【Domestic Papers】Harbin Institute of Technology (Shenzhen) --- A Review of ε-Ga₂O₃ Films: Fabrications and Photoelectric Properties

      Researchers from the Harbin Institute of Technology (Shenzhen) have published a dissertation titled "A Review of ε-Ga₂O₃ Films: Fabrications and Photoelectric Properties" in Materials.

Project support

      This work is financially supported by the National Natural Science Foundation of China (grant Nos.52102094), the Guangdong Basic and Applied Basic Research Foundation (grant Nos.2024A1515011678), and the Shenzhen Science and Technology Innovation Committee (grant Nos.JCYJ20220531095017039, KJZD20240903095400001).

Abstract

      Gallium oxide (Ga₂O₃), as an ultra-wide bandgap semiconducting material, has attracted extensive research interest in recent years. Owing to its outstanding electrical and optical properties, as well as its high reliability, Ga₂O₃ shows great potential in power electronics, optoelectronics, memory devices, and so on. Among all the different polymorphs, ε-Ga₂O₃ is the second most thermally stable phase. It has a hexagonal crystal structure, which contributes to its isotropic physical properties and its suitable growth on low-cost commercial substrates, such as Al₂O₃, Si (111). However, there are far fewer research works on ε-Ga₂O₃ in comparison with the most thermally stable β phase. Aiming to provide a comprehensive view on the current works of ε-Ga₂O₃ and support future research, this review conducts detailed summarizations for the fabrication processes of ε-Ga₂O₃ thin films and the photoelectrical properties of ε-Ga₂O₃-based photodetectors. The effects of different deposition parameters on film phases and qualities are discussed. The forming mechanisms of ε phase prepared by chemical vapor depositions (CVDs) and physical vapor depositions (PVDs) are analyzed, respectively. Conclusions are made concerning the relationships between film microstructures and properties. In addition, strategies for further improving ε-Ga₂O₃ film performance are briefly summarized.

Summary

      In summary, ε-Ga₂O₃ films can be fabricated via either CVD or PVD techniques. In the case of CVD, substrates play an important role as a base for ε phase formation. Suitable temperatures, pressures, and precursor ratios are required for ε-phase nucleation and pure phase growth. In the case of PVD, doping is a necessary condition for the formation of the ε phase. The quality of film is further affected by substrate type, deposition temperatures, and oxygen pressures. The reported photoelectrical properties of ε-Ga₂O₃-based photodetectors vary a lot. In general, defects, including grain boundaries, doping-induced lattice distortions, and oxygen vacancies, can generate trapping energy levels and obstacles for the migrations of carriers, leading to low responsivity and slow response of the ε-Ga₂O₃ films. The optimization of electrode structures and the introduction of passivation layers can strongly enhance the responsivity and detectivity of ε-Ga₂O₃ films. Special electrode materials can generate build-in electric field in ε-Ga₂O₃ films to achieve ultrafast responses. Despite the findings of current research, the formation mechanisms of ε-Ga₂O₃ films remain unclear. Especially, ε-Ga₂O₃ films without doping have never been successfully synthesized via PVD techniques. The simultaneous improvements of the photoelectrical properties (responsivity, PDCR, detectivity, responding time, etc.) of ε-Ga₂O₃ are still very difficult to attain. Moreover, many other properties and potential applications of ε-Ga₂O₃, such as power performance for electronic devices and high electron mobility properties for RF devices, are rarely investigated. It can be expected that, with further understanding of the fundamentals of ε-Ga₂O₃, the films’ properties can be enhanced significantly and specifically tuned to meet the requirements of a great range of applications.