Functionalization of GaN Nanowire Sensors With Metal Oxides: An Experimental and DFT Investigation

Abstract

The detection of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules by GaN nanowire sensors which were functionalized with various metal oxides have been comprehensively studied with device fabrication, characterization and modeling with first-principles calculations based on density functional theory (DFT). In this work, GaN nanowires were prepared on Si substrate by standard top-down fabrication process and then fabricated into resistor based chemical sensors. The surface of GaN nanowires were functionalized by three metal oxides: TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , ZnO and SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> for analysis. The UV illuminated device characterization results indicated that the devices with TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> functionalization exhibited the highest response toward NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas. It showed quick response (240s) and recovery (280s) process with strong NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> selectivity. In modeling, the oxide functionalized GaN in contact with NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecule was designed and geometrically optimized. Simulation results indicated that TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> functionalization enabled the most energy favorable surface for NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> adsorption among the three metal oxides. In addition, the electronic properties of these oxide functionalized GaN have been studied in terms of the total density of states (TDOS) and projected density of states (PDOS), indicating an excellent agreement with the above-mentioned experimental measurements. Furthermore, the effect of environmental humidity on the adsorbate-nanocomposite interaction has been simulated and studied. Overall, the metal oxide functionalization significantly enhances the performance of GaN gas sensors and selecting an appropriate oxide will optimize the detection.