Thickness Dependent Nano-Crystallization in Ge₂Sb₂Te₅ Films and Its Effect on Devices

A thickness dependent nano-crystallization in ultra-thin Ge 2 Sb 2 Te 5 films was studied by in-situ exothermal and isothermal electrical resistivity measurements. When the film thickness is below 20 nm, an exponential increase of the crystallization temperature with decreasing film thickness was found. Furthermore, the crystallization speed was decreased with decreasing film thickness. Based on Kissinger plot and Johnson–Mehl–Avarami (JMA) model, the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier and the Avrami coefficient was investigated. The crystallization activation energy increased from 2.86 eV in 30 nm thick film to 4.66 eV in 5 nm thick film. The increasing of activation energy was explained by a thickness dependent interfacial energy model and was believed as the main reason for the increasing crystallization temperature. In the films thinner than 10 nm, the two-dimensional nucleus growth corresponding to an Armani coefficient n <1 was proposed to attribute to the decreasing crystallization speed. In addition, the effects of this nano-crystallization on line-type phase change devices were discussed.