AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA8
An Investigation of the Surface Reaction Mechanisms of Alternating-Grown Ordered Layers: CdS on ZnSe(100)

Tuesday, November 3, 1998, 4:20 pm, Room 308

Session: Semiconductor Surface Chemistry
Presenter: M. Han, Columbia University
Authors: M. Han, Columbia University
Y. Luo, Columbia University
J.E. Moryl, Columbia University
R.M. Osgood, Jr., Columbia University
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In previous experiments we have found that ordered layers of CdS on ZnSe(100) substrate were formed by a binary surface reaction sequence using gas-phase dosing of the substrate with (CH@sub 3@)@sub 2@Cd and with H@sub 2@S. This system provides a prototypical example of the interplay between reaction and desorption in such a binary sequence. In this talk we report a careful investigation of the surface chemistry using TPD. The experiments were performed in a UHV chamber, which is also equipped with AES, LEIS and LEED. Analysis of the TPD spectra show clearly that adsorbed DMCd irreversibly dissociates on ZnSe(100)-c(2x2) surface, and that the previously reported self-limiting reaction results from a methyl-termination of the surface. At ~ 370K, DMZn desorbs from this surface due to a methyl exchange reaction. This desorption temperature is independent of coverage, indicating a first-order reaction. In addition, at high DMCd exposures, the adsorption and desorption process leads to replacement of surface Zn by Cd. The experiments have also examined the reaction of the methyl-terminated surface with H@sub 2@S. This reaction is also self-limiting, yielding desorbed CH@sub 4@ and forming a sulfur-hydride-terminated surface. Studies of surfaces formed by more than one binary reaction sequence showed that the alternating growth surfaces are also terminated with either methyl group or sulfur-hydride. The methyl-passivated surface above the first layer preferentially desorbs methyl radicals at ~390K instead of metal-alkyl species. For the sulfur-hydride-terminated surface the recombinative reaction of HS species causes desorption of H@sub2@S at 480K. In this case, the symmetric peak shape and its shift to lower temperature with increasing the coverage suggest a second-order reaction mechanism. In more general terms the above results indicate that the relative strengths of bonding energy for methyl-metal(II) and metal-VI element play an important role in the surface reactions.