Abstract: Stoichiometry of SiO₂/Si interfacial regions. I. Ultrathin oxide films

Abstract

Nonstoichiometric interfacial transition regions located between thermally grown silicon oxide films and silicon substrates have been identified by variation in silicon and oxygen x‐ray photoelectron spectral data.1 Changes in Si 2p binding energies, and line intensities and linewidths of both Si 2p and O 1s photoelectron lines are detected in spectra of oxide films less than 25‐Å thick and are each consistent with a silicon‐rich transition region. The composition and width of the nonhomogeneous oxide transition regions in ultrathin, thermally grown silicon oxide films are quantitatively evaluated in the present study using Si 2p binding energy and intensity data.Homogeneous reference films are used to relate Si 2p binding energies with oxygen content of the films. The determination of a mean escape depth for Si 2p photoelectrons from oxide films provides a relationship between Si 2p line intensities and oxide film thickness. Changes in Si 2p binding energies and intensity data are used to identify changes in ultrathin oxide film composition as a function of film thickness. Extrapolation of experimental Si 2p binding energy and intensity data to a film thickness of 2.5 Å provides the composition of a first layer of oxide film. The composition of the silicon oxide transition region and its width are determined by fitting the experimental data to a set of generated theoretical curves. These curves are derived from calculations of the changes in Si 2ρ binding‐energy maxima as a function of the width of a transition region.Ultrathin silicon oxide samples less than 30 Å thick were generally formed on p‐type, 2‐Ω cm silicon substrates with both (100) and (111) orientations, although n‐type and 10−3 Ω cm silicon and surfaces formed by cleavage were also studied. Oxide films were grown at oxidation temperatures ranging from 23° to 850 °C and with oxidants which included O2, O2+N2, air, and boiling aqueous solutions [concentrated HNO3, H2O2 (30%), deionized water].Nonstoichiometric transition regions were identified in all thermally grown ultrathin silicon oxide films. These transition regions were indistinguishable as a function of oxidation conditions, oxidant, or substrate dopant, or type. The transition regions were resolvable from x‐ray photoelectron data only as a function of substrate orientation. Estimates of parameters characterizing ultrathin oxide transition regions are summarized in Table I. It was determined that oxide transition regions, which are approximately 14 Å thick, contain about 1015 nonoxidized Si–Si bond cm−2. Ultrathin oxides formed on (100) oriented substrates have narrower, less steeply graded transition regions than do oxides on (111) oriented substrates. The first oxide layer is more completely oxidized in oxides on (100) oriented substrates.