Crystallization Kinetics And Structure Thresholds Of Ge-Te

Ge-Te based chalcogenides play an important role in both the infrared optical transimission and phase change memory,i.e.,Ge₂₀Te₈₀,which has a good structural stability,can be used in the infrared optical transmission,and Ge₅₀Te₅₀ that with ultra-fast crystallization speed can be employed in phase change memory.It implies that,composition change can significantly affect the physical properties of Ge-Te binary,especially for the crystallization kinetics.We first studied the crystallization of of three chalcogens.And then,we revealed the crystallization kinetics of Ge_xTe_100-x together with the flash DSC and proper viscosity models.It was also studied other physical properties and structures for researching the thresholds in Ge-Te binary.Lastly,the glass transition in Te-based phase-change materials ad the fragile-to-strong behaviors were investigated and discussed.The main contents of this work are summarized as follows:1?By the methods of the Flash DSC and conventional DSC,we studied the crystallization kinetics of three chalcogens with the VFT viscosity model.It was found that,the fragilities of S?Se?Te are 126?67.5?60,the decoupling coefficients are0.545?0.718?0.67,and the maximum crystal growth rates(U_max)are 9.7×10^-3?1.3×10^-6?0.47 m s^-1.Such results are in line with previous experience,i.e.,the Ediger equation.2?By using the Flah DSC,it is observed a strong non-Arrhenius temperature dependent viscosity.We considered two viscosity models for estimating the crystal growth kinetics coefficient.The results showed that the MYEGA model was more suitable to describe the temperature dependent viscosity and the crystal growth kinetics for supercooled liquid Ge₅₀Te₅₀.The glass transition temperature and fragility were estimated to be 432.1 K and 130.7,respectively.The temperature dependent crystal growth rates,which were extrapolated by MYEGA model was in line with the experimental results that were measured by in situ TEM at a given temperature.The crystal growth rate reached a maximum of 3.5 m s^-1 at 790 K.These results based on ultrafast DSC with the MYEGA model offer a revelation for crystallization kinetics of supercooled liquid Ge₅₀Te₅₀.3?We used the Flash DSC and conventional DSC to investigate the crystallization kinetics of Ge_xTe_100-x in a wide compositional range?15?x?55?.The crystallization kinetics of first crystalline phase in Ge_xTe_100-x were estimated and it was found the span of crystallization speed in supercooled liquid can be reached as large as 10¹⁵ m s^-1.Component dependences of maximal crystallization speed(U_max)and fragility were investigated,revealing the component in x=20.4 has the lowest U_max of 1.22×10^-3 m s^-11 with smallest fragility of 42.2,and the component in x?50 possesses the largest U_maxax of 3 m s^-1.This evidenced that,Ge₅₀Te₅₀ is the most suitable phase-change material for information storage and Ge₂₀Te₈₀ is the best media for information transparency in Ge-Te binary.Moreover,a tri-counter pattern was carried out for estimating the crystal growth rate directly at any temperature?supercooled liquid region?and any component?15?x?55?.In addition,we first found a peculiar component Ge₂₂Te₇₈ with terrible thermal properties,i.e.,phase separation,low crystallization temperature,ultrahigh fragility and anomalous crystallization kinetics.More importantly,together with the crystallization kinetics parameters of other glass formers,it was found a specific relation between reduced glass temperature(T_rg)and U_max for which can be benefit to simplify material screenings and performance optimizations.4?Thermal behaviors and configurations of the as-deposited Ge_xTe_100-x film at a wide compositional range?12.5?x?85?have been investigated.It was found that,Ge_xTe_100-x exhibits one-or two-step crystallization behavior with a complicated competition between GeTe and Te?or Ge?phases.The composition dependences of crystallization temperature and activation energy for the first crystallization,as well as cut-off absorption edge,have been revealed.It exhibits four singularities at x=20.4(Ge₂₀Te₈₀),x=33(Ge₃₃Te₆₇),x=49(Ge₅₀Te₅₀),and x=22(Ge₂₂Te₇₈).By means of Raman spectra analysis,the structural sketches for the amorphous Ge_xTe_100-x films that is helpful to understand the evolution of crystallization behavior were built up.It is evident that,the presences of homopolar Te-Te and heteropolar Ge-Te bonds?especially in the corn-sharing structure?are the reasons for the structural and thermal stability of Ge-deficient Ge_xTe_100-x.GeGe₄ tetrahedron occurs when x>50 and effectively enhances the stability for Ge-rich films.5?Glass transitions of two Te-based phase-change materials were studied by modulated DSC.It was found that,both Ge₂Sb₂Te₅ and Ge₅₀Te₅₀ are marginal glass formers where?T?=T_x-T_g?is less than 2.1 ^oC when the heating rate below 3 ^oC min^-1.The fragilities of Ge₂Sb₂Te₅ and Ge₅₀Te₅₀ can be estimated as 46.0 and 39.7,respectively,around glass transition temperature,implying that a fragile-to-strong transition would be presented in such Te-based PCMs.Such fragile-to-strong transition behaviors in chalcogenides is benefit to solve the contradiction of well amorphous thermal stability and fast crystallization speed in phase-change materials.In addition,it was found an abnormal"three-step relaxation model"in the quenched Ge₁₅Te₈₅.This anomalous relaxation phenomenon is also presented in some metallic glasses that with the fragile-to-strong transition behavior.