Experimental and Computational Studies of Ge-rich and Se-substituted GeSbTe Phase-change Memory Materials for Flexible Integrated Circuit Applications

Abstract

Flexible electronics are a key requirement for the full realisation of the internet of things, a network of interacting everyday objects with smart capabilities. Many of these objects are required to be lightweight, portable and flexible, therefore, it follows that there is a requirement for a suitable flexible memory technology. This thesis focuses on the implementation of phase-change memory (PCM) for this purpose. PCMs store information in their states, which can be amorphous, crystalline or some mix of the two. Here, both simulation and experimental methods are used to realise flexible PCMs that are suitable for integration into low-cost and easy to fabricate flexible integrated circuits. Furthermore, phase-change materials with increased crystallisation temperatures are of particular interest as they allow higher temperature manufacturing processes to be used without affecting the initial as-deposited amorphous device state. This can provide a simple route to a write-once phase-change memory.

Here, germanium rich and selenium substituted modifications to the archetypal Ge2Sb2Te5 composition are investigated for application as both non-volatile write-once and rewritable memories in flexible electronics. Thin films of Ge7Sb2Te5 and Ge2Sb2Se4Te1 compositions are deposited and their electrical, optical and structural properties explored as a function of temperature using four-point probe electrical testing, Raman spectroscopy, and X-ray diffraction. A computational model, that combines electrothermal and phase-change simulations, is also developed to enable the design and exploration of the likely switching behaviour of device architectures suitable for use in low-cost flexible electronics applications. The phase-change model uses a Gillespie cellular automata (GCA) approach that has been extended from previously reported implementations to include the capabilities for more physically realistic simulation of thermal boundary resistances, electrical conductivity and thermal conductivity; as well as an important extension to enable the modelling of Se-substituted phase-change materials. Subsequently, this model was used to demonstrate a fully rewritable PCM using the Ge2Sb2Se4Te1 composition on a flexible substrate suitable for integration into flexible integrated circuits, where devices sizes and excitation pulses are significantly larger than in conventional PCM architectures.

Finally, PCM devices using both the Ge7Sb2Te5 and Ge2Sb2Se4Te1 compositions were fabricated, in both crossbar and pore-cell configurations, and their switching properties experimentally investigated. Devices fabricated with Ge7Sb2Te5 were shown to be suitable for write-once application, whereas the use of Ge2Sb2Se4Te1, with its relatively slow crystallisation rate, enabled rewritable devices to be successfully achieved.