High-throughput synthesis and screening of chalcogenide thin films for phase-change memory

The limitations of Flash memory as an electronic storage medium have driven the development of newtechnologies. Amongst these, Phase-Change Random Access Memory (PCRAM) has emerged as a viable replacement for Flash due to its greater number of write cycles and faster write speeds. However, while phase-change materials have been known for over 50 years interest has only picked up over the past decade. This has created a gap in understanding of the structural and functional properties of these materials, which is only now being addressed one material at a time. The research presented here introduces a high-throughput (HT), combinatorial approach to the synthesis and screening of phase-change chalcogenide glasses. This approach focused on the screening of properties relevant to the use of chalcogenides as potential PCRAM materials. Starting with the seminal Ge:Sb:Te system, a HT workflow was developed that utilised Raman spectroscopy and X-ray diffraction to study the structure of both its amorphous and crystalline phases. The crystallisation temperature and electrical resistivity were chosen as pertinent functional properties to be evaluated under this approach. Using HT physical vapour deposition, thin film libraries of the largest reported Ge:Sb:Te compositional space were synthesised, covering the majority of the ternary space. HT software tools enabled the analysis of the film’s structural evolution before and after crystallisation, revealing the formation of 5 distinct phases after annealing at 200 ◦C. These tools also facilitated the analysis of functional properties along six pseudobinary lines, four of which are novel to the literature. Specifically, the GeTe–Sb and GeSb–Te lines were most useful towards understanding the relationship between the structure and function in the Ge:Sb:Te system. The HT approach was applied to the less studied N-doped Ge:Sb:Te system, for which the most extensive compositional space yet was also synthesised. After annealing at 300 ◦C two types of materials, related to N-Sb2Te3 and N-Ge2Sb2Te5, were identified through the first systematic Raman and XRD analyses of this system. Their functional properties were studied along pseudobinary lines and the results contrasted to those of Ge:Sb:Te. This led to the conclusion that N-Ge:Sb:Te materials would be more suitable for PCRAM use than Ge:Sb:Te in high temperature applications. A novel parametric testing platform, the Integrated Microelectrode Testing System (IMTS), was developed in order to characterise the electrical threshold switching field of both Ge:Sb:Te and N-Ge:Sb:Te materials, resulting in the first systematic analysis of this parameter on both systems. In the process, a model to calculate the dielectric constant of Ge2Sb2Te5 using I-V data from the IMTS was derived. Overall, the results of these experiments established the validity of the high-throughput approach as a means to produce and evaluate phase-change chalcogenides more quickly and as reliably as traditional materials research techniques