au ) were wire bonded and mounted onto an array board chamber. The devices were biased under 5V DC. Photoconductivity measurements, humidity testing and NO₂ gas testing (1ppm and 10ppm concentration) were conducted. The sensors were illuminated under constant UV LED illumination throughout the duration of the testing at two wavelengths, 265 nm and 365 nm and three irradiance levels, 5mW, 30mW and 60mW. In general, the In₃O₂ GaN NW sensors where characterized by low sensitivity levels, whereas the In₃O₂/Au GaN NW sensors achieved excellent sensitivity levels. Fig.1 shows typical dynamic responses to 1ppm NO₂ of the In₃O₂/Au GaN NW sensors under illumination at 265 nm and 365 nm, respectively, and Fig.2 the response at various relative humidity (RH) levels. An increase in resistance upon exposure to NO₂ is observed, confirming the oxidizing nature of the NO₂ gas. At this NO₂ concentration level, the highest response value (29%) was obtained under (5mW, 265 nm) illumination, whereas the sensitivity under (5mW, 365nm) illumination was lower. For (30mW, 265nm) illumination, the sensitivity was 23% and for (30mW, 365nm) illumination it dropped to 15%. This phenomenon has been observed in the past by other authors also, and although its origins are not yet understood unambiguously, it may be caused by a higher rate of phonon energy relaxation at the surface at higher wavelengths, thereby reducing the effectiveness of chemisorption at the surface.

In conclusion, room-temperature low-power photoactivated In₃O₂/Au GaN NW sensors with excellent levels of sensitivity at elevated relative humidity were designed, fabricated and tested.