Manuscript received December 11;
1996;
revised May 2;
1997. This work was supported by the U.S. Army under the microelectronics cooperative agreement under DAAL019 523 530. The authors are with the Department of Electrical Engineering and Joint Program for Advanced Electronic Materials;
University of Maryland at College Park;
College Park;
MD 20742 USA. Publisher Item Identifier S 0018-9480(97)06001-8;