British Reseachers Create Simulations that More Accurately Predict Properties of GaN

Researchers from University College London (UCL) worked with teams at
Daresbury Laboratory and the University of Bath to reveal the complex
properties of gallium nitride using computer simulations. Accurate predictions
of these properties can help make better blue LEDs and predict their output
before actual fabrication.

LEDs employ two layers of semiconductors: a conduction layer with electrons
available and a layer with positive charges or holes. When an electron and a
hole meet, they emit a photon (light particle). A cristalline film of a
particular material-GaN for blue LEDs-is grown and then doped. Dopants donate
an extra positive or negative charge to the material.

GaN, the key material for blue LEDs, has a large energy gap between
electrons and holes (known as a wide bandgap). The wide bandgap is essential
for tuning the emitted photons to produce blue light. Doping to donate mobile
negative charges in the material proved to be easy. However, donating positive
charges from GaN failed completely. The innovation, which won the inventors of
the blue LED the Nobel Prize for physics last year, required doping GaN with
unexpectedly large amounts of magnesium.

“While blue LEDs have now been manufactured for over a decade,”
said John Buckeridge (UCL Chemistry), the study’s lead author, “there has
always been a gap in our understanding of how they actually work, and this is
where our study comes in. Based on what is seen in other 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 GaN, at least a
hundred atoms of magnesium have to be added. It’s technically extremely
difficult to manufacture GaN crystals with so much magnesium in them, not to
mention that it’s been frustrating for scientists not to understand what the
problem was.”

The team published details of their findings in the journal Physical Review
Letters. The team used highly sophisticated computer simulations to accurately
predict the unusual behavior of doped GaN at the atomic level. While a quantum
mechanical model can make accurate predictions about perfect crystals with
repeating patterns of atoms, such a model has difficulty dealing with defects
which do not fit the repeating pattern of atoms. The computer simulations for
accurate prediction of GaN crystals with some defects requires use of
supercomputers because of the large numbers of atoms and their interactions.

“To make an accurate simulation of a defect in a semiconductor such as
an impurity, we need the accuracy you get from a quantum mechanical
said David Scanlon (UCL Chemistry), a co-author of the article.

“Such models have been widely applied to the study of perfect crystals,
where a small group of atoms form a repeating 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.”

The team solved the the issue with an approach pioneered in another Nobel
Prize winning research: hybrid quantum and molecular modeling, the subject of
2013’s Nobel prize in Chemistry. The new models simulate different parts of a
complex chemical system with different levels of theory. Some previously
unexplained experimental results about the behavior of GaN now fit with the new

Richard Catlow (UCL Chemistry), one of the study’s co-authors said, “Our
simulation shows that the behavior of the semiconductor is much more complex
than previously imagined, and finally explains why we need so much magnesium to
make blue LEDs successfully.”

“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,”
Catlow said. “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.”

“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,”
Catlow added.

Aron Walsh of Bath Chemistry noted, that the team is looking forward to
using the new simulations to investigate the properties of heavily defective
GaN and help develop alternative doping strategies to improve the efficiency of
solid-state lighting.

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