1. Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, College of Engineering, Seoul National University 1 , Gwanak-ro 1, Gwanak-gu, Seoul 08826, South Korea
2. Sandia National Laboratories 2 , Livermore, California 94550, USA
3. Department of Electrical and Computer Engineering, Texas A&M University 3 , College Station, Texas 77843, USA