Zheng,

I suspect you may be experiencing surface induced damage due to direct
diffusion into silicon.

If your process flow permits, try growing a layer of oxide(say 3000angs, wet)
before diffusing. Strip the oxide after that.

Deep diffusion is generally problematic, hence the above solution may not be a
perfect one.

Goodluck.

Robert S. Okojie
RSO6948@hertz.njit.edu