June 22 (Mon) @ 11:00am: "V-defect Engineered Green III-Nitride Light Emitting Diodes," Alejandro Quevedo, ECE PhD Defense

Location: Engineering Science Bldg (ESB), Room 1001

Abstract

Blue InGaN-based light emitting diodes (LEDs) are exceptionally efficient, with peak wall-plug efficiencies (WPEs) exceeding 80 %. This has enabled a revolution in solid-state lighting, with blue LEDs and their phosphor-coated white-light counterparts replacing their inefficient predecessors in households, automotives, projectors, and displays around the world. LEDs use 90 % less energy and last much longer than traditional incandescent bulbs and the decline of US residential electricity consumption seen in the 2010s has been attributed to their wide adoption in US households. More recently, they have enabled new technologies including visible light communication, quantum sensing, biomedical disinfection, and photonic integrated circuits.

InGaN-based LEDs rapidly decrease in efficiency for longer wavelength green – red light emitters, resulting in what is known as the “Green Gap”. Many factors go into the poor efficiency of long-wavelength LEDs but polarization-related effects are most significant. The group III-nitrides are polar materials; the polarization discontinuities between InGaN and other III-nitride cladding layers worsen with increasing In-content, and higher In-composition InGaN is required for long wavelength emission. One effect this has is increasing internal barriers to carrier injection within LED devices. This reduces device performance by increasing forward operating voltage and reducing WPE. This makes addressing the Green Gap an important area of research for expanding LED efficiencies and applications.

V-defects are morphological defects that are commonly observed during the growth of III-nitride LEDs. They are hexagonal pyramid-shaped depressions with six semipolar sidewalls. The sidewalls of V-defects are of interest because polarization-induced barriers to carrier injection are lower when injecting through the semipolar plane, leading to reduced forward voltages and higher WPE for long wavelength LEDs. This has led to significant research around V-defects over the last decade and wide adoption of V-defect engineering by the LED industry.

In this work, we provide a novel approach to V-defect formation, size, and density control in LEDs grown by metal-organic vapor deposition (MOCVD). We investigate V-defect propagation through the active region of devices and how to maintain the size of these defects after their opening. We provide an analysis of carrier transport around V-defects to address concerns around carrier leakage through the threading dislocation which nucleates each V-defect. Through active region engineering of V-defect LEDs, we achieve a peak external quantum efficiency (EQE) of 44 % and a peak WPE of 38 %. These results help provide a pathway to resolving the Green Gap through V-defect and active region engineering. This work advances scientific understanding of V-defect formation and propagation during III-nitride epitaxy, and how they may be better implemented in long wavelength LEDs.

Alejandro Quevedo Is a Ph.D. candidate in the Electrical and Computer Engineering Department at UC Santa Barbara, advised by Prof. Steven DenBaars and Prof. James Speck. He received a B.S. in Physics and Mathematical Sciences from UC Santa Barbara in 2019. His research focuses on high efficiency III-nitride green, yellow, and red micro-LEDs.

Hosted By: ECE Professor Steven DenBaars

Submitted By: <alejandromquevedo@ucsb.edu>