Comment on `Electronic Properties of Red P-Type Tl2S5 Single Crystals'

Recently, Gamal et al. [Chin. Phys. Lett. 22 (2005) 1530] reported the results of electrical conductivity, Hall effect and thermoelectric measurements on p-type Tl₂S₅ single crystals. From the experimental data for the temperature dependence of differential thermoelectric power, Gamal et al. determined thevalues of 2.66×10^-41kg and 2.50×10^-41kg, respectively, for the effective masses of electrons and holes in p-type Tl₂S₅, which are about ten orders of magnitude smaller than the free electron mass (9.11×10^-31kg). We argue that the anomalously small values obtained for the effective mass of chargecarriers in Tl₂S₅ have no physical significance.     

Temperature-Dependent Electron Transport in In0.5Ga0.5 P/GaAs Grown by MOVPE

Hall effect measurements in undoped In0.5Ga0.5 P/GaAs allo grown by metal organic vapour-phase epitaxy （MOVPE） have been carried out in the temperat.ure range 15-350K. The experimenta.1 results are analysed using a two-band model including conduction band transport calculated using an iterative solution of the Boltz- mann equation. A good agreement was obtained between theory and experiment. The impurity contents of In0.5Ga0.5 P/GaAs alloy, such as donor density ND, acceptor density NA and donor activation energy εD, were also determined.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Ionic conductivity of NTE materials c- Zr1−xErxW2O8−x/2

It is found that the c- Zr_1−xEr_xW₂O_8−x/2 solid solutions which are well known to have isotropic NTE properties clearly exhibit oxygen-ionic conductance and their ionic conductivities are measured to be about 10⁻⁴ S cm⁻¹ at 673 K, which is comparable to that of ceria-based solid electrolytes.     

The large anisotropy of the magnetic and transport properties in the Ba5Co5ClO13 single crystal

In this Letter, the single crystals of Ba₅Co₅ClO₁₃ were grown by the flux method successfully. Their structure, magnetic and transport properties were studied. A large anisotropy of the magnetic and transport properties has been detected in this compound. Below the TN∼108 K, the magnetic susceptibility exhibits an antiferromagnetic peak in χc and an upturn transition in χab. We suggest that this behavior is consistent with the competition of the ferromagnetic (FM) intra-blocks coupling and antiferromagnetic (AFM) inter-blocks coupling in this compound. The temperature dependence of the resistivity displays a hump in ρab and a kink in ρc around TN, suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the three-dimensional (3D) variable range hopping (VRH) mechanism.     

Power factors of late rare earth-doped Ca3Co2O6 oxides

Polycrystalline samples of (Ca_1−xR_x)₃Co₂O₆ with R = Gd, Tb, Dy and Ho at x=0-0.1 were synthesized and the effects of rare earth substitution on their thermoelectric properties were investigated. In the high-temperature region, the rare earth substitution resulted in an increase in the Seebeck coefficients (S), and the S values increased with decreasing ionic radius of rare earth elements in the order Gd³⁺>Tb³⁺>Dy³⁺>Ho³⁺. In contrast, the influence of rare earth substitution on the electrical resistivity was small. The high-temperature power factor was thereby improved by the late rare earth substitutions, particularly those with Ho³⁺ for Ca²⁺. For the Ho-doped samples (x≤0.05), the power factor was significantly improved by increasing Ho concentration.     

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.     

Leakage current study of Si1−xCx embedded source/drain junctions

This work investigates the impact of various processing parameters on the leakage current of in situ P-doped Si_1−xC_x embedded source/drain (S/D) junctions, i.e., the carbon content x (%) and the thermal budget used either before or after the selective epitaxial deposition. It is shown that while the area leakage current density, generated by defects in the depletion region is not affected by the epitaxial process or the strain in the substrate, the perimeter leakage current density (J_P) increases with x. From the stronger reverse bias dependence of J_P, it is derived that a higher electric field exists along the junction periphery. This is confirmed by capacitance-voltage (C-V) measurements, demonstrating a higher p-well B doping density for increasing x. It is believed that this originates from the strain dependence of the B diffusivity in the p-well region. No evidence of electrically active extended defect formation was found, so it is expected that the off-state current of embedded Si_1−xC_x S/D nMOSFETs will not be adversely affected by the selective epitaxial deposition.     

