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.

Wafer bonding techniques, which are very different from epitaxial growth techniques, can be used not only for the fabrication of micro-electromechanical systems (MEMS), silicon on insulator (SOI), and Si-based device integration, but have recently been applied to the achievement of high-quality homojunctions and heterojunctions in the photoelectric field. That is, carrier transport at the interface of the wafer-bonded junction should be unimpeded and carrier recombination at the bonded interface should be restrained. For Si/Si wafer bonding, although a high bonding strength and a bubble-free bonded interface are needed for the fabrication of the MEMS and SOI, a perfect Si/Si bonded interface which is expected to be bubble-free, oxide-layer-free, and dislocation-free is needed for the achievement of high-performance photoelectric devices, such as Ge/Si single-photon avalanche photodiodes. On the other hand, for Ge/Si heterogeneous hybrid integration (high lattice mismatch), threading dislocations (TDs) in the Ge film can be eliminated by low-temperature heterogeneous wafer bonding, due to the lower diffusion rate of misfit dislocations (MDs) at the Ge/Si bonded interface. This is very different from epitaxial growth, in which high-density TDs form in the integrated Ge layer due to the threading of MDs at high-temperature. In this paper, we review the wafer bonding of Si-based semiconductors based on different bonding methods. The advantages and disadvantages of different bonding methods are pointed out for comparison. We focus on the illustration of the fabrication of Si/Si and Ge/Si wafer pairs with TD-free, bubble-free, and oxide-layer-free bonded interfaces. Finally, the outlook for the development of Si/Si and Ge/Si wafer bonding and devices based on the wafer bonding technique is considered. We trust that this work may provide guidance for the low-temperature heterogeneous hybrid integration of different group materials with ultrahigh lattice mismatch, such as GeSn on Si and III–V materials on Si.

Phosphorus (P) and Boron (B) co-doping effects at the nanoscale in Si nanocrystals/SiO₂ multilayers have been studied in the present work. Several interesting experimental results are achieved which are in contrast to the case in bulk-Si and the previous observations on the doped Si nanocrystals. It is found that all the co-doping samples are n-type regardless of B doping ratios. The P doping efficiency in Si NCs is higher than B dopants, and it can be improved via B co-doping with suitable levels. Raman and ESR spectra indicate that the different occupation preferences of P and B in Si NCs are responsible for the interesting co-doping behaviors. It looks like that the electronic structures and the physical properties of Si NCs can be modulated via the impurities co-doping approach.