Researchers at the University of Illinois Urbana-Champaign (UIUC) have successfully fabricated blue light-emitting diodes (LEDs) down to an unprecedented 250 nanometers (nm) in size, a critical step for nextgeneration technologies like ultra-high-resolution displays and advanced optical communication. However, their study, published in Applied Physics Letters, reveals a significant challenge: a sharp "efficiency cliff" when these LEDs are scaled to submicron dimensions.
The research team, led by Professor Can Bayram from the Department of Electrical and Computer Engineering and the Nick Holonyak, Jr. Micro and Nanotechnology Laboratory, employed a top-down fabrication approach using electron beam lithography on QST substrates. The resulting submicron LEDs, some as small as 250nm by 250nm, initially demonstrated promising electrical characteristics, including ideal forward voltage and remarkably low leakage current density. This suggested that the chemical treatments and passivation techniques used were effective in recovering sidewall damage from the etching process.
Despite these positive indicators, the study found a dramatic decrease— approximately 70%—in peak external quantum efficiency (EQE) as the LEDs were scaled from 2 micrometers (µm) down to 250 nm. This "efficiency cliff" is a surprising outcome, particularly as the EQE remained relatively stable for devices scaled from 20 µm down to 2 µm.
"Our findings show that the conventional sidewall passivation methods, which work well for micro-LEDs, are not sufficient when we push the dimensions into the submicron realm," said Professor Bayram, an Intel Alumni Endowed Faculty Scholar and director of the Innovative Compound semiconductoR LABoratory (ICORLAB). "The issue appears to be that as the LED mesa size becomes comparable to the distance carriers can diffuse laterally, the impact of the sidewalls and any associated non-radiative recombination becomes overwhelmingly dominant, even with current state-of-the-art passivation."
The research indicates that at these ultrasmall scales, the proportion of the LED's active region affected by sidewall defects significantly increases. While the team achieved good recovery of sidewall damage, evidenced by low leakage currents, the standard passivation (an atomic layer deposited bilayer of aluminum oxide and silicon dioxide) could not sufficiently suppress surface recombination in these submicron devices.
This study underscores a critical hurdle for the practical implementation of top-down fabricated submicron LEDs. While the successful fabrication of 250nm LEDs with good electricals is a promising starting point, overcoming the efficiency cliff is paramount. "These results call for a fundamental rethinking of how we manage sidewall effects in these tiny light emitters," Professor Bayram added. "Novel passivation strategies, potentially involving new materials or techniques to prevent lateral carrier diffusion, will be essential to unlock the full potential of submicron LEDs for future technologies."
The work was carried out in the Micro and Nanotechnology Laboratory and Frederick Seitz Materials Research Laboratory Central Research Facilities at the University of Illinois Urbana-Champaign. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0001558. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
About the Innovative Compound semiconductoR LABoratory (ICORLAB) at UIUC: The ICORLAB, led by Prof. Can Bayram, focuses on the research and development of advanced semiconductor materials and devices for applications in optoelectronics, electronics, and energy.
Contact: Professor Can Bayram Department of Electrical and Computer Engineering Nick Holonyak, Jr. Micro and Nanotechnology Laboratory University of Illinois at UrbanaChampaign Email: cbayram@illinois.edu Phone: +1 (217) 300-0978 Webpage: https://icorlab.ece.illinois.edu
J. Lee, Y. C. Chiu, and C. Bayram, “Efficiency Cliff in Scaling InGaN Light-emitting Diodes Down to Submicron,” Applied Physics Letters 126, 242101 (2025). https://doi.org/10.1063/5.0257758 DOI: 10.1063/5.0257758