Temperature-dependent Resistivity Measurements Research Articles

Investigation of the phase change mechanism of Ge6Sn2Sb2Te11

Thin films of Ge6Sn2Sb2Te11 were synthesized and compared to the well-known unsubstituted phase change material (PCM) Ge8Sb2Te11. In situ X-ray diffraction (XRD) and temperature dependent sheet resistance measurements evidenced a significant decrease of the phase change temperature from 144 °C for Ge8Sb2Te11 to 112 °C for Ge6Sn2Sb2Te11. The resistance measurements also revealed an intermediate step during the phase change. Detailed in situ transmission electron microscopy (TEM) and (XRD) investigations on structural ordering phenomena suggest that this intermediate step is associated with the disorder of structural vacancies on the cationic sites stable up to 130 °C. Annealing the sample beyond 130 °C leads to a subsequent ordering of vacancies and thus to the formation of a metastable primitive trigonal phase with vacancy layers. At ∼240 °C - ∼300 °C, a transition to the stable phase is observed. For the first time, an in plane movement of bi-layer defects is observed by in situ TEM as a result of a self-ordering mechanism. These findings represent new insights into the transition process on the nanoscale and suggest that tin substituted PCMs may represent promising candidates for multi-level data storage applications.

Vanadium Pentoxide Nanofibers as IR sensors for Bolometer Application

The main aim of this work is to report an alternative technique of creating vanadium pentoxide (V2O5) based uncooled infrared (IR) detector, by a state-of-the-art V2O5 nanofibers, manufactured by facile and economical electrospinning process. The nanofibers were thermally and electrically characterized to determine their bolometric performance. The nanofibers show maximum voltage responsivity of (ℜv ) 6987.3 V/W at 100 mA DC bias, under normal room temperature and pressure condition. Nanofibers show very good thermal response (τs ) and recovery time (τr ) when subjected to a periodic On-Off cycle of IR lamp (150W) illumination. Temperature dependent resistance measurement reveals that nanofibers are exhibiting semiconductor to metallic phase transition at 67°C with maximum TCR% -1.6%/K at the transition temperature. V2O5 nanofibers characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman Spectroscopy confirms their crystallinity and elemental composition. The optical band gap of the nanofibers is analyzed by UV-Visible spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images confirms their microstructural dimensions and surface homogeneity. The entire analysis reaffirms the suitability of V2O5 nanofibers as one of the futuristic sensing material for IR imaging applications. Figure 1

Crystal growth and characterization of bulk Sb2Te3 topological insulator

The Sb2Te3 crystals are grown using the conventional self flux method via solid state reaction route, by melting constituent elements (Sb and Te) at high temperature (850 °C), followed by slow cooling (2 °C/h). As grown Sb2Te3 crystals are analysed for various physical properties by x-ray diffraction (XRD), Raman Spectroscopy, Scanning Electron Microscopy (SEM) coupled with Energy Dispersive x-ray Spectroscopy (EDAX) and electrical measurements under magnetic field (6 Tesla) down to low temperature (2.5 K). The XRD pattern revealed the growth of synthesized Sb2Te3 sample along (00l) plane, whereas the SEM along with EDAX measurements displayed the layered structure with near stoichiometric composition, without foreign contamination. The Raman scattering studies displayed known ( and ) vibrational modes for the studied Sb2Te3. The temperature dependent electrical resistivity measurements illustrated the metallic nature of the as grown Sb2Te3 single crystal. Further, the magneto—transport studies represented linear positive magneto-resistance (MR) reaching up to 80% at 2.5 K under an applied field of 6 Tesla. The weak anti localization (WAL) related low field (±2 Tesla) magneto-conductance at low temperatures (2.5 K and 20 K) has been analysed and discussed using the Hikami—Larkin—Nagaoka (HLN) model. Summarily, the short letter reports an easy and versatile method for crystal growth of bulk Sb2Te3 topological insulator (TI) and its brief physical property characterization.

