High Temperature Resistivity Measurements Research Articles

Melt-grown semi-insulating Mn:β-Ga2O3 single crystals exhibiting unique visible absorptions and luminescence

Several acceptor dopants have been explored in β-Ga2O3 to produce semi-insulating substrates and epitaxial films. Fe and Mg make up the majority of research thus far; however, other transition metals provide potential alternatives for optimized performance. β-Ga2O3 bulk single crystals were grown by the Czochralski and vertical gradient freeze methods with a nominal dopant concentration of 0.25 at. % Mn. Ultraviolet-visible-near infrared spectroscopy and photoluminescence revealed polarization- and orientation-dependent optical absorptions (pleochroism) coupled with an orange luminescence. All samples were electrically insulating, on the order of 109–1011 ohm cm at room temperature, indicative of acceptor doping. Actual dopant concentrations of the intentionally doped transition metal and background impurities were determined via glow discharge mass spectrometry, indicating the macroscale segregation behavior. High-temperature resistivity measurements indicated an experimental acceptor level of 1.7 ± 0.2 eV. Hydrogenation of samples resulted in an increase in the orange luminescence and O–H stretching modes observable in the infrared spectrum. Density functional theory calculations were performed to determine the likely site-occupancy and acceptor level of Mn in the bandgap.

Orbital hybridisation effects in B2 phase Cr doped Co2MnAl

We investigate here the magnetic, transport, local structural and electronic properties of Co2MnCr x Al (x = 0, 0.05, 0.1 and 0.2). Our results show that all the compounds stabilise in B2 phase and are ferromagnets. The results reveal disorder at the structural and magnetic levels. X-ray absorption near edge structure (XANES) analysis reveal signature of antisite disorder between Mn and Al atoms with equal ratio. The electronic structure calculations suggest enhancement in the half metallicity, localisation of electrons at the Fermi level and an increment in density of states with doping. The combined results of electronic structure calculations and XANES studies suggest transfer of electrons to the Co site. The results of high temperature resistivity measurements suggest the conduction electrons are undergoing transition from delocalisation to weak localisation to activated behaviour with Cr doping. The extended x-ray absorption spectroscopic analysis shows that the local structure around Mn atom is different from the global structure as obtained from the x-ray diffraction results. The behaviour of the edge region is in line with the trend as obtained from the compositional analysis. We observe link between the hybridisation of 3d like states at the Mn, Cr sites with that at the Co site and the transport properties. This could help in understanding the unusual decrement in the lattice parameter with doping. These results reveal the role of local structure in understanding the physical properties of such systems.

Temperature dependent striction effect in a single crystalline Nd2Fe14B revealed using a novel high temperature resistivity measurement technique

We studied the temperature dependence of resistivity in a single crystalline Nd2Fe14B using a newly developed high temperature probe. This novel probe employs mechanical pin connectors instead of conducting glue/paste. From warming and cooling curves, the Curie temperature was consistently measured around T c = 580 K. In addition, anomalous discrete jumps were found only in cooling curves between 400 and 500 K, but not shown in warming curves. More interestingly, when the jumps occurred during cooling, the resistivity was increased. This phenomenon could possibly be due to a temperature dependent striction effect induced by the re-orientation of magnetic domains well below the Curie temperature. Further microscopic study is needed to confirm this effect.

High-temperature electrical resistivity measurements of hcp iron to Mbar pressure in an internally resistive heated diamond anvil cell

ABSTRACTElectrical and thermal transport properties of iron are of great importance for understanding the thermal evolution and dynamics of the Earth’s core, of which iron is a major component. However, the reported values of iron’s conductivity at core conditions are still highly controversial. In this study, we employed an internally resistive heated diamond anvil cell (IHDAC) technique that reduces temperature heterogeneity inside the heated sample, and revisited the electrical resistivity of hexagonal closed packed (hcp) iron up to 110 GPa and 2500 K. Our results are in complete agreement with the reported values of the high pressure-temperature electrical resistivity of hcp iron obtained by using the laser-heated diamond anvil cell technique, supporting high iron conductivity at the Earth’s core conditions. The high pressure-temperature resistivity measurement in an IHDAC employed here would be a more suitable method to examine more resistive iron alloys at the Earth’s core conditions.

