Recovering Near-band-edge Ultraviolet Light Emission And Absorption In SnO₂ And In₂O₃ Via Nano-engineering

Exploitation of wide-bandgap oxides has been a time-enduring research theme as a result of their rich functionalities and technological applications. Among the functional oxides, SnO₂ and In₂O₃ are extensively applied as transparent conducting thin films in the fields of gas sensors, solar cells, liquid crystal display and so on due to their excellent optical and electrical properties. The even-parity symmetry of the conduction-band minimum and valence-band maximum states in SnO₂ and In₂O₃ prohibits the band-edge radiative transition and light emission, which is called dipole-forbidden rule. In other words, near-band-edge light emission or absorption can not be realized theoretically. Therefore, there was little achievements on the optical properties of SnO₂ and In₂O₃, which has hindered their potential use in optical applications, such as light-emitting diodes（LEDs） and photodetectors. With the development of nanotechnology, people have found that the band structure and the associated optical transitions in nanostructured materials can be significantly different from those of the single crystal counterparts. Recently, a lot of researchers began to study the nanostructures of SnO₂ and In₂O₃. Ultraviolet（UV） emission in the nanostructures of SnO₂ and In₂O₃ were observed as a result of breaking the dipole-forbidden rule. These results indicated that nano-engineering method apparently is more efficient to break the dipole-forbidden rule and recoveringnear-band-edge UV light emission and absorption. In this paper, we fabricated the hybrid SnO₂ and In₂O₃ films with the nanocrystals embedded into the amorphous matrix by the radio frequency（rf） magnetron sputtering method and annealing process.Near-band-edge UV light emission and absorption were observed from the hybrid SnO₂ and In₂O₃ films.SnO₂ thin films with a thickness of 1600 nm were deposited on quartz substrates at room temperature using pure argon（Ar） as the working gas by using the rf magnetron sputtering method, which was followed by annealing procedure. The results of XRD, Raman Spectra and TEM indicated that SnO₂ nanocrystals were embedded in the SnO₂ amorphous matrix after annealing at 400 ℃ to form a SnO₂nanoparticle/amorphous hybrid film.. We investigated the PL properties of the SnO₂ thin films. The band-edge UV emission was observed from the hybrid film due to the hybrid structure breaking the dipole-forbidden rule of bulk SnO₂. This hybrid SnO₂ film was then deposited on a p-type Ga N substrate to form a SnO₂ hybrid film-based LED and a band-edge UV electroluminescence（EL） was observed. Our results suggest that this easy and effective approach may find extensive application in the field of optoelectronics, displays and solid-state lighting.Based on the achievement of SnO₂, we deposited In₂O₃ films on quartz substrates with the In₂O₃ nanocrystals embedded into the amorphous In₂O₃ matrix. The near-band-edge UV emission and absorption were observed in the hybrid In₂O₃ films.In order to apply In₂O₃ to UV photoelectric field, the hybrid In₂O₃ film was deposited on the p-Ga N/sapphire wafer to form an In₂O₃ /p-Ga N heterojunction photodiode. The photodiode showed an obvious rectifying behavior in a current–voltage measurement and a narrow-band UV photoresponse at the near-band-edge region under back-illumination conditions. This result further illustrated that the dipole-forbidden rule can be broken by the tailored hybrid structure, and realized near-band-edge UV light emission and absorption. Electronic structure calculations based on the first-principles method demonstrate that the breaking of dipole-forbidden transition rule is derived from the surface states of In₂O₃ nanocrystals.