Local bonding environments of Si–OH groups in SiO2 deposited by remote plasma-enhanced chemical vapor deposition and incorporated by postdeposition exposure to water vapor

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Abstract
We have deposited thin films of SiO2 by remote plasma-enhanced chemical vapor deposition and have identified similar infrared (IR) spectroscopic signatures of Si–OH groups incorporated during either film growth, or the cooling down process in the deposition chamber. These films can also be hygroscopic and, on postdeposition exposure to atmospheric water vapor, they show changes in the IR spectra associated with the incorporation of additional Si–OH groups. These changes are (i) the development of a new symmetric feature, centered at about 3350 cm−1, within the asymmetric O–H stretching band generated during growth and/or cooling down; (ii) the development of a new spectral feature at 925 cm−1; and (iii) a shift in the Si–O bond-stretching band to higher wavenumber. We show that the first two changes in the IR spectra are due to near-neighbor Si–OH bonding groups that result from the reaction between water vapor and the Si–O–Si bonds of the SiO2 host network. These spatially correlated Si–OH groups have different spectral features, due to relatively strong hydrogen bonding interactions, from the randomly distributed Si–OH groups that are incorporated initially during film growth and/or cooling down. The shift in the frequency of the Si–O stretching band derives from a preferential reaction of water with strained and highly reactive Si–O–Si bonding groups, i.e., those with the smallest Si–O–Si bond angles which are attacked by water vapor, resulting in the formation of near–neighbor pairs of Si–OH bonding groups.

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