Ga₂O₃ has emerged as a promising ultrawide bandgap semiconductor for numerous device applications owing to its excellent material properties. In this paper, we present a comprehensive review on major advances achieved over the past thirty years in the field of Ga₂O₃-based gas sensors. We begin with a brief introduction of the polymorphs and basic electric properties of Ga₂O₃. Next, we provide an overview of the typical preparation methods for the fabrication of Ga₂O₃-sensing material developed so far. Then, we will concentrate our discussion on the state-of-the-art Ga₂O₃-based gas sensor devices and put an emphasis on seven sophisticated strategies to improve their gas-sensing performance in terms of material engineering and device optimization. Finally, we give some concluding remarks and put forward some suggestions, including (i) construction of hybrid structures with two-dimensional materials and organic polymers, (ii) combination with density functional theoretical calculations and machine learning, and (iii) development of optical sensors using the characteristic optical spectra for the future development of novel Ga₂O₃-based gas sensors.

ITO/Ag/ITO (IAI) trilayer films were deposited on glass substrate by radio frequency magnetron sputtering at room temperature. A high optical transmittance over 94.25% at the wavelength of 550 nm and an average transmittance over the visual region of 88.04% were achieved. The calculated value of figure of merit (FOM) reaches 80.9 10^-3 Ω^-1 for IAI films with 15-nm-thick Ag interlayer. From the morphology and structural characterization, IAI films could show an excellent correlated electric and optical performance if Ag grains interconnect with each other on the bottom ITO layer. These results indicate that IAI trilayer films, which also exhibit low surface roughness, will be well used in optoelectronic devices.

The impurity-associated optical transitions in a piezoelectric core–shell nanowire were studied using the density matrix formalism. In the calculations, four electron states were considered using a finite-difference algorithm combined with a variational approach. An obvious blueshift in the optical spectrum, especially for the transition from the free electron to impurity-bound ground states, was predicted to result from the intrinsic Stark effect induced by the piezoelectric field. The effects of the optical transition between impurity-bound states on the absorption coefficient and the refractive index of wurtzite nanowires were enhanced as the piezoelectric polarization becomes stronger. The findings are helpful for guiding further experiments on the linear and nonlinear optical properties of piezoelectric nanowires.

Ga₂O₃/Ag/Ga₂O₃-laminated films with high electrical conductivity and ultraviolet (UV) transparency were achieved by radio frequency magnetron sputtering at room temperature (RT) on quartz glass. The influence of annealing temperature and ambient on the structural, electrical and optical properties of Ga₂O₃/Ag/Ga₂O₃-laminated films were investigated in detail. As the annealing temperature increases, the optical bandgap of the Ga₂O₃-laminated films widens. The Ga₂O₃/Ag/Ga₂O₃-laminated films exhibited good photoelectric performance with a figure-of-merit (FOM) value of 5.83 × 10⁻³ Ω⁻¹, a sheet resistance of 12.55 Ω/sq, a transmittance of 95.15% at 325 nm, and an average transmittance of 77.56% (250~300 nm). All these results suggest that RT-fabricated Ga₂O₃/Ag/Ga₂O₃-laminated films show great potential in UV transparent conductive electrodes for UV optoelectronic devices and in flexible electronics.