Study of phase-change materials for low-energy devices

This PhD thesis presents recent advancements in phase change materials, a family of materials employed in the so-called phase-change memories (PCMs) for neuromor- phic applications. PCMs exhibit favorable energy consumption compared to other memory types. Nonetheless, the fabrication of nanoscale PCM devices faces several challenges, such as resistance drift and atomic migration under cycling. Recently, it has been proposed that these issues can be alleviated by designing phase-change heterostructures (PCHs), in which the phase-change material is confined by a care- fully chosen material. The main aim of this work is to explore the feasibility of this approach and to show how it can be implemented in actual devices. The thesis is structured into several chapters, starting with a broad overview of PCMs and PCHs, emphasizing both their current limitations and the promising developments that have emerged over recent years. Chapter 2 provides a discussion of the theoretical and computational aspects of ab-initio simulations. Subsequently, chapter 3 explores novel PCHs based on two important phase-change materials, Ge2Sb2Te5 and GeTe, both confined by TiTe2. My findings suggest that these het- erostructures can be realized experimentally and exhibit good switching properties. Chapter 4 focuses on a PCH made of pure Sb confined by TiTe2. In recent years, there has been growing interest in using pure Sb as an elemental phase-change mate- rial. Unlike GeTe or Ge2Sb2Te5, it has a simple stoichiometry, offering clear benefits from both an experimental and a practical standpoint. The results demonstrate that this Sb-based PCH displays ultrafast recrystallization properties. Chapter 5 details my computational contribution to a project conducted at RWTH Aachen University in the research group led by Professor Matthias Wuttig. It addresses the study of thin Bi films with varying thickness, with particular attention to Peierls distortions and bonding mechanisms. The findings indicate that the properties of Bi can be tuned by altering the thickness of the film and that the changes are governed by Peierls distortions and by the transition from covalent bonding to metavalent bonding. Finally, Chapter 6 provides an account of the collaborative work with STMicroelec- tronics, involving TCAD simulations of Ge2Sb2Te5-based memory cells. Specifically, models were developed and calibrated to reproduce the behavior of the device and to elucidate various characteristic phenomena. In conclusion, this thesis improves the understanding of PCHs as a novel device concept for in-memory and neuromorphic computing and proposes new strategies to optimize PCH-based devices, highlighting substantial potential for further research in this field.