Oxidation of Hydrogenated Si(111) by a Radical Propagation Mechanism

Abstract

The reactivity of the hydrogenated Si(111) surface toward H2O and O2 was investigated in order to elucidate the mechanism of oxidation of the first silicon bilayer in air. Density functional theory calculations were performed to identify elementary reaction steps and their corresponding activation energy barriers. The perfect surface is unreactive toward H2O and O2 at room temperature as deduced from the high energy barriers found. However, isolated Si dangling bonds, (Si3)Si·, surrounded by SiH groups, readily react with O2 (but not with H2O), producing a silanone intermediate of the form (Si2O)SiO· where one of the silicon back bonds is oxidized. This intermediate is reached after a series of elementary steps with very small activation energy barriers. In the next step, the oxygen atom of the silanone group inserts into a Si–Si back bond, and the surface silicon dangling bond is regenerated as a (SiO2)Si· moiety in which the silyl group has two oxidized back bonds. This initiates a surface chain reaction in which the oxidized silyl group abstracts a hydrogen atom from a neighboring SiH thus producing a new Si dangling bond that is oxidized by O2 in the next step of the chain reaction. This radical propagation mechanism explains the two-dimensional oxide growth in air and the lack of surface SiOH groups. Therefore, the oxidation of the H–Si(111) surface requires the presence of radicals in air that, upon reaction with the hydrogenated surface, produce silicon dangling bonds where the oxidation begins and propagates by hydrogen abstraction from nonoxidized neighboring SiH groups.

Ga-assisted growth of GaAs nanowires on silicon, comparison of surface SiOx of different nature

“…It is well established that Si surfaces naturally undergo oxidation even at room temperature, simply by exposing them to air. The same oxidation process takes place also in the case of hydrogen passivated surfaces [25][26][27]. As a consequence, what was claimed as oxide-free surface had most probably oxidized to Si þ 1 (i.e.…”

“…Native oxide is a thin layer of oxide formed by the natural exposure of a Si wafer to air; it follows the surface roughness of the underlying silicon substrate and it grows monolayer by monolayer [25,35,27]. The chemical composition of native oxide depends on its thickness.…”

“…1 , characteristic of sub-stoichiometric oxide. As a remark, interstitial oxide is present in every type of oxides on silicon since it represent the initial step of oxidation of every silicon surface, being followed by oxide of different composition, depending on the processing [27]. In the following, FTIR spectrum of Si substrate is used as a tool to investigate the physical properties that influence the early stages of growth of self-catalyzed GaAs nanowires.…”

The role of surface passivation for efficient and photostable PbS quantum dot solar cells

“…Prior reports have recognized the deleterious effects of hydroxide species on the (photo)stability of other organic and inorganic PV technologies [29][30][31] , whereas alkyl-terminated ligands, such as oleic surfactants on QDs, can benefit the stability 32 . We tested the photostability of non-encapsulated QD solar cells soaked under simulated AM1.5G illumination in the N2-filled glove box, to eliminate additional degradation effects due to oxidation from the environment.…”

Effects of the Formulations of Silicon‐Based Composite Anodes on their Mechanical, Storage, and Electrochemical Properties

“…[1,77] Moreover,t he fact that the highest loss of conductivity for the SiÀHc omposite occurred during the first few days of storage could be due to the presence of al arge quantity of labile SiÀH bonds, whichc an form the SiO 2 layer. [78][79][80] These findings demonstrate that the modified Si composite electrode can be processed and stored for al ong period with only ar elatively small change in itse lectronic conductivity.T herefore, it is relevant to determine the potential impact of the modification on its electrochemical performance with storage time foru se as an anode in aL i-ion half-cell.…”