Transcript Oxidation of GaN for light conversion: ab initio thermodynamic modelling Adam J.

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: [email protected]: URL: http://www.bath.ac.uk/csct
Background – GaN in LEDs
Benefits of improved 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.
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*. Solarpowered LED lanterns are currently being trialled in the developing
world, boosting health and education as well as reducing
dependence on kerosene.
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.
Photo: Rouzbeh Delavari http://www.flickr.com/photos/rouzwawi/2667783454/
Problems
• GaN is grown on expensive sapphire substrate
*Goldemberg et al (2004)
lm/W
Incandescent
Halogen
Compact
Fluorescent
Mercury Vapour
13
15
55
Operating
time (hrs)
1,200
2,500
10,000
Typical price ($)
38
24,000
15.00
Home LED bulb*
65
>25,000
15.00
Experimental LEDs
100+
???
???
0.60
3.00
3.00
• 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