Enhanced infrared sensing properties of vanadium pentoxide nanofibers for bolometer application

The main aim of this work is to report an alternative technique of creating vanadium pentoxide (V2O5) based uncooled infrared (IR) detector, by a state-of-the-art V2O5 nanofibers, manufactured by facile and economical electrospinning process. The nanofibers were thermally and electrically characterized to determine their bolometric performance. The nanofibers show maximum voltage responsivity (Rv) 6987.3 V/W at 100 mA DC bias, in a normal room temperature and pressure condition. Nanofibers show very good thermal response (τs) and recovery time (τr) when subjected to a periodic On-Off cycle of IR lamp (150 W) illumination. Temperature dependent resistance measurement shows that nanofibers are exhibiting semiconductor to metallic phase transition at 67 °C with maximum temperature coefficient of resistance (TCR%) − 1.6%/K at the transition. V2O5 nanofibers characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman Spectroscopy confirms their crystallinity and elemental composition. The optical band gap of the nanofibers is analyzed by UV–Visible spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images confirms their microstructural dimensions and surface homogeneity. The entire analysis reaffirms the suitability of V2O5 nanofibers as one of the futuristic sensing material for IR imaging applications.

A fully automated temperature-dependent resistance measurement setup using van der Pauw method.

The van der Pauw (VDP) method is widely used to identify the resistance of planar homogeneous samples with four contacts placed on its periphery. We have developed a fully automated thin film resistance measurement setup using the VDP method with the capability of precisely measuring a wide range of thin film resistances from few mΩ up to 10 GΩ under controlled temperatures from room-temperature up to 600 °C. The setup utilizes a robust, custom-designed switching network board (SNB) for measuring current-voltage characteristics automatically at four different source-measure configurations based on the VDP method. Moreover, SNB is connected with low noise shielded coaxial cables that reduce the effect of leakage current as well as the capacitance in the circuit thereby enhancing the accuracy of measurement. In order to enable precise and accurate resistance measurement of the sample, wide range of sourcing currents/voltages are pre-determined with the capability of auto-tuning for ∼12 orders of variation in the resistances. Furthermore, the setup has been calibrated with standard samples and also employed to investigate temperature dependent resistance (few Ω-10 GΩ) measurements for various chalcogenide based phase change thin films (Ge2Sb2Te5, Ag5In5Sb60Te30, and In3SbTe2). This setup would be highly helpful for measurement of temperature-dependent resistance of wide range of materials, i.e., metals, semiconductors, and insulators illuminating information about structural change upon temperature as reflected by change in resistances, which are useful for numerous applications.

Anisotropic magnetic properties of the ferromagnetic semiconductor CrSbSe3

Single crystals of CrSbSe$_3$, a structurally pseudo-one-dimensional ferromagnetic semiconductor, were grown using a high-temperature solution growth technique and were characterized by x-ray diffraction, anisotropic, temperature- and field-dependent magnetization, temperature-dependent resistivity and optical absorption measurements. A band gap of 0.7 eV was determined from both resistivity and optical measurements. At high temperatures, CrSbSe$_3$ is paramagnetic and isotropic with a Curie-Weiss temperature of $\sim$145 K and an effective moment of $\sim$4.1 $\mu_B$/Cr. A ferromagnetic transition occurs at $T_c$ = 71 K. The $a$-axis, perpendicular to the chains in the structure, is the magnetic easy axis, while the chain axis direction, along $b$, is the hard axis. Magnetic isotherms measured around $T_c$ do not follow the behavior predicted by simple mean field critical exponents for a second order phase transition. A tentative set of critical exponents is estimated based on a modified Arrott plot analysis, giving $\beta\sim$0.25, $\gamma\sim$1.38 and $\delta\sim$6.6.

Enhancement of Electrical Resistivity in Nickel Doped ZnO Nanoparticles

A systematic investigations on the structural, morphological and electrical properties of Zn1-xNixO (x = 0.00, 0.03 and 0.05) nanoparticles synthesized via sol-gel auto combustion technique. The prepared samples were characterized by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM). X-ray diffraction pattern shows the formation of single phase with hexagonal wurtzite structure. The lattice parameter of Ni doped ZnO is slightly greater than that of un-doped ZnO nanoparticles. The crystalline size of prepared nanoparticles is found to be in 30 to 32 nm. SEM image shows that the grains are in nanometre range which confirms the nanocrystalline nature of present samples. The temperature dependent DC electrical resistivity measurements have been carried out in the temperature range of 303-573 K. The DC electrical resistivity was found to increase with increase in Ni2+ concentration into ZnO matrix. The resistivity decreases with increasing temperature which interpreted semiconducting nature of ZnO nanoparticles.

Electrical characterization and examination of temperature-induced degradation of metastable Ge0.81Sn0.19 nanowires.

