Study On Measurement And Calculation Of The Anisotropic Thermal Conductivity Of Phase-change Film Materials

Phase change memory(PCRAM)is considered to be a strong competitor of next-generation memory due to its fast erasing and writing speed,low power consumption,and continuously shrinkable size.However,thermal crosstalk between adjacent cells has become the main obstacle to its large-scale commercialization.In-depth study of the heat transport characteristics of phase change materials is very important to solve this problem.The current research on the thermal conductivity of phase change materials is limited by the lack of simple and reliable in-plane thermal conductivity testing methods for thin films,and most of them are treated as isotropic materials.This processing method is partial and inaccurate,because the thermal conductivity of the actual phase change films has significant anisotropy,and the anisotropy is controlled by the vacancies and the interface in the structure.In view of the above problems,this paper selects Ge₂Sb₂Te₅ films with significant contribution of vacancies to the phase transition process and GeTe/Sb₂Te₃ superlattice films with significant interface effects as the research objects.The regulation effects of vacancies and interfaces on anisotropic thermal conductivity are studied respectively.And this paper carries out first-principles calculations to reveal the deep physical mechanism of the regulation,in order to fully and accurately understand the heat transport characteristics of phase change film materials,and provide theoretical guidance for solving the PCRAM thermal crosstalk problem.The main results obtained are as follows:(1)A series of Ge₂Sb₂Te₅ films with different ordering degrees of vacancies are tested,revealing the regulation of the vacancies to the anisotropic thermal conductivity of Ge₂Sb₂Te₅ films:as the degree of ordering of vacancies increases,the out-of-plane thermal conductivity and in-plane thermal conductivity of Ge₂Sb₂Te₅ films both increase,and the anisotropy of thermal conductivity also increases.Several crystalline Ge₂Sb₂Te₅ models with different vacancy states are constructed,and their phonon characteristics are obtained through first-principles calculations,revealing that the low-frequency migration of the vibration mode of Te atoms is the main reason for the increase in the thermal conductivity of Ge₂Sb₂Te₅ during the ordering of vacancies.Further research finds out that the optical branch phonons plays a major role in the increase of the anisotropy of the thermal conductivity.(2)The thermal conductivity of a series of GeTe/Sb₂Te₃ superlattice films with different numbers of periods has been tested,uncovering the regulation of the interface to the anisotropic thermal conductivity of the GeTe/Sb₂Te₃ superlattice film:the phonons with different motion directions are influenced by the interface to the different degree,thus the out-of-plane thermal conductivity and in-plane thermal conductivity of superlattice films vary with the number of interfaces,but the anisotropy of the thermal conductivity of superlattice is greater than that of its component materials.The phonon spectrum,phonon density of states,and partial density of states of the superlattice are obtained through first-principles calculations,revealing that the interface scattering caused by the mismatch of the lattice vibration modes of the component materials is the the main reason for the decrease of the thermal conductivity of the superlattice.Further research infers that the anisotropy of the thermal conductivity of superlattice is mainly affected by the optical vibration modes of Sb and Te atoms.