On the crystallization of amorphous selenium films: Thermal effects and photoeffects

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Abstract
We have characterized the morphology and crystallography of interface cylindrites and filamentary crystals, formed in the crystallization of amorphous Se films, more definitively. In ’’ideal’’ cylindrites, each of the radiating arms is in a particular crystallographic orientation such that the c axis makes a characteristic angle ϑ, often but not always 90°, with the radial direction. In ’’nonideal’’ cylindrites the orientation may be fixed within an arm, but varies from arm to arm. The effects of varying cylindrite structure on the optical extinction behavior were noted and interpreted. The arms are thinner, and the greater their frequency of branching, the lower the temperature of crystallization; further, in low‐temperature growth the arms may develop pronounced curvature. Filamentary crystals form preferentially at interarm boundaries and especially at arm branching points. Thin filaments grown at room temperature were single crystals with c axes perpendicular to and [110] directions parallel with the filament axes. Frequently cylindrites were generated by the impingement of filaments on the interface or the free surface. The rates of elongation, ?F, of one type of filament were about 101 times greater than the thickening rates ?IC, but about 103 times less than the radial growth rates ?IC of the interface cylindrites. However, the temperature dependence of ?F and ?IC are described by activation energies which are about the same, 32.7±1.5 kcal/mole. Photoillumination with white light sharply increases both ?IC and l̇F while not appreciably affecting ?IC in the highest‐purity films. Such illumination also markedly increases the rate of heterogeneous nucleation and the rate of impingement of filament tips on the free surface, but it produces no measurable homogeneous nucleation.