August 5, 2015: Scientists at UCL, in collaboration with teams at the University of Bath and Daresbury Laboratory, have revealed why blue LEDs are so difficult to manufacture. Using sophisticated computer simulations, they discovered that the main culprit are the complex properties of blue LEDs? main component?gallium nitride.
Although blue LEDs were commercialised about two decades ago, they came into the limelight when the inventors of the development of new forms of energy saving lighting with the help of blue LEDs were honored with the 2014 Nobel Prize in Physics.
Two layers of semiconducting materials make an LED. One of these layers has negative charges or electrons, and the other layer has positive charges or holes. When a voltage passes through, an electron and a hole meet at a junction, and light particle or photon is emitted.
The main component of blue LEDs is gallium nitride, which has a large energy gap between electrons and holes. This gap is important in tuning the energy of the photons which emit the blue light. While doping to donate negative charges in the substance is easy, donating positive charges is difficult. The invention that won the Nobel Prize of 2014 made doping possible with magnesium.
John Buckeridge of UCL Chemistry, and the lead author of the study, said that the industry always failed to understand how blue LEDs work, and hence this study was conducted. ?To donate a single mobile hole in gallium nitride, at least a hundred atoms of magnesium need to be added. Technically it is very difficult to manufacture gallium nitride crystals with so much magnesium in them,? said John Buckeridge.
The study that has been published in Physical Review Letters, reveals the root cause of the problem. The study examined the behaviour of the doped gallium nitride at the atomic level with the help of sophisticated computer simulations.
?To simulate a defect or an impurity in the semiconductor material, the team needed the accuracy that comes from a quantum mechanical model,? explains David Scanlon (UCL Chemistry), a co-author of the paper. ?Such models are usually applied to study crystals, where a small group of atoms form a pattern. Introducing a defect that breaks the pattern presents a conundrum, which required the UK?s largest supercomputer to solve. Indeed, calculations on very large numbers of atoms were therefore necessary but would be prohibitively expensive to treat the system on a purely quantum-mechanical level,? he added.
The team applied hybrid quantum and molecular modeling to find the problem. ?The simulation revealed that when a magnesium atom is added, it replaces a gallium atom but does not donate the positive charge to the material,? said Richard Catlow (UCL Chemistry), one of the study?s co-authors. ?So, to provide a lot of energy to release the charge 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,? he added. The simulations explained the unique behaviour of gallium nitride.
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