According to foreign media reports, scientists at the University of South Carolina 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.
A team at the University of South Carolina began developing small interconnected UV LED arrays in the early 2000s.The first generation UV LED has a pixel diameter of 25 microns and comprises a 10×10 array, which improves optical output power, provides ultra-high reliability, and reduces current crowding, mainly by measuring series resistance and auxiliary current expansion.
Recently, the team studied the structure again for deep UV light sources, focusing on the thermal impedance and the effect of pixel size on the transmitted power.
According to Richard Floyd, a spokesman for the team, reducing the size of the deep UV LED to less than 20 microns would not increase the difficulty of most production steps and would not require specialized equipment to do so.For example, the team used standard Photoresist Masking and Karl Suss MJB-3 Mask Aligner for all the lithography steps.
However, to ensure uniformity across the wafer, the team optimized lithography exposure and development times below 20 microns, as well as ohmic contact annealing conditions at the P-pole.
The team compared the 90-micron diameter single-pixel LED reference to three different array structures: 36 15-micron diameter pixel arrays, 81 10-micron diameter pixel arrays, and 324 5-micron diameter pixel arrays, all with 5-micron gaps.
Using on-wafer measurements (using 500ns pulses and a duty cycle of 0.05% to minimize device heat), a single 5 micron (diameter) pixel with an aluminum heat sink with a drive current of 10.2kA·cm-2 was found to have a peak brightness of 291W·cm-2, 30 times the brightness of the aforementioned LED reference.
Results from three different arrays show that reducing pixel size improves performance thanks to superior heat removal technology that eliminates thermal droop.Compared with the single chip reference, the output power of the 324 pixels array with a diameter of 5 microns is increased by more than 5 times in continuous wave mode and by more than 15 times in pulse mode.
In this study, the team found that the external quantum efficiency of the interconnected micropixel array does not decrease as the pixel size decreases. Therefore, the researchers are confident that they can develop smaller deep UV leds without compromising the device performance.
However, it should be noted that the results also show a slight decrease in thermal impedance as the device size is further reduced, but only this index is affected.
Now, the University of South Carolina team is working on a process to enhance the efficiency of light extraction, with the hope of using independent electrons to control these pixels.
It is reported that the research breakthrough will help promote UVC leds to better compete with mercury lamps in applications requiring high doses of deep ultraviolet irradiation.After all, mercury lamps have raised concerns about their toxicity in some applications, such as mask disinfection.
Although the current UVC LED is not as good as mercury lamp in terms of photoelectric conversion efficiency, cost performance and life expectancy, continuous technological breakthroughs will eventually promote the process of UVC LED replacing mercury lamp.
