Conduction Phenomena in Rutile Single Crystals

Application of voltage along the c axis in rutile single crystals produces an increase in conductivity after a ``delay'' period. During this period, which may amount to several days at room temperature, the current decays as a result of trapping. The subsequent rise in current leads to thermal breakdown unless the current is limited externally. The destruction of the insulating properties reflects a change in the bulk, brought about by an injection of positive space charge from the anode. This charge, believed to be holes, is injected by field emission due to a steepening anode fall and allows the electron current to rise exponentially. The anode fall is approximately 10−3 mm wide and sustains fields of 106 V/cm. Trapping of electrons and holes produces two types of color centers, which are distinguishable by their color and migration in the applied field.