Structural and electronic properties of the spin-filter material CrVTiAl with disorder

The effects of chemical disorder on the electronic properties of the spin-filter material CrVTiAl are investigated experimentally and theoretically. Synchrotron X-ray diffraction experiments on bulk CrVTiAl and the associated Rietveld analysis indicate that the crystal structure consists primarily of a mixture of a partially ordered B2 phase, a fully disordered A2 phase, and a small component of an ordered L21 or Y phase. High temperature resistivity measurements confirm the existence of a bandgap. First-principles, all-electron, self-consistent electronic structure computations show that the chemically disordered A2 and B2 phases are metallic, while the spin-filter properties of the ideal Y-type phase are preserved in the presence of L21 disorder (Cr and V mixing). The Hall coefficient is found to decrease with increasing temperature, similar to the measured increase in the conductivity, indicating the presence of thermally activated semiconductor-like carriers.

Magnetic and structural properties of Co2MnSi based Heusler compound

The influence of antisite disorder occupancies on the magnetic properties of the half-metallic Co2MnSi compound was studied by experimental techniques and first-principles calculations. The neutron diffraction studies show almost equal amount of Mn and Co disorders of 6.5% and 7.6%, which was found to be in good agreement with density functional theory (DFT) calculations of the stable Co2MnSi system with the corresponding disorders. DFT studies reveal that antiferromagnetic interactions introduced by Mn disorder lead to a reduction of the net magnetic moment. The results are discussed in conjunction with neutron diffraction and magnetization measurements. Magnetotransport measurements revealed a positive magnetoresistance for bulk Co2MnSi, which decreases as temperature increases. A Curie temperature of ∼1014 K was determined for the compound by high-temperature electrical resistivity and dilatometry measurements.

Structural, magnetic, electrical and electrochemical properties of SrCoO2.5, Sr9Co2Mn5O21 and SrMnO3 compounds

SrCo1−xMnxO3−d (0 ≤ x ≤ 1) were synthesized by conventional solid state reaction method. We found that Mn substitution hinders the decomposition of carbonate at 900 °C and increases the synthesis temperature of SrCo1−xMnxO3−d to 1200 °C. Three different main phases emerge as the Mn content is increased; SrCoO2.5, Sr9Co2Mn5O21 and SrMnO3. Structural properties of the samples were investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Valance states of the transition metal (TM) ions were estimated based on lattice parameters and ionic radii of TM ions. High temperature resistivity measurements show hysteresis upon heating and cooling and is attributed to oxygen release without any endothermic/exothermic activity. The spin configurations of Co and Mn ions in SrCoO2.5, Sr9Co2Mn5O21 and SrMnO3 were estimated by Curie-Weiss fitting of χ-T curves. Electrochemical measurements show two anodic/cathodic peaks indicating two main types of oxygen sites in the unit cell.

Electrically Active Defects in Neutron Irradiated HPSI 4H-SiC X-ray Detectors Investigated by ZB-TSC Technique

Electrically active defects in high-purity semi-insulating (HPSI) 4H-silicon carbide (4H-SiC) X-ray detectors have been characterized before and after neutron irradiation by zero-bias thermally stimulated current (ZB-TSC) and high-temperature resistivity measurements. The ZB-TSC measurements prior to irradiation reveal nine hole traps in the energy range of 0.22–1.16 eV and two electron traps at $\text{E}_{c} -0.35$ eV and $\text{E}_{c} -0.44$ eV. For the same case, high-temperature resistivity measurements yield thermal activation energy in the range of 1–1.3 eV. It is considered that the hole traps at $\text{E}_{v}+ 0.83$ eV and $\text{E}_{v }+ 1.16$ eV along with defects located near the activation energies are possibly responsible for the substrate compensation. A poor resolution of 11.8-keV full-width at half-maximum (at 40 keV of 152Eu) with un-irradiated HPSI 4H-SiC detectors is attributed to the charge trapping and polarization effects. After neutron irradiation at the fluence of $10^{11}$ n/cm2, seven new traps ( $\text{E}_{v}+ 0.68$ eV, $\text{E}_{v} + 1.08$ eV, $\text{E}_{v} + 1.22$ eV, $\text{E}_{c} -0.72$ eV, $\text{E}_{c} - 0.9$ eV, $\text{E}_{c} - 1.04$ eV, and $\text{E}_{c} - 1.1$ eV) are identified with no significant changes in the activation energies. This indicates that the substrate resistivity is unaffected by the neutron-irradiation-induced defects. No considerable changes in the X-ray spectral response are noted at $10^{11}$ n/cm2.

