Hematite $\left(\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)$ is known for poor electronic transport properties, which are the main drawback of this material for optoelectronic applications. In this study, we investigate the concept of enhancing electrical conductivity by the introduction of oxygen vacancies during temperature treatment under low oxygen partial pressure. We demonstrate the possibility of tuning the conductivity continuously by more than five orders of magnitude during stepwise annealing in a moderate temperature range between 300 and 620 K. With thermoelectric power measurements, we are able to attribute the improvement of the electrical conductivity to an enhanced charge-carrier density by more than three orders of magnitude. We compare the oxygen vacancy doping of hematite thin films with hematite nanoparticle layers. Thereby we show that the dominant potential barrier that limits charge transport is either due to grain boundaries in hematite thin films or due to potential barriers that occur at the contact area between the nanoparticles, rather than the potential barrier within the small polaron hopping model, which is usually applied for hematite. Furthermore, we discuss the transition from oxygen-deficient hematite $\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_{3-x}$ towards the magnetite $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ phase of iron oxide at high density of vacancies.

• Received 19 July 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.1.065407