Blue LEDs use two layers of semiconducting materials (insulating materials which can be made conduct electricity in special circumstances). One has mobile negative charges, or electrons, available for conduction, and the other positive charges, or holes. When a voltage is applied, an electron and a hole can meet at the junction between the two, and a photon (light particle) is emitted.
The desired properties of a semiconductor layer are achieved by growing a crystalline film of a particular material and adding small quantities of an 'impurity' element, which has more or fewer electrons taking part in the chemical bonding (a process known as 'doping'). Depending on the number of electrons, these impurities donate an extra positive or negative mobile charge to the material.
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 for the in ventors of blue LEDs, required doping it with large amounts of magnesium.
"While blue LEDs have now been manufactured for over a decade," explained John Buckeridge (UCL Chemistry), lead author of the study, "there has always been a gap in our understanding of how they actually work, and this is where our study comes in. Naïvely, 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