Instrument for stable high temperature Seebeck coefficient and resistivity measurements under controlled oxygen partial pressure

The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO2) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO2-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O2 sensor and the transient behavior of the resistance as a proxy. A pO2 range of 10−25–100 atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt–Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO2 on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. An approximately 11 % increase in power factor over a pO2 range of 10−19–10−8 atm at 973 K for the donor-doped single crystals is observed.

High pressure study of B12As2: Electrical transport behavior and the role of grain boundaries

Using a diamond anvil cell, the evolutions of alternate-current impedance spectra and direct- current resistivity in B12As2 have been investigated up to 51.9 GPa. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The grain boundary resistance shows a relatively smaller contribution to the total resistance above 16.8 GPa. By using the double-Schottky barrier model, the space charge potential was obtained. A pressure-induced inversion of charge defect concentration in the space charge layer was found at 20.7 GPa. The high-temperature resistivity measurements indicate that the transport activation energy is determined by defect energy levels in the band gap.

Crystal growth and characterization of the narrow-band-gap semiconductors OsPn₂ (Pn = P, As, Sb).

Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

Structure of liquid Cu–Sb alloys by ab initio molecular dynamics simulations, high temperature X-ray diffraction, and resistivity

Structure of Cu–Sb melts has been studied by ab initio molecular dynamics simulations, high-temperature X-ray diffraction and resistivity measurements. Over the whole concentration range, heterogeneous coordination numbers are larger than that of homogeneous atoms. This indicates preferential Cu–Sb coordination in Cu–Sb melts. A drop is observed in maximum position of simulated Sb–Sb partial distribution functions around Cu75Sb25, which reveals the rapid increase of Sb–Sb coordination. Around eutectic melts, main peak splitting is observed in both structure factor and simulated total pair distribution functions, which reveals the co-existence of Cu–Sb heterogeneous and Sb–Sb clusters. Abnormal changes in temperature coefficient of resistivity are observed around pure Sb and in compound-forming range, which are well interpreted as reinforcement of Peierls distortion and Cu3Sb compound clusters, respectively. Structural inhomogeneity that results from atomic size effect also has been discussed by analyzing concentration dependence of Warren–Cowley parameters and concentration correlation functions.

Effects of size and oxygen annealing on the multiferroic behavior of bismuth ferrite nanoparticles

We have compared the multiferroic behavior in different-sized BiFeO3 nanoparticles (11–29 nm) prepared by a solution evaporation method. The effects of oxygen vacancies on the behavior of phase pure BiFeO3 have been studied for particles prepared under two different annealing environments, air and oxygen. We find that BiFeO3 nanoparticles show weak ferromagnetism at room temperature which increases with decreasing particle size. However, the effects are determined not to be related to the presence of more oxygen vacancies, since oxygen annealing increases the magnetic moment in this system. In dielectric studies, we observe clear evidence for magnetoelectric coupling with the imaginary part of dielectric constant exhibiting a clear anomaly around the magnetic transition temperature (T ~ 560 K). In high temperature resistivity measurements insulating and activated resistivity behavior is observed with the activation energies being lower for T < TN. The increased ferromagnetism in smaller particles is explained in terms of suppression of the antiferromagnetic spin spiral whereas oxygen annealing actually leads in our case to an improvement of the crystallinity.

Characterization of 4H-SiC Epitaxial Layers and High-Resistivity Bulk Crystals for Radiation Detectors

Defect and electrical characterization of bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates is reported. Optical microscopy, electron beam induced current (EBIC) imaging, current-voltage ( $I$ – $V$ ) measurements, thermally stimulated current (TSC) spectroscopy (94 K–620 K), Hall effect, and van der Pauw measurements have been conducted for characterization and defect correlation studies. Both epitaxial layers exhibited relatively shallow levels related to Al, B, $L$ - and D-centers. Deep level centers in the n-type epitaxial layer peaked at ${\sim} 400$ K ( $E_{a} \sim 1.1$ eV), and ${\sim} 470$ K were correlated with $IL_{2}$ defect and 1.1 eV center in high-purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ${\sim} 290$ K ( $E_{a} = 0.82\hbox{--}0.87$ eV) that was attributed to $IL_{1}$ and HK2 centers, and at ${\sim} 525$ K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band-gap. Results of EBIC and optical microscopy showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layer. The $I$ – $V$ characteristics of the devices on SI epitaxial layer exhibited steps corresponding to the ultimate trap filling of deep centers. The high-temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep-level electron trap centers. The responsivity of the n-type epitaxial 4H-SiC detector in the soft X-ray energy range is reported for the first time.

