A Hall coefficient investigation of ferromagnetic Ga1−xMnxAs layers on (100) GaAs substrates

Ferromagnetic Ga_1−xMn_xAs epilayers with Mn mole fraction in the range of x≈2.2-4.4% were grown on semi-insulating (100) GaAs substrates using the molecular beam epitaxy technique. The transport properties of these epilayers were investigated through Hall effect measurements. The measured hole concentration of Ga_1−xMn_xAs layers varied from 4.4×10¹⁹ to 3.4×10¹⁹ cm⁻³ in the range of x≈2.2-4.4% at room temperature. From temperature dependent resisitivity data, the sample with x≈4.4% shows typical behavior for insulator Ga_1−xMn_xAs and the samples with x≈2.2 and 3.7% show typical behavior for metallic Ga_1−xMn_xAs. The Hall coefficient for the samples with x≈2.2 and 4.4% was fitted assuming a magnetic susceptibility given by Curie-Weiss law in a paramagnetic region. This model provides good fits to the measured data up to and the Curie temperature T_c was estimated to be 65, 83 K and hole concentration p was estimated to be 5.1×10¹⁹, 4.6×10¹⁹ cm⁻³ for the samples with x≈2.2 and 4.4%, respectively, confirming the existence of an anomalous Hall effect for metallic and insulating samples.     

Thermoelectric performance of ternary Bi-Sb-Ag alloys prepared by mechanical alloying and subsequent pressureless sintering

The alloys with the general formula of Bi₈₅Sb_15−xAg_x (x=0, 1, 3, 5, 7) were prepared by mechanical alloying and subsequent pressureless sintering (Bi₈₅Sb₁₅ alloy was used for comparison). Their transport properties involving electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The maximum absolute value of Seebeck coefficient (120 μV/K) was found at 160 K in the alloy Bi₈₅Sb_15−xAg_x (x=3). The figure-of-merit of alloy Bi₈₅Sb_15−xAg_x (x=1) reached a maximum value of 2.16×10⁻³ K⁻¹ at 219 K, which is as large again as that of the reference sample Bi₈₅Sb₁₅.     

Preparation, structure and oxide ion conductivity in Ce6−xYxMoO15−δ (x=0.1-1.4) solid solutions

The Ce_6−xY_xMoO_15−δ solid solution with fluorite-related structure have been characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), IR, Raman, scanning electric microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods. The electric conductivity of samples is investigated by Ac impedance spectroscopy. An essentially pure oxide-ion conductivity of the oxygen-deficiency was observed in pure argon, oxygen and air. The highest oxygen-ion conductivity was found in Ce_5.5Y_0.5MoO_15−δ ranging from 5.9×10⁻⁵ (S cm⁻¹) at 300 °C to 1.3×10⁻² (S cm⁻¹) at 650 °C, respectively. The oxide-ion conductivities remained stable over 80 h-long test at 800 °C. These properties suggested that significant oxide-ionic conductivity exists in these materials at moderately elevated temperatures.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Effect of iron doping on the characterization and transport properties of calcium phosphate glassy semiconductors

X-ray diffraction (XRD), differential scanning calorimeter (DSC), density (d) and dc conductivity (σ) of the glasses in Fe₂O₃-CaO-P₂O₅ system were reported. The dc conductivity in the temperature range 303-453 K was measured. The overall features of these XRD curves confirm the amorphous nature of the present samples. The density of glasses increases from 2.750 to 2.892 g/cm³ with increasing Fe₂O₃ content as a result of a strengthening of cross-linking within glass network. The glass temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The glasses had conductivities ranging from 10⁻⁹ to 10⁻⁵ Sm⁻¹ at temperatures from 303 to 453 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Structure, microstructure and ionic conductivity of the solid solution LiTi2−xSnx(PO4)3

