Elsevier

Current Applied Physics

Volume 30, October 2021, Pages 77-84
Current Applied Physics

Electrical and structural properties of VO2 in an electric field

https://doi.org/10.1016/j.cap.2021.05.010Get rights and content

Highlights

  • Joule heat affects the metal-insulator transition (MIT) and the structural-phase transition (SPT) of VO2.

  • The Tc of VO2 is shifted towards a lower temperature in an external electric field.

  • A metallic phase of VO2 is observed in the only rutile (or M2) phase.

  • An external field drives valence electrons to a conduction band of an insulating VO2.

  • MIT, SPT, and pre-edge peak shift at V K edge do not simultaneously occur at a uniform temperature.

Abstract

We examined the electrical and local structural properties of a VO2 film at different electric fields using electrical resistance and x-ray absorption fine structure (XAFS) measurements at the V K edge in the temperature range of 30–100 °C. The Tc value of the metal-to-insulator transition (MIT) during both heating and cooling decreases with electric field. When the electric field exceeds a certain value, the MIT becomes sharper due to Joule heating. The MIT, the structural phase transition (SPT), and the pre-edge peak transition of the VO2 do not congruently occur at a uniform temperature. A metallic VO2 is observed in only the rutile (or M2) symmetry. An electric field induces a substantial amount of conduction electrons in insulating VO2. Simultaneously measured resistance and XAFS reveal that Joule heating caused by an external electric field significantly affects the MIT and SPT of VO2.

Introduction

Vanadium dioxide (VO2) is a traditional metal-to-insulator (MIT) transition material [[1], [2], [3]]. A typical Tc value of the MIT of VO2 is approximately 68 °C [4], and it is sensitive to pressure [5,6], doping [[7], [8], [9], [10], [11], [12]], and structural disorder and strain [[13], [14], [15], [16], [17], [18]]. The MIT of VO2 is induced by external parameters, including heat [[1], [2], [3], [4]], electric fields [[17], [18], [19], [20], [21], [22], [23]], and magnetic fields [24], and it is accompanied by a first-order structural phased transition (STP). The resistivity ratio of the insulating to metallic phases is approximately a fourth order of magnitude and the MIT is quite sharp, particularly in a single crystal VO2 [25]. Researchers have dedicated tremendous efforts toward elucidating the origin of the MIT [[26], [27], [28], [29]] and using VO2 in practical applications, including transistors [21,[30], [31], [32]], batteries [[33], [34], [35], [36], [37]], smart windows [38,39], memristors [[40], [41], [42], [43]], switches [[44], [45], [46], [47], [48]], and sensors [[49], [50], [51], [52], [53]]. The dramatic change of MIT has been ascribed to the SPT based on the fact that the MIT occurs congruently with the SPT near the same temperature. However, neither conventional band theory nor the V–V dimerization model can predict the bandgap of ~0.65 eV of the insulating VO2 [[54], [55], [56]]. Furthermore, a recent study showed the sharp MIT features of VO2 with a substantial amount of structural disorder, which does not have regular V–V dimers [57]. The Mott-Hubbard insulator model, a strongly correlated electron model, can explain the bandgap of VO2 [28,58,59]; however, it is limited to explaining the sharp and dramatic MIT features of VO2, as opposed to the MIT of Ti2O3, which is triggered by impurity levels [60]. Kim et al. suggested that the MIT could be induced by changes in the charge carrier types, such as holes and electrons in insulating and metallic phases, respectively [28]. Other researchers have shown that structural strain induces the MIT of VO2 [61]. The trigger of the MIT of VO2 remains controversial in the Mott-Hubbard model. The Mott-Hubbard insulator model and the structural-driven Peierls transition model have been used to elucidate the bandgap of the insulating phase and the MIT features of VO2, respectively [26]. VO2 experiences an SPT from an insulating M1 phase to a metallic rutile phase via an M2 phase when heated from room temperature (RT). Recently, several researchers have suggested the existence of an M3 phase, which is a metallic M2 phase [28,59]. In practical applications of VO2, the Tc value, sharpness, and resistance jump size of the MIT are the most important parameters. Since the MIT of VO2 is sensitive to external parameters, including electric field, doping, size, and structural disorder and distortion [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]], the MIT features can be artificially engineered by modifying the external parameters.

