1. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China
2. Wuxi Suntech Power Co., Ltd., Wuxi, Jiangsu 214208, China
3. Laboratoire de Physique des Interfaces et des Couches Minces, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, 91128 Palaiseau, France
4. Optics Valley Laboratory, Wuhan, Hubei 430074, China