Hall voltage reversal and structural phase transition in VO2 thin films

In this work, we investigated the nanoscale conduction and charge transport characteristics of epitaxial VO2 thin films around the metal-insulator transition (MIT) using the Hall transport measurement and conduction atomic force microscopy. Unlike the conventional oxides, the VO2 thin films show unique transport characteristics. First, the dominant carrier type shows a critical change from electron to hole during the MIT sequence (cooling sequence) or from hole to electron during the reverse MIT sequence (heating sequence). Second, the carrier density measured during the MIT sequence is higher than that measured during the reverse MIT sequence, evidenced with a clear thermal hysteresis. Third, the volume fraction (area percentage) of the nanoscale high-conduction phase also shows a thermal hysteresis, evidenced with a larger volume fraction of the high-conduction region in the MIT sequence than the reverse MIT sequence. The first-principles calculations indicate that the dominant carrier is the hole in the monoclinic phase, while it is the electron in the rutile phase, suggesting that the unique charge transport characteristics are attributed to the structural phase transition. Our work provides a deep insight into the nanoscale conduction and charge transports in VO2 thin films.