Atomic Layer Deposition of Metal Phosphates

Abstract

Since the introduction of lithium iron phosphate (LFP) as an electrode material for Li-ion batteries (LIB’s), metal phosphates in general have become increasingly important. The strong covalency of the P-O bonds typically results in a very good thermal and structural stability, which is one of the main reasons for the success of LFP as a LIB electrode. However, the applications of these metal phosphates are multi-fold. Apart from LIB’s, they have been proven to be promising towards e.g. electrocatalytic water splitting, biocompatible coatings or even as protective coatings for luminescent materials.With these applications in mind, the research towards the deposition of these materials also becomes increasingly important. Atomic Layer Deposition (ALD) has emerged as a deposition technique with unique nanotailoring capabilities compared to conventional methods, particularly owing to its high thickness and compositional control. Down-scaling micro-electronic devices according to Moore’s law has already increased interest in the unique capabilities ALD can offer. However, in addition to using ALD for the fabrication of such devices, it can also offer a unique way to create model systems to gain a fundamental insight for a broad range of applications.From this it can be seen why the combination of both research fields, i.e. ALD of metal phosphates, is gaining a lot of interest over the past years. Since the first metal phosphate has been deposited in 1969, a big challenge has been to find a suitable phosphate precursor. Today, the most popular choice proves to be trimethyl phosphate, i.e. TMP. It has allowed for the deposition of a variety of phosphates (e.g. aluminium phosphate, lithium phosphate, iron phosphate, etc.), but more research is required to further extent this research field. To stimulate this research, a comprehensive study on the currently (un)available literature is needed. It would e.g. allow to get a better understanding on the effect of different phosphate precursors and/or various reaction mechanisms during ALD can be better understood, as well as the potential applications of these materials (such as lithium ion battery applications). As such a study on ALD of metal phosphates was so far not yet available, we hope that this work can create a view that is of interest towards various scientific fields, and help to guide future thin film researchers.