Study On Electron Phase Separation And Charge Transport Of Vanadium Dioxide Thin Films

Vanadium dioxide?VO₂?is a typical Mott insulator.Due to its rich and unique physical properties and many potential applications,it has been a hot research topic in the field of strongly correlated electronic materials in recent years.There is an insulator-metal transition in VO₂ at about 341 K.When the phase transition occurs,the resistivity of VO₂ decreases sharply by about 3–4 orders of magnitude.During the insulator-metal phase transition,vanadium dioxide is also accompanied by structural phase transition from low-T monoclinic phase to high-T rutile phase.Since the phase transition temperature of VO₂ is very close to room temperature,it has great device application potentials.Most of the descriptions on the metal-insulator transition?MIT?behaviors of vanadium dioxide in previous studies were based on the assumption that the microstructure of the whole sample is homogeneous.Therefore,the measurement of the overall behavior of the sample is considered to be sufficient to reflect its intrinsic properties.However,this assumption is likely to be questioned for known strongly correlated electronic material systems,especially transition metal oxides.Because in these transition metal oxide systems,the inhomogeneity of physical properties and the phenomenon of electronic phase separation are quite common.VO_2,as one of the strongly correlated electronic oxides,has been widely studied in the past.In the phase transition process,two-phase coexistence occurs within a wide temperature range.Although it will be of great valuable to study and understand the two-phase coexistence,it has not been well studied especially on the electron transport and conductivity in nanoscale.Understanding the phenomenon of electron phase separation in nanoscale and clarify the corresponding mechanism of charge transport can not only deepen the understanding of the basic physical properties of VO₂,but also help to expand its future application in novel nano electronic devices.In this paper,we will mainly use the conductive atomic force microscopy?CAFM?technique,combined with Hall effect measurement,to carry out systematic and deep studies on the electronic phase separation phenomena,charge transport characteristics and how to regulate the electronic phase in nanoscale of VO₂ thin film.The main research contents and results are as follows:1.Study on the nanoscale conductive behaviors of VO₂ thin films deposited on sapphire substrates by using the CAFM technique.?1?There exists coexistence of nanoscale high-conduction and low-conduction phases in VO₂ thin films over a broad temperature range?303-353 K?across the insulator-metal transition temperature.Even at low temperature?303 K?or high temperature?353 K?,the high-conduction or low-conduction regions will not disappear.?2?The area fraction of the nanoscale high-conduction phase of the high conductive phase and the scale of the high conductive region will increase with the increase of temperature.The current-voltage?I-V?curves of the high conductive phase measured at different temperatures show that the current circuit of the high conductive phase does not have good metal conductive behavior.However,it is more similar to the conductive characteristics of semiconductor.?3?By fitting the I-V curves measured in current circuit of the high-conduction region,it reveals that the charge transport behavior at low measurement temperature is dominated by space charge limited current mechanism.However,it changes to be dominated by Schottky emission at high measurement temperature.2.Study on the charge transport characteristics of VO₂ thin films by using Hall effect measurements.?1?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?.?2?The carrier density measured during the MIT sequence is higher than that measured during the reverse MIT sequence,evidenced with a clear thermal hysteresis of?10 K.The area percentage of the nanoscale high-conduction phase also shows a thermal hysteresis,evidenced with a larger area percentage of the high-conduction region in the MIT sequence than the reverse MIT sequence.These results imply that there is a close relationship between the area percentage of the high conduction region and the carrier density.?3?The first-principles calculations indicate that the dominant carrier is hole in the monoclinic phase,while it is electron in the rutile phase,suggesting that the unique charge transport characteristics are attributed to the structural phase transition.3.Control of the nanoscale conductive phase of VO₂ thin films by UV irradiation.?1?Using 365 nm UV light,we successfully realized the reversible regulation of the area percentage of the nanoscale high-conduction and low-conduction phases of the VO₂ thin films.?2?Through a comparative study on the characteristics of the conductive phases of VO₂thin films under the irradiation of 940 nm infrared light and 365 nm UV light,it is revealed that the critical mechanism to control the changes of the conductive phase should be the generation and removal of oxygen vacancies in the VO₂ thin films,rather than the free electrons excited from the valence band to the conduction band by light.