SOLID STATE COMMUNICATIONS, vol. 88, no. 5, page. 361 - 363, 1993-11
APPLIED PHYSICS LETTERS, vol. 61, no. 13, page. 1516 - 1518, 1992-09-28
JOURNAL OF THE KOREAN PHYSICAL SOCIETY, vol. 26, no. 2, page. 137 - 141, 1993-04