Going Green with cubic GaN
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Mar 10, 2014
Conventional green LEDs are plagued by strong internal electric fields, which impair recombination and make it difficult to address droop. The solution is switch to growth on the cubic phase of GaN, which is free from internal fields and has a smaller bandgap, making it easier to reach longer wavelengths. By MARK DURNIAK and CHRISTIAN WETZEL from RENSSELAER POLYTECHNIC INSTITUTE
Increasing ultraviolet efficiency with nano-patterned sapphire
Mar 05, 2014
Switching from conventional patterning of sapphire to a nano-scale variant trims epitaxial growth times and materials costs while boosting extraction efficiency. By JIANCHANG YAN, PENG DONG, AND JUNXI WANG from the INSTITUTE OF SEMICONDUCTORS AT THE CHINESE ACADEMY OF SCIENCES.
The evolving GaN VCSEL
Mar 03, 2014
Building a GaN VCSEL is far harder than making one from GaAs, but progress is being made through the introduction of different types of mirrors, alternative current injection schemes and new crystal orientations. BY DANIEL FEEZELL FROM THE UNIVERSITY OF NEW MEXICO.
Bringing the inverter onto the chip
Feb 24, 2014
Monolithic integration of LEDs and transistors is helping to usher in a new era for solid-state luminaires, where the emitter and its control electronics are united on single chip. By Zhongda Li and co-workers from The Smart Lighting Engineering Research Centre at Rensselaer Polytechnic Institute.
Addressing the weakness of GaN transistors
Feb 17, 2014
Researchers reveal how to slash dynamic resistance, minimise interface traps and identify the origin of current collapse. BY RICHARD STEVENSON
Turbocharging channels with compounds
Feb 11, 2014
To maintain the improvements in efficiency that traditionally result from shrinking transistor dimensions, foundries will soon have to replace silicon channels with those based on higher mobility semiconductors. This move, a monumental upheaval for the silicon industry, could be easiest to implement by turning to engineered wafers with separate layers for the p-type and n-type transistors, argues Lukas Czornomaz from the Advanced Functional Materials Group at IBM Zurich.
Uniting the strengths of LEDs and lasers
Jan 09, 2014
Optical designers are keen to combine the high-intensity, directional output of a laser with the low cost and broad spectral output of an LED. These wishes can now be fulfilled, due to the development of a novel LED featuring a parabolic mirror, claims Bill Henry from InfiniLED.
Photoluminescence pinpoints Auger as the cause of LED droop
Jan 02, 2014
The debate on droop has been enriched by compelling evidence that Auger is to blame. Strong support for this includes our novel experimental data that shows that carriers produced in a green quantum well interact via Auger processes to generate higher energy carriers, which populate and recombine in an ultraviolet well, explain Michael Binder, Bastian Galler, Roland Zeisel and Anna Nirschl from Osram Opto Semiconductors.
LED droop: Overwhelming evidence for Auger
Dec 20, 2013
At the heart of the debate over the origin of droop is the question: Auger or not Auger? Circumstantial evidence for Auger has been mounting, and now this is joined by a ‘smoking gun’, the observation of hot Auger electrons in electro-emission. Detailing their novel experiment and its interpretation are a UCSB-Ecole Polytechnique partnership involving Claude Weisbuch, James Speck, Justin Iveland, Marco Piccardo, Lucio Martinelli and Jacques Peretti.
Solid-state lighting: Are laser diodes the logical successors to LEDs?
Dec 18, 2013
It is expensive to manufacture a GaN laser, and its peak efficiency is not that impressive. So why is this device, rather than the LED, being touted as the future of solid-state lighting? RICHARD STEVENSON investigates.
Wafer bonding creates record-breaking four-junction cell
Dec 17, 2013
To prevent the formation of efficiency sapping defects, conventional multi-junction cells are built with lattice-matched materials. But this restriction can be lifted with wafer-bonding, which enables the fabrication of a four-junction cell with record-breaking efficiency, say Rainer Krause and Bruno Ghyselen from Soitec and Frank Dimroth from the Fraunhofer Institute for Solar Energy Systems.
