Reaction of Hyperthermal Oxygen Ions with Self-Assembled Monolayers and Silicon Oxide Thin Films

In order to understand the degradation pathways suffered by protective coatings and polymeric satellite materials in the low-earth orbit (LEO) space environment, reactions of hyperthermal oxygen ions with self-assembled monolayers (SAMs) and silicon oxide thin films are studied under ultra-high vacuum (UHV) conditions.

The scattered ionic products are collected with energy-, angle- and mass-resolved detection when 5-40 eV O⁺ ions impact an alkanethiolate SAM. X-ray photoelectron spectroscopy (XPS) is used to measure the resulting erosion yield and degree of oxidation in the hydrocarbon layer. To learn about the site-specificity to hydrogen abstraction in this system, SAM layers are grown for which the hydrogen atoms located on the C-12, C-11, or C-10 positions of 1-dodcanethiol are substituted with deuterium atoms. By comparing the yields of OH to OD emerging from these three isotopomers, it is found that hyperthermal O⁺ ions initially abstract only H(D)-atoms bound to the top two carbon atoms within the SAM layer. In addition, scanning tunneling microscopy (STM) images of the irradiated SAM layer reveal that 5-eV O⁺ ions attack the film predominantly near domain boundaries. In contrast, large defect-free surface domains show considerable stability against 5-eV O⁺ bombardment.

The reaction dynamics of hyperthermal O₂⁺ with a SiOx/Si(001) thin film is also studied. Isotopic labeling helps to identify the mechanisms leading to the anionic reaction channels. O^– signals are composed of dissociative scattering and sputtering products, whereas O₂^– signals arise from nonreactive scattering, recoil abstraction and symmetric substitution channels. The complex dynamics associated with ion-beam oxidation of Si(001) by 5-120 eV O⁺ and O₂⁺ are discussed. The cross section for oxygen incorporation is found to depend strongly on the conditions under which the underlying oxide layer was grown; the kinetic energy of the incorporating ion; and whether the incident ion is atomic or molecular oxygen.