Newcastle University

Boron-Hydrogen interactions in diamond

Diamond, with a band-gap of 5.5 eV is a good electrical insulator. However the addition of boron, with one fewer valence electron than carbon, leads to hole-mediated p-type semi-conduction.

In B-doped diamond exposed to hydrogenation, the electrical conductivety can e completely removed, and it is thought that this relates to formation of B-H pairs. This passivation process is an inference based upon electrical, optical and SIMS experiments, but direct observation of B-H complexes has not yet been made.

Fig 1: B-H complex charge density plot.

This is where modelling can play a significant role. The structure of a B-H pair is not as simple as other H-passivation processes. Calculations suggest that the H atom lies in a three-member ring configuration partially bonded both to the B acceptor and one of its carbon neighbours. This can be viewed in some sense to resemble a complex of a negatively charged boron acceptor and a positively charge interstitial hydrogen donor.

A combined experiment and modelling examination of the temperature dependence of the regeneration of acceptors by the release of hydrogen is good additional evidence for this defect, but also for another deeper trap.

Fig 2: B-H-B complex.

Fig 3: B-H-H-B complex.

We have also investigated other properties of B-aggregates, and boron-multi-hydrogen complexes implicated in a type-inversion, so of which is presented elsewhere.