Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

Energy parameters of two-electron tin centers in PbSe

Relations that make it possible to use an experimentally measured temperature dependence of carrier concentration to determine the Hubbard energy U and temperature dependence of the Fermi level F for two-electron tin centers in lead selenide are derived. A study of Pb_1?x?ySn_xNa_ySe solid solutions shows that their Fermi level in the temperature region 100–600 K lies below the valence band top E _v and that their F(T) dependences are linear, with extrapolation to T = 0 yielding E _V ?F = 210±10 meV. The Hubbard energy of the two-electron tin centers in PbSe is found to be U = ?80±20 meV.     

Technology and electronic properties of PHEMT AlGaAs/In y(z)Ga1 − y(z)As/GaAs compositionally graded quantum wells

Graded In _y Ga_{1 ? y} As quantum well epitaxial technology is developed for engineering the band potential profile. The crystal structure of the samples is clarified by high-resolution X-ray diffraction. The influence of quantum-well bending on the crystal and electron transport properties is studied on one- and two-side δ-doped Al_0.23Ga_0.77As/In _y Ga_{1 ? y} As/Al_0.23Ga_0.77As PHEMT heterostructures. The highest InAs content gradient reached is 1.2%/nm for the mean InAs content y = 0. 2. Optimization of the InAs content grading leads to an increase in the electron mobility and concentration. This effect is related to the straightening and deepening of the quantum-well potential profile. In addition, the electron wavefunction shifts toward the quantum-well center, thus reducing electron scattering.     

Relation between Seebeck Coefficient and Lattice Parameters of (Ca2?y Sr y CoO3) x CoO2

The effect of dopant content on the Seebeck coefficient of the Sr²⁺-doped (Ca_2?y Sr _y CoO₃) _x CoO₂ (y?=?0 to 0.4) system was investigated. This system can be described as a misfit layered structure of (CoO₂) and (Ca_2?y Sr _y CoO₃) layers, which have different b lattice parameters denoted by b ₁ and b ₂, respectively. Due to the solubility limit of Sr in (Ca₂CoO₃) _x CoO₂, systematic investigations were available only up to y?=?0.2. Nevertheless, the structural uniqueness enabled partially quantitative analysis of the correlation between the crystal structure and Seebeck coefficient of the system. Substitution of Sr for Ca leads to lattice expansion which accompanies an anisotropic change of the lattice parameters. Among the lattice parameters considered, the increase in the lattice parameter b ₂ of the insulating layer was larger than the change of any other lattice parameter, which induced in-plane stress in the conducting layer. As a result, as the b ₁/b ₂ misfit ratio of (Ca_2?y Sr _y CoO₃) _x CoO₂ is decreased, the Seebeck coefficient also decreases. Practical guidance for selecting dopants to enhance thermoelectric performance is proposed.     

Heteroepitaxial silicon-carbide nitride films with different carbon sources on silicon substrates prepared by rapid-thermal chemical-vapor deposition

Cubic crystalline silicon-carbon nitride (Si_1−x−yC_xN_y) films have been grown successfully using various carbon sources by rapid-thermal chemical-vapor deposition (RTCVD). The characteristics of the Si_1−x−yC_xN_y films grown with SiH₃CH₃, C₂H₄, and C₃H₈ are examined and compared by x-ray photoelectron spectroscopy (XPS) spectra, scanning electron microscopy (SEM) images, and transmission electron microscopy (TEM) patterns. The XPS spectra show that the differences of chemical composition and chemical-bonding state are co-related to the C bonding type of the different carbon source. The SEM images and TEM analysis indicate that the better Si_1−x−yC_xN_y film can be obtained using C₃H₈ gas as the carbon source. In addition, correlations between the growing stages to the microstructure of the cubic-crystalline Si_1−x−yC_xN_y films have been illustrated in detail.     

