The nature of the acceptor centre in semiconducting diamond

A precision study has been made of the temperature dependence of the Hall effect and resistivity in five carefully selected natural semiconducting diamonds. The donor and acceptor concentrations in each sample have been calculated from the Hall effect data, and the acceptor concentrations are shown to correlate excellently with the intensities of the acceptor infrared absorption spectrum in each diamond. The behaviour of the resistivity is characteristic of a single acceptor centre, and the much lower activation energy observed at low temperatures is attributed to the tunnelling of electrons from ionized to neutral acceptor centres. The aluminium concentration in each crystal has been measured by slow neutron activation analysis and found to be substantially lower than the acceptor concentration in every case. Aluminium therefore cannot form the acceptor centre as was previously thought. Boron is the only group III element that neutron activation analysis has not eliminated as a possible acceptor, and the results of other workers point to boron as a likely candidate. The revised interpretation, that the acceptor centre in semiconducting diamond is definitely not aluminium and probably boron, is discussed, and its significance to previous publications is critically considered.