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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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 Osramï¿½s Andreas Lï¿½ffler and Michael Binder.
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.