Substitutional Nitrogen

  

Figure 8.4: Structure of the proposed VO_sN_s defect. This does not have the required bonding or gap states to behave as a shallow donor defect. The cube marks the (100) directions, corners on ideal lattice sites. The N-O distance in Å.

It was previously proposed by members of our group that a substitutional N_sO_i pair could form the core of a STD [212]. The problem here is that the substitutional defect seems to be a rare defect. However, in analogy with the VO₂ centre, we examined a substitutional split N_s-O_s pair. This, however, was found to possess a deep level, with no bonding between the N_s and O_s atoms. The addition of extra O_i in neighbouring bond centres did not provide sufficient strain to push the gap state up to give a shallow donor level. A diagram of the structure is shown in Figure 8.4.