Real-Time X‑ray Photoemission Spectroscopy Study of Si(001)-2×1 Exposed to Water Vapor: Adsorption Kinetics, Fermi Level Positioning, and Electron Affinity Variations

The great advantage of X-ray photoemission spectroscopy, when performed in real time, e.g., during the reaction of a gas with a surface, is the possibility of monitoring in a single experiment both the chemical aspects (adsorption kinetics, bond formation) and the physical ones (Fermi level positioning, variations in the electron affinity). In the present study we examine the reaction of water with Si(001)-2×1 at room temperature in real time not only because water, ubiquitous in (ultra) high-vacuum systems, is the main source of surface defects controlling the surface Fermi level, but also because water-saturated silicon may become an interesting starting surface in the atomic layer deposition of dielectrics on silicon. The question of water adsorption on silicon Si(001)-2×1 is renewed under the following four perspectives: (1) We propose an original kinetic analysis of the water uptake using an integrated form of the precursor model differential equations, underlying a dependence on pressure. (2) We perform a thorough analysis of the Fermi positioning within the band gap due to water-related surface defects as a function of water coverage and for four different doping types and levels. (3) We follow the changes in the surface dipole as a function of coverage, with considerations of the dissociation channels. (4) Using seven different n and p doping levels, we extract the electron affinity at saturation, a useful parameter to know if heterostructures are built upon the water-covered surface. In addition to an applicative view, the present data can be a benchmark for theoretical calculations such as molecular dynamics, surface defect energy, and work function calculations.