Atomic layer deposition of Hafnium-nitride and Lanthanide nitrides

The rapid growth of computer technology over the last three decades has been largely driven by advances in silicon based materials and processes, which have enabled the development of smaller, faster, lower power transistors. However, we are rapidly approaching fundamental limits of silicon technology and new materials are required to enable future advances. Considerable research has been carried out on developing metal oxides based on materials such as hafnium (Hf) and rare-earth (RE. Such as Gd, La, Nd & Pr) elements, these oxides could be used to replace the native silicon oxide currently used in microelectronics, allowing further device scaling. In contrast to these oxides, little work has been reported about their nitride counterparts, which is perhaps partly due to the challenge associated with their production using conventional-chemistry or thin-film techniques. However, Hf- and RE- nitrides display a wide range of useful electronic and magnetic properties, which are potentially exploitable in a variety of electronic, spintronic and optoelectronic devices. In conventional electronic integrated circuits, nitrides could be used alongside their oxide counterparts, acting as diffusion barriers, nucleation layers or electrical contacts. While in advanced spintronic devices, their magnetic properties could be exploited to enable the production of transistors that operate using the quantum properties of electrons; opening up a new era in computing. In this project, Hf and RE -nitrides will be investigated, using atomic layer deposition (ALD) to produce nitride thin films, and a range of advanced characterisation techniques to study their structural and electrical properties. The ALD process involves exposing a heated surface (usually silicon) to alternating pulses of two complementary precursor gases; one of these is a metal-containing molecule and the other a reactant (such as water or ammonia for oxide or nitride deposition respectively). The precursors are absorbed onto the surface and undergo chemical reactions with the previously absorbed surface molecules, by-products of the reactions are carried away from the surface by a vacuum system leaving a pure oxide, nitride or metal film on the surface. To avoid gas phase reactions between the active gases, inert gas purges are introduced between exposures. As the deposition surface can only accommodate a limited number or precursor molecules, ALD growth is self-limiting, which means that each pair of precursor pulses deposits about one layer of atoms. As a result, ALD allows a very high level of control over films thickness and also produces extremely uniform films (even on high-aspect ratio etched structures), which are essential for advanced microelectronic devices. A number of transition metal-nitrides, most notably titanium and tungsten, have been successfully deposited by ALD. In the first part of this project we will investigate the ALD and properties of Hf-nitride, this material is of technological significance as it complements Hf-oxide, which is expected to be a key material in the production of integrated circuits in the near future. In the second part of this project, we will investigate a number of RE-nitrides, these are expected to have a wide and continuous range of useful electronic and magnetic properties. This ambitious multidisciplinary project will seek to develop a reliable thin film deposition method, which is readily scalable to mass production manufacturing. It will also seek to gain a greater understanding of the physical and electronic properties of these unusual materials.