2, DSI, Singapore, , Singapore
Our objective was to investigate new ways to tune the properties of chalcogenides. We investigated two different chalcogenide materials, Ge2Sb2Te5 and Agx(Sb2S3)1-x. The Ge2Sb2Te5 material was investigated in a phase change random access memory cell. The effect of a voltage pulse on the cell resistance was measured whilst the cell was in the SET (crystalline) state. At voltage levels below that necessary to RESET (amorphise) the cell, we found that the cell resistance increased during the first 7 ns of the voltage pulse. On longer time scales the cell’s resistance returned to the its initial resistance. Our analysis shows that the resistance transient is not due to capacitive effects and can be explained by a model that includes both phonon scattering and charge carrier excitation. We find that this non-phase change switching effect is highly repeatable and can be cycled more than 109 times. We believe, therefore, that this effect could present a route to performing high speed volatile memory switching and non-volatile phase change switching in the same physical memory cell.
The electric field also has a substantial impact on the crystallisation temperature of Agx(Sb2Te3)1-x. The crystallisation temperature of Sb2Te3 is lowered by adding up to 7% Ag into the amorphous material. We find that Ag ions are readily driven into the amorphous Sb2Te3 structure by applying an electric voltage to Ag electrodes that interface the Sb2Te3 film. Sb2Te3 crystallises in to an orthorhombic structure with an Sb-S bond length of 2.54 Å . Our X-ray absorption measurements showed that the Ag-S bond length in Ag doped Sb2Te3 is 2.34 Å. Since the Ag-S bonds are shorter than the Sb-S, they are likely to be more stable and this implies that smaller crystal nuclei are likely to form in Ag doped Sb2Te3. This theory is supported by the an observed reduction in the crystal nuclei incubation time when Ag ions are forced into the Sb2Te3 structure with an applied electric field of 200 kV/m . These results show that applying electric fields can be used to enhance the crystallisation kinetics of Ag-doped Sb2Te3.
In conclusion, the results presented in this poster show that electric fields can play an important role in crystallisation of chalcogenides and the effects should be exploited in memory and computational processing technologies.
This work was funded my the Singapore Ministry of Education under the auspices of projects MOE2017-T2-1-161 “Electric-field induced transitions in chalcogenide monolayers and superlattices” and T1MOE1703 “Advanced Intelligent Materials (AIM)”. We are also grateful for EXAFS beam time at SPring-8 BL01B1, proposal number: 2014A1244.
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