The thermal chemistry of model organosulfur compounds on gallium arsenide (110)

Organothiols and related organosulfur compounds hold promise for use as sel f-assembled ultra-thin electron beam resists. II-VT material growth precurs ors, and monolayer passivation layers. We report the results of temperature -programmed reaction and desorption (TPR/D) studies of the surface chemistr y of three model organosulfur compounds, CH3SH, (CH3S)(2) and (CH3)(2)S, on the (110) face of gallium arsenide. Our measurements indicate that each of these species interacts strongly with the GaAs(110) surface. The first mon olayer of CH3SH desorbs at similar to 300 K, with a desorption wave peak te mperature (extrapolated to the zero coverage limit) corresponding to a deso rption activation energy of 0.81 eV. This value is high compared to similar to 110 K for the second layer, suggesting a chemical interaction between t he thiol and the surface, despite the fact that the monolayer feature in th e TPR/D spectra exhibits first-order kinetics, and the molecule is observed to desorb intact. The behavior of (CH3S)(2) is markedly different; the dis ulfide decomposes upon adsorption (or during the TPR/D temperature ramp) an d desorbs predominantly as (CH3)(2)S at similar to 500 K, perhaps as the re sult of a concerted associative/dissociative desorption process that leaves sulfur at the surface. This result is also evidence that adsorbed CH3SH an d (CH3)(2)S do not assume equivalent surface-bound forms and supports the i dea that the hydrogen atom of the thiol remains in close association with t he adsorbed parent molecule, while the disulfide undergoes dissociative ads orption. Finally, our results indicate that (CH3)(2)S, like CH3SH, molecula rly adsorbs/desorbs. The desorption activation energy, extrapolated to zero coverage, is 0.79 eV, consistent with a strong adsorbate-substrate interac tion. The similarity of this value to that for CH3SH suggests that a specif ic sulfur-surface interaction dominates the desorption kinetics at low cove rage. In the monolayer regime, with increasing exposure, the monolayer deso rption feature in the TPR/D spectra broadens and exhibits strong asymmetry, and ultimately develops a lower temperature feature, indicating complex ki netics consistent with coverage-dependent phase behavior. Finally, by compa rison of the measured desorption energies of the organosulfur compounds wit h those of the alkyl halides, for which the adsorption on GaAs(110) has bee n extensively investigated, an estimation of the chemical and physical cont ributions to the molecule-surface interaction is obtained. Our analysis sug gests that both types of interactions play important roles in the adsorptio n energetics. (C) 2000 Elsevier Science B.V. All rights reserved.