The now ubiquitous LED has been used in electrical components since 1962, and can be found everywhere from car headlights to barcode scanners. Blue LEDs are required to create white light, which makes LEDs suitable for smartphone and computer screens and bright, highly efficient, long-lasting light bulbs. But it wasn’t until 1994 that the first blue LEDs were made, earning the three inventors the 2014 Nobel Prize in Physics.
So what is it that makes blue LEDs so difficult to manufacture?
Using computer simulations, scientists at UCL, in collaboration with the University of Bath and Daresbury Laboratory, looked at the properties of the main component in blue LEDs, gallium nitride, to better understand just why blue LEDs are so hard to make.
LEDs are made of two layers of semiconducting materials – one with negative charges, or electrons, available for conduction, and the other positive charges, or holes. When a voltage is applied, an electron and a hole meet at the junction between the two and a particle of light (photon) is emitted.
‘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. Naively, based on what is seen in other common semiconductors such as silicon, you would expect each magnesium atom added to the crystal to donate one hole. But in fact, to donate a single mobile hole in gallium nitride, at least a hundred atoms of magnesium have to be added. It’s technically extremely difficult to manufacture gallium nitride crystals with so much magnesium in them, not to mention that it’s been frustrating for scientists not to understand what the problem was.’ Said lead author John Buckeridge.
Co-author Richard Catlow, also of UCL, explains, ‘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. In fact, to provide enough energy to release the charge will require heating the material beyond its melting point. Even if it were released, it would knock an atom of nitrogen out of the crystal, and get trapped anyway in the resulting vacancy. 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’.
You can view the paper here.
For all things blue, take a look at this blog on colour.
By Simon Frost