Research On Heat Transfer Characteristics Of GeTe/Bi₂Te₃ Superlattice Phase-change Materials

The applications of Phase change random access memory（PCRAM） are severely constrained by the factors such as considerably large programming current and the thermal disturbance between adjacent storage units. These problems can get improved through optimizing the heat transfer characteristics of phase change materials, for example, Ge Te/Sb2Te3 superlattice phase-change material with low thermal conductivity can improve the power consumption and the thermal crosstalk of PCRAM. But the related research on heat transfer characteristics of superlattice phase-change materials has just started, thus the heat transfer characteristics of superlattice materials and how to get a lower thermal conductivity still need further study.As Bi₂Te₃ and Sb2Te3 share the same rhombohedral crystal structure, replacing Sb atoms with Bi weakens the cohesion and enhances the crystallization, this thesis puts forward the Bi-based Ge Te/Bi₂Te₃ superlattice phase-change materials, and systematically studies the change tendency of the thermal conductivity of superlattice phase-change materials through the construction of experimental measuring platform and the first principles calculation. The findings make Bi-based Ge Te/Bi₂Te₃ superlattice materials to be promising candidates as phase change materials, which can reduce the programming current and also suppress the thermal disturbance between adjacent storage units in PCRAM.A platform for thin film thermal conductivity measurement based on 3ω method has been set up and improved. Through measuring the thermal conductivities of Si O2 film and silicon substrate, the good accuracy and stability of the measurement platform have been fully confirmed. The thermal conductivities of phase-change Ge Te/Bi₂Te₃ superlattice materials are studied by the self-setup 3ω measurement platform, the results show that the thermal conductivities of Ge Te/Bi₂Te₃ superlattices are lower than the thermal conductivities of Ge Te/Sb2Te3 superlattices, and their thermal conductivities both drop firstly and rise afterwards with increasing interfaces. Additionally, Ge Te/Bi₂Te₃ superlattice can achieve a lower thermal conductivity than traditional Ge2Sb2Te5 phase-change materials. The effects of film thickness have also been studied, and the results show that the thermal conductivities of Ge Te/Bi₂Te₃ superlattices reduced with the decrease of the film thickness.Through the simulation by first principle theory, the phonon properties and the thermal conductivities of Ge Te/Bi₂Te₃ superlattice materials were calculated, the results are in good agreement with the measurement of 3ω method, namely the thermal conductivities of Ge Te/Bi₂Te₃ superlattice materials show downward trends in comparison with Ge Te/Sb2Te3 superlattice materials. We preformed contrast calculation to confirm the stronger interface scattering caused by the introduction of Bi atoms in Bi-based Ge Te/Bi₂Te₃ superlattice materials is the main reason for the decrease of thermal conductivity.