AVS 56th International Symposium & Exhibition
    Electronic Materials and Processing Thursday Sessions
       Session EM-ThA

Invited Paper EM-ThA6
Growth and Process Technologies for High Efficiency InGaN-Based Light-Emitting Diodes

Thursday, November 12, 2009, 3:40 pm, Room B1

Session: Quantum Structures and Nitrides Devices
Presenter: J.-I. Chyi, National Central University, Taiwan
Authors: J.-I. Chyi, National Central University, Taiwan
H.-C. Lin, National Central University, Taiwan
Correspondent: Click to Email
Raising external quantum efficiency has always been the focused area of research in InGaN blue/green light-emitting diodes (LEDs). In the early 90s, breakthroughs in material growth and p-type doping restored the heat of pursuing high brightness InGaN LEDs. It was, however, several other key technology advancements that made solid-state lighting era a reality. Among these technologies, patterning sapphire substrate and semiconductor surface is one of the most essential one. In this report, we present three different patterning techniques, which emphasize different aspects of patterning technology and show how they impact the external quantum efficiency as well as other characteristics of InGaN/GaN quantum well light-emitting diodes. We report the epitaxial growth of GaN on patterned sapphire substrates (PSSs) with micro-lens of three different geometric shapes by metal-organic chemical vapor deposition. Growth mode analysis shows that micro-lens with sharp tips prohibit the nucleation and growth of GaN on their top and lead to a wider lateral growth region with low dislocation density. The external quantum efficiency (at 20 mA) of LEDs is improved by nearly 20% by using this technique. However, the residual strain instead of dislocation density in the GaN buffer layer plays a heavier role in the external quantum efficiency of the light-emitting diodes. Although the PSS approach has been shown effective and gives the freedom of pattern design, it requires extra photolithographic steps that make it more complicated and costly. We propose a maskless wet-etching method to prepare patterned sapphire substrates, namely naturally etched sapphire substrates (NESSs). At 20 mA, nearly 20% improvement in external quantum efficiency is achieved even the LEDs have already an indium-tin oxide transparent contact layer and a roughened surface. It has also been shown that the uniformity of device performance across the wafer is not a concern when using this technique. The surface patterning techniques reported so far involve spatially with the active region of LEDs and make the patterning process very critical in maintaining device performance as well as reliability. To avoid this problem, we propose to pattern the dicing streets around the LED chips where no electrical contacts are present. Forming a triangular lattice consisting of dry-etched circular holes with a diameter/periodicity of 3/3 μm on the dicing street. The external quantum efficiency (at 20 mA) of the LEDs is increased by about 13%. Meanwhile, the forward voltage increases only 0.05 V and improved reliability is observed as expected. In addition, a novel growth technique for improving internal quantum efficiency will also be presented. During the growth of quantum wells, a growth interruption is introduced at each InGaN to GaN interface while having trimethylindium (TMIn) and NH3 continue flowing into the reactor. Photoluminescence, X-ray diffraction, atom force microscopy, and high-resolution transmission electron microscopy indicate that the treatment leads to a smoother InGaN surface and InGaN/GaN interface with substantial decrease in V-shape defects density, compared to the samples without the treatment. The external quantum efficiency of 525 nm green LEDs prepared by this process is increased by as much as 43%.