Aug 20, 2010
Nitrides at the Nanoscale
Light-emitting diodes (LEDs) based on gallium nitride (GaN) have already achieved enormous commercial success, and have the potential to revolutionise home and office lighting, providing light sources which are at least five times more efficient than conventional light bulbs. This could result in significant reductions in energy usage and greenhouse gas emissions, since, surprisingly, 20% of the electricity generated in the UK is used for lighting.
However, despite the increasing ubiquity of these devices, their operation remains poorly understood. Compared to more common semiconductors, such as gallium arsenide (GaAs), nitride materials are simply riddled with defects, with dislocation densities as high as 109 cm-2 being common in commercial LEDs. Dislocation densities a million times lower would be expected to cause rapid device failure in GaAs-based LEDs due to non-radiative carrier recombination at dislocation sites, but blue-emitting nitride devices are long-lived and highly efficient. Only by understanding GaN-based devices' apparent immunity to vast defect densities can we hope to improve device efficiencies still further, and to tackle the many challenges which remain before LED lighting becomes a widespread reality.
The active region of a blue-emitting LED typically consists of several indium gallium nitride (InGaN) quantum wells. Some nanoscale features of the quantum wells are believed to prevent carrier diffusion in the plane of the quantum well, preventing carriers from reaching dislocation cores, thus avoiding non-radiative recombination. Fluctuations in the indium content in the quantum well have often been suggested as the origin of this carrier localisation. However, atom probe tomography (APT) of quantum well structures has provided a three-dimensional compositional map of blue-emitting quantum wells with nanometre resolution, revealing that InGaN in brightly-luminescent devices is a random alloy, free from significant indium content fluctuations.
The hunt is now on for the real reason why nitride LEDs work so well, and evidence from a range of techniques suggests several possibilities, including localisation of holes at randomly formed indium-nitrogen-indium chains, and localisation of electrons at slight fluctuations in quantum well thickness. By combining data from multiple microscopy techniques to give a complete understanding of the structure and properties of these amazing materials, we can foresee an ever-brighter future for nitrides.
The researchers presented their work in Journal of Physics D: Applied Physics.
About the author
Dr Rachel Oliver is a Royal Society University Research Fellow at the University of Cambridge. Her collaborative research with the OPAL EPSRC National Atom Probe Facility at the University of Oxford resulted in the world's first atom probe tomography images of a nitride semiconductor. Her current research focuses on the nanoscale structure of nitride materials, with the dual aims of understanding and optimising current devices and of developing new nanostructures and thus new device concepts.