Maintaining Moore’s Law: The role of III-Vs as a logical successor
Dec 11, 2013
If reductions in the dimensions of the transistor are going to go hand-in-hand with increases in its performance and a trimming of its power consumption, silicon channels will have to be replaced with higher mobility materials, such as III-Vs. But will this happen, and if so, when? What has to be done to usher in these new materials? And if III-Vs are to make an impact in microprocessors, will they be there to stay? Richard Stevenson puts these questions, plus several more, to analyst Dean Freeman from Gartner Research.
Cuprous halides: The key ingredient for cheap, ultra-efficient LEDs?
Nov 26, 2013
To propel widespread uptake of solid-state lighting, LEDs must be cheaper and more efficient. One way to do that is to switch the material used to make these devices from nitrides to cuprous halides, which have incredibly high exciton binding energies and can be grown on silicon substrates, argue Doyeol Ahn from the University of Seoul, Korea, and Seoung-Hwan Park from Catholic University of Daegu, Korea.
Isn’t it time to get serious about standards?
Nov 18, 2013
Spec sheets can indicate identical compositions of a layer in a particular structure produced by different suppliers. But in practice there will be variations associated with in-house measurements, calibration samples, and data interpretation. Far greater consistency is possible, however, with an expanding portfolio of true reference samples that are already available, argues Kris Bertness from NIST.
Diminishing droop with superior electron-blocking layers
Nov 15, 2013
It is very tricky to come up with a watertight explanation for the cause of droop. However, it is certainly possible to combat this mysterious malady, which causes LED efficiency to decrease at high current densities, by: turning to better electron-blocking layers made from InAlN; and improving the injection of holes into the wells, plus their distribution throughout the active region, argues to Jae-Hyun Ryou from the University of Houston, Russell Dupuis and P. Douglas Yoder from Georgia Institute of Technology and Fernando Ponce from Arizona State University.
Building brighter and cheaper UV LEDs
Nov 06, 2013
It is far more challenging to make a bright, cheap ultra-violet LED than one emitting in the blue. But success is promised with a transparent contact layer, reflective electrodes, photonic structures and growth on silicon, says Hideki Hirayama from RIKEN.
Pursuing pathways to widespread adoption of solid-state lighting
Oct 28, 2013
The solid-state lighting revolution will be spurred by plummeting LED costs and improvements to the quality of emitted light. Success on these fronts could be aided by refinements to existing technologies; the introduction of GaN-on-silicon LEDs; a move to colour mixing of red, green and blue LEDs; and a switch from LEDs to lasers at the primary lighting source. All options were discussed at the International Conference on Nitride Semiconductors. Richard Stevenson reports
Cranking up the efficacy of Green LEDs
Oct 21, 2013
Green LEDs don't deliver the same level of performance as their red and blue cousins. However, by decreasing the current density with a larger chip and optimising growth conditions to reduce dark spots it is possible to close that gap with LEDs that hit 190 lumens per watt at a 100 mA drive current, says Osram’s Andreas Löffler and Michael Binder.
Slashing defect densities in nitride films grown by high temperature ammonia MBE
Oct 17, 2013
Grown in an MOCVD reactor, nitride films tend to yield fewer defects than when they are formed in MBE chambers. But this gap in material quality can disappear with high-temperature ammonia MBE, which produces epitaxial structures with outstanding electrical characteristics, argues Alexey Alexeev and Stanislav Petrov from SemiTEq.
An elemental change to laser design
Oct 07, 2013
Today's telecom lasers are plagued with Auger-related losses, which drive down efficiency and make device cooling mandatory. The solution: Switch to an active region with alloys featuring a bismuth content of more than 10 percent, claim members of the European team BIANCHO.
Displaying 1 - 20 of 1614