Effect of incident flux on surface concentrations and condensation coefficients when growth and vaporization involve mobile surface species

Abstract

Kinetic analysis of condensation and vaporization processes which proceed through a mobile, adsorbed precursor state shows that the relative concentration c_s/c_se of mobile surface molecules depends upon both the vapor saturation ratio P/P_e and the condensation or vaporization coefficient α: c_s/c_se=α+(P/P_e)(1−α). The result may affect previous treatments of two‐dimensional nucleation and of spiral ledge dynamics in which it was assumed that c_s/c_se =P/P_e. When condensation or vaporization proceeds through a mobile surface layer, condensation coefficients of unity may be approached, but not attained. If surface migration is rapid, the condensation–vaporization coefficient α will change with vapor saturation only if there is a change in the concentration of surface sites at which mobile molecules are incorporated, in the rate constant for incorporation of mobile molecules at these sites, or in the surface residence time. When α is small, the temperature dependence of α yields information on the difference between the activation energy for incorporation and the depth of the molecule–surface potential well. If condensation or vaporization occurs primarily from spiral ledges originating at screw dislocations, condensation and vaporization coefficients will decrease as the system is taken from large oversaturations or large undersaturations towards equilibrium. The slopes of hillocks or pits formed at screw dislocations will decrease concurrently. The results are applied to vaporization and condensation of arsenic and antimony.