Many previous studies have reported on observations of surface charge in an insulating VO2 [62]. One common finding is that when an external electric field with a larger than the threshold value is applied to insulating VO2, the MIT of VO2 can be induced [20,22,23]. In an electric field, the charge carrier density of the VO2 was substantially increased, and the VO2 remained in a metallic phase even near 0 K [62]. This suggests that a sufficiently strong electric field can induce the electrons of the V 3d orbital to jump into a conduction band, thereby keeping VO2 in a metallic phase [22,23]. In a direct-current (DC) electric field, conduction charges percolate through an ultra-fine path where the electric potential is lower than that of the surroundings [20,22,63]. A small current in a micro-channel creates heat, which induces an SPT of the VO2 in the channel area because the Joule heating can raise the temperature of the channel above the Tc value [61,63]. Once a micro-channel of VO2 enters a metallic phase, the current is instantaneously increased, resulting in a significant increase in electric power. Previous studies have reported that the temperature of a current micro-channel on VO2 is approximately 230 K higher than the set temperature, due to Joule heating [63,64]. Specifically, the heat propagates to the adjacent areas of the micro-channel, leading to the expansion of the metallic areas, the creation of new channels, and an increased current [65]. An avalanche effect caused by the mutual assistance of the current and Joule heating can drive VO2 from an insulating phase to a metallic phase in an electric field, even at a fixed external temperature. The Joule heating effect can be one of the most important parameters in a practical device application of VO2, because such applications involve an electric device that is operated in an external electric field.

In this study, we examined the structural and electrical properties of VO2 in different electric fields by using in-situ x-ray absorption fine structure (XAFS) and resistance measurements. X-ray diffraction (XRD) is a standard tool used to determine the crystallinity of a crystal. However, XRD is limited to determining the structural properties of disordered and amorphous systems. Transmission electron microscopy (TEM) is also widely used to characterize atomic orderings within small regions. However, in-situ TEM measurements have a number of issues, including accurately controlling of temperature and applied electric field on the same region of TEM measurements. XAFS is a unique local probe for capturing the local structural and chemical properties around a selected species atom [[66], [67], [68]]. XAFS detects the local structural properties around V atoms as well as the local density of the states of the V 3d orbital while an electrical resistance measurement monitors changes in the electrical properties of VO2 in an electric field. We observed that the Tc value shifted toward a significantly lower temperature in an external field compared to the value without the field. The local structural properties around the V atoms of VO2 also changed when varying the electrical properties in an external electric field. Notably, the current density of VO2 increased substantially in an external field, even at RT.

Section snippets

Experimental details

To investigate an MIT and an SPT of VO2 in electric fields, b-oriented VO2 films were synthesized on α-Al2O3 (0001) substrates using a DC-sputtering deposition method from a vanadium target with a purity of 99.95%. The b-axis of the VO2 film is parallel to the direction of the film surface and the lattice constant of b is ~4.507 Å [63]. The base vacuum of the growth chamber was below 10−6 Torr and the working pressure was approximately 10−3 Torr during the deposition of VO2 film. The substrate

Results and discussion

When DC electric field is applied in the ac-plane of the VO2 film with a thickness of ~1300 Å, as illustrated in the inset of Fig. 1 (b), electric current mainly runs perpendicular to the b-axis of the VO2. Examining the temperature-dependent electrical resistance in the applied voltage of 0.5 V yields that the Tc-MIT values of MIT are 79 °C and 71 °C for the heating and cooling processes, respectively, as shown in Fig. 1 (a). The Tc-MIT value of 79 °C during heating is larger than the typical T

Conclusions

The electrical properties of a VO2 film with the applied voltage of 0.5 V–20 V are directly compared to the structural properties obtained using in-situ resistance and XAFS measurements at the V K edge. The measurements show that Tc-MIT, Tc-Pre, and Tc-SPT do not simultaneously occur at the uniform temperature. This means that the MIT, the local density of states, and the SPT of VO2 are correlated to each other; however, they are still distinct phenomena. An external electric field can drive

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was conducted under the auspices of the Basic Science Research Program through the National Research Foundation of Korea government grant funded by the Ministry of Education (No. 2017K1A3A7A09016390, No. 2020K1A3A7A09080403) and the research funds of Jeonbuk National University in 2019. XAFS data were collected at beamline 20-BM of APS in USA and beamline 8C of PLS II in Korea. This research used resources of APS, an Office of Science User Facility operated for the U.S. Department of

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