Structure of laser-crystallized Ge2Sb2+xTe5 sputtered thin films for use in optical memory

Abstract

The structure of laser-crystallized thin films of ${\mathrm{Ge}}_{\mathrm{2}}{\mathrm{Sb}}_{\text{2+x}}{\mathrm{Te}}_{\mathrm{5}}\text{\hspace{0.05em}(0.0<x⩽1.0)}$ formed by the sputtering method were identified by x-ray diffraction studies to be composed of two phases: one phase is chiefly NaCl type crystal with a lattice constant of about 6 Å and a composition corresponding to ${\mathrm{Ge}}_{\mathrm{2}}{\mathrm{Sb}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{5}};$ the other phase comprises a small amount of an amorphous component such as Sb metal. Results of the Rietveld and the whole-powder-pattern fitting analyses show good agreement when assuming that (i) the 4(a) site is wholly occupied by only Te, (ii) the 4(b) site is randomly occupied by Ge or Sb atoms, and (iii) a little less than 20% of the 4(b) site is always vacant independent of the x value. The above results and the fact that halo noise rises with x increasing from 0.0 to 1.0 indicate a more precise model of crystal structure as follows. That is, Sb atoms added beyond the stoichiometric ratio, ${\mathrm{Ge}}_{\mathrm{2}}{\mathrm{Sb}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{5}},$ never fill up the vacancies of the 4(b) site in the NaCl type structure; the excess Sb atoms will remain in the amorphous state and concentrate, for example, at the grain boundary. The authors conclude that the amounts of the amorphous component produced through the crystallization process predominantly determine the crystallization rates and the critical temperatures of Ge–Sb–Te amorphous films, reportedly that they show a gentle and continuous dependence on the compositional deviation from the ${\mathrm{GeTe–Sb}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{3}}$ pseudobinary line.