Photoinduced Material Transport and Phase Separation in Amorphous Chalcogenide Films

The chapter presents a comprehensive review of experimental studies of photoinduced mass transport and phase separation in amorphous arsenic chalcogenides performed by the authors. AFM studies demonstrate that illumination of the amorphous As2S2 film by a tightly focused, linearly polarised oblique laser beam of sub-bandgap energy results in a giant photoexpansion of the film. At above-bandgap illumination, the giant photoexpansion is observed along with lateral mass transport of the film material from the irradiated area. This leads to the formation of a pit on the film surface primarily due to a gradient optical force that makes the material move from strongly illuminated towards less illuminated areas. Simultaneously, a protrusion, shifted along the beam direction, is formed on the film surface due to the scattering force acting in the forward direction. For amorphous As2(S,Se)3-based films doped with foreign atoms, laser-induced photosoftening and enhanced diffusion results in the formation of semiconductor nanocrystals in the laser spot with the participation of dopant atoms. The effects of photosoftening and mass transport and related mobility of species in the amorphous films finally resulting in the localised formation of nanocrystals are governed mainly by nonthermal mechanisms. The presence of nanocrystals is verified by Raman and photoluminescence spectroscopy. Localised photooxidation of arsenic on the surface of amorphous chalcogenide films confirmed by Raman spectroscopy is considered in view of the effect of the laser light energy and the type of the network modifier.