GERMANIUM SUPERSATURATION DURING THE CRYSTALLIZATION OF AMORPHOUS TE-GE-SN THIN-FILMS

The structure and phase relations of Te-Ge-Sn thin films are examined with application to erasable optical storage media. Free energy data from the literature predict that the region of the Te-Ge-Sn phase diagram between Ge, Sn, and the TeGe-TeSn pseudobinary consists of one two-phase field [alpha-Ge and Te50 (GexSn1-x)50] and one three-phase field (alpha-Ge, beta-Sn, and TeSn). Electron diffraction from five different Te-Ge-Sn films annealed at 623 K experimentally confirms this prediction. One composition from the two-phase field is deposited as a tri-layer film with the structure 150 nm SiO2/75 nm Te36.3Ge47.4Sn16.3/150 nm SiO2 on a grooved disk substrate, and the microstructure resulting from low-power (12 mW) CW and higher-power (approximately 50 mW) pulsed laser exposure is studied by transmission electron microscopy and electron diffraction. Of particular significance is that laser-induced crystallization produces a single-phase structure consisting of the Te-Ge-Sn compound phase which is supersaturated with respect to the excess Ge. This supersaturation leads to a disordering of the equilibrium NaCl-type structure of this phase. Crystallization of a micron-sized amorphous spot on a approximately 200 ns time scale occurs by a diffusionless process. The fast erase times required by a phase-change optical recording application can thus be achieved in off-stoichiometric compound compositions by way of a nonequilibrium crystallization process.