Phase Change Memory Materials via Non-Aqueous Electrodeposition into Nano-structured Templates

This project is concerned with developing non-aqueous electrochemical methods and suitably tailored reagents to facilitate spatially selective electrodeposition of binary (e.g. In2(Se,Te)3, Sb2(Se,Te)3, Ge(Se,Te)) and the ternary chalcogenide materials (e.g. Ge2Sb2Te5, doped Sb2Te3) for applications in solid-state phase change memory (PCM). The key objectives are to demonstrate successful deposition of the target materials inside nano-scale (down to 2 nm) confined cell structures and to establish the effect of down-scaling pore size on the deposition process. Successful electrodeposition of well-defined compound semiconductor alloy compositions of these types will provide a significant new enabling technology which could also have a major impact on the other applications requiring semiconductor alloy deposition on a nano-scale. Using non-aqueous solvents (such as MeCN, propylene carbonate or chlorofluorocarbons) will bring several advantages over aqueous processes:(i) the use of a much wider range of reagents which can be tailored to the application;(ii) access to more reactive alloy compositions;(iii) a wider range of deposition potentials,while these solvents are more readily available, less expensive, much more easy to purify and less viscous (important for penetrating narrow. high aspect-ratio pores) than for example ionic liquids.These chalcogenide alloys are of major interest for phase change memory (PCM) materials - an emerging technology for non-volatile memory which is expected to compete with (and even replace) FLASH memory in specialist and everyday consumer electronics. Production of these alloys by electrodeposition could bring several advantages over current methods of production (mainly PVD), since it allows spatially selective deposition (since the materials are only deposited on the electrode surface), filling the pores of the templates from the bottom, hence enabling complete filling even of very narrow nanopores - leading to a very significant reduction of the dimensions of each individual cell, and hence potentially much higher cell density. In turn this will lead to faster switching between the crystalline and non-crystalline phases, leading to smaller devices and greater energy efficiency. To achieve these targets requires a multidisciplinary approach involving several key contributions: (i) to develop (and refine) new tailored molecular compounds (electrochemical reagents) with elements from the p-block (gallium, indium, germanium, antimony) in combination with groups containing the chalcogens i.e. the elements selenium and tellurium; (ii) to use these as reagents for the growth of the binary & ternary alloy materials by electrochemical deposition into nano-structured silica or alumina templates comprised of very narrow parallel pores with well-defined diameters between 1000 nm and 2 nm; (iii) characterisation of the deposited materials to determine the element ratios present (composition), their crystal structures, and phase change properties;(iv) deposition of the 'best' compositions into well-defined pores on a chip array to allow switching of the arrays memory cells in an actual device, hence demonstrating the true potential of this new approach.The team of investigators brings together a complementary and internationally unique set of skills and expertise to achieve these targets, while the input from our Project Partners, Ilika Technologies Ltd will add considerable value to the project.

The research described in this proposal will develop non-aqueous electrodeposition as an enabling technology for the controlled growth of binary and ternary semiconductor alloys inside nano-structured templates. The target application is solid-state phase change memory. The work has potential in the short to medium term to have very considerable impact in the field and is likely to attract much interest to companies engaged in materials discovery for a wide range of applications, in addition to those semiconductor companies in the private sector working on PCM manufacture, as indicated in the attached Letter of Support from Ilika Technology Ltd. PCM is a very timely and topical area of research since it is an emerging technology that has the capacity to compete with FLASH memory in specialist and consumer electronic devices. One of the key stumbling blocks at present is the difficulty in down-scaling the memory cell size to the targets set out in the Semiconductor Roadmap - our method has a really good chance of achieving this, since the deposition can only occur at the electrode surface - i.e. at the bottom of the nano-pores. Furthermore, electrodeposition allows control over the thickness of the deposit, simply by switching off the potential. Thus, growing the alloys to a specific (regular) depth inside very narrow pores using our approach is entirely feasible. Key targets are to develop reagents which can be used to demonstrate successful deposition of the target compositions via the non-aqueous electrochemical method, and to demonstrate that these materials can be grown inside the pores of nano-structures of diameter 1000 to 2 nm. Phase change media feature strongly in the new EPSRC-supported Centre for Nanostructured Photonic Metamaterials in the University of Southampton. Discussions with colleagues there have already indicated considerable interest in the electrodeposition of PCMs as an enabling technology and we will be active in exploiting new opportunities arising in this area too. In addition to the IP arising from this work (which we intend to protect through patent applications and exploit with advice and expertise from staff in the Research & Innovation Services in the University of Southampton), the project has the potential to deliver very high impact publications - internationally leading journals such as Nature Materials, Advanced Materials, JACS and Angewandte Chemie will be targeted. The relevance of the topic of this research project and their end applications mean that it lends itself extremely well to public (consumer) dissemination. The investigators have established track records in outreach and the 'promotion of science' to non-specialist audiences and will be active in pursuing these dissemination pathways e.g. through talks and workshops for schools, science fairs etc., articles in newsletters, Education in Chemistry, Chem. & Eng. News etc. Support in the preparation of suitable news items and flyers will be provided by our marketing expert in the Faculty (C. Mills).