STRUCTURAL STUDY OF THE ANNEALING OF ALKYLSILOXANE SELF-ASSEMBLED MONOLAYERS ON SILICON BY HIGH-RESOLUTION X-RAY-DIFFRACTION

X-ray interferometry/holography was utilized to investigate the profil e structures of n-hexadecyltrichlorosilane self-assembled monolayers ( SAM's), chemisorbed onto the SiOx surface layer of Ge/Si multilayer su bstrates fabricated by molecular beam epitaxy, in their initial ''as d eposited'' form and throughout an annealing process. This approach per mitted an unambiguous determination of the electron density profiles o f the various layers within the inorganic multilayer substrate and the chemisorbed SAM overlayer, including the smoothness/roughness of the interfaces between adjacent layers, to relatively high spatial resolut ion without any prior assumptions. The corresponding in-plane structur es of these various forms of the SAM's were investigated by high Delta Q-resolution X-ray diffraction. The interpretation of the latter resu lts was greatly facilitated by comparison with analogous kinematical s tructure factors calculated from molecular dynamics computer simulatio ns of an ensemble of alkyl chain molecules ''chemisorbed'' onto a plan ar surface as a function of in-plane molecular density and temperature . Our results indicate that the initial ''as deposited'' and ''high-te mperature'' forms of these SAM's consist of small domains of highly ti lted chains (relative to the normal to the monolayer plane) within a p ositionally-disordered distorted hexagonal in-plane lattice, consisten t with the dominance of chain-chain interactions. Upon cooling and dry ing, substantial structural reorganization takes place within the SAM and the SiOx substrate surface layer resulting in the predominant ''an nealed'' form of these SAM's consisting of larger domains of much less tilted chains within a positionally-disordered hexagonal in-plane lat tice, consistent with the dominance of chain headgroup-surface interac tions. While such annealing produces only a modest increase in the in- plane interchain correlation length, the so-''annealed'' SAM's are the n structurally stable over the 293-363 K temperature range investigate d, consistent with their intradomain structure.