Reversible Phase Transition in Gete through Resonance Bond Swiching

Abstract

Phase change materials, especially those along the GeTe-Sb2Te3 tie-line (GST), have been widely used in applications of memory devices. Using these materials, information can be stored as optical and electrical change contrast arising from thermally induced structural transitions between the crystalline and amorphous phases. Amorphous phase is obtained by heating the material of crystalline phase upon melting temperature and cooling it fast enough so that it solidify in the amorphous phase. On the other hand, crystalline phase is obtained by heating the material of amorphous phase during relative longer time (a few ns) than amorphization time within appropriate temperature range between crystallization temperature and melting temperature. Contrary to this relatively small difference of heating condition, their physical properties are extremely different from each other. Besides, their fast switching properties and rather good reversibility show promising characteristics for phase change materials as a candidate material for universal memory as well as a most intriguing topic in material society. Despite of these specific material properties, the fundamental reasons for this reversible and fast transition are still in a debate and needs to be understood more considerably as a view point of local structure and orbital environment. Many of the previous studies for understanding phase change mechanism concentrated in comparison of orbital structure between the crystalline phase and amorphous phase. However, the investigation on the modified crystalline phases which is weakly perturbed by thermal variation or external stress could show possible phase transition path. In this study, using uni-axial stress (bending stress), the crystalline GeTe is modulated to have reversible structure deformation which gives rise to drastic change on optical property with resonance bond breaking. Modulated GeTe also shows reversible electrical change caused by orbital rearrangement and irreversible electrical change caused by defect formation. A stress which is relatively smaller thermodynamic perturbation than heat also can be an important variable in phase change mechanism and it might support the reason why both of set and reset state in phase change memory contain various of chemical states.