Abrupt Change In Resistivity Research Articles

VO2 low temperature deposition and terahertz transmission modulation

Recently, the applications of vanadium dioxide film (VO2) in terahertz functional devices have attracted much attention because VO2 has a remarkable response to THz wave, In this work BK7 glass a material highly transparent to both THz and optical band is adopted as a substrate. High-quality VO2 film is deposited on a BK7 substrate using low temperature magnetron sputtering technology. The crystallinity and microstructure of the thin film are investigated by X-ray diffraction and atomic force microscopy. The results indicate that the as-deposited film crystallizes directly into single-phase VO2 with (011) preferred orientation and compact nanostructure. Under a heating-cooling cycle, the film undergos a metal-insulator transition with an abrupt resistivity change reaching more than 4 orders of magnitude. Terahertz transmission modulation is characterized by terahertz time domain spectrum, and a giant modulation depth of 89% is obtained. Due to the high transparence and the huge modulation effect, the VO2/BK7 can be widely used for THz devices such as modulators and switches.

The VO2 interface, the metal-insulator transition tunnel junction, and the metal-insulator transition switch On-Off resistance

Transition metal compounds showing a metal-insulator transition (MIT) show complex behavior due to strongly correlated electron effects and offer attractive properties for nano-electronics applications, which cannot be obtained with regular semiconductors. MIT based nano-electronics, however, remains unproven, and MIT devices are poorly understood. We point out and single out one of the major hurdles preventing MIT-electronics: obtaining a high Off resistance and high On-Off resistance ratio in an MIT switch. We show a path toward an MIT switch fulfilling strict Off and On resistance criteria by: (1) Obtaining understanding of the VO2-interface, a protoypical MIT material interface. (2) Introducing a MIT tunnel junction concept to tune switch resistances. In this junction, the metal or insulating phase of the MIT material controls how much current flows through. Adapting the junction's parameters allows tuning the MIT switch's Off and On resistance. (3) Providing proof of principle of the junction and its switch resistance tuning capability, experimentally in two forms. (4) Showing theoretically how stringent Off and On resistance specifications can be fulfilled. The prototypical VO2 MIT results in an abrupt change in bulk electrical resistivity at ∼68 °C. We show that the VO2 MIT manifests itself in an abrupt interfacial transition of current across a VO2-barrier interface forming a tunnel junction. In a first tunnel junction form, a two orders of magnitude abrupt change in contact resistivity induced by the bulk MIT is shown in VO2-metal contact structures. VO2-metal contact properties are discussed in detail, and the work function of VO2 is found to be 5.2eV(25 °C)−5.3eV(90 °C). In a second junction form, an abrupt change in tunneling current of up to an order of magnitude caused by the bulk MIT is shown to be present in VO2-insulator-metal capacitor structures with atomic layers deposition (ALD) Al2O3 and HfO2 barrier layers. The capacitors show the feasibility of using the MIT to switch a component to a high Off resistance state. Current and capacitance-voltage characteristics of the capacitors are analyzed as well as voltage or field dependent MITs at VO2 interfaces. The abrupt change in current across the VO2 interface is shown to be driven by the change in free carriers in bulk VO2 across the MIT.

Resistive switching characteristics of maghemite nanoparticle assembly on Al and Pt electrodes on a flexible substrate

Resistive switching characteristics of maghemite (γ-Fe2O3) nanoparticle assembly were investigated in structures of top-electrode (Al,Pt)/γ-Fe2O3-NPs (∼ 30 nm-thick)/bottom electrode (Al,Pt) on a flexible polyethersulfone substrate. The assembled NP layer with Al electrodes showed both unipolar and bipolar switchings with abrupt resistance change in multiple levels associated with formation and sequential rupture of conducting filaments, which is ascribed to Fe enrichment by the interfacial reaction. On the other hand, the NP layer with Pt electrodes exhibited memristive switching with hysteresis in current–voltage characteristics dependent on bias polarity, gradually changing the resistance with respect to bias conditions, and preserved resistance until a new state was developed by subsequent biasing.

A Review on the Regulation of Magnetic Transitions and the Related Magnetocaloric Properties in Ni-Mn-Co-Sn Alloys

In Ni-Mn-X (X =In, Sn, Sb) ferromagnetic shape memory alloys, a ferromagnetic transition from paramagnetic to ferromagnetic austenite and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite occur in some particular composition ranges, in which abundant physical properties have been observed by the abrupt change of magnetization and resistivity around their transition temperatures in these alloys. Therefore, tuning the martensitic transformation temperature (TM) and enlarging the working-temperature interval for Ni-Mn-X (X=In, Sn, Sb) alloys, are of great importance. In the present paper, we will focus on the effect of external factors, including pre-deformation, annealing, and high pressure annealing, on the magnetic transitions and the related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloys. Our approaches and the main results in this particular field will be reviewed.

