How dissociated fragments of multiatomic molecules saturate all active surface sites—H₂O adsorption on the Si(100) surface

Abstract

A fundamental question for the adsorption of any gas molecule on surfaces is its saturation coverage, whose value can provide a comprehensive examination for the adsorption mechanisms, dynamic and kinetic processes involved in the adsorption processes. This investigation utilizes scanning tunneling microscopy to visualize the H2O adsorption processes on the Si(100) surface with a sub-monolayers (<0.05 ML) of chemically-reactive dangling bonds remaining after exposure to (1) a hydrogen atomic beam, (2) H2O, and (3) Cl2 gases at room temperature. In all three cases, each of the remaining isolated single dangling bonds (sDB) adsorb and is passivated by either of the two dissociation fragments, the H or OH radical, to form a surface Si–H and Si–OH species. A new adsorption mechanism, termed ‘dissociative and asynchronous chemisorption’, is proposed for the observation presented herein. Upon approaching a sDB site, the H2O molecule breaks apart into two fragments. One is chemisorbed to the sDB. The other attaches to the same or the neighboring passivated dimer to form a transition state of surface diffusion, which then diffuses on the mostly passivated surface and is eventually chemisorbed to another reactive site. In other words, the chemisorption reactions of the two fragments after dissociation occur at different and uncorrelated time and places. This adsorption mechanism suggests that a diffusion transition state can be an adsorption product in the first step of the dissociative adsorption processes.

Trimethylamine Probes Isolated Silicon Dangling Bonds and Surface Hydroxyls of (H,OH)-Si(001)

“…The liberated CH 3 radical could react locally by breaking a Si–Si bond or it could diffuse farther away until it finds an IDB distant, at an average distance of 2–3 nm, given the IDB coverage of a few hundredths of ML. Such a long-distance mechanism is suggested for water adsorbates on (H,OH)-Si(001) …”

“…Such a longdistance mechanism is suggested for water adsorbates on (H,OH)-Si(001). 11 3.2. HREELS in the 105−160 K Temperature Interval.…”

“…In fact, the presence of an interfacial layer of SiO 2 in between the HKO/Si stack limits the highest possible gate stack capacitance . Moreover, starting from (H,OH)-Si(001), electrically active surface defects, IDBs, are accessible to molecules (including organic ones) that could passivate them (see refs , ). The (H,OH)-Si(001) surface could also be suitable for area-selective ALD (ASD), as it could be alkylated via the reaction of an aminosilane with a hydroxyl.…”

Atomic and molecular functionalisation of technological materials: an introduction to nanoscale processes on semiconductor surfaces

“…In closely related work, the Lin group [21] investigate molecular adsorption to single dangling bond sites on the passivated Si(001) surface. It is commonly thought that a dissociative adsorption reaction cannot occur on Si(001) surfaces where two reactive sites are not immediately adjacent.…”

“…It is commonly thought that a dissociative adsorption reaction cannot occur on Si(001) surfaces where two reactive sites are not immediately adjacent. However, Chang et al [21] show that H 2 O molecules dissociatively adsorb at single dangling bond sites that are spatially separated by distances of several unit cells or more. They explain this phenomena by a process they call asynchronous chemisorption where part of the adsorbate is first captured into a meta-stable surface site and is then able to diffuse and chemisorb at some available dangling bond site nearby.…”