The Nasicon compounds with the composition LiTi_2−xSn_x(PO₄)₃ (x=0-1.8) were synthesised by the solid state reaction. Their structures were determined from X-ray powder diffraction using Rietveld analysis. All the compositions present the space group R-3c. The refinements show that the Ti and Sn cations are statistically distributed over the same position while the Li ones are exclusively located on the M1 site. The lattice constants a and c exhibit linear variation over the whole composition range. The bond lengths are in accordance with those of other Nasicon structures. The SEM micrographs of the samples show relative porous microstructures. The ionic conductivity is about 10⁻⁴-10⁻⁵ S cm⁻¹; for the activation energy, a typical value of 0.32 eV is obtained for x=0.6 composition whereas significant deviation from linearity in the temperature dependence of the dc conductivity, is observed for the Sn-rich ones. This tendency is discussed along with the structural features.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Dielectric properties and conductivity in CuO and MoO3 doped borophosphate glasses

A novel set of glasses of the type (B₂O₃)_0.10-(P₂O₅)_0.40-(CuO)_0.50−x-(MoO₃)_x, 0.05≤x≥0.50, have been investigated for dielectric properties in the frequency range 100 Hz-100 kHz and temperature range 300-575 K. From the total conductivity derived from the dielectric spectrum the frequency exponent, s, and dc and ac components of the conductivity were determined. The temperature dependence of dc and ac conductivities at different frequencies was analyzed using Mott's small polaron hopping model, and the high temperature activation energies have been estimated and discussed. The observed initial decrease in conductivity (ac and dc) and increase in activation energy with the addition of MoO₃ have been understood to be due to the hindrance offered by the Mo⁺ ions to the electronic motions. The observed peak-like behavior in conductivity (dip-like behavior in activation energy) in the composition range 0.20-0.50 mol fractions of MoO₃ may be due to mixed transition effect occurring in the present glasses. The temperature dependence of frequency exponent, s, has been analyzed using different theoretical models. It is for the first time that the mixed transition metal ion (TMI) doped borophosphate glasses have been investigated for dielectric properties and conductivity over wide temperature and frequency ranges and the data have been subjected to a thorough analysis.     

Photoluminescence investigation of ferromagnetic Ga1−xMnxN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x ≈ 0.1-0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga_1−xMn_xN layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga_1−xMn_xN layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10⁻⁴, 2.7 × 10⁻⁴, 3.4 × 10⁻⁴ eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga_1−xMn_xN layers increased, the temperature dependence of the band-gap energy was clearly reduced.     

Femtosecond laser-induced damage of gold films

Single- and multi-shot ablation thresholds of gold films in the thickness range of 31-1400 nm were determined employing a Ti:sapphire laser delivering pulses of 28 fs duration, 793 nm center wavelength at 1 kHz repetition rate. The gold layers were deposited on BK7 glass by an electron beam evaporation process and characterized by atomic force microscopy and ellipsometry. A linear dependence of the ablation threshold fluence F_th on the layer thickness d was found for d ≤ 180 nm. If a film thickness of about 180 nm was reached, the damage threshold remained constant at its bulk value. For different numbers of pulses per spot (N-on-1), bulk damage thresholds of ∼0.7 J cm⁻² (1-on-1), 0.5 J cm⁻² (10-on-1), 0.4 J cm⁻² (100-on-1), 0.25 J cm⁻² (1000-on-1), and 0.2 J cm⁻² (10000-on-1) were obtained experimentally indicating an incubation behavior. A characteristic layer thickness of L_c ≈ 180 nm can be defined which is a measure for the heat penetration depth within the electron gas before electron-phonon relaxation occurs. L_c is by more than an order of magnitude larger than the optical absorption length of α⁻¹ ≈ 12 nm at 793 nm wavelength.     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

