Mesoscale structure in phase change material (PCM) GST225

Phase change materials (PCM) were discovered by Stan Ovshinsky and his collaborators [S.R. Ovshinsky et al PRL, 21,1450 (1968)] in the 1960’s These materials are amenable to rapidly, repeatedly and reproducibly transition from crystalline to amorphous phases. The chalcogenide Ge₂Sb₂Te₅ or GST225 is an archetypical and important example. Because, of the large contrast in electrical and optical properties of these two highly switchable but stable phases, PCMs are widely used as the active storage media in compact optical disks (DVD and DVD-R/RW) and for electronic, non-volatile random access (PCRAM) devices. We studied Phonon and the microscopic structure of GST225; a natural super-lattice type ordering in the growth habit of the material was observed. Electron microscopy reveal the atomic scale arrangement to be a 2-dimensional layered stacked structure along the orthogonal direction with submicron (~250 nm) scale periodicity. Room temperature Raman spectra, variations in chemical composition and results of electron microscopy will be presented.