PNNL

Abstract

We report the synthesis of silicon-doped hematite (Si:a-Fe2O3) single crystals via chemical vapor transport, with Si incorporation on the order of 1019 cm-3. The conductivity, Seebeck and Hall effect were measured in the basal plane between 200 and 400 K. Distinct differences in electron transport were observed above and below the magnetic transition temperature of hematite at ~265 K (the Morin transition, TM). Above 265 K, transport was found to agree with the adia-batic small-polaron model, the conductivity was characterized by an activation energy of ~100 meV and the Hall effect was dominated by the weak ferromagnetism of the material. A room temperature electron drift mobility of ~10-2 cm2 V-1 s-1 was estimated. Below TM, the activation energy increased to ~160 meV and a conventional Hall coefficient could be de-termined. In this regime, the Hall coefficient was negative and the corresponding Hall mobility was temperature-independent with a value of ~10-1 cm2 V-1 s-1. Seebeck coefficient measurements indicated that the silicon donors were fully ionized in the temperature range studied. Finally, in the doped species we observe a very broad infrared absorption fea-ture centered near 0.4 eV that does not appear to be hydroxyl related but is tentatively assigned to photon-assisted small-polaron hops. These results are discussed in the context of existing hematite transport studies.