Metal-semiconductor Transition Temperature Research Articles

ELECTRICAL SWITCHING IN GLASS-CERAMIC STRUCTURES BASED ON VANADIUM DIOXIDE

The changes in electric current during the switching of vanadium dioxide-based glass-ceramic samples from the off-state to the on-state and vice versa were studied in this paper. The delay in the transition from the off-state to the on-state when a switching voltage U0 is applied to the glass-ceramic sample is related to the time tdon required to heat it to the metal-semiconductor phase transition temperature in VO2. The delay time tdon decreases with increasing U0. It was found that the delay of the reverse transition from on-state to off-state is controlled by the current cord within which the VO2 crystallites of the glass-ceramics are in the metal phase. The delay time t doff of this transition determines the time during which the current cord exists in the VO2 glass-ceramic sample after the voltage U0 is switched off. The value of tdoff increases as U0 increases.

Influence of a Thin V2O3 Spacer on Interlayer Interactions in Fe-Ni/V2O3/FeNi Film Structures

In this paper, we explore the suggestions of the results of experimental studies on low-dimensional layered systems in FeNi/V2O3/FeNi film structures. The multifunctional material V2O3 is used as an interlayer between the magnetically active FeNi layers. The films were obtained by ultrahigh vacuum magnetron sputtering on a glass substrate with a base size of 10−10 Torr. It has been found that for V2O3 films, the decrease in the metal–semiconductor transition temperature increases significantly. Magnetic characteristics were studied on the MPMS-XL SQUID magnetometer. The exchange effect occurs both in the region where the oxide has a magnetic order and in the paramagnetic region. The latter is due to the effect of the magnetic panel in the oxide package. The phenomenon of oscillation of the exchange field occurs depending on the phenomenon of intermediate observation observed experimentally.

Investigation on transport property of heterostructure in Bi/Bi2Sr2CaCu2O8+δ composite film

The heterostructure of bismuth (Bi) and Bi2Sr2CaCu2O8+δ (Bi2212) is considered as a potential type of quantum material. The strain-engineering controls the band structure in the composite films, which is applied in the high-temperature quantum spin Hall materials. In this work, Bi/Bi2212 composite film is synthesized using a two-step method to reduce the fabrication cost. The microstructure and transport properties of the composite films are characterized and analyzed. Experimental results demonstrate that the Bi/Bi2212 composite film obtains the high-quality crystallinity and excellent connectivity. The transport property measurement results show that the band structure changes due to the coupled effects of the strain and electric field. The origin is the compensation between the electron and hole charge carriers. The above band gap change leads to a shift in the metal-semiconductor transition temperature. Functionally coupled composite films exhibit new properties in magnetoresistance devices.

Effect of Bi-doping on structural, magneto-caloric and magneto-resistive properties of La0.67- xBixCa0.33MnO3 perovskites

We report the doping effect of Bi on structural, magnetic and magneto-resistive properties of nanocrystalline La0.67-xBixCa0.33MnO3 (x = 0, 0.05, 0.1) (LBCMO) samples. Rietveld refinement of the X-ray diffraction (XRD) patterns revealed that all the three samples crystallize into orthorhombic crystal structure having Pbnm space group. The average crystallite size was calculated using Debye Scherer's formula and the crystallite size varies from 25 to 30 nm. Scanning electron microscopy (SEM) images shows that the particles are spherical in nature and distributed uniformly. The temperature dependent magnetization data infers that all the three samples undergo paramagnetic to ferromagnetic (PM-FM) phase transition with transition temperature (Tc) ~245 K, 250 K and 230 K upon cooling. La0.67Ca0.33MnO3 (LBCMO1), La0.62Bi0.05Ca0.33MnO3 (LBCMO2) samples exhibit second order magnetic phase transition, whereas La0.57Bi0.1Ca0.33MnO3 (LBCMO3) sample exhibit first order like transition. The maximum magnetic entropy changes |ΔSMmax| were calculated for nanocrystalline LBCMO samples using thermodynamic relations and the values were 3.73 Jkg−1K−1, 4.72 Jkg−1K−1 and 4.47 Jkg−1K−1 respectively, where field change was μoH = 5 T. As a consequence of temperature dependence of electrical resistivity, data shows metallic behavior before metal-semiconductor transition temperature (TMS) and semiconducting behavior after TMS. The magnetic field dependent electrical resistivity data reveals a huge change in resistivity near TMS and the maximum magnetoresistance (MR) value around ~60% for an applied field of 5T around the TMS for LBCMO3 sample. The coexistence of magnetocaloric effect and moderately high value of magnetoresistance makes the studied samples promising for multifunctional device applications near its critical temperature.

