Chalcogenide thin film materials for next generation data storage

Abstract

Data can be stored in the form of amorphous and crystalline marks within a chalcogenide thin film. Commonly Ge₂Sb₂Te₅ is used as the active data storage material to record these two different phases. Changes in phase are induced by controlled heating and cooling with laser radiation or an electric current. The research reported here shows, for the first time, that phase changes are possible in two new materials: BiSbTe and GaLaS. A further, known, phase change system, GeSbTe, has been used to test combinatorial deposition techniques and high throughput characterisation methodologies. An optical system, the static tester, was developed to test the time necessary for phase transitions. The system, capable of automated operation, can sequentially characterise the phase change kinetics of composition spread, thin film, samples. The GeSbTe system was investigated, as a function of composition, using combinatorial deposition methods. This showed that combinatorial methods can be applied to phase change materials and allowed new material characterisation over the whole ternary system. Using combinatorial methods, the electrical sheet resistance of the amorphous material is shown to be correlated to the tellurium concentration and is thought to be due to correlated increases in lone-pair defect charge trapping centres. The material’s resistivity can change by more than an order of magnitude by increasing the Te content from 20 at.% to 50 at.%. Conversely, in the amorphous phase, the refractive index was shown to decrease with increasing Te proportion and this has been related to a decrease in the material’s polarizability. The Sb:Te binary system has been doped with Bi using two different methods; sputter deposition from a composite target and using combinatorial, thermal evaporation deposition of elemental targets. New results have shown that Sb₈Te₂ can be doped with up to 13at.% Bi and still exist in a stable amorphous state. A novel chalcogenide material consisting of gallium, lanthanum and sulphur was shown, for the first time, to phase change. The crystallisation time of Ga:La:S:Cu material was found to be 150ns and dependent on the Cu proportion. Increasing the Cu by 30 at.% increased the crystallisation time to 350ns. The electrical resistivity of these materials was approximately 4Ωm. This allows efficient Joule heating and electrical switching was demonstrated. A finite element analysis has shown that this material can be amorphised with a current of just 0.4µA in comparison to 2.5mA required for a similar volume of Ge₂Sb₂Te₅. Therefore Ga:La:S:Cu shows potential as a future electrical phase change data storage material.

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ORCID for Daniel Hewak: orcid.org/0000-0002-2093-5773

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