Impact of disorder on optical reflectivity contrast of epitaxial Ge₂Sb₂Te₅ thin films

Abstract

Classification of the optical reflectivity contrasts of single-phase, epitaxial Ge2Sb2Te5 thin films with respect to the vacancy arrangements.

Phase change thin films for non-volatile memory applications

“…6(a)). 50,92,93 These differences in optical reectivity can be explained by changes in the free carrier absorption due to electron localization effects. Fig.…”

“…The formation of regularly spaced vacancy layers in epitaxially grown GST thin lms (phase II) is associated with the occurrence of additional diffraction peaks in XRD q-2q measurements compared to the expected XRD pattern of GST thin lms containing phase I. 61,93 These additional diffraction peaks can be regarded as superstructure reections due to the vacancy layer substructure within the (pseudo)cubic lattice. In general, the resulting crystal structure becomes closely related to that of the stable trigonal phase of GST, which yields only minor differences in 2q values of the corresponding XRD diffractograms.…”

“…9(a) featuring only the cubic GST225(111) and cubic GST(222) reections corresponds to an epitaxial cubic GST thin lm which is characterized by random distribution of vacancies (phase I) and thus no superstructure reections appear. This phase can be obtained in an elegant way by ns-laser crystallization of amorphous GST thin lms deposited on crystalline Si(111) substrates or by laser induced recrystallization of trigonal GST225 thin lms, 83,93 while GST225 phase II can be produced by fs-laser irradiation. 61 The fabrication process of phase I ensures that no vacancy ordering is present, whereas during preparation methods involving thermal annealing on longer timescales partial vacancy ordering might occur.…”

Applications of Phase Change Materials in Electrical Regime From Conventional Storage Memory to Novel Neuromorphic Computing

“…However, the subsequent ordering of the vacancies into vacancy layers gradually improves the structural order in the cation sublattice in the GST225 materials, which vanishes the localized states and causes a transition from an insulating to a metallic state. Unlike its electrical properties, the optical properties difference is likely due to the changes in the free carrier absorption caused by the electron localization effects [23], [38], [39]. As revealed from the corresponding electrical measurements, the vacancy order phase (the second phase) was found to have a slightly higher carrier concentration than the disordered phase (the first phase).…”

The impact of vacancies on the stability of cubic phases in Sb–Te binary compounds

“…The crystalline phase utilized in GeSbTe-based PCMs that are currently in use is the cubic metastable phase, which contains large quantities of vacancies [6][7][8] . The vacancies in the Ge/Sb sublattice are intrinsic 9 and have been demonstrated to play an important role in phase transitions and related properties [10][11][12][13][14][15][16][17] .…”