1. School of Electrical Engineering , Korea Advanced Institute of Science and Technology (KAIST) , 291 Daehak-ro, Yuseong-gu , Daejeon 34141 , Republic of Korea
2. Department of Electrical Engineering and Computer Sciences , University of California , Berkeley , CA 94720 , USA
3. Department of Nanodevice Technology , National Nanofab Center (NNFC) , 291 Daehak-ro, Yuseong-gu , Daejeon 34141 , Republic of Korea