Metastable germanium-tin alloys are promising materials for optoelectronics and optics. Here we present the first electrical characterization of highly crystalline Ge0.81Sn0.19 nanowires grown in a solution-based process. The investigated Ge0.81Sn0.19 nanowires reveal ohmic behavior with resistivity of the nanowire material in the range of ∼1 × 10-4Ω m. The temperature-dependent resistivity measurements demonstrate the semiconducting behavior. Moreover, failure of devices upon heating to moderate temperatures initiating material degradation has been investigated to illustrate that characterization and device operation of these highly metastable materials have to be carefully conducted.

Non-destructive reversible resistive switching in Cr doped Mott insulator Ca2RuO4: Interface vs bulk effects

A non-destructive reversible resistive switching is demonstrated in single crystals of Cr-doped Mott insulator Ca2RuO4. An applied electrical bias was shown to reduce the DC resistance of the crystal by as much as 75%. The original resistance of the sample could be restored by applying an electrical bias of opposite polarity. We have studied this resistive switching as a function of the bias strength, applied magnetic field, and temperature. A combination of 2-, 3-, and 4-probe measurements provide a means to distinguish between bulk and interfacial contributions to the switching and suggests that the switching is mostly an interfacial effect. The switching was tentatively attributed to electric-field driven lattice distortions which accompany the impurity-induced Mott transition. This field effect was confirmed by temperature-dependent resistivity measurements which show that the activation energy of this material can be tuned by an applied DC electrical bias. The observed resistance switching can potentially be used for building non-volatile memory devices like resistive random access memory.

Structural and electrical properties of VO2/ZnO nanostructures

We examined the structural and electrical properties of uniformly-oriented VO2/ZnO nanostructures. VO2 was deposited on ZnO templates by using a direct current-sputtering deposition. Scanning electron microscope and transmission electron microscope measurements indicated that b-oriented VO2 were uniformly crystallized on ZnO templates with different lengths. VO2/ZnO formed nanorods on ZnO nanorods with length longer than 250 nm. X-ray absorption fine structure at the V K edge of VO2/ZnO showed M1 and R phases of VO2 at 30 and 100 °C, respectively, suggesting structural-phase transition occurring between the two temperatures. Temperature-dependent resistance measurements of VO2/ZnO nanostructures revealed metal-to-insulator transition at 65 °C and 55 °C during a heating and a cooling, respectively, regardless of ZnO length. Asymmetry behavior of resistance curves from VO2/ZnO nanostructure during a heating and a cooling was attributed from a strong bond of VO2 and ZnO.

Multiple electrical phase transitions in Al substituted barium hexaferrite

Barium hexaferrite is known to be a very good ferromagnetic material. However, it shows very good dielectric properties, i.e., the dielectric constant is comparable to that of the ferroelectric material. However, its crystal symmetry does not allow it to be a ferroelectric material. Hence, the electrical properties have revived the considerable research interest on these materials, not only for academic interest, but also for technological applications. There are a few reports on temperature dependent dielectric behavior of these materials. However, the exact cause of dielectric as well as electrical conductivity is yet to be established. Hence, Al (very good conducting material) substituted barium hexaferrite (BaFe12−xAlxO19, x = 0.0–4.0) has been prepared by following the modified sol-gel method to understand the ac and DC electrical properties of these materials. The crystal structure and parameters have been studied by employing the XRD and FTIR techniques. There are two transition temperatures, which have been observed in the temperature dependent ac dielectric and DC resistivity measurement. The response of dielectric behaviors to temperature is similar to that of the ferroelectric material; however, the dielectric polarization is due to the polaron hopping, which is evident from the DC resistivity analysis. Hence, the present observations lead to understand the electrical properties of barium hexaferrite. The frequency dependent dielectric dispersion can be understood by the modified Debye model. More interestingly, the dielectric constant decreases and DC resistivity increases with the increase in the Al concentration, which has the correlation between bond length modifications in the crystal due to substitution.

