1. Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, and SKC Co., Ltd., #460 Chonhung-ri, Songgo-ub, Chonan-city, Chungchongnam-do 330-836, Korea, Optoelectronic Materials Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Korea, and Advanced Energy Technologies Department, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720-8168