Crystallization of amorphous selenium films. I. Morphology and kinetics

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Abstract
We report a study of the morphology and kinetics of crystallization of films of a‐Se vapor deposited on mica substrates. Morphological examination was facilitated by the complete dissolution of the amorphous component of the films by CS2, while the crystalline component was left essentially intact. Crystallization begins (in the temperature range 30–100°C) with the formation and radial growth of crystalline aggregates, with a cylindrite (two‐dimensional analog of spherulite) morphology, at the mica‐film interface. Filamentary crystals stem from the less regular regions of these cylindrites and grow upwards through the film. Upon reaching the vicinity of the free surface, some of the filamentary tips develop into cylindrites which grow in and parallel with the free‐surface plane. The interface cylindrites were about 500 Å thick and their presence apparently is responsible for the phenomenon of ``visual'' darkening often seen in a‐Se films. They are composed of small crystal domains each oriented so that the Se chain axes are parallel with the interface and perpendicular to the disk radius. In the regular morphology, favored by higher‐temperature crystallization, the domains are stacked into lamellae which extend radially from the inner regions to the outer edge of the cylindrites. These lamellae are interconnected by less regularly oriented domains and are much thicker and less regular in cross section than the crystals formed by linear polymers. The radial growth rates of the regular interface cylindrites in films of the highest purity were constant in time and quite reproducible at a given temperature. They are among the highest rates reported for a‐Se and about two orders of magnitude higher than the corresponding rates for free‐surface cylindrites. Their temperature dependence is described by u = 6 × 1015 exp[(− 32.7 kcal/mole)/ RT] cm/sec.

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