A Structural and Optical Insight on Ge-Sb-Te based Nano-composites

Electronic memory and computing devices currently rules our digital lives, creating and consuming more than 10^21 bytes of data per year. This amount is expected to grow exponentially, questing for a so-called “hardware revolution”. Bit size-reduction governed the development of solid-state-memory and semiconductor technologies in the last decades. Yet, the innovative concept of an “universal memory” emerged for transcending the size-node. Indeed, this novel system combines high-speed computation and high-density storage skills. Memory devices based on Phase-Change Materials (PCMs) may serve to the scope, defying to scale reduction and both having a competitive erase/read speed with respect to the primary memory systems -as Static/Dynamic Random Access Memory (SRAM, DRAM)- and the large capacity of non-volatile secondary/tertiary storage systems -as Solid State Disk (SSD), Hard Disk Drive (HDD), Digital Video Disc (DVD). The appeal of Phase-Change Materials arises from their ability to rapidly and reversibly switch between the amorphous and crystalline states, via optical or electrical pulses. Interestingly, the two structural phases own significantly different physical properties, in particular in terms of reflectivity and conductivity. Many binary or ternary compounds display phase-change features, still Ge-Sb-Te (GST) alloys are the prominent members of this class of materials and are largely employed in industry. However, the main drawback is the PCMs relatively high operation power consumption, in the form of energy required for the phase transformation and of energy dissipation through -primarily- thermal diffusion. This thesis follows a GST dimensional strategy for power optimization -while maintaining advanced performances- for future integration in novel memory devices. The followed approach includes the case of 2-D highly-textured superlattice structures -of alternately deposited GeTe bilayers and Sb2Te3 quintuple layers- and of 0-D Ge2Sb2Te5 nanoparticles. Chapter 1 reviews the state-of-the-art of Phase-Change Materials, that stimulated the research questions addressed in the present work. Extended X-Ray Absorption Spectroscopy (EXAFS) -briefly presented in the first part of Chapter 2- is a powerful experimental tool for investigating a material’s atomic structure. As described in Chapter 3, one of the two allotrope crystalline phases of (GeTe)-(Sb2Te3) superlattices is revealed in details via EXAFS measurements performed at the Ge and Sb K-edges. The emerged structural picture is commented in light of the proposed models in literature, advising a power-saving yet over-simplified switching process occurring in the superlattice structure. Chapter 4 tackles the problem of power consumption by experimentally demonstrating the energy boost on the optical phase-change process occurring in 0-D Ge2Sb2Te5 nanoparticles. Here, a stable but reversible transition from the crystalline to the amorphous state of nanoparticles is induced with a single low-fluence femtosecond laser pulse. Thermodynamic, optical and structural considerations corroborate the experimental evidence. The laser source together with the setup used for the optical measurements are described in the second part of Chapter 2. The optical arrangement -conceived for time-resolved measurements- led to follow also the relaxation pathways of photo-excited nanoparticles below the threshold fluence for permanent amorphization. The results of this study are unveiled in Chapter 5. The ultrafast dynamics are compared to theoretical simulation and modelled with a phenomenological rate equation. Remarks on the resulting time-scales and the underlying interaction mechanisms -questioning the nature of the resonant bonding- close the Chapter.