In the field of semiconductor lighting, deep ultraviolet LED (UVC LED) has attracted much attention because of its wide application in sterilization, air purification, water treatment and other fields. However, its low external quantum efficiency (EQE) has long limited the commercialization process. Recently, a Chinese research team has made a major breakthrough in sapphire substrates by optimizing the preparation strategy of aluminum nitride (AlN) template layers, which has greatly improved the performance of UVC LEDs and promoted the development of the industry in the direction of high efficiency and low cost.
Ⅰ Technical bottlenecks and research background of UVC LEDs
UVC LEDs are made of aluminum gallium nitrogen (AlGaN) materials, but due to lattice mismatch and thermal mismatch between AlGaN and sapphire substrates, high-density dislocations and interface defects occur in the materials. These structural defects not only affect the crystal quality, but also reduce the carrier recombination efficiency, which significantly limits the luminous efficiency of the device.
To overcome these challenges, the researchers are working to improve the quality of AlN as a template layer to reduce the dislocation density and improve the photoelectric conversion performance of UVC LEDs.
Ⅱ Innovative preparation strategies for AlN templates
The research team used three different methods to prepare the AlN buffer layer on the sapphire substrate and achieved a crack-free, stress-controlled, high-quality AlN film with the help of metal-organic chemical vapor deposition (MOCVD) process.
1. AlN-I Method: Epitaxial Layer Regrowth (ELO)
The defect density was reduced by the three-dimensional growth mechanism, but the dislocation density was still as high as 5.1×10⁹/cm² and the surface roughness was 6.69 nm.
2. AlN-II Method：Sputtering nucleation layer （sp-NL）+ high-temperature annealing
In this method, a 12nm sputtering nucleation layer (sp-NL) was deposited on the sapphire substrate, and the structure was optimized by high-temperature annealing, so that the dislocation density was reduced to 2.8×10⁹/cm², and the surface roughness was reduced to 1.21nm.
3. AlN-III method: indium (In) doping enhances two-dimensional growth
Indium (In) doping was introduced on the basis of the AlN-II method, and the indium atom was used to reduce the Ehrlich–Schwoebel barrier, promote the two-dimensional growth of AlN, and alleviate the lattice mismatch stress. As a result, the dislocation density was further reduced to 1.5×10⁹/cm² and the surface roughness was only 0.49 nm.
Figure :(a) Epitaxial structures of AlN-I, (b) AlN-II, and (c) AlN-III. (d) Schematic diagram of the growth conditions of AlN-III and (e) the potential mechanism of indium modulation.
Ⅲ Indium doping mechanism and UVC LED performance improvement
Indium doping is a key innovation of the AlN-III method. Since the indium-nitrogen (In-N) bond energy is much weaker than that of the aluminum-nitrogen (Al-N) bond energy, the indium atom can promote the diffusion of the AlN surface to form a flatter film, while stretching the AlN lattice and reducing the mismatch between AlGaN and the template.
Experimental data show that the UVC LED (LED-C) based on AlN-III template is superior to other methods in terms of electrical and optical performance.
Table: Measured forward voltage (VF), light output power (LOP) and external quantum efficiency (EQE) of LED-A, LED-B and LED-C
The LED-C sample achieved a breakthrough with a lower forward voltage (6.07V), an increase in optical output power to 19.1mW, and an EQE of 4.1%.
Ⅳ Technical significance and industrialization prospects
By optimizing the preparation strategy of AlN template, this study not only significantly improved the luminous efficiency of UVC LED, but also provided key technical support for environmentally friendly germicidal light sources. This high-performance UVC LED has great potential for applications in medical, food processing, air purification, and other fields.
Ⅴ Future directions
Although indium doping has significantly improved the performance of UVC LEDs, the lattice mismatch problem has not been completely solved, and future research can further optimize the material growth process and improve the stability of the device. In addition, reducing manufacturing costs and improving the yield of large-scale production will be important challenges for commercialization.
Ⅵ Industry dynamics and market prospects
1. Research progress at home and abroad
- The iGaN team of USTC proposed a maskless lithography technology based on deep ultraviolet micro-LED arrays, which greatly improved the luminous efficiency of micro-LEDs.
- Zhou Shengjun's team from Wuhan University improved the electro-optical conversion efficiency of deep ultraviolet LEDs by 5.5% through AlGaN-based ultra-thin tunnel junction technology.
2. Market prospect and enterprise layout
- Nichia Chemical expects deep UV LED WPE to reach 7.5% in 2024, and the optical power of 280nm equipment is 150mW.
- Crystal IS has a single-chip optical output power of 160mW at 265nm and 500mA.
- Violumas 265nm UVC LED 130mW, L70 lifespan up to 13,000 hours.
- Zhongke Lu'an 265-280nm 30-35mW products were mass-produced, with an electro-optical conversion efficiency of 6%.
- The optical power of UVLEDTEK is 40-50mW at 270-280nm 100mA, and the optical power maintenance rate is 90% after 1000 hours of aging.
- The external quantum efficiency of Zhixin semiconductor exceeds 7%, and the optical power of 40mil chip exceeds 200mW.
- Annsemi 275nm 30mW@100mA, L70 estimated life span is more than 10,000 hours.
Ⅶ Conclusion
The technological breakthrough of the Chinese research team in the field of deep ultraviolet LED has not only promoted the development of academic research, but also enhanced the feasibility of industrial application. With the continuous optimization of manufacturing processes, UVC LEDs are expected to play an important role in more fields, providing more efficient technical support for human health and environmental protection.
This study not only lays an important foundation for the commercial application of UVC LED, but also points out the direction of technological innovation in the semiconductor lighting industry. In the future, with the continuous investment and technological progress of industry giants, high-efficiency, long-life, and low-cost UVC LED solutions will become the mainstream of the market, pushing the entire industry into a new stage of development.
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