Single-crystalline Vanadium Dioxide Research Articles

Evaluation of various phase-transition materials for steep switching super-low power application in the latest technology node

Recently, a phase-transition field effect transistor (phase-FET) integrated with a phase-transition material (PTM) is attracting attention as a steep switching device, and attempts to solve the power consumption limitation of conventional CMOS using various PTMs are continuing. In order to utilize the phase-FET, it is essential to design an appropriate device and circuit characteristics by carefully considering the close correlation between the baseline-FET and the PTM and the hysteresis characteristics caused by the phase-transition. In this work, by integrating various PTMs according to the baseline FETs in various technology nodes, the logic and SRAM characteristics implemented the phase-FET were investigated, and the appropriate PTM and the electrical characteristic target required for the PTM were presented through systematic analysis. As a result, it was confirmed that single crystalline vanadium di-oxide (SC VO2) has characteristics suitable for constructing phase-FETs compared to other candidate PTMs in 32 nm planar FET, 7 nm Fin-FET, and 3 nm gate-all-around FET technology nodes. As a result of benchmarking with conventional-FET at each technology node, in the case of logic, the speed is 26.15% (32 nm), 20.26% (7 nm), and 20.05% (3 nm), and in the case of memory, standby power, read stability and write time are improved.

Phase management in single-crystalline vanadium dioxide beams

A systematic study of various metal-insulator transition (MIT) associated phases of VO2, including metallic R phase and insulating phases (T, M1, M2), is required to uncover the physics of MIT and trigger their promising applications. Here, through an oxide inhibitor-assisted stoichiometry engineering, we show that all the insulating phases can be selectively stabilized in single-crystalline VO2 beams at room temperature. The stoichiometry engineering strategy also provides precise spatial control of the phase configurations in as-grown VO2 beams at the submicron-scale, introducing a fresh concept of phase transition route devices. For instance, the combination of different phase transition routes at the two sides of VO2 beams gives birth to a family of single-crystalline VO2 actuators with highly improved performance and functional diversity. This work provides a substantial understanding of the stoichiometry-temperature phase diagram and a stoichiometry engineering strategy for the effective phase management of VO2.

Low-temperature wafer-scale fabrication of vertical VO2 nanowire arrays

Single-crystalline vanadium dioxide nanowires (VO2 NWs) have attracted significant interest due to their unique characteristics, which originate from the single-domain metal–insulator phase transition (MIT) property. However, the lack of facile technologies to produce vertical nanowire arrays (NAs) in a large area has limited the mass fabrication of VO2-based devices. Here, an antimony-assisted hydrothermal method is developed for the low-temperature production of wafer-scale vertical VO2 NAs on arbitrary substrates of glass, quartz, and silicon. Sb2O3 plays a key role in the controlled growth of pure VO2 (M1) by modulating the size, density, alignment, and MIT properties of VO2 NAs. Furthermore, the growth mechanism of vertical VO2 NAs is explained. In contrast to conventional fabrication technologies, the weak interaction between NA films and substrates enables a much easier transfer of VO2 NAs for various potential applications.

Negative differential resistance behavior in a single-crystalline vanadium dioxide nanobeam without epitaxial interfacial strain

We investigate the current-dependent negative differential resistance (NDR) behavior in a single-crystalline vanadium dioxide (VO2) nanobeam without epitaxial interfaces. VO2 nanobeams are grown out of the basal r plane of sapphire and then mechanically transferred onto an oxidized silicon substrate using a stamping transfer technique. Interestingly, compared to a previous report on an epitaxially grown VO2 nanobeam with a strong adhesion strain at interfaces, the transferred VO2 nanobeam without epitaxial interfaces exhibits distinctly different NDR phenomena. We observe three different NDR behaviors along with the appearance of an intermediate M2 phase in the current–voltage (I–V) characteristics of the transferred VO2 nanobeam device obtained using the current-sweeping mode. This result is well supported by temperature-dependent Raman spectra and corresponding optical microscope images showing the appearance of an intermediate M2 phase and the spatial distribution of insulating and metallic domains along the length of the transferred nanobeam without epitaxial interfaces, which is likely attributed to different surface stresses, including interfacial stress.

Self-assembly and horizontal orientation growth of VO2 nanowires.