Two-electron centers with negative correlation energy in Pb1−x SnxSe solid solutions

In Pb_1?x?y Sn_xNa_ySe solid solutions, the chemical potential F lies lower than the valence-band top E _v , and the dependences F(T) are linear, with the extrapolation to T = 0 K yielding a value E _V ? F ₀ = (210 ± 10) meV for solid solutions containing 0.3 at % of sodium and (250 ± 10) meV for those with 0.5 at % of sodium (F ₀ is the position of the chemical potential at T = 0 K). The dependences of F ₀ on the content of sodium are also linear, with the extrapolation to zero content yielding a value F ₀ = 162 ± 10 meV for solid solutions containing 2 at % of tin. The correlation energy U of two-electron tin centers in PbSe is independent of the content of tin and sodium and equal to U = (?65 ± 10) meV.     

Structural and Electrical Properties of Ta ax La(1−a)x O y Thin Films

Effect of annealing temperature on the characteristics of sol–gel-driven Ta _ax La_(1?a)x O _y thin film spin-coated on Si substrate as a high-k gate dielectric was studied. Ta _ax La_(1?a)x O _y thin films with different amounts of a were prepared (as-prepared samples). X-ray diffraction measurements of the as-prepared samples indicated that Ta_0.3x La_0.7x O_y film had an amorphous structure. Therefore, Ta_0.3x La_0.7x O _y film was chosen to continue the present studies. The morphology of Ta_0.3x La_0.7x O _y films was studied using scanning electron microscopy and atomic force microscopy techniques. The obtained results showed that the size of grain boundaries on Ta_0.3x La_0.7x O _y film surfaces was increased with increasing annealing temperature. Electrical and optical characterizations of the as-prepared and annealed films were investigated as a function of annealing temperature using capacitance–voltage (C–V) and current density–voltage (J–V) measurements and the Tauc method. The obtained results demonstrated that Ta_0.3x La_0.7x O _y films had high dielectric constant (≈27), wide band gap (≈4.5 eV), and low leakage current density (≈10^?6 A/cm² at 1 V).     

Determination of the composition of multicomponent chalcogenide semiconductors by X-ray fluorescence analysis

The composition of As _x (Ge _y Se_{1 ? y} )_{1 ? x} glassy alloys is quantitatively determined by measuring the X-ray fluorescence spectrum of a Ge_0.2As_0.4Se_0.4 reference alloy. The atomic fractions of arsenic, germanium, and selenium are calculated from the X-ray fluorescence spectra and the x _RFA = f(x) and y _RFA = f(y) dependences are plotted. These dependences make it possible to determine the composition of the glasses with an accuracy of ±0.0005 for x and y. This procedure is effective for finding the concentration of the tin impurity in Pb_{1 ? x} Sn _x Se crystalline solid solutions. However, it is impossible to determine the content of tellurium in Te _x (As _y Se_{1 ? y} )_{1 ? x} glassy alloys because the alloy components have significantly different X-ray fluorescence characteristics.     

Effect of Surface Preparation on Mechanical Properties of Ni Contacts on Polycrystalline (Bi1?x Sb x )2(Te1?y Se y )3 Alloys

We present a comparison of different surface preparation techniques for hot-extruded (Bi_1?x Sb _x )₂(Te_1?y Se _y )₃ alloys prior to Ni plating and their consequences on adhesion. The surface preparation is carried out by a relatively fast and well-controlled electrochemical etching procedure that provides morphologies with homogeneous flat surfaces on a scale of tens of micrometers. This procedure is not optimal for Ni metallization since it does not ensure enough roughness on the surface. Applying wet chemical etching with hypochlorite and nitric acid after the electrochemical etching changes the morphology, and the surface roughness increases, as evidenced by scanning electron microscopy and atomic force microscopy. Tensile tests carried out on n-type and p-type 1-mm-thick specimens, covered with a 12-??m Ni layer followed by Sn-Pb soldered joints, confirm a significant improvement in the ultimate adhesion strength, with fracture occurring within the thermoelectric material. Use of this two-step surface treatment, after slicing and prior to metallization, leads to maximum adhesion strength of around 38?MPa for either type of thermoelectric material. These results are consistent with the morphological changes observed by scanning electron microscopy on the surface after chemical etching.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

