1. Surface and Microanalysis Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-8371, Micron Technology, Inc., 8000 S. Federal Way, P.O. Box 6, MS 1-717, Boise, Idaho 83707-0006, and SVTC Technologies, 2706 Montopolis Drive, Austin, Texas 78741