Blue semipolar InGaN microcavity light-emitting diode with varying cavity lengths from 113 to 290 nm

Significance 

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.

Blue semipolar InGaN microcavity light-emitting diode with varying cavity lengths from 113 to 290 nm

About the author

Joonho Back received his Ph.D. in Electrical and Computer Engineering at the University of California, Santa Barbara in 2021 under the guidance of Professor Shuji Nakamura. He received his M.S. in Electrical and Computer Engineering at University of California, Santa Barbara in 2017 and B.S. in Electronic Engineering in Dongguk University, Seoul, Korea in 2013.

His research has focused on the micro-cavity light-emitting diodes (MC-LEDs), III-nitride based vertical-cavity surface-emitting lasers (VCSELs) for various visual light applications, such as near-eye displays, lighting, optical sensing, and data communications. He has designed and grew the III-nitride optoelectronic devices using the MOCVD. Also, he has developed the fabrication process to demonstrate the ultra-short MC-LEDs.

About the author

Mr. Vincent Rienzi is a graduate student at the University of California, Santa Barbara pursuing a Ph.D. in Materials under the supervision of Professor Shuji Nakamura. He received his B.S. in Materials Science and Engineering (MS&E) at Lehigh University in 2018, where he was awarded the 2018 MS&E’s Kahn Memorial Award and 2018 College of Engineering’s Bill Hardy award, which are awarded to the most outstanding senior in the MS&E department and College of Engineering, respectively. After being granted Lehigh’s Presidential Scholarship, he received his M.Eng. in MS&E with Professor Siddha Pimputkar as his advisor in 2019 while studying corrosion resistance of various metals as a suitable autoclave material for the bulk crystal growth of ammonothermal InN. His research interests are on the crystal growth, fabrication, and characterization of compound semiconductor materials and devices for novel functionality, improved performance, and sustainability.

His current research focuses on developing III-Nitride semiconductors spanning the spectral regime from UV to IR for a variety of solid state lighting devices, such as microLEDs, lasers, microcavity-LEDs, and single photon sources, for a multitude of applications, including optical quantum information processing, near-eye displays, disinfection, and visible light communication.

About the author

Matthew S. Wong received his Ph. D. in Materials at the University of California, Santa Barbara (UCSB) under the guidance of Professor Steven P. DenBaars. Currently, he is postdoctoral researcher in the Materials department at UCSB under the guidance of Professors Shuji Nakamura and Steven P. DenBaars. His main research focus has been on micro-light-emitting diodes and lasers.

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About the author

Dr. Hongjian Li received his bachelor’s degree from Wuhan University in 2009 and obtained his PhD degree from Institute of Semiconductor, Chinese Academy of Science in 2014. He works as project scientist/postdoctoral at SSLEEC, Materials Department, University of California, Santa Barbara. He focuses on the MOCVD growth of highly efficient InGaN-based micro-LEDs and laser diodes. He has filed more than 10 US patents and published over 35 peer-reviewed journal articles.

About the author

Dr. Steven P. DenBaars is a Professor of Materials and Electrical and Computer Engineering at the University of California Santa Barbara. Prof. DenBaars has been very active in entrepreneurship, having helped co-found several start-up companies in the field of photonics and electronics. His research focuses on growth and fabrication of compound semiconductor devices. He is a leading pioneer in the development of III-Nitride semiconductors for solid state lighting, micro-LED displays, UV photonics, power switching, and RF electronics. He received the IEEE Fellow award in 2005, member of the National Academy io Engineers 2012, and National Academy of Inventors in 2014. He has authored or co-authored over 980 technical publications, 350 conference presentations, and over 185 patents.

About the author

Claude Weisbuch is a semiconductor physicist. He is now an emeritus “Directeur de Recherche” at the Centre National de la Recherche Scientifique (CNRS) at Ecole Polytechnique, France and a distinguished professor in the materials department of the University of California at Santa Barbara, He has been at Bell aboratories Murray-Hill, USA  (1979-1981), then at Saint-Gobain and Thomson-CSF (now Thales), France. He was “directeur scientifique” (chief scientist) of Délégation Générale pour l’Armement (procurement), ministry of defense, France, from 1992 to 1998.

He founded in 2002 a high tech company, Genewave, Paris, devoted to fluorescence based molecular diagnostics systems, based on his work on light extraction in LEDs.

He has authored or co-authored more than 270 papers and 35 patents. He worked from 1993 on light extraction in LEDs through microcavities and photonic crystals, and since 2013 on fundamental processes in nitride LEDs.

About the author

Professor Shuji Nakamura is a Japanese-born American electronic engineer and inventor, specializing in the field of semiconductor technology. Professor Nakamura in Department of Materials and in the Electrical and Computer Engineering in the College of Engineering, University of California, Santa Barbara, also holds the Cree Endowed Chair in Solid State Lighting & Displays. Professor Nakamura is regarded as the inventor of the blue LED, a major breakthrough in lighting technology. He is a recipient of the 2014 Nobel Prize for Physics “for the invention of efficient blue light-emitting diodes, which has enabled bright and energy-saving white light sources.” He is a fellow of the U.S. National Academy of Engineering, National Academy of Inventors, and the National Inventors Hall of Fame. He is a recipient of numerous international honors including the 2006 Millennium Technology Prize, The Harvey Award (2009), 2014 Order of Culture Award in Japan, the 2015 Global Energy Prize, the Technology & Engineering Emmy Award (2012), the 2018 Zayed Future Energy Prize, the 2020 National Academy of Science (NAS) Award for the Industrial Application of Science, and the 2021 Queen Elizabeth Prize for Engineering.

Reference

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 nmApplied Physics Express, 14(4), 042003.

Go To Applied Physics Express

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