Anomalous PL brightening caused by partial Auger recombination of a (D, X) complex during an impact ionization avalanche in n-GaAs

Partial Auger recombination of the (D⁰, X) bound-exciton in n-GaAs at 4.2 K during an impact ionization avalanche under an applied pulse voltage has been investigated. The bright photoluminescence (PL)-pattern observed by applying the pulse voltage, characterizes well the formation of a current density filament. The observation of the bright PL spectra inside the current density filament gives a quite new result concerning the partial Auger recombination process of the (D⁰, X) complex, leaving the neutral donor in the excited states.     

Energy band gap and oscillator parameters of Ga4Se3S single crystals

The optical properties of the Bridgman method grown Ga₄Se₃S crystals have been investigated by means of room temperature, transmittance and reflectance spectral analysis. The optical data have revealed an indirect allowed transition band gap of 2.08 eV. The room temperature refractive index, which was calculated from the reflectance and transmittance data, allowed the identification of the dispersion and oscillator energies, static dielectric constant and static refractive index as 21.08 and 3.85 eV, 6.48 and 2.55, respectively.     

A new combination of scattering mechanisms for transverse runaway (TR) of hot electrons

A new combination of energy and momentum scattering mechanisms has been found at which the transverse runaway (TR) of hot electrons takes place. Up to now only two combinations of scattering mechanisms at which TR occurred have been known. These two combinations were obtained by analytical solution of a complex integral equation at certain approximations. In the present work, using modern numerical methods, with no above-mentioned approximations, a solution of the integral equation for a new combination of scattering mechanisms has been found.In the work physical conditions responsible for dominance of corresponding scattering mechanisms are also analyzed.     

The effect of annealing process on the physical properties of La1−xNaxMnOy

The effect of Na doping and annealing time on the structure, electrical properties, magnetoresistance and thermopower properties has been investigated in perovskite La_1−xNa_xMnO_y (x=0.025, 0.075 and 0.1) systems. La_1−xNa_xMnO_y crystallizes in a single-phase rhombohedral structure. It is observed a simultaneous occurrence of the ferromagnetic to paramagnetic state and metallic to insulating state. In the meanwhile, a large negative magnetoresistance with low applied magnetic field is observed. In addition, ρ(T) curves for Na-doped samples exhibit another broad transition T_ms2 below T_ms. Such double peak behavior in the ρ(T) curve interpreted by the electronic inhomogeneity in the samples. The sign of S changes from positive to negative depending on composition. The values of Seebeck coefficient are small (in the microvolt range).     

Photoelectronic and electrical properties of Tl2InGaS4 layered crystals

Tl₂InGaS₄ layered crystals are studied through the dark electrical conductivity, space charge limited current and illumination- and temperature-dependent photoconductivity measurements in the temperature regions of 220-350 K, 300-400 K and 200-350 K, respectively. The space charge limited current measurements revealed the existence of a single discrete trapping level located at 0.44 eV. The dark electrical conductivity showed the existence of two energy levels of 0.32 eV and 0.60 eV being dominant above and below 300 K, respectively. The photoconductivity measurements reflected the existence of two other energy levels located at 0.28 eV and 0.19 eV at high and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 330 K. The illumination dependence of photoconductivity is found to exhibit supralinear recombination in all the studied temperature ranges. The change in recombination mechanism is attributed to exchange in the behavior of sensitizing and recombination centers.     

