Crystal Structure, Electric Properties and Phase Transition of Phase Change Materials Ge₂Sb₂Se_5xTe_5−5x

Computers with solid state memories own their success in part due to phase change materials (PCMs). These are binary, ternary or even quaternary compounds, consisting of Ge, Sb and one or more chalcogen ions such as Te and Se. PCMs have been of interest for decades because their reversible transformation from crystalline to amorphous states of matter is coupled with vastly different electric and optical properties that are so uniquely suited to devices such as random access memories. PCM can be made into random access memories due to their vast differences in electrical and optical characteristics between amorphous and crystalline phases. In today’s PCM research, the central problem lies in finding a PCM with high performance in reading and writing data. This requires PCMs to have fast switching, low activation energy, and stable amorphous phases. The first PCM Ge2Sb2Te5 (GST-225) was discovered in 1987. After that many material systems have been proposed such as AgInSbTe, GeSbMnSn and AuTe2. Of these, GST-225 remains a promising candidate due to its unique physical properties. A key structural component to the transformation is the appearance of a rocksalt like structure that is intermediary to the hexagonal ground state and the amorphous state. The rocksalt structure is riddled with vacancies and acts as a conduit to the transition from the crystalline to the amorphous phase. However, the rocksalt like structure is prevalent in thin films only.

In this thesis, bulk Ge2Sb2Te5 (GST-225) was investigated to explore the structural and electronic properties with Se doping. Ge2Sb2Se5xTe5−5x (GSST-225) with varying degrees of Se doping were synthesized under two different conditions involving slow cooling (SC) and liquid nitrogen quenching (Q). It is shown that upon doping, liquid nitrogen quenched Ge2Sb2Se5xTe5−5x (GSST-225) exhibits a direct hexagonal-to-amorphous phase change above 𝑥 > 0.8 The rock-salt like structure appears as a second phase with a volume fraction that does not change as a function of the doping. The electric resistivity and optical reflectivity of GST-225 undergo changes with several orders of magnitude when it’s transformed between different phases. In this work, the phase change of GSST-225 is accompanied by a metal-to-insulator transition (MIT), with several orders of magnitude increase in the resistivity on approaching the amorphous state. A similar MIT is observed even without the phase change, in hexagonal crystals with doping levels above x > 0.8. Through crystallization kinetics study, we reported the re-crystallization onset temperature and activation energy of amorphous GSST. In our neutron diffraction experiments, PDF analyses were performed to research the local structure evolution across phase transition in Q-GSSTs. We reported the existence of local structures in amorphous GSST-225. These local structures have shapes like tetragon and feature Ge/Sb atoms siting at centers and Te/Se atoms siting at vertices.

From these results, GSST-225 is a suitable candidate for making random access memories. GSST-225 has fast phase transition and its amorphous phase is stable for data storage. It requires less activation energy to complete phase transition cycle. A more energy efficient type of random access memory can be made using this material.