Fabrication and Characterization of AlGaN-Based UV LEDs with a ITO/Ga₂O₃/Ag/Ga₂O₃ Transparent Conductive Electrode

A review on synthesis and applications of gallium oxide materials

Gallium oxide (Ga2O3), as a new kind of ultra-wide band gap semiconductor material, is widely studied in many fields, such as power electronics, UV - blind photodetectors, solar cells and so on. Owing to the advantages of its excellent performance and broad application prospects in semiconductor technology, Ga2O3 materials have attracted extensive academic and technological attention. This review mainly focuses on introducing the main liquid-phase synthesis methods of Ga2O3 nanoparticles, such as direct-precipitation, chemical bath deposition, hydrothermal, solvothermal, and sol-gel method, including the characteristics in process and advantages and disadvantages of these methods. Then, the effects of reaction conditions, such as pH, capping agent, aging and calcination conditions, on the morphologies and sizes of the precursor and the final products were elucidated. Moreover, the applications of Ga2O3 particles in the fields of catalysis, gas sensors, and other devices in current research on Ga2O3 nanomaterials are discussed with the description of the basic working principle and influence factors.

High Performance Broadband Ultraviolet A/Ultraviolet C Detector Based on Ga₂O₃/p-GaN Nanocomposite Film Fabricated by Magnetron Sputtering

Ultraviolet (UV) detector based on β-Ga₂O₃/p-GaN was fabricated in this paper. The growth process involved deposition of amorphous Ga₂O₃ layer by means of magnetron sputtering and conversion of β-Ga₂O₃. The obtained detector displays excellent UV sensing properties which covers Ultraviolet A(UVA)/Ultraviolet C(UVC) region with fast response. It will provide a new route to fabricate β-Ga₂O₃/p-GaN heterostructure for applications in broadband ultraviolet sensing.

Enhancing the light extraction efficiency for AlGaN-based DUV LEDs with a laterally over-etched p-GaN layer at the top of truncated cones

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet light-emitting didoes (DUV LEDs) can be enhanced by using truncated cone arrays with inclined sidewalls. In this work, the air-cavity-shaped inclined sidewall is applied and the p-GaN layer at the top of the truncated cone is laterally over-etched so that more light escape paths are generated for AlGaN-based DUV LEDs. The experimental results manifest that when compared with DUV LEDs only having the air-cavity-shaped inclined sidewall, the optical power for the DUV LEDs with laterally over-etched p-GaN at the top of the truncated cone is enhanced by 30% without sacrificing the forward bias. It is because the over-etched p-GaN makes little effect on the carrier injection and does not affect the ohmic contact resistance. Moreover, the simulation results show that the truncated cone with laterally over-etched p-GaN layer can enhance the LEE because the reduced p-GaN area can suppress the optical absorption and supplies additional light paths for DUV photos. Then, more light will be reflected into escape cones at the sapphire side.

Heteroepitaxial Growth of High-Quality and Crack-Free AlN Film on Sapphire Substrate with Nanometer-Scale-Thick AlN Nucleation Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200–270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

Electrical and Optical Properties of a Transparent Conductive ITO/Ga₂O₃/Ag/Ga₂O₃ Multilayer for Ultraviolet Light-Emitting Diodes

We fabricated an indium tin oxide (ITO)/Ga₂O₃/Ag/Ga₂O₃ multilayer as a transparent conductive electrode for ultraviolet light-emitting diodes (UV LEDs). The electrical and optical properties of the multilayer were improved by optimizing the annealing temperature of the ITO contact layer and the whole ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer, and the thickness of the ITO contact layer and Ag metal layer. After optimization, the sheet resistance and transmittance of the ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer was 3.43 Ω/sq and 86.4% at 335 nm, respectively. The ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer also exhibited a good ohmic contact characteristic with a specific contact resistance of 1.45×10⁻³ Ω·cm². These results show that the proposed ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer is a promising alternative as a p-type electrode for UV LEDs.

Impact of Oxygen Vacancy on the Photo-Electrical Properties of In₂O₃-Based Thin-Film Transistor by Doping Ga

In this study, amorphous indium gallium oxide thin-film transistors (IGO TFTs) were fabricated by co-sputtering. Three samples with different deposition powers of the In₂O₃ target, namely, sample A with 50 W deposition power, sample B with 60 W deposition power, and sample C with 70 W deposition power, were investigated. The device performance revealed that oxygen vacancies are strongly dependent on indium content. However, when the deposition power of the In₂O₃ target increased, the number of oxygen vacancies, which act as charge carriers to improve the device performance, increased. The best performance was recorded at a threshold voltage of 1.1 V, on-off current ratio of 4.5 × 10⁶, and subthreshold swing of 3.82 V/dec in sample B. Meanwhile, the optical properties of sample B included a responsivity of 0.16 A/W and excellent ultraviolet-to-visible rejection ratio of 8 × 10⁴. IGO TFTs may act as photodetectors according to the results obtained for optical properties.