Impurity band conductance through oxygen vacancy donor states in bismuth iron molybdate

Under chemical reduction at 620 K in an atmosphere of methanol vapor in a nitrogen carrier gas, bismuth iron molybdate [Bi3FeO4(MoO4)2] will give up lattice oxygen and form highly mobile vacancies. These vacancies act as donors and sit about 0.4 eV below the conduction band edge: The band gap is 2.7 eV. Because of the ability to remove up to 1% of the lattice oxygen and still maintain crystallographic stability, it is relatively easy to produce high donor carrier densities (up to 1020 cm−3). Under these conditions, and taking into account that this is a wide gap semiconductor with well compensated deep donor levels, it is possible to measure an electrical conductivity dominated by carrier movement through the impurity band. Both phonon assisted hopping and the transition to metallic impurity conduction have been observed. The crossover temperature from conduction dominated by the impurity band to conduction dominated by the conduction band was seen to increase from 125 to 380 K with increasing oxygen depletion.