An investigation of the surface reaction mechanisms of alternating-grown, ordered atomic layers: CdS on ZnSe(100)

Temperature-programmed desorption (TPD) is used for detailed investigation of the surface chemistry of a binary reaction sequence by sequential gas-ph ase dosing of a ZnSe(100) substrate with (CH3)(2)Cd and H2S. Analysis of th e TPD spectra shows that adsorbed DMCd irreversibly dissociates on a ZnSe(1 00)-c(2 x 2) surface at room temperature to form a fully methyl-terminated surface; this termination is responsible for the previously reported, self- limiting reaction. At similar to 370 K, DMZn desorbs from this surface due to a methyl-exchange reaction. This desorption temperature is independent o f coverage, indicating a first-order reaction. In addition, at high DMCd ex posures, the adsorption/desorption process leads to replacement of surface Zn by Cd. The experiments also examine the reaction of H2S with the methyl- terminated surface. This reaction, which is also self-limiting, forms a sul fur-hydride-terminated surface after the release of surface CH3 groups in t he form of CH4. Studies of surfaces formed by more than one binary reaction sequence are al so reported, which show that the alternating growth surfaces are terminated with either methyl groups or hydrogen. The methyl-passivated growth surfac e preferentially desorbs methyl radicals at similar to 390 K instead of the metal-alkyl species. For the sulfur-hydride-terminated surface the recombi native reaction of the HS species causes desorption of H2S at 480 K. In thi s case, the symmetric peak shape of the desorbed H2S signal and its shift t o lower temperature with increasing the coverage suggest a second-order rea ction mechanism. In more general terms, these results indicate that the rel ative strengths of bond for methyl-metal (II) and metal-VI element play an important role in the surface reactions. (C) 1999 Elsevier Science B.V. All rights reserved.