PhD Thesis Proposed Defense Session (Mr. Mohsen Vahedi)

Title: Phase-field model development for determining pressure and temperature of amorphous/crystallization in GST ternary alloy with structural defects

Presenter: Mohsen Vahedi

Supervisors: Dr.Mahdi Javanbakht, Dr. Mohammad Mashayekhi

Advisors: Dr. Hossein Jafarzadeh

Reviewers: Dr. Mohammad Silani, Dr. Saleh Akabarzadeh, Dr. Mehdi Salmani Tehrani

Abstract

Alloys based on Group 16 elements or chalcogens have a wide range of applications in thermal, electrical, and optical fields, which has led to extensive studies on their phase transitions. One of the most important materials in this category is the ternary GST alloy composed of Germanium (Ge), Antimony (Sb), and Tellurium (Te), which has a special application in phase-change memory devices. Research has been conducted extensively on this alloy through atomic simulations and experimental methods, focusing on its phase transition between crystalline and amorphous structures. Another method used in this thesis is the phase-field approach, which can effectively analyze the thermodynamics, kinetics, and structural changes of this material. Given the wide working temperature range of GST nanolayers (from ambient temperature to a melting point of 889 Kelvin), thermal strain and the driving force associated with it play a significant role in determining the driving force for crystallization-amorphization transitions. In this thesis, the driving force due to thermal strain during the transition is incorporated into the model. Additionally, previous models lack the capability to analyze issues under mechanical loads, especially high pressure, which makes it difficult to accurately predict crystalline or amorphous transition pressures. Here, by considering the dependency of the bulk modulus on hydrostatic pressure and phase type, the phase-field model is refined based on the Murnaghan equation of state, making the bulk modulus dependent on hydrostatic pressure. It is expected that with applied pressure and increased bulk modulus, the transition pressure will match the results from existing studies. Furthermore, existing research indicates that in structurally defective samples, the transition pressure is lower than that of defect-free samples. Here, specifically, by considering the nanovoid defect in the model, the transition pressure aligns with experimental and atomic simulation results. Another important point is that GST nanolayers, under pressure and within a low-temperature range (below 450 Kelvin) as well as higher temperatures, experience two distinct structural changes, indicating two different types of phase transition strains for this alloy. According to existing results, this has a significant impact on determining the transition pressure. Therefore, the strain of the phase transition in low-temperature and high-temperature regimes will be examined. Furthermore, existing research indicates that the crystallization temperature of GST nanolayers under applied voltage increases progressively as their thickness decreases, leading to sudden crystallization. This is expected to be influenced by size-dependent effects, the type of electrodes used, and the contact method, resulting in the distribution of temperature and internal stress. Therefore, by refining previous models, the prediction of sudden crystallization and the corresponding temperature, along with its dependency on dimensions, becomes feasible. Given the multi-physics model, which includes equations for phase-field transformation, elasticity, heat transfer, Poisson’s equation, and the Murnaghan equation of state, the COMSOL Multiphysics simulation software is used

Keywords: Phase-field method, GST alloy, crystal-amorphous phase transition, pressure-induced amorphization, structural defect, sudden crystallization