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China University of Science and Technology made important progress in semiconductor deep ultraviolet LED research

release time:2019-12-17publisher:

Recently, Sun Haiding and Long Shibing's group from the Microelectronics Institute of the University of Science and Technology of China have achieved important progress in the UV LED's luminescence performance by using the sapphire substrate chamfered angle to regulate the quantum well to achieve three-dimensional carrier confinement. Related research was published in Advanced Functional Materials under the title of "Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate".

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 type of deep ultraviolet light source adopts the light emitting diode (LED) light emitting principle, which has many advantages over the traditional mercury lamp. The most important advantage is that it does not contain toxic mercury. The implementation of the Minamata Convention indicates that the use of ultraviolet lamps containing mercury will be completely banned in 2020. Therefore, the development of a new type of environmentally friendly and efficient UV light source has become an important challenge facing people.

Deep ultraviolet LEDs (DUV LEDs) based on wide bandgap 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 depends mainly 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, and this efficiency is called internal quantum efficiency (IQE).

The research group of Professor Sun Haiding and Long Shibing of the University of Science and Technology of China University of Science and Technology cleverly adjusted the bevel angle of the sapphire substrate to greatly increase the IQE and luminous power of UV LEDs. The research group found that when the substrate's bevel angle is increased, the dislocations inside the UV LED are significantly suppressed, and the light emission intensity of the device is significantly improved. When the chamfered substrate reaches 4 degrees, the intensity of the device's fluorescence spectrum is increased by an order of magnitude, and the internal quantum efficiency has reached a record-breaking 90%.

Unlike traditional UV LED structures, the thickness of the potential wells and barriers in the light-emitting layer of this new structure, the multilayer quantum well (MQW), is not uniform. 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, which results 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 DUVLED structure. The carrier lifetime of this structure exceeds 1.60 ns, which is generally less than 1 ns in traditional devices. Further testing the chip's luminous power, the researchers found that its UV luminous power was more than twice that of traditional devices based on a 0.2-degree bevel substrate, as shown in the figure. 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 multi-quantum well and the influence of the unevenness of the potential well and barrier thickness on the luminous intensity and wavelength through theoretical calculations. The theoretical calculations and experiments have been in good agreement.

Figure: Photoluminescence spectra and device schematic diagrams of deep ultraviolet LEDs prepared on sapphire substrates with bevel angles of 0.2 and 4 degrees. The active area transmission electron microscope shows a high-resolution multiple quantum well structure and a comparison of output power.

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.

Researcher Sun Haiding, School of Microelectronics, University of Science and Technology of China is the first author and co-corresponding author of the paper. The project was completed in collaboration with Guo Wei and Ye Jichun, researchers from Ningbo Institute of Materials Science, Chinese Academy of Sciences, Professor 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 Technology. This research work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Chinese University of Science and Technology. Part of the sample processing technology was completed at the Micro-nano Research and Manufacturing Center of the University of Science and Technology of China.