Winegard Visiting Lectureship 2012
Sir Colin Humphreys, CBE, FRS, FREng
Professor & Director of Research
Department of Materials Science & Metallurgy
University of Cambridge
How Materials Science Can Help Solve Some Major World Problems
Thursday, March 22, 2012 | 1:00 to 2:00pm | BA 1130
It is widely believed that three of the major problems facing our world are shortage of energy, lack of drinkable water and global warming. This talk will assess the situation and then show how a closely related family of new materials can help to solve all three problems.
For example, if 40% of the world’s lighting was replaced by gallium nitride-based LEDs, this would enable over 600 power stations to be closed, saving substantial energy and carbon emissions. By adding aluminium to the gallium nitride, devices can be tailored to emit deep-ultraviolet light with a wavelength which damages the nucleic acid in bacteria and viruses, stopping them from reproducing and effectively killing them. This promises to be a low-cost way to purify water in the developing and developed world, saving millions of lives.
Finally the talk will indicate how some of the latest advances in materials science can improve our health.
Why are GaN LEDs So Bright when the Dislocation Density is So High?
Friday, March 23, 2012 | Noon to 1:00pm | WB 116
It has been said that GaN is the most important new semiconductor material since silicon. However, for the last fifteen years there has been a major controversy over why the InGaN quantum wells in GaN LEDs emit blue and green light with high efficiency even though the dislocation density is very high, typically 109 cm-2. Initially it was believed that this was because the InGaN decomposed into nanometer-size indium-rich clusters during the MOCVD growth. These clusters were believed to localise the carriers, thus preventing diffusion to dislocations. Such clusters were observed by electron microscopy. However it has been shown that electron beam damage could rapidly produce In-rich clusters, and that such clusters were not visible in low-dose electron microscopy. In addition, 3-D atom probe tomography has not revealed any In-rich clusters, and the In distribution is consistent with InGaN being a random alloy.
However, if In-rich clusters do not exist in the InGaN quantum wells what mechanism is localising the carriers? The answer is fascinating. High resolution electron microscopy and atom probe tomography have been used to reveal the detailed atomic structure of InGaN quantum wells. This experimentally observed structure has then been used as the input data for quantum mechanical modelling, which has revealed significant new insights into the light-emitting properties of GaN-based LEDs.