Submitted on: 02 January 2008.
Revised on: 08 March 2008.
Accepted on: 18 April 2008.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
Submitted on: 12 December 2005.
Revised on: 09 January 2006.
Accepted on: 27 January 2006.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
http://reply.papertrans.cn/1/69/6847/6847-33.png
http://reply.papertrans.cn/1/69/6847/6847-34.png
Submitted on: 09 September 2002.
Revised on: 06 October 2002.
Accepted on: 19 October 2002.
___________________CURRENT APPLIED PHYSICS---ELSEVIER
Submitted on: 04 October 2005.
Revised on: 31 January 2006.
Accepted on: 25 March 2006.
___________________CURRENT APPLIED PHYSICS---ELSEVIER