Musings

Well, as a newcomer to the field, and a theorist at that, I presently know very little about the CVD process, although I can state that the wired article has certainly been ‘sexed up’ (as the media here in the U.K. might put it) for the general audience. And Apollo Diamond are certainly not the only group capable of producing single-crystal CVD diamond, patent or not.

I can however comment on the “diamond-based integrated circuits” side of things. I have recently been working on the n-type conductivity problem in diamond. Now, we know that p-type conductivity is not (much of) a problem, due to substitutional boron having a ‘low’ activation energy of 0.38 eV in diamond.

However, the situation with regards to a suitable dopant for n-type behaviour is not so straightforward.

Nitrogen is readily incorporated into diamond, but has a deep donor level — its activation energy is about 1.7 eV.

Phosphorus doping of diamond has been an encouraging success story — It has taken nearly 10 years, but at least two groups (Japan, Belgium) can now claim 100% reproducibility in doping diamond with P and getting n-type activity with an activation energy of 0.6 eV. Some nice pn—junctions have been successfully fabricated from boron-doped and phosphorus-doped diamond by both of these groups.

Sulphur as an n-type donor has had a controversial history, with experimental and theoretical groups putting its donor level all over the place. Also, one of the first groups to report n-type activity with sulphur-doped diamond was later exposed as having been working with diamonds with non-negligible concentrations of boron, and that the activity was in fact p-type.

Now, my modelling calculations using density-functional theory predict that arsenic and antimony will have donor levels shallower than P, with activation energies of about 0.3—0.4 and 0.2—0.3 eV respectively. This follows from a simple argument treating the donor electron as experiencing a hydrogen-like (effective mass) potential, and with an increasing orbital radius as the atomic number of the dopant atom increases N, P, As, Sb…

Incidentally, my calculations predict sulphur (and singular S—H) to have a relatively deep single-donor level.

The problem with As and Sb doping lies (arsenic poisoning notwithstanding) in getting these large impurity atoms into the diamond lattice. However, even P has a negligible theoretical bulk solubility in diamond, but it is know to be readily incorporated into ⟨111⟩ growth sectors, via a mechanism that is currently not understood. Maybe As can undergo the same process, be incorporated into the bulk, and act as a shallow donor?

Mentioned in the article is a “novel way of inverting boron’s natural conductivity” in order to get n-type behaviour. This surely refers to the work by Teukam et al. on deuterated boron, recently published in Nature Materials. Now my calculations, and those of others in my group on hydrogenated boron defects in diamond do not support these claims, and indeed there is a lot of scepticism in the community concerning this paper.

So, to sum up, there’s a little way to go yet.

PS. When I visit your pages and see some ‘proper’ theory, I stop calling myself a theorist and use ‘modeller’ instead.
:o)