Transport Properties and Cationic Substitutions in Sr<Subscript>2</Subscript>IrO<Subscript>4</Subscript>

We realized cationic substitutions in Sr₂IrO₄ and measured resistivity, thermoelectric power, and the Hall coefficient. A two-carrier model, reflecting the presence of thermally activated carriers at high temperature, qualitatively explains the behavior of the Hall coefficient of Sr_1.95La_0.05IrO₄ in comparison with Sr₂IrO₄. Concerning the substitution of Ir by different transition metals, Pt with 5d orbitals does not affect the transport properties, contrary to Ti and Rh with 3d and 4d orbitals, respectively. This may be explained by strong spin–orbit coupling involved in Ir and Pt, in comparison with 3d or 4d transition metals.     

Thermoelectric Properties of B-Site Substituted LaRhO<Subscript>3</Subscript>

We have prepared Ni- and Mg-substituted LaRhO₃ and have measured the thermoelectric properties. Ni substitution decreases resistivity at room temperature, while Mg substitution decreases resistivity up to 5%, where the Mg substitution increases the carrier concentration, as confirmed by Hall coefficient measurements.     

Crystal Growth,Structure, and Stoichiometry of the Superconducting Pyrochlore Cd<Subscript>2</Subscript>Re<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report single-crystal growth of the superconducting pyrochlore Cd₂Re₂O₇ using a vapor transport technique. Several parameters of the growth conditions, including hot-zone temperature and starting stoichiometry, were varied in order to control the formation of ReO₂ inclusions, as confirmed by the electron microscopy, resistivity, and magnetic susceptibility measurements. The Rietveld refinement of x-ray (neutron) powder diffraction was found to be consistent with a cubic structure Fd3m with lattice constant a = 10.2250 (10.2358) Å and reduced coordinate of O1 = 0.3184 (0.3177) at 293 K (250 K). We also studied the oxygen stoichiometry by means of redox reactions, electron microprobe analysis (EMPA), and x-ray/neutron diffractions. Particularly, the neutron powder diffraction on the ¹¹⁴Cd-enriched specimens yielded an oxygen deficiency δ = 0.14 ± 0.03 solely at the O2 site, which was consistent with the EMPA results. The EMPA indicated that the oxygen deficiency is homogeneous in the bulk and in a range of 0.01 ± 0.18–0.23 ± 0.19.     

Properties of the Mo Back Contact for the Formation of a Thin-Film Photovoltaic Absorber

A Taguchi experimental design was used to find which deposition parameter has the most dominant effect on the electrical resistivity of molybdenum (Mo) films. Based on the most important parameter, the Mo films were further characterized by structural, electrical, and adhesive methods. Then, a copper indium gallium selenide (CIGS) thin film was fabricated by a two-stage process on the obtained Mo layer. The results show that working pressure had a dominant effect on electrical resistivity. The Mo films deposited at 1 mTorr and 2 mTorr exhibited compressive strain and dense polycrystalline microstructure, whereas those deposited at 3 mTorr and 4 mTorr exhibited tensile strain and an elongated grain with open boundaries. A Mo film with open porous structure, tensile strain, and lower resistivity was suitable for the formation of CIGS films. After selenization at 560°C, a single-phase chalcopyrite CIGS film with a layer of MoSe₂ at the Mo/CIGS interface was obtained.     

Anomalous thermoelectric power in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals

Based on data on the Hall coefficient, it is shown that the existence of potential barriers in the region of impurity conductivity of highly compensated Hg₃In₂Te₆ crystals is possible. The role of barriers in the anomalous behavior of transport phenomena is discussed qualitatively. Extremely large values of the thermoelectric power are related to the combination of thermoelectric powers of contact potentials for regions with different concentrations of electrons.     

Quasi-local states of Sn in Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> according to the studies of galvanomagnetic effects in classical and quantizing magnetic fields

The magnetic-field dependences of the Hall coefficient and transverse magnetoresistance are studied experimentally and theoretically in p-Bi₂Te₃ crystals doped heavily with Sn and grown by the Czochralski method in the case of both classical and quantizing magnetic fields as high as 12 T with the magnetic-field orientation along the C ₃ trigonal axis. The Shubnikov-de Haas effect and quantum oscillations of the Hall coefficient were measured at temperatures of 4.2 and 11 K. The six-ellipsoid Drabble-Wolfe model of the energy spectrum and the magnetic-field dependence of the Hall coefficient are used as the basis for the method for determining the Hall factor and Hall mobility. New evidence is obtained in support of the existence of the narrow band of impurity Sn states occupied partially with electrons against the background of the light-hole band spectrum. The parameters of impurity states are estimated including their energy (E _Sn ≈ 15 meV), the broadening (Γ « kT), and the radius of localization of the impurity state (R ≈ 30 Å).     