Compact Verilog-A model of phase-change RAM transient behaviors for multi-level applications

A new phase-change RAM (PCRAM) model is proposed in this paper for multi-level-cell (MLC) applications. The proposed model has been verified to reproduce the electrical behaviors well in various partial states. Moreover, it has been shown that the programming transient behaviors with various quenching times could be estimated using the proposed model. The new unified PCRAM model which describes three different regions of PCRAM with only one equation has three times smaller discrepancy between the model and measurement than the previous piecewise linear model. For the continuity in PCRAM resistance, the new model shows smooth transition at the boundaries between different regions, whereas some abrupt changes in PCRAM resistance were found in the piecewise model. The compactness with the simple parameter extraction, the continuity for the reliable simulation and the accuracy in the negative differential resistance region make the proposed model useful and suitable in developing the future MLC PCRAM circuits.

A high temperature diffraction-resistance study of chalcopyrite, CuFeS 2

The electrical, magnetic and structural properties of synthetic chalcopyrite, CuFeS 2, have been studied up to 873 K using DC resistance measurements performed in-situ during neutron powder diffraction experiments. Under ambient conditions the material adopts the accepted structural model for CuFeS 2 in the space group I4̄2 d, with the magnetic moment of the Fe 3+ cations aligned along [001]. The electrical resistivity is around 0.3 Ω cm under ambient conditions, consistent with semiconductor character, and decreases slightly with increase in temperature until a more abrupt fall occurs in the region 750–800 K. This abrupt change in resistivity is accompanied by a structural transition to a cubic zinc blende structured phase (space group F4̄3 m) in which Cu + and Fe 3+ cations are disordered over the same tetrahedral crystallographic sites and by a simultaneous loss of long-range magnetic order. The implications of these results are discussed in the context of previous studies of the chalcopyrite system.

Microthermal Stage for Electrothermal Characterization of Phase-Change Memory

This letter describes a novel experimental structure that captures the impact of rapid temperature transients and repetitive cycling on the thermal and electrical properties of Ge2 Sb2Te5 (GST). The microthermal stage dramatically improves the temporal resolution for heating and enables simultaneous thermal and electrical characterizations. Thermal conductivity measurements show phase transitions of GST accompanied by abrupt changes in electrical resistance. Repetitive cycling with durations down to 100 ns produces melt-quenched amorphous GST with the thermal conductivity 40% lower than that of crystalline GST. Recrystallization increases conductivity but not up to the value achieved by long-timescale bulk annealing. This is potentially because the rapidly recrystallized GST contains more disorder near the interface.

Magnetic and electronic transport properties of antiperovskite Mn 3Cu(Ge)N thin films

The antiperovskite structure Mn 3Cu(Ge)N thin films were grown on single crystal Si (100) substrates by facing target magnetron sputtering. It is found that the films exhibit (100) preferred orientation. Ge-doping does not change the antiperovskite structure but leads to obvious changes in surface morphologies, magnetic and electronic transport properties. As a function of temperature, the resistivity of the films shows a semiconductor-type behavior. Moreover, it is worth noting that an abrupt change of resistivity is observed and magnetic ordering of the films changes from ferrimagnetic (FI) to antiferromagnetic (AFM) due to Ge-doping. In addition, both the transition temperature ( T t) and Néel temperature ( T N) move towards higher temperature with increasing Ge content.

The Influence of Pt Doping on the Sensing and Conduction Mechanism of SnO2 Based Thick Film Sensors

The conduction mechanism in polycrystalline Pt doped SnO2 thick films is investigated by means of simultaneous DC and work function changes measurements under exposure to reducing gases (CO and H2) in different oxygen backgrounds. The results are showing that, in contrast to the case of polycrystalline undoped SnO2 thick films - where a continuous switch of the conduction mechanism from a depletion layer controlled model to an accumulation layer one was observed - the situation in case of Pt doping is different: one records an abrupt change in both resistance and work function changes, which may indicate the onset of the reduction of the material. The reduction of the material was further studied by a combination of DC resistance and catalytic conversion measurements.

On-Off Intermittency and Criticality in Early Stage Electromigration

We have studied here using high resolution resistance measurements the time and statistical behavior of abrupt resistance changes in a thin metal aluminum film undergoing electromigration. We reveal for the first time that early stage electromigration exhibits on-off intermittency. The intermittent resistance fluctuations are also shown to be scale invariant, an effect seen in the fluctuations of several physical systems including earthquakes, superconductor dynamics and stock markets. Finite size scaling of the resistance fluctuations demonstrates that they originate near a critical point.