Metal-semiconductor transition in the supercooled liquid phase of the Ge2Sb2Te5 and GeTe compounds

The ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and GeTe compounds are of interest for applications in phase change memories. In the reset process of the memory the crystal is rapidly brought above the melting temperature ${T}_{m}$ by Joule heating and then the liquid phase rapidly cools down leading to the formation of the amorphous phase. Since the liquid above ${T}_{m}$ is metallic and the amorphous phase is semiconducting a semiconductor-to-metal transition occurs in the supercooled liquid. Based on density functional simulations, we estimated the metal-semiconductor transition temperature ${T}_{\text{M-SC}}$ by monitoring the opening of a band gap in the supercooled liquid phase. Due to previous evidence on the importance of the van der Waals (vdW) interaction in describing the liquid phase of these materials, we used both the revised Vydrov-van Voorhis functional which includes vdW nonlocal interactions and the Perdew-Burke-Ernzerhof functional without vdW corrections. The estimated ${T}_{\text{M-SC}}$ is about 100--150 K higher with the former than with the latter framework for both compounds. By including vdW interactions the estimated ${T}_{\text{M-SC}}$ is closer to ${T}_{m}$ than to the glass transition temperature for both systems. The analysis of the structural properties as a function of temperature suggests a correlation between the metal-semiconductor transition and a Peierls distortion. However, the data support more a continuous structural transformation than the presence of a first order liquid-liquid phase change associated the metal-semiconductor transition.

The Effects of Oxygen Vatcancies on Electrical and Magnetic Properties of LaOBiSSe

LaO1−xBiSSe polycrystalline samples were successfully synthesized by conventional solid-state reaction method. The effects of oxygen vacancies on the crystal structure, magnetic properties under atmospheric and high pressure, and electrical property of LaOBiSSe were investigated. Polycrystalline samples LaO1−xBiSSe with x = 0–0.1 were well indexed to the tetragonal CeOBiS2-type structure with the space group P4/nmm. With increasing oxygen vacancy content, the crystal lattice contracted significantly along the c-axis, the semiconductor behavior was suppressed, and the metal-semiconductor transition temperature decreased, which is a necessary prerequisite for superconductivity in BiCh2-based (Ch = S and Se) system. However, oxygen vacancy level introduced in this work is insufficient to further contract the crystal lattice and increase carrier density. Even though external pressure (~ 1 GPa) was applied, no superconducting transition corresponding to LaO1−xBiSSe appeared except for that of manometer Pb. The reasons for the absence of superconductivity in LaO1−xBiSSe system were discussed.

Effect of Cr doping on the structural magnetic and transport properties of perovskite-like manganites

Structural, magnetic and transport measurements were performed on La0·7Sr0·3Mn1-xCrxO3 (0≤x ≤ 0.15) perovskite-like manganites in order to study the effect of the substitution on the Mn-site on the magnetotransport properties. All samples are found to be of single phase. The structural parameters obtained by using Rietveld refinement of X-ray diffraction data show that all samples crystallize with rhombohedral perovskite-type symmetry with R3‾c space group. The concentration dependence of the unit cell parameters indicates that chromium (or most of the chromium) has a trivalent state. The magnetization and electric measurements reveal that all the samples demonstrate a ferromagnetic to paramagnetic transition and a metallic to semiconductor one with increasing temperature. The Curie temperature TC and the resistivity transition temperature Tms shift to lower values with increasing Cr content. The correct expression for bandwidth W ~ TC is given. The temperature dependence of electrical resistivity supports the small polaron hopping model and variable range hopping conduction model above the metal–semiconductor transition temperature (Tms). The calculated hopping distance and hopping energy increased as rate of Cr content increased whereas density of states at Fermi level decreased.

Effects of microdefects and grain size on the phase transition properties of Nano-VO2(M)

Nano-VO2(M) samples were synthesized by annealing VO2(B) nanoparticles. The internal defects of nano-VO2(M) samples were characterized by positron annihilation spectroscopy, and the effects of microdefects and grain size on the phase transition properties were studied. The results indicate that 575 ​°C is the critical temperature for preparing pure phase VO2(M), and the morphology of the samples evolves gradually from nanorods to nanospheres with the increase of annealing temperature. With the increase of annealing temperature from 575 ​°C to 675 ​°C, the grain size of the nanospheres increases gradually, while the concentration and size of the microdefects decrease with the improvement of crystallinity. It is found that both the microdefects and grain size have opposite effects on the phase transition temperature of nano-VO2(M) samples. The grain size plays a leading role in the competition for the samples annealed at higher temperature, which makes the semiconductor-metal transition temperature lower.