Superconductivity in the Nb-Ru-Ge σ phase

We show that the previously unreported ternary $\sigma$-phase material Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ is a superconductor with a critical temperature of 2.2 K. Temperature-dependent magnetic susceptibility, resistance, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant $\gamma$ for Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ is 91 mJ mol-f.u.$^{-1}$K$^{-2}$ and the specific heat anomaly at the superconducting transition, $\Delta$C/$\gamma$T$_c$, is approximately 1.38. The zero-temperature upper critical field ($\mu_0$H$_{c2}$(0)) was estimated to be 2 T by resistance data. Field-dependent magnetization data analysis estimated $\mu_0$H$_{c1}$(0) to be 5.5 mT. Thus, the characterization shows Nb$_{20.4}$Ru$_{5.7}$Ge$_{3.9}$ to be a type II BCS superconductor. This material appears to be the first reported ternary phase in the Nb-Ru-Ge system, and the fact that there are no previously reported binary Nb-Ru, Nb-Ge, or Ru-Ge $\sigma$-phases shows that all three elements are necessary to stabilize the material. A $\sigma$-phase in the Ta-Ru-Ge system was synthesized but did not display superconductivity above 1.7 K, which suggests that electron count cannot govern the superconductivity observed. Preliminary characterization of a possible superconducting $\sigma$-phase in the Nb-Ru-Ga system is also reported.

Manipulation of dangling bonds of interfacial states coupled in GeTe-rich GeTe/Sb2Te3 superlattices

Superlattices consisting of stacked nano-sized GeTe and Sb2Te3 blocks have attracted considerable attention owing to their potential for an efficient non-melting switching mechanism, associated with complex bonding between blocks. Here, we propose possible atomic models for the superlattices, characterized by different interfacial bonding types. Based on interplanar distances extracted from ab initio calculations and electron diffraction measurements, we reveal possible intercalation of dangling bonds as the GeTe content in the superlattice increases. The dangling bonds were further confirmed by X-ray photoelectron spectroscopy, anisotropic temperature dependent resistivity measurements down to 2 K and magnetotransport analysis. Changes of partially coherent decoupled topological surfaces states upon dangling bonds varying contributed to the switching mechanism. Furthermore, the topological surface states controlled by changing the bonding between stacking blocks may be optimized for multi-functional applications.

Crystal growth and transport properties of Weyl semimetal TaAs

We report the single crystal growth and transport properties of a Weyl semimetal TaAs. Unsaturated large magnetoresistance of about 22 100% at 2 K and 9 T is observed. From the Hall measurement, carrier concentrations n = 4.608 × 1024 m−3 and p = 3.099 × 1024 m−3, and mobilities µp = 2.502 m2 V−1 s−1 and µn = 16.785 m2 V−1 s−1 at 2 K are extracted. The de Haas–van Alphen oscillations at 2 K and 9 T suggest the presence of a Fermi surface, and the quantum electronic parameters such as effective cyclotron mass and Dingle temperature were obtained using Lifshitz–Kosevich fitting. Temperature dependent resistivity measurements at different static magnetic fields suggest the formation of an insulating gap in the Weyl semimetal TaAs. An angle-resolved photoemission spectroscopy study reveals Fermi arc surface states with different shaped features such as a long elliptical contour around each point, a bowtie-shaped contour around each point, and a crescent-shaped feature near the midpoint of each line.

Effects of alkali metals on the formation of Tl-1212 type phase (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 with A = Li, Na and Rb

The effects of alkali metals A = Li, Na and Rb on (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 (Tl-1212) superconductors with x = 0–0.1 were studied. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed that the formation of Tl-1212 phase was favorable when the ionic radius of the alkali metal was closer to the radius of Tl1+. Ca0.3Sr0.7CuO2 was present as a major or minor phase in some of the samples. Scanning electron micrographs showed that the grain size increased with Li, Na and Rb substitutions. The temperature-dependent electrical resistance measurements showed metallic normal state behavior for all samples with onset transition temperature, T c onset between 96 and 105 K and zero-resistance temperature, T c zero between 92 and 99 K. AC susceptibility measurement showed superconducting transition, \({T_{{c_{{{\upchi}}}}{{'}}}}\) between 92 and 97 K. The peak temperature T p of the imaginary part of the susceptibility χ″ was the highest for Na and Rb with x = 0.1 samples (T p = 92 K). The inter-grain critical current density, J c at T p for x = 0 sample was J c (T p = 68 K) = 14 A cm−2 and for Li substituted sample with x = 0.1, J c (T p = 76 K) = 22 A cm−2. This work showed that Rb greatly enhanced the Tl-1212 phase formation due to the proximity of the Rb1+ and Tl1+ ionic radius. Na and Rb improved the transition temperature. Li, Na and Rb improved the inter-grain critical current density and increased the grain size of the samples.

