[1] Nanyang Technol Univ, Sch Mat Sci & Engn, Singapore 639798, Singapore
[2] Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore
[3] Nanyang Technol Univ, Sch Mech & Aerosp Engn, Singapore 639798, Singapore
[4] Univ Texas Austin, Microelect Res Ctr, Austin, TX 78712 USA