I got 99 problems...

With all of these developments and continuing research going into finding better ways and better precursors to make better thin films there are definitely going to be problems that are going to arise. For this post we are going to focus on thin films made from the group IV and V transition metals with nitrogen to made nitride thin films. There is a demand in industry for nitride thin films due the desirable qualities they possess, including how hard they are, long wear residence, high melting point and good chemical resistance. These qualities of nitride thin films makes them the major component in many applications, including  coatings for cutting and grinding tools, wear surfaces and semiconductors.

Group IV and V transition metals, Rf and Db are not looked at though as they are both highly radioactive and do not exist in nature.
One of the biggest problems when depositing thin films is contamination from other elements. When a nitride thin film has contamination from carbon and/or oxygen it reduces the properties of the thin film. Contaminations cause an increase in electrical resistance (which is a problem when depositing TiN films which need to be conductive) and also decrease the hardness of the film which will make it lose its ability to coat things. To overcome this issue, more research has gone into finding single source precursors. A single source precursor is a complex that will be deposited to form a thin film without the addition of any other complexes i.e the addition of a reducing agent or a nitrogen source (ammonia).

TDEAT, an example of a single source precursor used to deposit TiN thin films.
Another focus for future precursors is to be able to synthesis complexes in lower oxidation states (as discussed in Where did that plane get its nice coat?). A key example of this is TaN which was mentioned above. When in the 3+ state it forms the stoichiometric form which is conductive whereas if the +5 state is formed it dimerizes and forms the Ta₃N₅ which is insulating. It’s clear what issues can arise if the wrong oxidation state is formed. The same issue arises for other group IV and V transition metals when using lower oxidation state complexes. When in a lower oxidation state, the complexes have a tendency to dimerize like in the case of zirconium coordination complexes in a 3+ state. An idea to overcome this is to use bulky ligands. The bulkiness of the ligands will cause strict steric conditions which will prevent the complexes from dimerises and keeping the final thin film in the desired low oxidation state. 

Ti(III) complexes can be generated by using the stable nitroxyl radical TEMPO (2,2,6,6-tetramethylypiperidine-N-oxyl), an example of using cyclopentadienyl as a ligand
It is clear from the published literature available, that currently low oxidation state precursors for group IV and V nitride thin films are not widely or commercially available. There is a lot of attention going into research to find low oxidation state precursors, especially for TiN and TaN thin films for application in semiconductors. Although there is not been found yet on making precursors themselves, a lot as been done to find low oxidation state coordination compounds that could be carried forward in the future and applied as precursors. Key ligands that have emerged as potential precursors are hydrazine’s, amido containing ligands, cyclopentadienyl and guanidinates. The characteristic which is prominent in all of these ligands is their ability to stabilise the low valence nature of the transition metals in question as they are all electron rich. An example of the successful application of precursors which incorporate these ligands include the deposition of TiN films from 1,1-dimethylhydrazine which was also able to promote the reduction of Ti(+4) to Ti(+3). 

An example of the use of guanidinato ligands, [Ta(NEtMe)(N‑^tBu){C(N-ⁱPr)₂-(NEtMe)}₂] (RHS compound) was successfully used to deposit (+4) TaN thin films
Almost all of group IV and V have shown potential to be deposited as nitride thin films from low oxidation state precursors. The next step in the research to find this low oxidation state precursors will be to test if they have the right qualities to be used in CVD/ALD deposition to make the thin films.