According to foreign media reports, scientists at the University of South Carolina in the United States have developed the world’s smallest and brightest deep-ultraviolet LED with a chip size of only 5 microns and a wavelength of 281nm. The UVC LED array composed of these chips has a brightness of 361W·cm-2.
It is reported that at the beginning of this century, the research team of the University of South Carolina began to develop small UV LED interconnect arrays. The first generation of UV LED pixels have a diameter of 25 microns and form a 10×10 array, which improves the light output power, has ultra-high reliability, and reduces current congestion. It is mainly based on standard series resistance and auxiliary current expansion.
Recently, the team studied this structure again for the deep ultraviolet light source, focusing on the effect of thermal impedance and pixel size on the emission power.
According to team spokesperson Richard Floyd, shrinking the size of deep-ultraviolet LEDs to less than 20 microns will not increase the difficulty of most of the production steps, nor does it require special equipment to complete. For example, the research team used a standard photoresist mask (Photoresist Masking) and Karl Suss MJB-3 mask aligner (Mask Aligner) in all lithography steps.
However, in order to ensure the uniformity of the entire wafer, the research team optimized the exposure and development time of photolithography below 20 microns, and optimized the annealing conditions for the p-pole ohmic contact.
The research team compared a single-pixel LED reference object with a diameter of 90 microns with three different array structures: 36 pixel arrays with a diameter of 15 microns, 81 pixel arrays with a diameter of 10 microns, and 324 pixels with a diameter of 15 microns. For the 5-micron pixel array, the gaps are all 5 microns.
Left: 5-micron interconnected pixel array (the gap between pixels is 5 microns)
Right: The same array when the DC current is 60mA (Image source: University of South Carolina)
Through the on-wafer measurement method (using 500ns pulse and 0.05% duty cycle to minimize the heat of the device), it is found that a single 5 micron (diameter) pixel with an aluminum heat sink, when the driving current is 10.2kA·cm- At 2 o'clock, the peak brightness can reach 291W·cm-2, which is 30 times the brightness of the above-mentioned LED reference object.
The research results of three different arrays show that, thanks to the superior heat removal technology that eliminates thermal sagging, reducing the pixel size can improve performance. Compared with the single-chip reference, the output power of the 324 pixel arrays with a diameter of 5 microns is increased by more than 5 times in continuous wave mode, and the output power can be increased by more than 15 times in pulse mode.
Through this research, the research team found that the external quantum efficiency of the interconnected micro-pixel array will not decrease as the pixel size shrinks. Therefore, the researchers are confident that they can develop a smaller size without affecting the performance of the device. Deep UV LED.
But it should be noted that the research results also show that as the device size is further reduced, the thermal impedance will slightly decrease, but only this index will be affected.
Currently, the University of South Carolina research team is studying process technology to enhance the light extraction efficiency, hoping to use independent electrons to control these pixels.
It is reported that this research breakthrough will help promote UVC LEDs to better compete with mercury lamps in applications requiring high-dose deep ultraviolet irradiation. After all, in some application places such as mask disinfection, the toxicity of mercury lamps has caused a lot of concerns.
Although current UVC LEDs are still inferior to mercury lamps in terms of photoelectric conversion efficiency, cost performance, and lifespan, the continuous breakthroughs in technology will eventually promote the process of replacing mercury lamps with UVC LEDs.