Characterization of High-k Gate Dielectric with Amorphous Nanostructure

In the present study, Zr _x La_1?x O _y amorphous nanostructures were prepared by the sol–gel method such that the Zr atomic fraction (x) ranged from 0% to 70%. An analytical model is described for the dielectric constant (k) of Zr _x La_1?x O _y nanostructures in a metal–oxide–semiconductor (MOS) device. The structure and morphology of Zr _x La_1?x O _y film was studied using x-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Elemental qualitative analysis was performed using energy-dispersive x-ray spectra and a map that confirmed the findings. Preliminary information on the influence of thermal annealing on the morphological control of Zr _x La_1?x O _y amorphous nanostructures is presented. The dielectric constant of the crystalline Zr_0.5La_0.5O _y thin film is about 36. Electrical property characterization was performed using a metal–dielectric–semiconductor structure via capacitance–voltage and current density–voltage measurements.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

Study of the long-wavelength optic phonons in AlGaInP and AlGaInAs

The investigation of the lattice dynamics of (Al_xGa_1−x)_yIn_1−yP quaternary semiconductor alloys lattice matched to GaAs has been made by Raman scattering. The Raman spectra exhibit three-mode behavior depending on the composition. A modified random element isodisplacement (MREI) model is generalized to the III-V (A_xB_1−x)_1−yC_yD-type quaternary alloys, describing the behavior of the optical phonons. The calculated result of two quaternary mixed crystals, (Al_xGa_1−x)_yIn_1−yP and (Al_xGa_1−x)_yIn_1−yAs, is in good agreement to the experimental data.     

Electrochemical Deposition and Characterization of Thermoelectric Ternary (Bi x Sb1?x )2Te3 and Bi2(Te1?y Se y )3 Thin Films

Thermoelectric thin films of the ternary compounds (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ were synthesized using potentiostatic electrochemical deposition on gold-coated silicon substrates from aqueous acidic solutions at room temperature. The surface morphology, elemental composition, and crystal structure of the deposited films were studied and correlated with preparation conditions. The thermoelectric properties of (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ films, i.e., Seebeck coefficient and electrical resistivity, were measured after transferring the films to a nonconductive epoxy support. (Bi _x Sb_1?x )₂Te₃ thin films showed p-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively large negative potential with composition close to Bi_0.5Sb_1.5Te₃. In addition, Bi₂(Te_1?y Se _y )₃ thin films showed n-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively small negative potential, having composition close to Bi₂Te_2.7Se_0.3. In contrast to Bi₂Te_2.7Se_0.3 thin films, an annealing treatment was required for Bi_0.5Sb_1.5Te₃ thin films to achieve the same magnitude of power factor as Bi₂Te_2.7Se_0.3. Therefore, Bi₂Te_2.7Se_0.3 thin films appear to be good candidates for multilayer preparation using electrochemical deposition, but the morphology of the films must be further improved.     

On the stability of real exponential polynomials with interval-valued delays

We consider the family ? of exponential polynomials. $$f_T (z): = f_0 (z) + f_1 (z)e^{ - zT_1 } + f_2 (z)e^{ - T_2 } + \cdots + f_n (z)e^{ - zT_n } ,T : = T_1 \times T_2 \times \cdots \times T_n ,$$ where thef _k(z),k=0(1)n, are real polynomials under the degree restriction deg(f ₀)>deg(f _k),k=1(1)n, and theT _k,k=1(1)n, are intervals inR ₊. The functions in ? are characteristic functions of linear, retarded dynamical systems with constant coefficients and finitely many interval-valued discrete delays. A stability criterion for ? is expounded; ? is stable if (a) ? contains a stable member and (b) a certain functionalT:S(y) →; {0, 1} vanishes fory in a compact interval inR ₊. Here,S(y) is the boundary of a circular are regionS(y) in the complex plane derived fromf _T. Tools needed for a computer implementation are compiled.     