From weak to strong localization in a ferromagnetic high mobility 2DHG

Manganese modulation-doped two-dimensional hole systems confined in strained InAs/InGaAs/InAlAs heterostructures were investigated by low-temperature magnetotransport experiments. The study demonstrates quantized transport phenomena in the high field region, weak anti-localization in the low-field region and ferromagnetic ordering in the separated and insulating manganese-doped layer. A significant amount of manganese in the channel of inverted modulation-doped structures causes a strong localization effect with hysteretic-like abrupt resistance changes over several orders of magnitude at very low temperatures.

Change in the resistivity of Ge-doped Sb phase change thin films grown by chemical vapor deposition according to their microstructures

This study examined the effects of the composition and microstructure on the electric resistivity of Ge-doped Sb phase change thin films grown by cyclic plasma enhanced chemical vapor deposition. Ge and Sb layers were deposited sequentially to form either a GexSby mixture or Ge/Sb nanolaminated films. While the resistivity of the nanolaminated films was higher, the GexSby mixture showed a lower resistivity than the pure Sb film. This can be explained by the increase in carrier density of the alloy, as confirmed by first-principles calculations. An abrupt change in resistance accompanying a phase change was observed at ∼210 °C.

Effect of thermal processing on silver thin films of varying thickness deposited on zinc oxide and indium tin oxide

Silver films of varying thicknesses (25, 45, and 60 nm) were deposited on indium tin oxide (ITO) on silicon and zinc oxide (ZnO) on silicon. The films were annealed in vacuum for 1 h at different temperatures (300–650 °C). Four-point-probe measurements were used to determine the resistivity of the films. All films showed an abrupt change in resistivity beyond an onset temperature that varied with thickness. Rutherford backscattering spectrometry measurements revealed agglomeration of the Ag films upon annealing as being responsible for the resistivity change. X-ray pole figure analysis determined that the annealed films took on a preferential ⟨111⟩ texturing; however, the degree of texturing was significantly higher in Ag/ZnO/Si than in Ag/ITO/Si samples. This observation was accounted for by interface energy minimization. Atomic force microscopy (AFM) measurements revealed an increasing surface roughness of the annealed films with temperature. The resistivity behavior was explained by the counterbalancing effects of increasing crystallinity and surface roughness. Average surface roughness obtained from the AFM measurements were also used to model the agglomeration of Ag based on Ostwald ripening theory.

Electrical Resistance and Structural Changes on Crystallizaiton Process of Amorphous Ge-Te Thin Films

AbstractGeTe-Sb2Te3 pseudobinary compounds are attracting considerable attention as phase change materials for optical disk and phase change random access memory (PRAM). In these compounds, Ge2Sb2Te5 (GST) has been used for an optical disk memory such as DVD-RAM because the crystallization by laser beam heating is very fast (∼20ns). Recently, the GST has been much considered as material for PRAM and, therefore, the electrical resistance change due to crystallization and the phase change by applying an electrical current have been widely investigated. On the other hand, although GeTe compound has been suggested as the phase change material for the optical disk by Chen et al in 1986, the study focusing on the phase change material for PRAM is limited. Since GeTe is known to show the phenomenon of electrical switching, this compound has a potential of PRAM. In this study, the electrical resistance and crystalline structural changes on crystallization process in Ge-Te thin films were investigated.Films of amorphous Ge100–xTex (x : 46-94) with 200 nm thickness were deposited by sputtering of GeTe alloy target or co-sputtering of GeTe and Te targets on SiO2/Si substrates. In-situ electrical resistance measurements during heating process of these films were performed by two point probe method in a heating rate of 2∼50°C/min. X-ray diffraction (XRD) analysis was employed for the structural identification of thin films for 10-60° in 2′ using X-ray diffractometer with Cu-K. Transmission electron microscope (TEM) analysis was carried out to investigate the microstructure and to identify crystalline structure. The compositions of these films were confirmed by energy dispersive X-ray spectroscopy (EDS) attached TEM.All as-deposited Ge-Te films were confirmed amorphous by XRD and TEM. From the in-situ electrical resistance measurements, it is found that resistance change with crystallization process depends on the composition and the stoichiometric GeTe compound shows abrupt electrical resistance change at around 190 °C. The crystallization temperature of GeTe was higher than that of GST and resistance difference between the amorphous and the crystal was also larger. While the electrical resistance of GST film gradually decreased with increasing temperature after the crystallization at around 160 °C, that of GeTe film showed small temperature dependence after crystallization. It was found by X-ray measurement observation that the amorphous GeTe compound film crystallized first into a cubic state, and then into a stable rhombohedral state by further heating. The crystallization kinetics of Ge-Te thin films will be also presented.

