Although ultraviolet energy accounts for only 5% of sunlight, it is widely used in human life. At present, UV light applications include printing curing, coin anti-counterfeiting, skin disease treatment, plant growth light, and damage to the molecular structure of microorganisms such as bacteria and viruses. Therefore, it is widely used in air sterilization, water purification, and solid surface sterilization and disinfection.
The traditional ultraviolet light source generally uses the excited state of mercury vapor discharge to generate ultraviolet light, which has many defects such as high power consumption, large heat generation, short life, slow response, and potential safety hazards. The new deep ultraviolet light source uses the light emitting diode (LED) light emitting principle, which has many advantages over traditional mercury lamps. The most important advantage is that it does not contain toxic mercury. With the implementation of the Minamata Convention, it indicates that the use of mercury-containing ultraviolet lamps will be completely banned in 2020. Therefore, how to develop a new environmentally friendly and efficient ultraviolet light source has become an important challenge facing people. .
Deep ultraviolet LEDs (DUV LEDs) based on wide band gap semiconductor materials (GaN, AlGaN) have become the only choice for this new application. This all-solid-state light source system is small in size, high in efficiency, and long in life. Just a chip the size of a thumb cover, it can emit ultraviolet light that is stronger than a mercury lamp. The mystery of this mainly depends on the direct band-gap semiconductor material of group III nitrides: the electrons in the conduction band and the holes in the valence band recombine, thereby generating photons. The energy of the photon depends on the forbidden band width of the material. Scientists can precisely realize the emission of different wavelengths by adjusting the element composition in the ternary compound such as AlGaN. However, it is not always easy to achieve high-efficiency light emission of UV LEDs. Researchers have discovered that when electrons and holes recombine, photons are not always generated. This efficiency is called internal quantum efficiency (IQE).
The research group of Sun Haiding and Long Shibing of the School of Microelectronics, University of Science and Technology of China, and Guo Wei and Ye Jichun of the Ningbo Institute of Materials of the Chinese Academy of Sciences have discovered that in order to increase the IQE value of UV LEDs, a substrate that can be grown through AlGaN materials-sapphire Al2O3 is controlled by the beveling angle. The researchers found that when the beveling angle of the substrate is increased, the dislocations inside the UV LED are significantly suppressed, and the luminous intensity of the device is significantly improved. When the chamfered substrate reaches 4 degrees, the intensity of the fluorescence spectrum of the device is increased by an order of magnitude, and the internal quantum efficiency has reached a record-breaking 90%.
Different from the traditional UV LED structure, the thickness of the potential well and barrier in the multilayer quantum well (MQW) is not uniform in the light-emitting layer inside this new structure. With the help of high-resolution transmission electron microscopy, researchers were able to analyze quantum well structures at just a few nanometers on a microscopic scale. Studies show that at the substrate step, gallium (Ga) atoms will aggregate, which results in a localized energy band narrowing, and as the film grows, Ga- and Al-rich regions will extend to DUV LEDs. Surface, and twisted and bent in three-dimensional space, forming a three-dimensional multi-quantum well structure.
Researchers call this special phenomenon: the phase separation of Al and Ga elements and the localization of carriers. It is worth pointing out that, in the InGaN-based blue LED system, In is not 100% miscible with Ga, resulting in In and Ga-rich regions in the material, which results in local states and promoted loading. Radiative recombination of carriers. However, in AlGaN material systems, phase separation of Al and Ga is rarely seen. One of the important significance of this work is that the growth mode of the material is artificially adjusted to promote phase separation, and thus greatly improve the light-emitting characteristics of the device.
By optimizing the epitaxial growth adjustment on a 4-degree bevel substrate, the researchers explored an optimal DUV LED structure. The carrier lifetime of this structure exceeds 1.60 ns, which is generally lower than 1ns in traditional devices. Further testing the chip's luminous power, the researchers found that its ultraviolet luminous power was more than twice that of traditional devices based on a 0.2-degree bevel substrate. This is more certain proof that AlGaN materials can achieve effective phase separation and carrier localization. In addition, the experimentalists also simulated the phase separation phenomenon inside the AlGaN multiple quantum wells and the effects of the unevenness of the potential well and barrier thickness on the luminous intensity and wavelength through theoretical calculations. The theoretical calculations are in good agreement with the experiments.
The research results were jointly completed by Professors Dai Jiangnan and Chen Changqing of Huazhong University of Science and Technology, Professor Zhang Zihui of Hebei University of Technology, and Professor Boon Ooi and Professor Iman Roqan of King Abdullah University of Science and Technology. Researchers believe that this research will provide new ideas for the development of highly efficient all-solid-state UV light sources. This idea does not require expensive patterned substrates or complicated epitaxial growth processes. And just relying on the adjustment of the bevel angle of the substrate and the matching and optimization of the epitaxial growth parameters, it is expected that the luminous characteristics of UV LEDs will be improved to a level comparable to that of blue LEDs, laying a test for large-scale applications of high-power deep UV LEDs And theoretical basis. The related results are titled "Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate" and published online at Advanced Functional Materials (DOI: 10.1002 / adfm. 201905445).