Study of electron capture by quantum dots using deep-level transient spectroscopy

The electron emission from and capture by quantum dots in InGaAs/GaAs p-n heterostructures were studied using deep-level transient spectroscopy in relation to the conditions of isochronous annealing with a switched-on or-off bias voltage (U _ra<0 or U _ra=0). The results indicate that, as a result of annealing with U _ra<0, a dipole that consists of charge carriers localized in the quantum dots and the ionized lattice defects is formed. The electrostatic potential of this dipole reduces the barrier for the electron emission from and capture by a quantum dot. If the annealing is performed at U _ra=0, no dipole is formed and the band offset is controlled by the conditions of heteroboundary formation during the structure growth. The dependence of the barrier height on the filling-pulse duration was also observed; this dependence is related to the manifestation of the Coulomb blockade for the electron capture by the ground and excited states of a quantum dot. The structures with quantum dots studied were grown by gaseous-phase epitaxy from metal-organic compounds using self-organization effects.     

Flicker noise in MOSFETs with gate-voltage-dependent mobility

Flicker noise measurements in MOSFETs at low drain bias are explained in terms of the dependence of the carrier mobility on the gate voltage of the form μ₀₀[1 + β(V_G ? V_T ? V₀)]^?1. Excellent agreement, both for the (I_d, V_g) characteristic and for the flicker noise, is obtained. The noise current spectrum is expressed in the normalized functions f(y₀, y₁) and f(y₀, y₁)/y₀ in terms of the bias parameters y₀ = β(V_g ? V_T) and y₁ = β(V_g ? V_T ? V_d).     

Self-ordering of Mg and O isoelectronic impurities in ZnSe

Self-ordering of isoelectronic magnesium and oxygen impurities in ZnSe is described. The formation of 1O4 Mg tetrahedral cells in ZnSe-enriched Mg_xZn_1?x O_ySe_1?y alloy (x ≥ 4y) in the ultradilute limit (1 × 10^?8 ≤ y ≤ 1 × 10^?3) of oxygen concentrations is predicted. Under certain conditions that are established for the temperatures of growth (300°C) and annealing (500°C), all of the oxygen impurity atoms should be surrounded by Mg atoms only. The origin of this phenomenon is the thermodynamic preference for Mg-O and Zn-Se bond formation compared to Mg-S and Zn-O bonds and a decrease in the elastic-stress energy after self-ordering of the alloy.     

Charge transport in silver chalcogenides in the region of phase transition

Data on the σ(T), R(T), and U(T) dependences in Ag₂Te, Ag₂Se, and Ag₂S in the region of the phase transition are analyzed. It is found that the phase transition in Ag₂Te is accompanied by a decrease in the electron concentration and this transition in Ag₂Se is accompanied by an increase in this concentration. The concentration of intrinsic charge carriers in Ag₂Te decreases by a factor of 4 as a result of the phase transition and increases by a factor of 2 in Ag₂Se. The effect of variation in the energy-band parameters in the region of phase transition on the electron mobility is considered. It is established that, in Ag₂Te and Ag₂S, electrons are scattered by optical phonons in the region of the phase transition, while electrons are scattered by acoustic phonons in the α and β phases. It is assumed that the anomalously large increase in σ and U in Ag₂S as a result of the phase transition is caused by an increase in the concentration n and a simultaneous decrease in σ _g and m _n ^* by a factor of about 2.     

Crystal structure and band gap of AlGaAsN

Quantum dielectric theory is applied to the quaternary alloy Al_xGa_1−xAs_1−yN_y to predict its electronic properties as a function of Al and N mole fractions. Results are presented for the expected crystal structure, minimum electron energy band gap, and direction in k-space of the band gap minimum for all x and y values. The results suggest that, for a proper choice of x and y, Al_xGa_1−xAs_1−yN_y could exhibit certain advantages over Al_xGa_1−xAs when utilized in field-effect transistor structures.