1. Centre for Device Thermography and Reliability, H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, U.K.
2. Centre for Diamond Science and Technology, University of Warwick, Coventry CV4 7AL, U.K.
3. Diamond Foundry, School of Physics and Astronomy, University of Cardiff, Cardiff CF24 3AA, U.K.
4. James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, U.K.
5. Cambridge Centre for Gallium Nitride, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, U.K.
6. Department of Physics, SUPA, University of Strathclyde, Glasgow G1 1XQ, U.K.