Formation of GaN-Ga₂O₃ surface heterostructures via femtosecond-laser-irradiation for sensitive and selective NO₂ sensing at elevated temperatures

Reliable detection of NO₂ gas, as one of the most toxic gases, is highly important for human health and environmental purposes. Gallium nitride (GaN) is a wide band gap material with high chemical stability. However, it shows poor sensing properties in pristine form. Herein, GaN films with different thicknesses were deposited using a self-designed hydride vapor phase epitaxy. It was found that the film with a thickness of ∼1 µm provided the best sensing results towards NO₂. Next, the femtosecond (FS) laser irradiation was applied to GaN film (∼ 1 µm) to locally induce partial oxidation, resulting in the formation of a GaN–Ga₂O₃ surface heterostructure. Various structural and surface characterizations confirmed the presence of crystalline Ga₂O₃ and increasing of surface roughness after the FS laser irradiation. The laser-treated GaN–Ga₂O₃ sensor exhibited enhanced sensitivity, selectivity and faster response and recovery times relative to pristine GaN film. The enhanced sensing behavior was attributed to the formation of GaN–Ga₂O₃ heterostructures acting as powerful sources of resistance modulation, along with an increased surface roughness, resulting in an increase in the density of active adsorption sites. Thus, the FS laser irradiation is a simple and scalable approach for tailoring the surface chemistry and electronic structure of GaN, resulting in overall improvement of sensing capabilities.