Growth and Structure of Water on SiO₂ Films on Si Investigated by Kelvin Probe Microscopy and in Situ X-ray Spectroscopies

Abstract

The growth of water on thin SiO 2 films on Si wafers at vapor pressures between 1.5 and 4 torr and temperatures between -10 and 21 o C has been studied in situ using Kelvin Probe Microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH) water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 °C and ice-like below 0 °C . 1 IntroductionAt ambient conditions all materials on earth are exposed to water vapor that produces films on their surfaces. The thickness and structure of this film is determined by the interaction forces between the surface and the adsorbed water molecules and has important implications for industrial, environmental and biological processes. One fundamental question is the range of the surface-induced modifications, if any, of the structure of water. In other words, how many layers are needed for water to reach its bulk structure? Not surprisingly the study of water at interfaces is a very active field, as demonstrated by the numerous review articles on this topic published in recent years. [1][2][3][4] Oxide surfaces are particularly important and have received considerable attention. 5 One of the most important oxides is amorphous SiO 2 because of its widespread presence in silicon technology and in natural minerals. [6][7][8] The surface of amorphous SiO 2 is usually modeled by a mixture of the (111) and (100) surfaces of hydroxylated β-cristobalite, which expose single and geminal hydroxyl groups, respectively. 9 On a fully hydroxylated (100) surface these groups are sufficiently close to each other that H-bonded networks can be formed. In the (111) surface the hydroxyl groups are more separated so that no H-bonds can form between them.The structure of water in contact with different SiO 2 surfaces has been studied both theoretically [10][11][12][13] and experimentally [14][15][16][17][18] . These studies have predicted an ordered hexagonal water layer on the hydroxylated surface of quartz (0001) 13 and a hexagonal ice-like structure on a fully hydroxylated β-cristobalite (100) surface. 10 However there is some controversy about the stability of such monolayer structures at room temperature. 12 No ice-like structure has been predicted for cristobalite (111) and the most favorable site for water molecules has been predicted to be a hollow site with the molecules forming 2 or 3 H-bonds with surface silanol groups. 11 Based on optical experiments an ice-like monolayer on amorphous SiO 2 has been proposed at room temperature and 10% relative humidity (RH). 14 Information on the structure of water films thicker than a monolayer is much scarcer. Sumfrequency ...

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

“…Contact potential difference measurements combined with XPS analysis under ambient pressure have shown that water molecules can be adsorbed on oxidized silicon with a significant increase of the contact potential (net positive charge or dipole on the surface) of about 400 meV, which essentially occurs between 2 and 5 water monolayers. 51 This result shows that H 2 O vapor adsorption can significantly decrease the barrier height in oxidized patches.…”

“…In addition, hydrophilic silicon oxide regions may also favor the physisorption of polar water molecules and contribute to modify the interface potential. 51 In order to get some insight in the dynamics of the molecular monolayer/SiO 2 /Si system, this work investigates the dipolar relaxation signature of chemisorbed and physisorbed polar moieties over a broad electrical bias and temperature range.…”

On the Transition from Bulk to Ordered Form of Water: A Theoretical Model to Calculate Adhesion Force Due to Capillary and van der Waals Interaction

“…It can be seen that there are two transitions in the adhesion force as shown in Eq. (11). The first transition is from solid-solid to solid-solid ?…”

“…After 10 % RH the monolayer starts to grow in multilayers and at about 30 % RH 3 monolayers are formed. The water will be considered as bulk only if the number of monolayers is greater than 3 [10,11]. So, water will be in an ordered form from 10 to 30 % RH.…”

“…On the other hand, experiments have been performed to measure the adsorption layers on surfaces using attenuated total reflection infrared spectroscopy [10]. Verdaguer et al [11] studied the growth and structure of water on SiO 2 by several techniques, like surface potential measurements using an atomic force microscope (AFM) and ambient pressure X-ray photoemission and near edge X-ray absorption fine structure spectroscopies (XPS and NEXAFS). The measurement results also followed the BET adsorption isotherm in a certain RH range which indicates the applicability of the model in that range.…”