Physical properties ofAxFe2−yS2(A=K, Rb, and Cs) single crystals

We successfully synthesized two new compounds Rb$_x$Fe$_{2-y}$S$_2$ and Cs$_x$Fe$_{2-y}$S$_2$ which were isostructural with K$_x$Fe$_{2-y}$Se$_2$ superconductor. We systematically investigated the resistivity, magnetism and thermoelectric power of $A_x$Fe$_{2-y}$S$_2$ ($A$=K, Rb and Cs) single crystals. High temperature resistivity and magnetic measurements show anomalies above 500 K depending on $A$ which are similar to $A_x$Fe$_{2-y}$Se$_2$. Discrepancy between ZFC and FC curves was observed in K$_x$Fe$_{2-y}$S$_2$ and Rb$_x$Fe$_{2-y}$S$_2$, while it disappears in Cs$_x$Fe$_{2-y}$S$_2$. Our results indicate the similar magnetism between $A_x$Fe$_{2-y}$S$_2$ and $A_x$Fe$_{2-y}$Se$_2$ at high temperature.

High temperature Seebeck coefficient and resistance measurement system for thermoelectric materials in the thin disk geometry

A versatile apparatus to measure the cross-plane Seebeck coefficient and the resistivity of bulk samples shaped as disks or thin plates, over a temperature range of 300 K-620 K with possible extension to higher temperatures, is presented. It is constructed from readily available equipment and instrumentation with parts that are easily manufactured. The Seebeck coefficient is measured over an average region of the sample under steady-state conditions. The sample resistance is measured using a four-point alternating current method and scaled to room temperature measurements with known geometry to calculate resistivity. A variety of sample shapes are supported. Most importantly, the support of the thin disk geometry allows for the very same samples to be used in a laser flash instrument. The design allows for rough vacuum, high vacuum, or purging with inert gases in the sample chamber. Measurements on thermoelectric ZnSb and a Ni reference material are presented.

Studies on transport properties of copper doped tungsten diselenide single crystals

During recent years, transition metal dichalcogenides of groups IVB, VB and VIB have received considerable attention because of the great diversity in their transport properties. 2H-WSe 2 (Tungsten diselenide) is an interesting member of the transition metal dichalcogenide (TMDC's) family and known to be a semiconductor useful for photovoltaic and optoelectronic applications. The anisotropy usually observed in this diamagnetic semiconductor material is a result of the sandwich structure of Se–W–Se layers interacting with each other, loosely bonded by the weak Van der Waals forces. Recent efforts in studying the influence of the anisotropic electrical and optical properties of this layered-type transition metal dichalcogenides have been implemented by doping the samples with different alkali group elements. Unfortunately, little work is reported on doping of metals in WSe 2. Therefore, it is proposed in this work to carry out a systematic growth of single crystals of WSe 2 by doping it with copper in different proportions i.e. Cu x WSe 2 ( x=0, 0.5, 1.0) by direct vapour transport technique. Transport properties like low and high temperature resistivity measurements, high pressure resistivity, Seebeck coefficient measurements at low temperature and Hall Effect at room temperature were studied in detail on all these samples. These measurements show that tungsten diselenide single crystals are p-type whereas doped with copper makes it n-type in nature. The results obtained and their implications are discussed in this paper.

Mg substitution in CuCrO 2 delafossite compounds

A detailed investigation of the series CuCr 1− x Mg x O 2 ( x = 0.0 – 0.05 ) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg 2+ for Cr 3+ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x ∼ 0.02 – 0.03 , indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr 2O 4 spinels as soon as x = 0.005 . The thermopower is discussed in the temperature-independent correlation function ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Thermally stimulated current and high temperature resistivity measurements of 4H semi-insulating silicon carbide

High purity semi-insulating 4H SiC single crystals have potential applications for room temperature radiation detectors because of the wide band gap and radiation hardness. To control carrier lifetime, a key parameter for high performance radiation detectors, it is important to understand the nature of the deep traps in this material. For this purpose, we have successfully applied thermally stimulated current (TSC) and high temperature resistivity measurements to investigate deep level centers in semi-insulating 4H SiC samples grown by physical vapor transport. High temperature resistivity measurements showed that the resistivity at elevated temperatures is controlled by the deep level with an activation energy of 1.56 eV. The dominant traps revealed by TSC measurements were at 1.1–1.2 eV. The deep trap levels in 4H-SiC samples, the impurity and point defect nature of TSC traps peaked at ∼106 K (0.23 eV), ∼126 K (0.32 eV), ∼370 K (0.95 eV), ∼456 K (1.1–1.2 eV) are discussed.