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 [a-Ge and Te₅₀ (Ge_xSni _x)₅₀] and one three-phase field (a-Ge, /?-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 Si0₂/75 nm Te3₆₃Ge47.₄Sn₁₆.3/150 nm Si0₂ on a grooved disk substrate, and the microstructure resulting from low-power (12 mW) CW and higher-power (—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 ~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.