Nucleation and Oriented Growth of Chemically Formed Sulphide and Oxide Films

Abstract

The kinetics of the sulphidation and oxidation of evaporated zinc and cadmium films in sulphur and oxygen have been investigated. Between 100 ° and 250 °C a rapid gas uptake occurs which may be described by k·log(t−t0). Subsequently, a slow reaction is observed, the time dependence being represented by a logarithmic expression or by a general power law of the type k·t1/n. Electron-optical and x-ray studies reveal a substantial effect of nucleation and recrystallization processes on the kinetics of the layer growth. The following phases of layer growth could be confirmed: (1) coating of the metal with a very thin (d ≈ 50 Å) nonoriented layer; (2) growth of oriented sulphide or oxide nuclei with diameters of 102–103 Å; (3) coalescence of these nuclei and formation of a coherent layer; in the case of CdS and CdO, formation of medium-size epitaxial crystallites; (4) formation of whiskers at elevated temperatures. (5) Measurements of zinc and cadmium single crystals gave similar results, but the reaction rate is much slower and the degree or orientation depends strongly on the pretreatment of the metal surfaces. The results show that nucleation and recrystallization processes affect considerably the kinetics of the formation of thin films on metals. From this it can be concluded that it is necessary to modify models of film growth, which in most cases are based on a homogeneous layer structure.