Oxidation of GaN for light conversion: ab initio thermodynamic modelling

1. Oxidation of GaN for light conversion: ab initio thermodynamic modelling Adam J. Jackson, Duncan Allsopp, Aron Walsh Doctoral Training Centre, Centre for Sustainable Chemical Technologies, University of Bath, BA2 7AY, UK. E-mail: a.j.jackson@bath.ac.uk: URL: http://www.bath.ac.uk/csct Benefits of improved LEDs Reduced energy consumption is widely seen as an essential goal to prevent a global environmental and energy crisis. Lighting makes up 19% of global energy consumption, and it is anticipated that LEDs will be able to offer around a 90% energy reduction compared with conventional incandescent lighting. It is estimated that over 2 billion people do not have access to affordable, efficient energy*. Solar-powered LED lanterns are currently being trialled in the developing world, boosting health and education as well as reducing dependence on kerosene. Background – GaN in LEDs GaN is a wide-bandgap semiconductor, notably used in high-efficiency bright blue LEDs. By doping and alloying with other materials, the semiconducting properties can be tuned for specific applications, using In for blue LEDs, while Al can be added to to push emitted light into the UV spectrum. Currently white LEDs are produced by adding a phosphor coating, which shifts and spreads the frequency range of emitted light. This comes at an efficiency cost, but opens a wider range of applications. *Goldemberget al (2004) Photo: RouzbehDelavarihttp://www.flickr.com/photos/rouzwawi/2667783454/ • Problems • GaN is grown on expensive sapphire substrate • Oxygen contamination during vapour deposition • Indium for alloying is rare, valuable and in-demand • Phosphor coating is inefficient U.S. Dept. of Energy (2010) except for *, based on advertised products on Amazon.com • Opportunities • Oxidation is a predictable, controllable reaction • Oxidation causes changes in semiconductor behaviour • Recent work shows that: • Oxide substrates are possible* • Oxide semiconducting properties are related** *Choi et al(2011) **Walsh et al (2011) • Methods • Ab-initio, all-electron DFT with GGA (PBEsol) as implemented by FHI-AIMS package • Periodic boundary conditions used to study bulk solid and surfaces using slab model • Thermodynamic modelling used to predict relationships between extent of oxidation, reaction conditions and electronic properties REFERENCES Choi, J. H. et al. (2011); “Nearly single-crystalline GaN light-emitting diodes on amorphous glass substrates”; N. Photon. 5, 763-769 Goldemberg, J., and Johansson, T.B. (2004). “World Energy Assessment: Overview, 2004 Update”; United Nations Development Program, United Nations Department of World Energy Council, New York U.S. Dept. of Energy, (2010). “Energy Savings Potential of Solid-State Lighting in General Illumination Applications 2010 to 2030” Walsh, A., Catlow, C., Miskufova, M. and Sokol, A. A. (2011). “Electron and hole stability in GaN and ZnO”; J. Phys.: Condens. Matter 23, 33217