Insights into the formation mechanisms of Si-OR monolayers from the thermal reactions of alcohols and aldehydes with Si(111)-H

Hydrogen-terminated Si(lll) reacts thermally at moderate temperatures with alcohols (RCH2OH) and aldehydes (RCHO) to form the corresponding Si-OCH2R f ilms. The films are characterized by Fourier transform infrared (FTIR) spec troscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microsco py (AFM). While FTIR and XP spectroscopies suggest that films of similar co mposition are formed, AFM and the relative chemical stabilities of the orga nic films show that the two reactions do not result in structurally identic al films. A mechanism for the reaction of the aldehyde with Si(111)-H is pr oposed which is analogous to the well-known hydrosilylation of aldehydes. T he reaction proceeds either by nucleophilic addition/hydride transfer or by a radical chain mechanism via adventitious radical initiation. The alcohol reaction is similar to the chemical etching of Si(lll)-H by water and shor t-chain alcohols. This reaction proceeds by nucleophilic attack followed by loss of dihydrogen. Traces of ammonium fluoride or water on the surface re sult in etching of the terraces on a time scale which is much faster than t he reaction of the alcohol but not of the aldehyde. This etching can be com pletely suppressed by the addition of chlorotrimethylsilane to the reaction mixture. This reagent quickly scavenges both water and fluoride from the s urface and reaction mixture. It is suggested that this may be a useful reag ent to scavenge undesirable nucleophiles during wet chemical modification o f Si(111)-H.