CF3I adsorption on InSb(001)

Abstract

The adsorption of CF₃I on the indium-rich InSb(001)-c(8 × 2) has been studied using primarily ultraviolet photoelectron spectroscopy (UPS) and the change in the work function, Δϕ, with some low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies. Adsorption at 110 K was molecular with precursor adsorption kinetics giving a constant sticking probability to saturation, which occurred at one monolayer. Multilayer adsorption was not expected, or observed, at this temperature. Δϕ, as a function of coverage was positive and showed a constant slope to about half coverage (Δϕ≈ 0.45 eV), where it changed to a smaller value, giving Δϕ≈ 0.7 eV at saturation. The molecule is thought to adsorb with the iodine end down. Desorption was found to be first order with an activation energy of ca. 38 kJ mol^–1 for ν= 10¹²– 10¹³ s^–1. The incident electron beams used for LEED and AES cracked the adsorbed molecule to chemisorbed C, I and F in a few seconds. The chemisorbed halogens so produced were desorbed by heating to 500 K, with a concomitant decrease in the surface indium concentration, indicating desorption of indium halides. However, the carbon concentration in the vicinity of the incident beam remained almost constant, even after heating to 500 K. The incident Hel, 21.2 eV, radiation affected the surface over 1000s of seconds by desorbing CF₃I molecules intact. The process, which is thought to be due to electron capture from the secondary electron background into the threshold resonance of CF₃I < 20 meV, stopped after ca. 1/3 of the monolayer had been desorbed. The reasons for molecular desorption under the action of secondary electrons from Hel radiation, but total cracking of the molecule under the 2 keV incident beam used for AES, are discussed.