Recently, the research group of Sun Haiding and Long Shibing of the School of Microelectronics of the University of Science and Technology of China made a breakthrough in the important progress of UV LED luminescence performance with regard to the use of sapphire substrate chamfer angle control quantum well to achieve three-dimensional carrier confinement.
Although ultraviolet rays account for only 5% of the energy in sunlight, they are widely used in human life. At present, ultraviolet light is widely used in water purification, light curing, sterilization and disinfection. Traditional ultraviolet light sources generally use the excited state of mercury vapor discharge to generate ultraviolet rays, which have many defects such as large heat generation, high power consumption, slow response, short life span, and potential safety hazards. The new deep-ultraviolet light source uses the light-emitting diode (LED) principle, which has many advantages over the traditional mercury lamp. Among them, the most important advantage is that it does not contain toxic mercury elements. The implementation of the "Minamata Convention" foretells that the use of mercury-containing ultraviolet lamps will be completely banned in 2020. Therefore, the development of a brand-new environmentally-friendly, high-efficiency ultraviolet light source has become an important challenge facing people.
As a result, deep ultraviolet light-emitting diodes (UV LEDs) based on wide-bandgap semiconductor materials (gallium nitride, aluminum gallium nitride) have become the best choice for this new application. This all-solid-state light source system is highly efficient, small in size, and has a long lifespan. It is just a chip the size of a thumb cover and can emit ultraviolet light stronger than a mercury lamp. The mystery here mainly depends on the direct band gap semiconductor material of III-nitride: when electrons in the conduction band recombine with holes in the valence band, photons are produced. The energy of the photon depends on the band gap of the material, so scientists can adjust the element composition in the ternary compound of aluminum gallium nitride (AlGaN) to achieve different wavelengths of light emission. However, it is not always that simple to achieve high-efficiency light emission of UV LEDs. Scientists have discovered that when electrons and holes recombine, photons are not always produced. This efficiency is called internal quantum efficiency (IQE).
Different from the traditional UV LED structure, the thickness of the potential well and the potential barrier in the light-emitting layer of this new type of structure, the multilayer quantum well (MQW), is not uniform. With the help of high-resolution projection electron microscopes, researchers were able to analyze quantum well structures of only a few nanometers at a microscopic scale. Studies have shown that gallium (Ga) atoms will aggregate at the steps of the substrate, which leads to a narrowing of the local energy band, and as the film grows, the Ga-rich and AI-rich regions will extend to the DUV LED The surface is distorted and bent in a three-dimensional space to form a three-dimensional multiple quantum well structure. Researchers call this special phenomenon: the phase separation of AI and Ga elements and the localization of carriers. It is worth pointing out that in the indium gallium nitride (lnGaN)-based blue LED system, ln and Ga are not 100% miscible, resulting in ln-rich and Ga-rich regions inside the material, resulting in a localized state and promoted loading. Radiation recombination of currents. But in the AlGaN material system, the phase separation of Al and Ga is rarely seen. One of the important meanings of this work is to artificially adjust the growth mode of the material, promote the phase separation, and thus greatly improve the light-emitting characteristics of the device.
This research will provide new ideas for the research and development of high-efficiency all-solid-state UV light sources. This idea does not require expensive patterned substrates and complex epitaxial growth processes. Relying only on the adjustment of the bevel angle of the substrate and the matching and optimization of epitaxial growth parameters, it is expected that the luminous characteristics of UV LEDs can be improved to a height comparable to that of blue LEDs, laying an experiment for the large-scale application of high-power deep UV LEDs. And theoretical basis.
