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Pure VO2, GexV1−xO2 (x < 0.2), and AlxV1−xO2 (x ≤ 0.02) exhibit above room temperature the well-known insulator-metal-transition (IMT) (at TIMT ≅ 340 K). In addition, AlxV1−xO2 exhibits an insulator-insulator-transition (IIT) at TIIT < TIMT. Both transitions can be induced either by external or by Joule heating. Both leave prominent marks on the I–V characteristics of the investigated samples, IIT at low currents and IMT at high currents. Reported herein is a comparative study of the I–V characteristics of needle-like pure and doped VO2 single crystals obtained with pulsed currents of various durations and with DC, in an attempt to separate thermal from non-thermal electric field effects. Identification of the Poole–Frenkel effect (a non-thermal electric field effect) in the I–V characteristics of the doped crystals may play an important role in a long-standing debate on the mechanism of IMT. Such an identification was obtained in this investigation for several Ge- and Al-doped VO2 crystals, so far only in narrow ranges of electric fields (<2000 V/cm) and pulse durations (≤1 ms) above which the non-linear conductivity of the crystals is dominated by Joule heating. [ABSTRACT FROM AUTHOR]

In addition to the well‐known insulator–metal transition above room temperature in pure VO2, AlxV1−xO2 exhibits an insulator–insulator transition at temperatures above room temperature and below the insulator–metal transition temperature, consisting of an increase in resistance by a factor ≤2. The wealth of static and dynamic domain structures discovered in the mixed‐metal‐insulator state of free‐standing, high‐quality, pure VO2 single crystals via optical microscopy synchronous with dc I–V tracing has inspired the use of this technique in these more complex materials. Reported herein are I(V) measurements conducted on crystals of AlxV1−xO2 (0.007≤ × ≤0.02) at various ambient temperatures (To). Prominent features (switching) on the I(V) loops are easily identified with the onset of the insulator–insulator and of the mixed‐metal‐insulator states, both induced by self‐heating (Joule heating). The switching results from instabilities of two different types of negative differential resistance regimes. The applied power at the onset of each switching scales with the excess temperature above ambient. For both transitions, optical microscopy synchronous with dc I–V tracing visualizes the expansion of each phase within the background upon the current increasing, and its contraction upon the current decreasing. [ABSTRACT FROM AUTHOR]

Reported herein is the mixed metal–insulator phase induced in needle‐like VO2 single crystals by direct current (DC) electric currents applied at ambient temperature; it appears in the current‐controlled negative differential resistivity (CC‐NDR) regime of the nonlinear I–V characteristic governed by self‐heating. In this regime, the I–V characteristic is stable for RL ≥ |dV/dI|max, (RL—the load resistance); for lower RL, portions of I–V are unstable and switching occurs between initial and final steady states (SSs). In high‐quality needle‐like VO2 single crystals (cross‐sectional area < 2 × 10−5 cm2) the SS NDR regime is "unique," whereas crossing from the insulating to the mixed phase I(V) is almost—or perfectly—smooth, the dissipation power (P = IV) versus current bends and this bend is accompanied by the appearance of insulating domains sliding along the metallic background in the sense of the current. The periodic domain emission, the variation of the dynamic parameters (domains' frequency and sliding velocity) over the steady‐state NDR regime, and the energetics of the process are discussed. These results show that the process of domain emission and sliding is fairly expensive at low frequencies but there is some evidence that the process becomes more efficient at higher frequencies. [ABSTRACT FROM AUTHOR]

VO2 is the prototype material for insulator–metal transition (IMT). Its transition at TIMT = 340 K is fast and consists of a large resistance jump (up to approximately five orders of magnitude), a large change in its optical properties in the visible range, and symmetry change from monoclinic to tetragonal (expansion by 1% along the tetragonal c-axis and 0.5% contraction in the perpendicular direction). It is a candidate for potential applications such as smart windows, fast optoelectronic switches, and field-effect transistors. The change in optical properties at the IMT allows distinguishing between the insulating and the metallic phases in the mixed state. Static or dynamic domain patterns in the mixed-state of self-heated single crystals during electric-field induced switching are in strong contrast with the percolative nature of the mixed state in switching VO2 films. The most impressive effect—so far unique to VO2—is the sliding of narrow semiconducting domains within a metallic background in the positive sense of the electric current. Here we show images from videos obtained using optical microscopy for sliding domains along VO2 needles and confirm a relation suggested in the past for their velocity. We also show images for the disturbing damage induced by the structural changes in switching VO2 crystals obtained for only a few current–voltage cycles. [ABSTRACT FROM AUTHOR]