High magnetoresistance in La0.5Nd0.15Ca0.25A0.1MnO3 (A = Ca, Li, Na, K) CMR manganites: Correlation between their magnetic and electrical properties

The influence of A-site cation disorder of monovalent alkali ions on structural and magneto-transport properties of colossal magnetoresistive (CMR) materials has been reported. All the compounds crystallize in Pbnm space group with orthorhombic symmetry. At low temperature, the samples exhibit a paramagnetic (PM) to ferromagnetic (FM) transition and a semiconductor to metallic one. The variation in both Curie temperature (Tc) and metal-semiconductor transition temperature (TMS) can be explained by the change in Mn4+ content and average A-site cation radius. A decrease in resistivity in the vicinity of TMS has been observed with an increase in the applied magnetic field. The values of Tc are close to TMS, signifying strong correlations between the magnetic and electrical properties. The electrical resistivity data in paramagnetic-semiconducting region (T >TMS) may be described by the adiabatic small polaron hopping (ASPH) model. Maximum magnetoresistance (MR%) has been observed in the vicinity of TMS.

Influence of copper on electrical conductivity of glass-ceramics based on vanadium dioxide

The results of studying the electrical conductivity σ for the glass-ceramic systems VO2 ‒vanadium-phosphate glass (VPG) and VO2‒VPG‒SnO2 with the additions of Cu and Cu2O are presented. It is found that the jump of conductivity at the temperature of metal-semiconductor phase transition (MSPT) in VO2 (Tt = 341K) takes place only for glass-ceramics containing not more than 5 wt% of these additives. When their content exceeds 5 wt%, the VO2 content decreases sharply according to the data of differential thermal analysis. The reason for this is the oxidation-reduction reactions in the liquid phase between Cu and VO2 at ceramics synthesis. These reactions result in the appearance of Magneli phases in glass‒ceramics composition. The phase transitions in Magneli phases V4O7, V5O9, and V6O13 are indicated by the temperature dependence of σ, as bends of the straight lines in coordinates log10σ ~ 1/T at the phase transition temperatures Tt of these phases: 240K, 150K, and 130K. The activation energy of σ for T> Tt is lower than for T <Tt, which indicates a change in the structure of energy zones inherent to MSPT.

Magnetic and Magneto-Transport Properties of the Sb Doping Mn Site in La0.67Ba0.33Mn1−xSbxO3 (0.03 and 0.07) Manganites

We investigate the effect of Sb5+ doping at Mn-site in La0.67Ba0.33Mn1−xSbxO3 (LBMO-Sbx) on the magnetic and magneto-electrical properties. The variation of the magnetization M versus temperature T, under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The resistivity and magneto-transport measurements are performed using standard four-probe assembly with and without magnetic fields. The temperature dependence of electrical resistivity shows that all samples undergo a sharp metal–semiconductor (M–SC) transition at a temperature (TM–SC), accompanying the ferromagnetic–paramagnetic transition. The peak resistivity ρmax is noted at the metal–semiconductor transition temperature (TM–SC) and lowering in TM–SC is observed for higher concentrations of Sb5+. The resistivity data have been analyzed in two parts. Firstly, in the metallic region below TM–SC the resistivity data is fitted with three degree polynomial. Secondly, in the semiconducting region above TM–SC data have been fitted with Small Polaron Hopping models. Above all, the magnetoresistance study showed a peak which has a high value around the M–SC transition temperature. The dependence of resistivity on the temperature and magnetic field data is used to deduce the magnetic entropy change.

Structural and magneto-transport properties of Li-substituted La0.65Ca0.35-xLixMnO3 (0 ≤ x ≤ 0.15) CMR manganites

With an aspect to understand the influence of cation mismatch on magneto-transport properties of colossal magnetoresistive (CMR) materials, a systematic study of La0.65Ca0.35-xLixMnO3 (0 ≤ x ≤ 0.15) manganites has been undertaken. The materials were synthesized by sol-gel Pechini process and their structures were confirmed by the analysis of XRD data using Rietveld refinement technique. A paramagnetic to ferromagnetic transition and a semiconductor to metallic one have been observed at low temperature. Both Curie temperature (Tc) and metal-semiconductor transition temperature (TMS) in the vicinity of Tc first increases with Li doping and then show a decreasing trend, which is justified on the basis of a competition between double-exchange and super-exchange mechanisms. It has been concluded that the electrical resistivity data in high-temperature paramagnetic-semiconducting region (T > TMS) may be described by the small polaron hopping model. Also, a large magnetoresistance (MR%) has been observed in the vicinity of metal-semiconductor transition temperature TMS, which is comparable to the MR% in the low-temperature region.

