A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. One particular LED technology called micron-sized LEDs (µLEDs) have attracted significant research attention as the most suitable next-generation displays owing to their numerous advantages, including wider color gamut and high wall-plug efficiency that are superior to those of organic LED and conventional liquid crystal displays. Commercial availability of µLEDs has progressed remarkably well despite the high investment costs involved. AlInGaP-based LEDs have been used for red, while InGaN-based LEDs for green and blue. Besides the high costs, the commercial application of µLEDs is still limited by several other reasons such as emission directionality and other issues associated with the color such as mixing, purity and stability.
Based on the previous findings, using single-mode emission InGaN-based microcavity LEDs (MC-LEDs) could potentially solve these issues. The suitability of MC-LEDs for display applications is attributed to their directionality, high spectral purity, and thermal stability compared to conventional LEDs. These advantages can be further attributed to the overlap between the cavity mode and the InGaN quantum well (QW) emission that are both of great importance in determining the spectral width and shape of the MC-LEDs. Notably, the efficient application of MC-LEDs requires higher light extraction efficiency (LEE), which can be achieved by shortening the cavity length. To date, however, cavity lengths less than 450 nm have not been reported.
Herein, researchers from the University of California, Santa Barbara: Dr. Joonho Back, Vincent Rienzi (PhD candidate), Dr. Matthew Wong, Dr. Hongjian Li, Professor Steven DenBaars, Professor Claude Weisbuch and Professor Shuji Nakamura studied the blue semipolar InGaN MC-LED with varying cavity lengths. The lengths varied from 113 nm (representing the shorted ever studied length) to 200nm. A semipolar (20-2-1) was chosen to fabricate the MC-LEDs with cavity lengths of 113, 205 and 290 nm to maintain the overlap between the cavity mode and InGaN QW emission desired to maintain the emission wavelength and high LEE. After the aperture etching process, a two-step sidewall treatment of MC-LEDs was performed: dipping in BHF to remove residues followed by submerging in H3PO4 at room temperature to eliminate sidewall damages due to reactive ion etching. Additionally, all measurements were carried out at room temperature for spectra measurements. The main objective was to prove the feasibility and applicability of the ultra-thin MC-LEDs. The research work is currently published in the journal, Applied Physics Express.
The research team demonstrated the possibility of attaining more accurate active layer positioning in the cavity. The antinode factor suggested the efficiency of the QW and cavity modes, and it depended on the QW position corresponding to the antinode of cavity modes. For the MC-LEDs with a cavity length of 113 nm at QW positions 46%, 60% and 75%, the corresponding peak external quantum efficiency (EQE) were 0.6%, 2.5% and 0%, respectively. Thus, it was expected to have the highest LEE of 35% at the QW position of 75%, but no emission was observed due to the excess current leakage due to the device fabrication process. On the other hand, MC-LED with a cavity length of 290 nm recorded the highest peak EQE of 6.7%. Furthermore, the peak wavelength was nearly constant at 430 nm for the current density ranging from 289 – 1868 A cm-2.
In summary, the authors demonstrated the feasibility of ultra-thin MC-LEDs in the nitride material system. By achieving the shortest ever cavity length of 113 nm and the highest light extraction efficiency, MC-LEDs are undoubtedly a fair bet for extending the application of displays. The study findings provided insights into ensuring a robust fabrication process that protects the integrity of the resulting devices by eliminating or minimizing associated damages. In a statement to Advances in Engineering, first author, Dr. Back said that their study will advance further the development of LED displays with directionality, spectral purity, and the high light extraction efficiency (LEE) thanks to the ultra-short cavity. He also commented that the MC-LEDs can be game changer when the device size of LEDs gets smaller than the pattern size of patterned sapphire substrate (PSS) since the high LEE is still feasible for MC-LEDs without the PSS.
Back, J., Rienzi, V., Wong, M., Li, H., DenBaars, S., Weisbuch, C., & Nakamura, S. (2021). Blue semipolar InGaN microcavity light-emitting diode with varying cavity lengths from 113 to 290 nm. Applied Physics Express, 14(4), 042003.