Parallel Band and Hopping Electron Transport in SrTiO₃

SrTiO₃ (STO) is a model system for studying oxide electronic devices. This work examines the electronic transport through a heterostructure comprising an acceptor (Fe)-doped STO layer on a donor (Nb)-doped STO substrate. This is done by fitting the steady-state current–voltage (I–V) curve measured at ambient temperature to numerical solutions of the drift-diffusion equations of itinerant (i.e., free) electrons and holes (band conduction) and localized electrons hopping through defect states within the bandgap (hopping conduction). The analysis shows that at reverse bias and small forward bias the current is carried mostly by hopping electrons that give rise to unexpectedly high currents. At forward bias above 1 V, most of the current is due to electron transport through the conduction band. The transition from hopping to band conduction occurs when the quasi Fermi levels of electrons and holes depart from the energy level of the defect states through which the electrons hop. These observations shed new light on the transport properties of STO-based devices at ambient temperatures wherein ionic defects such as oxygen vacancies are immobile and holes are trapped, for the most part.