The Fermi level in diamond

Diamond is a wide-band-gap material with large donor and acceptor ionization energies. In principle, at room temperature and below, the Fermi energy is pinned close to the donor or acceptor level, depending on which is present in the higher concentration. In semiconductors with shallow donors and acceptors the equilibrium charge states of defects are determined by the position of the Fermi level. However, in an insulating material like diamond we show that the calculated position of the Fermi level does not necessarily predict the correct charge state of a defect, and propose instead that the charge state is influenced by the proximity of the defect to a donor (or acceptor). Qualitatively this accounts for the dependence of the charge state on the concentration of isolated substitutional nitrogen and also explains why many optical centres can be present in two different charge states in the same diamond.