VO2 thin films for smart windows: Numerical study of the optical properties and performance improvement

Vanadium dioxide (VO2) have attracted tremendous interest in recent years due to their semiconductor-metal transition near room temperature. In this work, we report numerical simulation of the optical properties of VO2 thin layers which include the index of refraction, the coefficient of extinction, the reflectance and the transmittance. In this sense, we used the Model of Drude-Lorentz for calculating coefficients at from the optical parameters n and k using the determination of the dielectric constant ε (ω, T). Knowing that the semiconductor-metal transition temperature of VO2 is 68 °C, the thickness effect on the optical properties of VO2 thin films has been studied by transmittance simulations. Our results revealed a significant changes in optical properties of VO2 thin layers, which produces many interesting applications especially for smart windows.

Comparative study of La0.5Nd0.2Ca0.3–K MnO3 (x = 0.0 and 0.05) nanoparticles: Effect of A-cation size and calcination temperature

Abstract In order to study the effect of sintering temperature and A-site cation radius on the structural, magnetic and transport properties, a systematic investigation of La0.5Nd0.2Ca0.3–xKxMnO3 (x = 0.0 and 0.05) nanoparticles has been undertaken. Rietveld analysis of X-ray diffraction data confirmed the presence of orthorhombic symmetry with Pbnm space group for all the samples. A paramagnetic-ferromagnetic transition is shown by all our investigated compounds. The materials exhibit semiconducting behavior and the metal-semiconductor transition temperatures (TMS) are found to be close to Curie temperatures (Tc), indicating a strong correlation between their electrical and magnetic properties. Both Tc and TMS of K-doped samples were found to be higher than their undoped counterparts at same calcination temperatures. In the high temperature region, it was observed that the resistivity data was best fitted by SPH model.

Transmittance change with thickness for polycrystalline VO2 films deposited at room temperature

In this work, polycrystalline VO2 thin films have been prepared on BK7 glass substrates by DC magnetron sputtering at room temperature with a post-annealing treatment at 450 °C, and relative thickness dependence of the semiconductor-metal transition (SMT) properties have been systematically investigated. It revealed that the prepared VO2 films exhibited unique surfaces composed of particles with various sizes, which became larger with increased film thickness. Different luminous transmittance between the metallic phase and semiconductor phase can be obtained by changing the film thickness based on the optical interference theory. The optimum thickness of VO2 films was around 90 nm with high ΔTsol of 9.7% and relatively high Tlum of 36.4% (higher than theoretical 29.1%), which is a recommendable optical property for practical application. The critical SMT temperature fluctuated when film thickness increased, and the transition speed of the heating process got slower overall with thickness decreasing, which might be caused by mechanical stress between films and substrates. In addition, the transition speed of cooling process turned slower with thickness of VO2 films increasing, which can be ascribed to the slower heat dissipation of thicker films than that of thinner films during the cooling process. These results might give an indication for creating or designing potential materials based on the thickness dependence of phase transition properties of VO2 films.

A highly-efficient approach for reducing phase transition temperature of VO2 polycrystalline thin films through Ru4+-doping

A low semiconductor-metal transition (SMT) temperature (TSMT) is highly desirable for advancing practical applications of VO2. The doping with tetravalent metal elements attracts little attention as an effective approach for reducing the TSMT of VO2. Here we investigate the effect of Ru-doping on the microstructure and SMT characteristics of VO2 polycrystalline thin films. X-ray photoelectron spectroscopy reveals that Ru ions exist in the tetravalent oxidation state in Ru-doped VO2 thin films. X-ray diffraction patterns and Raman spectra indicate that substitutional Ru4+ ions decrease the crystallite size of films and induce the local tetragonal symmetry in the framework of monoclinic VO2 lattice. As a result, Ru4+-doping reduces the TSMT of VO2 polycrystalline thin films at the rate of 10.5 °C/at%. Moreover, the room temperature resistivity of thin films decreases from 9.98 Ω·cm for undoped VO2 to 0.63 Ω·cm for Ru-doped one with Ru atomic concentration of 2.32 at%, which is ascribed to the narrowed band gap and enhanced density of states of the V 3d band.