Thermoelectric properties of the Bi2−xYxRu2O7 (x=0-2) pyrochlores

Electrical conductivity and Seebeck coefficient for the Bi_2−xY_xRu₂O₇ pyrochlores with x=0.0,0.5,1.0,1.5,2.0 were measured in the temperature range of 473-1073 K in air. With increasing Bi content, the temperature dependence of the electrical conductivity changed from semiconducting to metallic. The signs of the Seebeck coefficient were positive in the measured temperature range for all the samples, indicating that the major carriers were holes. The temperature dependence of the Seebeck coefficient for the Y₂Ru₂O₇ indicated the thermal activation-type behavior of the holes, while that for the Bi_2−xY_xRu₂O₇ with x=0.0-1.5 indicated the itinerant behavior of the holes. The change in the conduction behavior from semiconductor to metal with increasing Bi content is consistent with the increase in the overlap between the Ru4d t_2g and O2p orbitals, but the mixing of Bi6s, 6p states at E_F may not be ruled out. The thermoelectric power factors for the Bi_2−xY_xRu₂O₇ with x=1.5 and 2.0 were lower than 10⁻⁵ W m⁻¹ K⁻² and those with x=0.0,0.5,1.0 were around 1-3×10⁻⁵ W m⁻¹ K⁻².     

DC and AC conductivities of (As2S3)100−x(AsSe0.5Te0.5I)x chalcogenide glasses

The DC and AC conductivities of samples from the system (As₂S₃)_100−x(AsSe_0.5Te_0.5I)_x, where x=0, 5, 10, 15, 20, 25, 30, 35, 50, 70 and 90 mol%, were measured as a function of temperature. Besides, the AC conductivities of the samples with x=10 and 30 were measured as a function of frequency from room temperature to the glass transition temperature. The DC conductivity dependence on temperature is of the Arrhenius type, whereas the value of the pre-exponential factor suggests the electrical conduction by localized states in the band tails and by localized states near the Fermi level. The small values of the conduction activation energy (10⁻²-10⁻¹ eV) obtained at higher frequencies suggest that the conduction in these materials is due to hopping of charge carriers between close defect states near the Fermi level.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La_1−xCa_xW_1/6O₂ and La_1−yW_1/6O₂ (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H⁺/D⁺ isotope exchange, and thermogravimetry in the temperature range 300-1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW_1/6O₂ with values reaching 3×10⁻³ S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.     

γ irradiation effect on the polarization and resistance of Li-Co-Yb-ferrite

The effect of γ irradiation on some physical properties of rare earth ferrite of the general formula Li_0.5+zCo_z Yb_xFe_2.5−2z−xO₄, (z=0.1, x=0.00, 0.025, 0.050, … , 0.200) is discussed. The temperature dependence of the polarization and resistance is studied in the range (300 K≤T≤700 K) at different frequencies (10 kHz≤f≤1 MHz). The relaxation time and the activation energy have been calculated before and after irradiation with γ rays doses of 1 and 3 Mrad. A comparison was made between the ac resistance before and after irradiation for the samples with (0.0≤x≤0.2). The results after irradiation with 1 Mrad γ rays showed that the resistance at the critical concentration decreases from 800 to 25 kΩ at room temperature. Furthermore, with increasing temperature the resistance ranged from R≈130 kΩ at T≈310 K to R≈0.13 kΩ at T≈640 K. Thus, it is possible to improve the conductivity of this type of rare earth ferrite materials to be used in technological applications at room as well as at high temperature.     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

Single crystal EPR and optical absorption study of Cr doped l-histidine hydrochloride monohydrate

EPR study of the Cr³⁺ ion doped l-histidine hydrochloride monohydrate single crystal is done at room temperature. Two magnetically inequivalent interstitial sites are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The zero field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations and the parameters are: D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.9108±0.0002, g_y=1.9791±0.0002, g_z=2.0389±0.0002, A_x=(252±2)×10⁻⁴ cm⁻¹, A_y=(254±2)×10⁻⁴ cm⁻¹, A_z=(304±2)×10⁻⁴ cm⁻¹ for site I and D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.8543±0.0002, g_y=1.9897±0.0002, g_z=2.0793±0.0002, A_x=(251±2)×10⁻⁴ cm⁻¹, A_y=(257±2)×10⁻⁴ cm⁻¹, A_z=(309±2)×10⁻⁴ cm⁻¹ for site II, respectively. The optical absorption studies of single crystals are also carried out at room temperature in the wavelength range 195-925 nm. Using EPR and optical data, different bonding parameters are calculated and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are: B=636, C=3123, Dq=2039 cm⁻¹, h=1.46 and k=0.21, respectively.