Control of Hole Density in Russellite Bi2WO6 via Intentional Chemical Doping.

Based on the fundamental design concept of modulating the valence band maximum of oxides and subsequent predictions through computational approaches, several lone-pair ns2-based p-type oxide semiconductors, such as Sn2+- or Bi3+-based complex oxides, have been developed. Thus far, the bandgap can be modified via tuning of the chemical composition, whereas the hole density cannot be intentionally controlled because of the poor chemical stability of Sn2+ and/or the formation of oxygen vacancies. The inability to control hole density prohibits the design and realization of emergent electronic devices based on p- and n-type oxide semiconductors. Herein, we report the control of hole density via intentional chemical doping in polycrystalline Bi2WO6. While the holes of polycrystalline Nb- or Ta-doped Bi2WO6 are strongly trapped by grain boundaries, the hole density obtained at high temperatures monotonically increases with the increase in the doping concentration. This study provides important insights into the development of practical p-type oxide semiconductors.

Bipolar Semiconducting Properties in α-SnWO4 Based on the Characteristic Defect Structure.

Diodes, memories, logic circuits, and most other current information technologies rely on the combined use of p- and n-type semiconductors. Although oxide semiconductors have many technologically attractive functionalities, such as transparency and high dopability to enable their use as conducting films, they typically lack bipolar conductivity. In particular, the absence of p-type semiconducting properties owing to the innate electronic structures of oxides represents a bottleneck for the development of practical devices. Here, bipolar semiconducting properties are demonstrated in α-SnWO4 within a 100 °C temperature window after appropriate thermal treatment. Comprehensive spectroscopic observations reveal that Sn4+ is present in p-type α-SnWO4 in a notably greater quantity than in n-type. This result strongly suggests that the Sn4+ substitutional defects on the W6+ sites contribute to hole-carrier generation in α-SnWO4. We also find that oxygen vacancies are initially formed in Sn-O-W bonds and migrate to W-O-W bonds with changes in semiconducting properties from p-type to n-type. These findings suggest useful strategies for exploring p-type oxide semiconductors and controlling their carrier type by utilizing the octahedral structure.