Effect of microwave treatment on current flow mechanisms in Au-TiB<Subscript>x</Subscript>-Al-Ti-n<Superscript>+</Superscript>-n-n<Superscript>+</Superscript>-GaN-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ohmic contacts

The temperature dependences of the contact resistivity ρ_c of Au-TiB_x Al-Ti-n⁺-n-n⁺-GaN-Al₂O₃ ohmic contacts have been studied before and after microwave treatment followed by nine-nonth room-temperature sample storage. The temperature dependences of ρ_c of initial samples were measured twice. The first measurement showed the temperature dependence typical of ohmic contacts; the repeated measurement in the temperature region above 270 K showed a ρ_c increase caused by metallic conductivity. After microwave treatment, the metallic conductivity in the ohmic contact is not observed. This is presumably associated with local heating of metal Ga inclusions under microwave irradiation and the formation, due to high chemical activity of liquid gallium, of compounds of it with other metallization components. In this case, the temperature dependence of ρ_c is controlled by ordinary charge transport mechanisms. After nine-nonth room-temperature storage, the temperature dependence of ?c is described by the tunneling mechanism of charge transport.     

Ball Mill Synthesis of Bulk Quaternary Cu<Subscript>2</Subscript>ZnSnSe<Subscript>4</Subscript> and Thermoelectric Studies

In this work, quaternary chalcogenide Cu₂ZnSnSe₄ (CZTSe) was synthesized using a mechanochemical ball milling process and its thermoelectric properties were studied by electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. The synthesis process comprises three steps viz., wet ball milling of the elemental precursors, vacuum annealing, and densification by hot pressing. The purpose of this is to evaluate the feasibility of introducing wet milling in place of vacuum melting in solid state synthesis for the reaction of starting elements. We report the structural characterization and thermoelectric studies conducted on samples that were milled at 300 rpm and 500 rpm. X-ray diffraction (XRD) analysis showed the existence of multiple phases in the as-milled samples, indicating the requirement for heat treatment. Therefore, the ball milled powders were cold pressed and vacuum annealed to eliminate the secondary phases. Annealed samples were hot pressed and made into dense pellets for further investigations. In addition to XRD, energy dispersive spectroscopy (EDS) studies were performed on hot pressed samples to study the composition. XRD and EDS studies confirm CZTSe phase formation along with ZnSe secondary phase. Electrical resistivity and Seebeck coefficient measurements were done on the hot pressed samples in the temperature range 340–670 K to understand the thermoelectric behaviour. Thermal conductivity was calculated from the specific heat capacity and thermal diffusivity values. The thermoelectric figure of merit zT values for samples milled at 300 rpm and 500 rpm are ～0.15 and ～0.16, respectively, at 630 K, which is in good agreement with the values reported for solid state synthesized compounds.     

Electrical characteristics of thin Ni2Si,NiSi, and NiSi2 layers grown on silicon

We have determined the resistivity, carrier concentration, and Hall mobility as a function of thickness (700–3000 Å) of Ni₂Si, NiSi, and NiSi₂ layers formed by vacuum annealing at 270÷v300°C, ≈ 400°C, and ≈ 800°C, respectively, of nickel films vacuum-deposited on a silicon substrate (111 n-type and 100 p-type Si ρ ≈ 1KΩ). The layer thicknesses were measured by 2 MeV⁴He⁺ backscattering spectrometry. The silicide phase was confirmed by x-ray measurements. The electrical measurements were carried out using van der Pauw configuration. We found the electrical transport parameters to be independent of the film thickness within the experimental uncertainty. The Hall factors were assumed to be unity. The majority carriers are electrons in NiSi and holes in Ni₂Si and NiSi₂. The resistivity values are 24±2, 14±1, and 34±2 μΩcm, the electron concentrations are 9±3, 10 and 7±1, and ≈ 2 × 10²² cm^?3, and the Hall mobilities are 3±1, ≈ 4.5 and 6, and ≈ 9 cm²/Vs for Ni₂Si, NiSi (〈100〉 and 〈111〉), and NiSi₂, respectively. The systematic error in the measured values caused by currents in the high resistivity substrate is estimated to be less than 6% for the Hall coefficient. The results show that Ni₂Si, NiSi, and NiSi₂ layers formed by a thin film reaction are electrically metallic conductors, a result which concurs with those reported previously (1) for refractory metal silicides. The Hall mobility increases with the Si content in the silicide. The electron concentration is lowest for NiSi₂ leading to the highest resistivity for the epitaxial phase of NiSi₂.     

