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Why blue LEDS are difficult to make
Saturday January 10, 2015 9:34 AM, IANS

Using sophisticated computer simulations, British scientists have uncovered the mystery of why blue light-emitting diodes (LEDs) are so tricky to make.

They have revealed the complex properties of their main component - gallium nitride.

"While blue LEDs have now been manufactured for over a decade, there has always been a gap in our understanding of how they actually work and this is where our study comes in," said lead author of the study John Buckeridge from University College London.

Blue LEDs were first commercialised two decades ago and have been instrumental in the development of new forms of energy saving lighting, earning their inventors the 2014 Nobel Prize in Physics.

The breakthrough required doping it with surprisingly large amounts of magnesium.

The key ingredient for blue LEDs is gallium nitride, a robust material with a large energy separation or "gap" between electrons and holes.

This gap is crucial in tuning the energy of the emitted photons to produce blue light.

But while doping to donate mobile negative charges in the substance proved to be easy, donating positive charges failed completely.

The breakthrough, which won the Nobel Prize, required doping it with surprisingly large amounts of magnesium.

"The simulation tells us that when you add a magnesium atom, it replaces a gallium atom but does not donate the positive charge to the material, instead keeping it to itself," explained co-study author Richard Catlow from UCL.

"Our simulation shows that the behaviour of the semiconductor is much more complex than previously imagined, and finally explains why we need so much magnesium to make blue LEDs successfully," Catlow added.

The study was published in the journal Physical Review Letters.


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