1. Key Laboratory of Physical Electronics and Devices of Ministry of Education and Shaanxi Provincial Key Laboratory of Photonics & Information Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, People's Republic of China
2. Solid-State Lighting Engineering Research Center, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, People's Republic of China
3. Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool L69 3GJ, United Kingdom