Manifestation of light and heavy electrons in the galvanomagnetic characteristics of Te-doped <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>0.88</Subscript>Sb<Subscript>0.12</Subscript> single crystals

The components of resistivity (ρ _ij ), Hall coefficient (R _ijk ), and magnetoresistance (ρ _{ij, kl} ) of n-Bi_0.88Sb_0.12 single crystals doped with tellurium to 0.01, 0.1, and 0.2 at % have been measured in the temperature range of 77–300 K. It is concluded that light and heavy electrons are involved in transport processes. The energy spacing between the bands of light and heavy electrons is found to be 40 meV, and the ratios of the effective masses and electron mobilities are estimated as m ₂^*/m _l ^* = 3 and b ≈ 0.16, respectively.     

Low Fatigue in Epitaxial Pb(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> on Si Substrates with LaNiO<Subscript>3</Subscript> Electrodes by RF Sputtering

Epitaxial PZT (001) thin films with a LaNiO₃ bottom electrode were deposited by radio-frequency (RF) sputtering onto Si(001) single-crystal substrates with SrTiO₃/TiN buffer layers. Pb(Zr_0.2Ti_0.8)O₃ (PZT) samples were shown to consist of a single perovskite phase and to have an (001) orientation. The orientation relationship was determined to be PZT(001)[110]∥LaNiO₃(001)[110]∥SrTiO₃ (001)[110]∥TiN(001)[110]∥Si(001)[110]. Atomic force microscope (AFM) measurements showed the PZT films to have smooth surfaces with a roughness of 1.15 nm. The microstructure of the multilayer was studied using transmission electron microscopy (TEM). Electrical measurements were conducted using both Pt and LaNiO₃ as top electrodes. The measured remanent polarization P _r and coercive field E _c of the PZT thin film with Pt top electrodes were 23 μC/cm² and 75 kV/cm, and were 25 μC/cm² and 60 kV/cm for the PZT film with LaNiO₃ top electrodes. No obvious fatigue after 10¹⁰ switching cycles indicated good electrical endurance of the PZT films using LaNiO₃ electrodes, compared with the PZT film with Pt top electrodes showing a significant polarization loss after 10⁸ cycles. These PZT films with LaNiO₃ electrodes could be potential recording media for probe-based high-density data storage.     

Effects of oxygen content on the structural,optical, and electrical properties of NiO films fabricated by radio-frequency magnetron sputtering

Nickel oxide (NiO) films were deposited on Corning glass substrate with variable (0–100%) oxygen content by radio-frequency sputtering. Effects of different oxygen content on the structural, optical, and electrical properties of NiO films were studied. X-ray diffraction showed that the NiO film deposited on substrate with 0% oxygen content resulted in a random polycrystalline structure and small grain size. The introduction of oxygen gas leaded to a (200) preferential orientation and larger grain size. The transmittance decreases with oxygen content due to the increase of oxygen interstitials in NiO films. The 0%-O₂ deposited NiO film has a tensile strain and a small band gap. Upon introducing 33%-O₂ content, the NiO film exhibits a compressive strain, increasing the bandgap. However, the compressive strain is released and gradually turns into tensile strain, which leads to the narrowing of bandgap with the increase of oxygen content. Hall measurement shows the obtained NiO is p-type and the resistivity decreases from 4.3 × 10⁵ Ω-cm to 5.02 Ω-cm with increasing oxygen content from 0% to 100%. The carrier concentration increases from 6.3 × 10¹⁴ cm⁻³ to 4.6 × 10¹⁸ cm⁻³ and the mobility decreases from 26 cm²/V-s to 0.26 cm²/V-s for the NiO films deposited with oxygen content increasing from 50% to 100%. X-ray photoelectron spectroscopy showed that the Ni⁺³/Ni⁺² ratio is the origin of p-type NiO and the ratio increases from 1.32 to 2.63 by increasing the oxygen content from 0% to 100%, which caused more defects, oxygen interstitials and nickel vacancies.     

Preparation of Bi0.5Sb1.5Te3 Thermoelectric Materials by Pulse-Current Sintering Under Cyclic Uniaxial Pressure

The effects of applying cyclic uniaxial pressure during the pulse-current sintering process on the crystal alignment and thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ were investigated. Sintering was performed at 673 K using pulse-current heating under 70 MPa or 100 MPa of cyclic uniaxial pressure. x-Ray diffraction patterns and electron backscattered diffraction analyses showed that application of the cyclic uniaxial pressure enhanced crystal grain orientation. The texture consisted of flattened crystal grains stacked in the thickness direction of the sintered materials. The hexagonal c-plane strongly tended to align in the direction perpendicular to the uniaxial pressure. Owing to the crystal alignment, the Hall mobility in the direction perpendicular to the uniaxial pressure became larger than that of equivalent samples prepared with a constant uniaxial pressure. As a result of the increase in Hall mobility, the resistivity of the material was decreased while the equivalent Seebeck coefficient was maintained and the power factor was improved.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Theoretical Study of Electronic Structure and Thermoelectric Properties of Doped CuAlO<Subscript>2</Subscript>

