AVS 54th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP40
Effect of Water Vapor on Monolayer Features Produced by Pulsed 248-nm Irradiation of Cleaved Alkali Halide Surfaces1

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Session: Surface Science Poster Session
Presenter: K. Kimura, Washington State University
Authors: K. Kimura, Washington State University
S.C. Langford, Washington State University
J.T. Dickinson, Washington State University
Correspondent: Click to Email

Low doses of energetic electrons can produce monolayer etch pits on alkali halide cleavage surfaces.2 Similarly, UV laser radiation can roughen steps and produce monolayer islands and pits on NaCl at fluences well below the macroscopic damage threshold.3 Step edges on NaCl are highly vulnerable to erosion due to high densities of kinks. In NaCl, step erosion is dramatically accelerated in the presence of 10-5 Pa water vapor.4 In this work, we compare atomic force microscopy (AFM) images of NaCl, KCl, and KBr cleavage surfaces exposed to pulsed 248-nm laser radiation in vacuum. Since atmospheric water vapor affects all these surfaces, AFM was performed under dry nitrogen. Prior to laser exposure, the principal features are straight, monatomic cleavage steps. Fifty 248-nm pulses at 100 mJ/cm2 are sufficient to produce monolayer islands and to roughen cleavage steps in ultrahigh vacuum on all three materials. Island-free zones are observed on both the upper and lower terraces along cleavage steps. The island densities and the width of the island free-zones vary significantly with laser fluence. Islands appear to be formed from material eroded from the steps. Similar laser exposures in the presence of 10-5 Pa water vapor produce monolayer etch pits, rather than islands, on KBr and KCl. Higher laser fluences are required to produce pits on NaCl. Calibrated quadrupole mass spectrometry on the products emitted during laser irradiation shows emission intensities consistent with that required to form the etch pits. In the presence of 10-5 Pa water vapor, the detected intensities are typically several times higher than in ultrahigh vacuum. Islands and etch pits are also observed on surfaces exposed to low doses of 2 keV electrons; again, these features are dramatically affected by the presence of water vapor. Although mechanism for the effect of water vapor is not clear, the dissociative adsorption of water at surface halogen vacancies is expected to play a role.

1This work was supported by the U.S. Department of Energy-under Grant DE-FG02-04ER-15618.
2B. Such, P. Czuba, P. Piatkowski, and M. Szymonski, Surf. Sci. 451(1/3), 203-207 (2000)
3K. H. Nwe, S. C. Langford, and J. T. Dickinson, J. Appl. Phys. 97, 043501 (2005).
4K. H. Nwe, S. C. Langford, and J. T. Dickinson, J. Appl. Phys. 97, 043502 (2005).