Important progress in the luminous efficiency of ultraviolet light-emitting diodes (UV LEDs)
By improving the material crystal quality, designing and epitaxially growing a new active region quantum well structure, and monolithically integrating a photomultiplier converter into the traditional LED device structure, the electro-optical conversion efficiency of the semiconductor deep ultraviolet LED (DUV LED) with a wavelength of 280nm was increased to more than 20%.
UV LEDs are an environmentally friendly and energy-saving alternative to mercury lamps. This device is also known for its small size and long life. By changing the aluminum content in the quantum well of aluminum gallium nitride (AlGaN), a wide-bandgap semiconductor material, UV LEDs can cover the spectral range from 210 nm to 360 nm. In addition to replacing mercury lamps in the above applications, UV LEDs can also be used in other applications due to their small size, high efficiency, and continuously adjustable wavelength, making them more marketable. According to market research and analysis, the market size of UV LEDs will grow from US$500 million in 2019 to US$1 billion in 2023, or even higher.
The study found that we can take a variety of innovative approaches to improve efficiency, mainly including: reducing defects and dislocations in the epitaxial layer; improving n-type and p-type doping efficiency to form a conductive film for improved current injection efficiency; designing device structure to improve light extraction efficiency and conversion efficiency; and using a novel AlGaN-based quantum well and barrier in the active light-emitting area.
We grew the heterostructures of our UV LEDs on sapphire, a low-cost transparent substrate. We avoided using single-crystal AlN substrates because they are too expensive. The drawback of sapphire is that it has a lattice and thermal expansion mismatch with nitrides, which results in poor crystal quality and is not conducive to radiative recombination of carriers in the active region. To address this issue, we turned to patterned sapphire substrates with pyramidal structures on the surface and epitaxially grew high-quality AlN films using lateral overgrowth. Based on the half-width of the X-ray diffraction rocking curve, we inferred that the dislocation density of the film was less than 3 x 108 cm-2. In addition, we performed reciprocal space tests, which showed that the epilayer stress was fully released. Based on these findings, we know that reducing dislocations can suppress non-radiative recombination in the active region of UV LEDs and ultimately improve the radiative recombination efficiency of these devices.
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