The doping level dependence of thermoelectric properties of delafossite CuAlO₂ has been investigated in the constant scattering time (τ) approximation, starting from the first principles of electronic structure. In particular, the lattice parameters and the energy band structure were calculated using the total energy plane-wave pseudopotential method. It was found that the lattice parameters of CuAlO₂ are a = 2.802 Å and c = 16.704 Å, and the internal parameter is u = 0.1097. CuAlO₂ has an indirect band gap of 2.17 eV and a direct gap of 3.31 eV. The calculated energy band structures were then used to calculate the electrical transport coefficients of CuAlO₂. By considering the effects of doping level and temperature, it was found that the Seebeck coefficient S(T) increases with increasing acceptor doping (A _d) level. The values of S(T) in our experiments correspond to an A _d level at 0.262 eV, which is identified as the Fermi level of CuAlO₂. Based on our experimental Seebeck coefficient and the electrical conductivity, the constant relaxation time is estimated to be 1 × 10^?16 s. The power factor is large for a low A _d level and increases with temperature. It is suggested that delafossite CuAlO₂ can be considered as a promising thermoelectric oxide material at high doping and high temperature.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Enhancement of Spin–Orbit Torque by Strain Engineering in SrRuO3 Films

Complex oxides with 4d/5d transition metal ions, e.g., SrRuO₃, usually possess strong spin–orbit coupling, which potentially leads to efficient charge-spin interconversion. As the electrical transport property of SrRuO₃ can be readily tuned via structure control, it serves as a platform for studying the manipulation of charge-spin interconversion. Here, a factor of twenty enhancement of spin–orbit torque (SOT) efficiency via strain engineering in a SrRuO₃/Ni₈₁Fe₁₉ bilayer is reported. The results show that an orthorhombic SrRuO₃ leads to a higher SOT efficiency than the tetragonal one. By changing the strain from compressive to tensile in the orthorhombic SrRuO₃, the SOT efficiency can be increased from an average value of 0.04 to 0.89, corresponding to a change of spin Hall conductivity from 27 to 441 × ħ/e (S cm⁻¹). The first-principles calculations show that the intrinsic Berry curvature can give rise to a large spin Hall conductivity (SHC) via the strain control, which is consistent with the experimental observations. The results provide a route to further enhance the SOT efficiency in complex oxide-based heterostructures, which will potentially promote the application of complex oxides in energy-efficient spintronic devices.     

Enhancement in Thermoelectric Properties of TiS<Subscript>2</Subscript> by Sn Addition

A series of Sn added TiS₂ (TiS₂:Sn _x ; x = 0, 0.05, 0.075 and 0.1) were prepared by solid state synthesis with subsequent annealing. The Sn atoms interacted with sulfur atoms in TiS₂ and formed a trace amount of misfit layer (SnS)_1+m(TiS_2?δ)_n compound with sulfur deficiency. A significant reduction in electrical resistivity with moderate decrease in the Seebeck coefficient was observed in Sn added TiS₂. Hence, a maximum power factor of 1.71 mW/m-K² at 373 K was obtained in TiS₂:Sn_0.05. In addition, the thermal conductivity was decreased with Sn addition and reached a minimum value of 2.11 W/m-K at 623 K in TiS₂:Sn_0.075, due to the impurity phase (misfit phase) and defects (excess Ti) scattering. The zT values increased from 0.08 in pristine TiS₂ to an optimized value of 0.46 K at 623 K in TiS₂:Sn_0.05.     

Effect of ZnO Nanoparticles on the Sintering Behavior and Physical Properties of Bi<Subscript>0.5</Subscript>(Na<Subscript>0.8</Subscript>K<Subscript>0.2</Subscript>)<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> Lead-Free Ceramics

Sintered Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ + x wt.% ZnO nanoparticle (BNKT–xZnO_n) ceramics have been fabricated by conventional annealing with the aid of ultrasound waves for preliminary milling. Because of the presence of the liquid Bi₂O₃–ZnO phase at the eutectic point of 738°C, the sintering temperature decreased from 1150°C to 1000°C, and the morphology phase boundary of BNKT–xZnO_n ceramics can be clarified by two separated peaks at (002)_T and (200)_T of 2θ in the x-ray diffraction (XRD) patterns. The improvement of ferroelectric properties has been obtained for BNZT–0.2 wt.% ZnO_n ceramics by the increase of remanent polarization up to 20.4 μC/cm² and a decrease of electric coercive field down to 14.2 kV/cm. The piezoelectric parameters of the ceramic included a piezoelectric charge constant of d ₃₁ = 78 pC/N; electromechanical coupling factors k _p = 0.31 and k _t = 0.34, larger than the values of 42 pC/N, 0.12 and 0.13, respectively, were obtained for the BNKT ceramics.