Synthesis, crystal structure and thermoelectric properties of IrSn1.5Te1.5 -based skutterudites

The mixed anion skutterudite IrSn_1.5Te_1.5 has been synthesized and characterized by X-ray powder diffraction, thermopower and electrical resistivity measurements. Structural analysis reveals that Sn and Te order in layers perpendicular to the [111] direction of the skutterudite unit cell, and a distortion of the anion sublattice is evident. The thermopower (S) is 160 μV/K at room temperature, while the electrical resistivity (ρ) is . The effects of chemical substitutions on the Ir site (Ru, Pd) and Sn site (In, Sb) have been investigated. The power factor (S²/ρ) was found to improve with In substitution but, at 0.9 μW/K² cm, is too small for this material to be useful for thermoelectric applications.     

Transport and magnetic properties of disordered LixVyO2 (x=0.8 and y=0.8)

The magnetic and electron transport properties of rhombohedral Li_xV_yO₂ (x=0.8 and y=0.8) are studied. The dc susceptibility of Li_xV_yO₂ can be well fitted to the modified Curie-Weiss law, which verified the paramagnetic ground state. The magnetic hysteresis and ac susceptibility also confirm this paramagnetism. The Li_xV_yO₂ exhibits semiconducting behavior, which is explained by thermal activated process at high temperature and variable-range hopping mechanism at low temperature. Anderson localization plays an important role in both the electron transport behavior and the magnetic behavior due to the site disorder between the Li⁺ ion and V⁴⁺ ion.     

Highly cation-deficient manganese perovskite, La1−xMn1−yO3 with x=y

Oxidative (δ>0) nonstoichiometry in the perovskite ‘LaMnO_3+δ’ has been known to be manifested not with O interstitials but rather with cation vacancies of equal amounts at the two cation sites, La and Mn, i.e. La_1−xMn_1−yO₃ with x=y. Here, we report the fabrication of samples with record-high cation-vacancy concentrations (x>0.12 or δ>0.4) by means of a variety of high-pressure oxygenation techniques. Linear (negative) dependence of the cell volume on x was observed within the whole x range investigated, down to 56.9 Å³ (per formula unit) for a sample oxygenated at 5 GPa and 1100 °C using Ag₂O₂ as an excess oxygen source. With increasing degree of cation deficiency in La_1−xMn_1−xO₃, the ferromagnetic transition temperature was found to follow a bell shape with respect to x exhibiting a maximum of ∼250 K about x≈0.1. For moderately oxygenated samples large magnetoresistance effect was evidenced.     

The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Dark electrical conductivity and photoconductivity of Ga4Se3S layered single crystals

Ga₄Se₃S layered crystals were studied through the dark electrical conductivity, and illumination- and temperature-dependent photoconductivity in the temperature region of 100-350 K. The dark electrical conductivity reflected the existence of two energy states located at 310 and 60 meV being dominant above and below 170 K, respectively. The photoconductivity measurements revealed the existence of another two energy levels located at 209 and 91 meV above and below 230 K. The photoconductivity was observed to increase with increasing temperature. The illumination dependence of photoconductivity was found to exhibit linear and supralinear recombination above and below 280 K, respectively. The change in recombination mechanism was attributed to the exchange in the behavior of sensitizing and recombination centers.     

Transport and magnetotransport properties of Mn-doped InxGa1−xAs/GaAs quantum well structures

We report on the transport, magnetotransport and magnetic properties of In_0.17Ga_0.83As quantum well in GaAs δ-doped by Mn. At low temperatures, the anomalous Hall effect was observed which detects the spin-polarized carriers. Negative magnetoresistance was found at low temperatures, which became positive at high temperature.     

Beating patterns in the oscillatory magnetoresistance of an AlGaN/GaN heterostructure

Magneto-transport measurements have been carried out on a modulation-doped Al_0.22Ga_0.78N/GaN heterostructure in a temperature range between 1.5 and 25 K with a rather high carrier density, 1.1×10¹³ cm⁻². Striking beating patterns in magnetoresistance vs magnetic field are observed in the vicinity of a special temperature. Theoretical simulation is performed and the comparison between numerical simulations and the experimental data reveals that the beating patterns are due to the interference of the magneto-intersubband scattering and the SdH oscillator of first subband.