Study on Electrical Properties and 1064nm Pulse Laser Damage of Vanadium Oxide Thin Film

Vanadium oxide thin film which has a reversible semiconductor-metal phase transition has been obtained by reactive ion-beam sputtering and subsequent annealing in Ar gas. Micro-analysis shows that this thin film is homogeneous and compact, its spheric grain size is about 50nm and it is composed of VO2 and V2O5. Electrical properties testing indicate that its phase transition temperature is near 60°C, abrupt change of resistivity before and after phase transition is approximate to 3 orders of magnitude, temperature coefficient of resistance (TCR) is about -0.0393K-1 at 25°C and activation energy is about 0.3006eV at the range of low temperature. Using a pulse laser beam with wavelength of 1064nm and pulse width of 10ns, laser damage threshold was obtained to be 20.1mJ/cm2. Damage spot morphology of the film was also researched carefully to discover its laser damage mechanism. All results above prove that this thin film is a perfect thermo-sensitive material that can be used for uncooled infrared detector and laser protection.

Cr2−xTixO3 (x ≤ 0.5) as CH3COCH3 sensitive resistors

Fine powders of Cr2−xTixO3 (x=0–0.5, CTO) were prepared by combustion technique, followed by heat treatment at 1000°C in ambient air. The XRD results showed the presence of a solid solution of TiO2 in Cr2O3 as the major phase along with CrTiO3 as the minor phase for x≥0.4. EDAX studies on Cr2−xTixO3 (x=0.1 and 0.3) suggested the composition of the oxides close to the theoretical values. The XPS studies showed the presence of Cr3+ and Ti4+species on the surface. Ti4+ inclusion in the eskolait Cr2O3 had significant influence on the activation energy for conduction (Ea) at 100–700°C in air. The commercial Taguchi sensors coated with CTO, when subjected to acetone in ambient air, showed an abrupt change in resistance within 10s and took the same time for recovery at 400°C. Different humidity levels were achieved by various water vapor buffers thermostated at room temperature where sensor response decreased nearly by 40% at 80% RH. The stability test indicated the first order exponential decay to a constant sensor potential value of 4.7. The origin of the gas response was attributed to the surface reaction of (CH3)2CO to form an intermediate geminal diol, followed by formation of an unstable ethanoyl radical and then final product CH3COOH, which was desorbed as a gas at 400°C.

Comparative Analysis of VO2 Thin Films Prepared on Sapphire and SiO2/Si Substrates by the Sol–Gel Process

VO2 thin films were successfully grown on sapphire and SiO2/Si substrates by the sol–gel process. The VO2 phase was well formed during simplified low pressure annealing in oxygen. The films prepared on sapphire directly crystallized to the VO2 phase without passing through intermediate phases with increasing the annealing temperature, resulting in highly [010]-oriented films on Al2O3(1010) substrate. In contrast, the polycrystal films grown on SiO2/Si reached the final VO2 phase with passing through several phases. Mixed phases existed at the interface between the film and substrate. For the films on sapphire, the phases with low-valent vanadium appeared drastically at the interface region along the depth, whereas the phase of the polycrystal film slowly changed into a low valence state at the initial stage of the interface and then returned to a high value. And both films showed an abrupt change in resistivity at the different transition temperature. The property of the interface may affect the crystallization and the metal–insulator transition of the films.

Growth of VO 2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO 2) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO 2 film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO 2. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V 2O 5 and V 3O 7) could not be eliminated from stoichiometric VO 2. Single-phase monoclinic VO 2 growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.

Thermal stability of spin-transfer switching in CPP-GMR devices

Spin-transfer switching (STS) was investigated in CoFe/Cu/CoFe current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) devices. The devices showed magnetoresistance ratio of 0.65±0.04% at room temperature. Abrupt resistance changes due to STS were clearly observed by applying short current pulses. We evaluated thermal stability factors of the small Co–Fe free layer cell from pulse duration dependence of switching current and from static magnetization processes. The thermal stability factors obtained from the two methods were in good agreement.

Influence of ambient atmosphere on metal-insulator transition of strained vanadium dioxide ultrathin films

The effect of ambient atmosphere on metal-to-insulator transition (MIT) in strained vanadium dioxide (VO2) ultrathin films (7–8nm) grown epitaxially on TiO2 (001) single crystal substrate by pulsed laser deposition was investigated by varying the ambient oxygen pressure and substrate temperature with the intention being to control arbitrarily the MIT events of strained VO2 ultrathin films, including the MIT temperature and the resistivity change. When reducing the ambient oxygen pressure, the MIT temperature remained almost constant, whereas the change of resistivity during MIT tended to be smaller. Contrary, varying the substrate temperature resulted in the variation of the MIT temperature with keeping the abruptness of transition. The lower the substrate temperature, the lower the MIT temperature, and in addition the MIT temperature ∼290K with keeping the abrupt change of resistivity is found to be the lowest compared with previously reported values. The reduction of the MIT temperature was found to correlate with shortening c-axis length when decreasing the substrate temperature, indicating the significant strain effects. Thus these results highlight that controlling appropriately the ambient atmosphere enables us to modulate arbitrarily the MIT events of strained VO2 thin films near room temperature.