Physical properties of neodymium tin oxide pyrochlore ceramics

AbstractIn this work, physical properties of neodymium tin oxide pyrochlore ceramics prepared by solid state reaction technique are investigated by means of X-ray diffraction, scanning electron microscopy, ultraviolet-visible light (UV-Vis) spectrophotometry and temperature dependent electrical resistivity measurements. The pyrochlore is observed to have a cubic FCC crystal lattice with lattice parameter of 10.578 Å. The planes of the cubic cell are best oriented in the [2 2 2] direction. From the X-ray, the UV-Vis spectrophotometry and the electrical resistivity data analysis, the grain size, strain, dislocation density, optical and thermal energy band gaps, localized energy band tail states and resistivity activation energies are determined and discussed. The pyrochlore is observed to have an optical energy band gap of ~3.40 eV. This value corresponds to 365 nm UV light spectra which nominates the neodymium tin oxide pyrochlore ceramics for the use as UV sensors.

Growth and characterization of BaZnGa

We report the growth, structure and characterization of BaZnGa, identifying it as the sole known ternary compound in the Ba–Zn–Ga system. Single crystals of BaZnGa can be grown out of excess Ba–Zn and adopt a tI36 structure type. There are three unique Ba sites and three M = Zn/Ga sites. Using DFT calculations we can argue that whereas one of these three M sites is probably solely occupied by Ga, the other two M sites, most likely, have mixed Zn/Ga occupancy. Temperature-dependent resistivity and magnetization measurements suggest that BaZnGa is a poor metal with no electronic or magnetic phase transitions between 1.8 and 300 K.

Effect of chemical pressure on the electronic phase transition in Ca1−xSrxMn7O12 films

We demonstrate how chemical pressure affects the structural and electronic phase transitions of the quadruple perovskite CaMn7O12 by Sr doping, a compound that exhibits a charge-ordering transition above room temperature making it a candidate for oxide electronics. We have synthesized Ca1−xSrxMn7O12 (0 ≤ x ≤ 0.6) thin films by oxide molecular beam epitaxy on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. The substitution of Sr for Ca results in a linear expansion of the lattice, as revealed by X-ray diffraction. Temperature-dependent resistivity and X-ray diffraction measurements are used to demonstrate that the coupled charge-ordering and structural phase transitions can be tuned with Sr doping. An increase in Sr concentration acts to decrease the phase transition temperature (T*) from 426 K at x = 0 to 385 K at x = 0.6. The presence of a tunable electronic phase transition, above room temperature, points to the potential applicability of Ca1−xSrxMn7O12 in sensors or oxide electronics, for example, via charge doping.

Elevated transition temperature in Ge doped VO2 thin films

Thermochromic GexV1−xO2+y thin films have been deposited on Si (100) substrates by means of reactive magnetron sputtering. The films were then characterized by Rutherford backscattering spectrometry (RBS), four-point probe electrical resistivity measurements, X-ray diffraction, and atomic force microscopy. From the temperature dependent resistivity measurements, the effect of Ge doping on the semiconductor-to-metal phase transition in vanadium oxide thin films was investigated. The transition temperature was shown to increase significantly upon Ge doping (∼95 °C), while the hysteresis width and resistivity contrast gradually decreased. The precise Ge concentration and the film thickness have been determined by RBS. The crystallinity of phase-pure VO2 monoclinic films was confirmed by XRD. These findings make the use of vanadium dioxide thin films in solar and electronic device applications—where higher critical temperatures than 68 °C of pristine VO2 are needed—a viable and promising solution.

Characterization of Primary Carrier Transport Properties of the Light-Harvesting Chalcopyrite Semiconductors CuIn(S1–xSex)2

We report the carrier transport properties of CuIn(S1–xSex)2 (0 ≤ x ≤ 1), a promising chalcopyrite semiconductor series for solar water splitting. A low concentration Mg dopant is used to decrease the carrier resistivity through facilitating bulk p-type transport at ambient temperature. Temperature-dependent resistivity measurements reveal a four-order magnitude decrease in bulk electrical resistivity (from 103 to 10–1 Ohm cm) for 1% Mg-doped CuIn(S1–xSex)2 as x increases from 0 to 1. Hall effect measurements at room temperature reveal p-type majority carrier concentrations that vary from 1015 to 1018 cm–3 and mobilities of approximately 1–10 cm2 V–1 s–1. These results provide insights into the fundamental carrier transport properties of CuIn(S1–xSex)2 and will be of value in optimizing these materials further for photoelectrochemistry applications.