Single-crystalline vanadium dioxide (VO2) nanostructures have attracted an intense research interest recently because of their unique single-domain metal-insulator phase transition property. Synthesis of these nanostructures in the past was limited in density, alignment, or single-crystallinity. The assembly of VO2 nanowires (NWs) is desirable for a “bottom-up” approach to the engineering of intricate structures using nanoscale building blocks. Here, we report the successful synthesis of horizontally aligned VO2 NWs with a dense growth mode in the [1-100]quartz direction of a polished x-cut quartz surface using a simple vapor transport method. Our strategy of controlled growth of VO2 NWs promisingly paves the way for designing novel metal-insulator transition devices based on VO2 NWs.

Probing the photothermally induced phase transitions in single-crystalline vanadium dioxide nanobeams

Using Raman spectroscopy, we demonstrated photothermally induced crystallographic phase transitions of vanadium dioxide (VO2) nanobeams clamped to and free-standing on a substrate. Compared to the temperature-dependent Raman measurements, the laser-power-dependent Raman characteristics provide substantial evidence for the photothermal origin of the phase transitions of the VO2 nanobeams. The laser power necessary to cause phase transitions in the free-standing nanobeam was approximately eight times smaller than the laser power used in the substrate-clamped nanobeam. Our study will enhance the understanding of the complex phase transitions of strongly correlated oxides and thereby provide a foundation for engineering desirable properties in novel devices.

Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide

Single crystalline vanadium dioxide (VO${}_{2}$) nanobeams offer an ideal material basis for exploring the widely observed insulator--metal transition in strongly correlated materials. Here, we investigate nonequilibrium carrier dynamics and electronic structure in single crystalline VO${}_{2}$ nanobeam devices using scanning photocurrent microscopy in the vicinity of their phase transition. We extracted a Schottky barrier height of $\ensuremath{\sim}$0.3 eV between the metal and the insulator phases of VO${}_{2}$, providing direct evidence of the nearly symmetric band gap opening upon phase transition. We also observed unusually long photocurrent decay lengths in the insulator phase, indicating unexpectedly long minority carrier lifetimes on the order of microseconds, consistent with the nature of carrier recombination between two d-subbands of VO${}_{2}$.

One-pot synthesis of carbon-coated VO2(B) nanobelts for high-rate lithium storage

Uniform carbon-coated single crystalline vanadium dioxide (VO2(B)@C) nanobelts were successfully prepared by using a facile one-pot hydrothermal approach. Sucrose plays a dual role in this hydrothermal process, namely as a carbon precursor for the carbon shell and, as a reductant to reduce V2O5 to VO2(B). The thickness of the carbon coating layer is tunable from 3.0 to 6.9 nm by changing the ratio of the precursors. Although a high carbon content can improve the electrical conductivity of VO2(B)@C nanobelts, a thick carbon coating layer would block the lithium ion diffusion. The optimal thickness is found to be 4.3 nm (carbon content: 6.6 wt%), where the cathode displays superior performance with highly reversible specific capacities, good cycling stabilities and excellent rate capabilities (e.g. 100 mA h g−1 at 12.4 C).

Stoichiometry Engineering of Monoclinic to Rutile Phase Transition in Suspended Single Crystalline Vanadium Dioxide Nanobeams

Coexisting monoclinic M(1) (insulating) and rutile (metallic) domains were observed in free-standing vanadium dioxide nanobeams at room temperature. Similar domain structures have been attributed to interfacial strain, which was not present here. Annealing under reducing conditions indicated that a deficiency of oxygen stabilizes the rutile phase to temperatures as low as 103 K, which represents an unprecedented suppression of the phase transition by 238 K. In a complementary manner, oxygen-rich growth conditions stabilize the metastable monoclinic M(2) and triclinic T (or M(3)) phases. A pseudophase diagram with dimensions of temperature and stoichiometry is established that highlights the accessibility of new phases in the nanobeam geometry.

