Sputtering of Molecules From ZnS, CdS, and FeS 2
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SPUTTERING OF MOLECULES FROM ZnS, CdS, AND FeS 2 *
S. Nikzad**, W.F. Calaway, M.J. Pellin, C.E. Young, and D.M. Gruen, Materials Science, Chemistry, and Chemical Technology Divisions, Argonne National Laboratory, Argonne, IL 60439, and T.A. Tombrello, Division of Physics, Mathematics, and Astronomy, Caltech, Pasadena, CA 91125 It has been the general consensus in the sputtering community that molecules are sputtered from a clean metal surface in much smaller quantities than atoms. Some of the existing models further suggest that the sputter yield of molecules should be even lower from a compound than from a pure metal when the constituents of the molecule do not reside on neighboring sites in the solid. In our experiments, neutral species sputtered from single crystals of ZnS, CdS, and FeS 2 by a 3 keV Ar+ beam have been observed by laser photo-ionization followed by time-of-flight mass spectrometry. While the atomic metal (Fe, Zn, Cd) and S2 were the predominant species observed, substantial amounts of S, FeS, Zn 2 , ZnS, Cd 2 , and CdS were also detected. The experimental results demonstrate that molecules represent a larger fraction of the sputtered yield than was previously believed from secondary ion mass spectrometry experiments. In addition, our data suggest that the sputtered molecules are not necessarily formed from adjacent atoms in the solid. The applicability of various sputtering models will be discussed in the light of these results.
INTRODUCTION Although sputtering of atoms from metal targets has been an area of extensive research in the past three decades, the understanding of the sputtering mechanism for molecules is, at best, in its infancy. Because of competing processes of formation, dissociation, and ejection, the sputtering of molecules is a much more complicated process than the sputtering of atoms. With the exception of a few studies [1-4], sputtered molecules generally have been detected by secondary ion mass spectrometry (SIMS) [5] or optical emission of the excited state molecules [6,7]. Theoretical studies of the subject are also in the initial stages [8-12]. Models of the process can be divided into two major categories. In the 'Direct Emission' model, molecules are formed in the solid and are ejected via single Collision [8] or double Collisions (8,10]. In the 'Recombination' model, molecules are formed above the surface [9,13]. Molecular dynamics calculations of sputtered molecules have been performed primarily by Garrison and Winograd and support the recombination model [13,14]. With only few experimental results, it remains an open question as to whether molecules form as a result of simultaneous ejection of neighboring atoms from the solid or because of a molecular bond that exists in the solid before sputtering. This question can be best addressed by sputtering experiments using a binary target, where the constituents of the sputtered molecules may not occupy * work supported by the US Department of Energy, BES-Materials Science, under contract W-31-109-ENG-38 **Present Address:
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