【International Papers】Growth and properties of Sn-doped sol-gel Ga₂O₃ thin films

      Researchers from the University of Canterbury have published a dissertation titled "Growth and properties of Sn-doped sol-gel Ga₂O₃ thin films" in Journal of Vacuum Science & Technology A.

Corresponding Author

      Martin Allen is a Professor in electrical engineering at the University of Canterbury. He has a BSc (Hons) degree in Physics from the University of Bristol, UK, an MSc in Plasma Physics from the University of Oxford, UK, and a PhD in Electrical Engineering from the University of Canterbury.

      His current research interests include the fundamental properties and commercial device applications of metal oxide semiconductors, particularly those that are optically active in the UV spectrum.  

Background

      Wide bandgap transparent semiconducting oxides (TSOs) are an interesting class of electronic material combining high optical transparency with strong electrical conductivity—properties that are usually mutually exclusive in nature. Ga₂O₃ is one such TSO with an ultrawide bandgap (E_g ∼ 4.8 eV) that extends its transparency into the ultraviolet-A and ultraviolet-B regions. It also has a very high thermal stability (m.p. ∼1900 °C) and one of the highest-known semiconductor breakdown strengths (∼8 MV cm⁻¹). Consequently, Ga₂O₃ has attracted significant interest for a wide range of applications, including high-efficiency power electronics, high-operating temperature electronic devices, and deep-ultraviolet solar-blind photodetectors. Ga₂O₃ has six different crystal phases, α, β, γ, η, ɛ, and κ, with the monoclinic β phase being the most thermodynamically stable and also the most studied, with some research also conducted on amorphous Ga₂O₃. 

      Ga₂O₃ thin films are usually produced using vapor deposition techniques such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and chemical vapor deposition. These techniques involve the use of equipment that is expensive to operate and in the case of Ga₂O₃ suffer from low growth rates due to the formation of volatile suboxides.

Abstract

      Ga₂O₃ is an ultrawide bandgap semiconductor with a very high thermal stability and breakdown strength that is attracting significant interest for use in high-efficiency power electronics and solar-blind UV photodetectors. However, Ga₂O₃ films are usually expensive to produce by physical vapor deposition due to the creation of volatile suboxides that can significantly limit the achievable growth rates. In this work, a low cost, solution-based sol-gel technique was used to rapidly produce Ga₂O₃ films. High quality (100% transparent, 0.3 nm root-mean-squared roughness) Sn-doped Ga₂O₃ films of up to 500 nm thickness were grown by sol-gel on c-plane sapphire substrates. The resulting films were amorphous and exhibited sufficient electrical conductivity for the fabrication of PtO_x/Pt Schottky diodes with four orders of magnitude rectification (at ±10 V). Post-growth annealing of the films in N₂ at temperatures higher than 600 °C introduced polycrystalline β-Ga₂O₃ phases but did not produce any improvement in electrical conductivity. This work demonstrates the ability of the highly scalable sol-gel technique to rapidly produce low-cost Sn-doped Ga₂O₃ films and the first Schottky diode devices fabricated on intentionally doped sol-gel Ga₂O₃ thin films.

Conclusion

      Highly transparent and very smooth single and multilayer amorphous Ga₂O₃ films of up to 500 nm thickness were produced on c-plane sapphire substrates using a rapid spin-on sol-gel technique. Sn-doped electrically conducting films were produced by introducing SnCl₂ into the Ga(NO₃)₃⋅xH₂O/EtOH precursor solution. An optimal Sn-doping percentage of 0.01 at. % was used for the fabrication of PtO_x/Pt Schottky diodes that were characterized by rectifying ratios of four orders of magnitude. Post-growth annealing at temperatures of 600 °C and above introduced β-Ga₂O₃ phases into the films but did not produce any significant improvement in electrical conductivity, which decreased at temperatures of 800 °C and above, most likely due to Al diffusion from the c-plane sapphire substrates. This works demonstrates the potential of the sol-gel technique to produce low-cost Sn-doped Ga₂O₃ films and electrical devices such as Schottky diodes.