Intrinsic Optical Properties of Vanadium Dioxide near the Insulator−Metal Transition

We studied the insulator-metal transition (IMT) in single-domain, single crystalline vanadium dioxide (VO(2)) microbeams with infrared microspectroscopy. The unique nature of such samples allowed us to probe the intrinsic behavior of both insulating and metallic phases in the close vicinity of IMT, and investigate the IMT driven by either strain or temperature independently. We found that the VO(2) insulating band gap narrows rapidly upon heating, and the infrared response undergoes an abrupt transition at both strain- and temperature-induced IMT. The results are consistent with recent studies attributing the opening of VO(2) insulating band gap to a correlation-assisted Peierls transition.

Growth and Electrical Properties of Vanadium-Dioxide Thin Films Fabricated by Magnetron Sputtering

Single crystalline vanadium-dioxide (VO2) thin lms on sapphire (0001) have been grown by using a radio-frequency (rf) magnetron sputtering technique. The dependences of the resistivity change and the hysteresis width of the VO2 thin lms on the oxygen ow ratio [= O2 100 Ar+O2 (%)] at a xed deposition temperature of 550 C have been studied. The VO2 lm grown at an oxygen ow ratio of 3.85 % showed a metal-to-insulator transition around 65 C with a resistivity ratio as high as 4.3 103 and a hysteresis width of 6 C. Moreover, the VO2 lm, thermally cycled of up to 100 times across the transition temperature, was found to be stable; i.e., no noticeable changes in the magnitude of the resistivity and in the shape of the hysteresis curve were observed.

A new route to single crystalline vanadium dioxide nanoflakes via thermal reduction

Large-area single-crystalline vanadium dioxide nanoflakes were first fabricated via a thermal reduction method in a tube furnace. The sample was characterized by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results show that VO2 nanoflakes are single-crystalline with a monoclinic structure. The VO2 nanoflakes have a width of 200–300 nm, a thickness of 50–100 nm, and a length up to 1–2 μm. It is found that single-crystalline VO2 nanoflakes show a novel and complicated 5–7-step Li-storage behavior for an insertion amount of <0.6 mol lithium per mol of VO2.

Investigation of transition metal oxides by ultrasonic microscopy

Abstract Ultrasonic microscopy is now well established in non-destructive testing and has become a versatile method for investigating solid materials. It may serve both as imaging technique for large scale inspection of sample homogeneity and also as measuring device for a local determination of elastic properties. Our investigations of single crystalline vanadium dioxide nicely demonstrate this capability of flexible use and are exemplary for the beneficial effect of ultrasonic microscopy on materials research, in general. Here we focus on three different subjects: the detection of odd phases in as-grown single crystals, the inspection of structural transformations accompanying the metal–insulator transition and the investigation of elastic properties performed on small crystallites.

Molybdenum substitutional doping and its effects on phase transition properties in single crystalline vanadium dioxide thin film

Molybdenum (Mo) doped vanadium dioxide thin films were synthesized using a Mo striped vanadium (V) target during pulsed laser ablation process. The film structure was characterized by high resolution x-ray diffraction, x-ray rocking curve and Rutherford backscattering/channeling measurements. The results show that the full width at half magnitude of the x-ray rocking curve is as narrow as 0.0074°, comparable to that of the (0001) sapphire substrate, 0.0042°, in this study. The ratio of the aligned-to-random backscattered yield reaches 5%, implying that the growth is that of the single crystalline epitaxy. The result of angular scans for both V and Mo atomic channelings reveals that Mo atoms successfully take sites of the V sublattice as a substitutional dopant. It has been noted that the degradation of the phase transition properties of the film upon doping is closely related to the conductivity in the semiconductor phase.

Thermochromic VO 2 thin films studied by photoelectron spectroscopy

Thermochromic vanadium dioxide films which are discused as “intelligent” window coatings were deposited by reactive rf-sputtering. The films are grown on quartz polycrystalline and unidirectional. Photoelectron spectroscopy (XPS, UPS) was applied to study the density of states distribution in the valence band regime below and above the phase transition temperature. It is shown that the band structure of the polycrystalline films bears resemblance to single-crystalline vanadium dioxide to a large extent, e.g. bandgap, valence band energy position, and valence band structure and width in the semiconducting phase. In contrast to single crystals, e.g. the metal-semiconductor transition in the films occurs in a broad temperature range of about 20 to 30 K, dependent mainly on deposition temperature. Correlations between optical, electronical, and structural properties of the film are evident.