Microstructural properties, conduction mechanism, dielectric behavior, impedance and electrical modulus of La0.6Sr0.2Na0.2MnO3 manganite

La0.6Sr0.2Na0.2MnO3 manganite was synthesized using sol–gel method. XRD pattern with Rietveld refinement indicates that sample crystallizes in the rhombohedral structure with $$R\overline{3}c$$ space group. The electrical conductivity analysis shows a metal–semiconductor transition temperature at TMS = 280 K for the sample. Dielectric constants decrease with frequency and their behaviors have been investigated according to the Maxwell–Wagner theory of interfacial polarization. An appropriate electrical equivalent circuit was used to analyze the Nyquist plots, and the results show that the conduction mechanism of the synthesized manganite is mainly due to the grain boundary contribution. The modulus analysis shows the presence of electrical relaxation phenomenon and non-Debye nature for the sample. The activation energy deduced from the conductivity analysis matches very well with the values estimated from the relaxation time and the grains boundary resistances. This indicates that relaxation process and electrical conductivity are attributed to the same defect.

Temperature vs. impedance dependencies of neutron-irradiated nanocrystalline silicon carbide (3C-SiC)

At the present work, impedance spectroscopy of nanocrystalline silicon carbide (3C-SiC) has been investigated as a function of temperature. Nanocrystalline 3C-SiC particles irradiated by neutrons (2 × 1013 n cm− 2s− 1) up to 20 h. Impedance of neutron-irradiated nanocrystalline 3C-SiC has been studied at the temperature range of 100–400 K. Impedance spectra of the nanocrystal have been comparatively studied before and after neutron irradiation. The natures of conductivity and the metal—semiconductor transition temperature (TMS = 250 K, 325 K, and 370 K at the various frequencies) have been defined from the complex impedance spectroscopy. Polarization of nanocrystalline 3C-SiC increased corresponding to neutron irradiation duration. The mechanism of all effects observed in the experiments has been given in the work.

Effect of A-site disorder of heavier rare earth ion on structural, magnetic and transport properties of lanthanum based chromium manganite perovskite system

In order to study the effect of A-site cation mismatch on the structural, magnetic and transport properties, a systematic investigation of La0.6R0.1Ca0.3Mn0.9Cr0.1O3 (A = La, Eu and Ho) has been undertaken. Rietveld analysis of powder x-ray diffraction data revealed that all the compounds crystallize in the orthorhombic symmetry with Pbnm space group. Temperature dependent magnetization studies showed that all our investigated compounds exhibit a paramagnetic-ferromagnetic transition at low temperature with Curie temperature (Tc) decreases with decrease in average A-site ionic radius ( ). The materials show metal–semiconductor transition at low temperature and the values of metal–semiconductor transition temperature (TMS) are found to be much lower than Curie temperatutre (Tc). It has been concluded that the conduction mechanism was dominated by small polaron hopping model in the high temperature paramagnetic semiconducting region.

Impact of low titanium concentration on the structural, electrical and dielectric properties of Pr0.75Bi0.05Sr0.1Ba0.1Mn1−xTixO3 (x = 0, 0.04) compounds

In this work, we report the effect of Ti-doping on the structural, electrical and dielectric properties of Pr0.75Bi0.05Sr0.1Ba0.1Mn1−xTixO3 (x = 0 and 0.04) manganites synthesized by sol–gel route. X-ray structural analysis using Rietveld refinement technique reveals that our samples are single phase and crystallize in the orthorhombic system with Pbnm space group. These structures were characterized by impedance spectroscopy in the frequency range from 40 to 1 MHz and temperatures between 80 and 400 K. DC conductivity analysis shows that our samples disclose a semiconductor behaviour with a semiconductor–metal transition temperature TMS at 380 and 360 K for x = 0 and 0.04 respectively. This result shows that the TMS temperature can be controlled by varying the titanium content. AC conductivity (σAC) results show a considerable sensitivity to both temperature and frequency. The impedance plots were well described by an equivalent circuit model taking into account the contributions of semiconductor grains and insulating grain boundaries in the conduction mechanism. Furthermore, a non-Debye type of relaxation in our compounds was confirmed. In addition, we have demonstrated that the dielectric relaxation is hidden by the DC conduction mechanism.