Thermal and electron driven chemistry of CCl4 on oxidized Si(100)

Abstract

Dosed on oxidized Si(100) at 100 K, carbon tetrachloride adsorbs and desorbs without dissociation. The monolayer desorbs at 135 K, 10 K lower than the multilayer. This unusual behavior is attributed to stronger interactions between condensed CCl4 molecules than between CCl4 and SiO2. Irradiation with either low-energy (⩽50 eV) electrons or Mg Kα x rays causes C–Cl cleavage. For 50 eV incident electrons, the decay of the CCl4 temperature programmed desorption peak area occurs with an effective cross section of (2.0±0.1)×10−16 cm2. The same cross section characterizes the ejection of CCl (CCl+) during electron irradiation. After low electron fluences, C2 and C3 molecules desorb reflecting both electron-induced C–Cl bond dissociation and C–C bond formation. At 2.5 eV incident electron energy, the cross section is still high—10−17 cm2. Electron activation is attributed to a combination of impact ionization and electron attachment mechanisms.

Low-Energy Electron-Induced Oligomerization of Condensed Carbon Tetrachloride

“…Tetrachloroethylene has been previously identified as an electrondriven reaction product of 1-2.5 monolayers of CCl 4 on SiO 2 . 35,36 B 45 hexachloroethane (Figure 3) is identified as a CCl 4 radiolysis product that desorbs from the Mo(110) surface at ∼210 K. The identification of C 2 Cl 6 as a radiolysis product of CCl 4 is further corroborated by its desorption temperature (∼210 K) being higher than that of CCl 4 (∼140 K) and C 2 Cl 4 (∼190 K). This observation is consistent with the boiling point trends of the following three species: 349 (CCl 4 ), 394 (C 2 Cl 4 ), and 458 K (C 2 Cl 6 ).…”

“…We attribute the premature formation of multilayers to the formation of three-dimensional islands of CCl 4 on the Mo(110) surface. Islanding effects have been previously invoked to explain thermal desorption data for CCl 4 on Si(100), 35,36 Fe(110), 42 and GaAs(110). The radiolysis products were identified by the following: (1) comparing the m/z for observed desorption features in thermal desorption data to fragments in known mass spectra; (2) invoking trends in multilayer desorption temperatures and in boiling point temperatures; and (3) comparing the results of postirradiation temperature-programmed desorption experiments to the results of previous γ-radiolysis and low-energy electron radiolysis studies.…”

“…In addition to being previously identified as a γ-radiolysis product of CCl 4 , 20,[22][23][24] hexachloroethane has been previously identified as a low-energy electron-driven reaction product of CCl 4 on clean, hydrogen precovered, and oxygen precovered Si(100). 35,36 We attribute the formation of a miniscule amount of C 2 Cl 6 in the absence of electron irradiation to either photon-or surfaceinduced reactions. We base this conjecture on previous photochemistry 46,47 and surface chemistry 42,48,49 studies of CCl 4 .…”

“…[32][33][34] The electron-driven chemistry of CCl 4 adsorbed onto Si(100) has been investigated by using electron-stimulated desorption (ESD), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). 35,36 Following low-energy (50 eV) electron irradiation (fluence ) 1.6 × 10 16 cm -2 ), postirradiation temperature-programmed desorption data were used to identify C 2 Cl 2 (130 K), C 2 Cl 4 (162 K), and C 2 Cl 6 (180 K) as electron-induced reaction products of one to two monolayers of CCl 4 on clean Si(100). 35 In addition, on oxidized Si(100), it was determined that a C 3 Cl x species was formed in low quantities at electron fluences g5 × 10 16 cm -2 .…”

“…35 In addition, on oxidized Si(100), it was determined that a C 3 Cl x species was formed in low quantities at electron fluences g5 × 10 16 cm -2 . 36 Post-irradiation reflectance absorbance infrared spectroscopy (RAIRS) data were used to identify CO, CO 2 , HCl, and C 2 Cl 4 as radiolysis products of CCl 4 coadsorbed with water. [37][38][39] According to the proposed mechanism, the initial radiolysis of CCl 4 and H 2 O mixtures leads to the formation of trichloromethyl radicals ( • CCl 3 ) and dichlorocarbene (:CCl 2 ).…”