**Acknowledgements**

This work was carried out at the University of Hong Kong, City University of Hong Kong and Pusan National University. The authors would like to thank H.W. Choi for the opportunity to work on laser micromachining for GaN LEDs. This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023418).
