Thermal chemistry of CH ₃ on Si/Cu(100)

Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH ₃ (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal, decomposition of SiH ₄ at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu ₂Si structure. These Cu ₂Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH ₃) ₃SiH desorbed near 200 K from CH ₃/Si/Cu-(100) prepared with a low Si concentration. With increasing Si concentration a (CH ₃) ₃SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH ₃) ₃SiH at 200 and 420 K were believed to be due to the thermal reaction of CH ₃ with the low Si density and high Si density (i.e., Cu ₂Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu ₂Si phase covered the surface, only the 420 K peak was present during CH ₃/Si/Cu(100) TPD. Results also suggested that (CH ₃)-Si and possibly some (CH ₃) ₂Si intermediates predominated on the surface below room temperature, and (CH ₃) ₃-Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH ₃) ₃SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).