Thermoelectric Properties of Ca3Co4-x Ga x O9+δ Prepared by Thermal Hydro-decomposition

The polycrystalline samples of Ca₃Co_4-x Ga _x O_9+δ (0 ≤ x ≤ 0.15) were prepared by a simple thermal hydro-decomposition method. The high density ceramics were fabricated using a spark plasma sintering technique. The crystal structure of calcined powders was characterized by x-ray diffraction. The single phase of Ca₃Co_4-x Ga _x O_9+δ was obtained. The scanning electron micrograph illustrated the grain alignment perpendicular to the direction of the pressure in the sintering process. The evidence from x-ray absorption near edge spectra were used to confirm the oxidation state of the Ga dopant. The thermoelectric properties of the misfit-layered of Ca₃Co_4-x Ga _x O_9+δ were investigated. Seebeck coefficient tended to decrease with increasing Ga content due to the hole-doping effect. The electrical resistivity and thermal conductivity were monotonically decreased with increasing Ga content. The Ga doping of x = 0.15 showed the highest power factor of 3.99 × 10^-4 W/mK² at 1,023 K and the lowest thermal conductivity of 1.45 W/mK at 1,073 K. This resulted in the highest ZT of 0.29 at 1,073 K. From the optical absorption spectra, the electronic structure near the Fermi level show no significant change with Ga doping.     

Investigation of MBE-Growth of Mid-Wave Infrared Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Undoped mid-wave infrared Hg_1?xCd_xSe epitaxial layers have been grown to a nominal thickness of 8–14 μm on GaSb (211)B substrates by molecular beam epitaxy (MBE) using constant beam equivalent pressure ratios. The effects of growth temperature from 70°C to 120°C on epilayer quality and its electronic parameters has been examined using x-ray diffraction (XRD) rocking curves, atomic force microscopy, Nomarski optical imaging, photoconductive decay measurements, and variable magnetic field Hall effect analysis. For samples grown at 70°C, the measured values of XRD rocking curve full width at half maximum (FWHM) (116 arcsec), root mean square (RMS) surface roughness (2.7 nm), electron mobility (6.6?×?10⁴ cm² V^?1 s^?1 at 130 K), minority carrier lifetime (～?2 μs at 130 K), and background n-type doping (～?3?×?10¹⁶ cm^?3 at 130 K), indicate device-grade material quality that is significantly superior to that previously published in the open literature. All of these parameters were found to degrade monotonically with increasing growth temperature, although a reasonably wide growth window exists from 70°C to 90°C, within which good quality HgCdSe can be grown via MBE.     

The Effect of Sn Orientation on Intermetallic Compound Growth in Idealized Sn-Cu-Ag Interconnects

The work reported here explores the influence of crystal orientation on the growth of the interfacial intermetallic layer during electromigration in Cu||Sn||Cu solder joints. The samples were thin, planar Sn-Ag-Cu (SAC) solder layers between Cu bars subject to a uniaxial current. Electron backscatter diffraction (EBSD) was used to characterize the microstructure before and after testing. The most useful representation of the EBSD data identifies the Sn grain orientation by the angle between the Sn c-axis and the current direction. The tested samples included single-crystal joints with c-axis nearly parallel to the current (“green” samples) and with c-axis perpendicular to the current (“red” samples). At current density of 10⁴ A/cm² (steady-state temperature of ~150°C), an intermetallic layer grew at an observable rate in the “green” samples, but not in the “red” ones. A current density of 1.15 × 10⁴  A/cm² (temperature ~160°C) led to measurable intermetallic growth in both samples. The growth fronts were nearly planar and the growth rates constant (after an initial incubation period); the growth rates in the “green” samples were about 10× those in the “red” samples. The Cu concentrations were constant within the joints, showing that the intermetallic growth is dominated by the electromigration flux. The measured growth rates and literature values for the diffusion of Cu in Sn were used to extract values for the effective charge, z ^*, that governs the electromigration of Cu. The calculated value of z ^* is significantly larger for current perpendicular to the c-axis than along it.     

Characterization of <Emphasis Type="Italic">n</Emphasis>-Type and <Emphasis Type="Italic">p</Emphasis>-Type Long-Wave InAs/InAsSb Superlattices

The influence of dopant concentration on both in-plane mobility and minority carrier lifetime in long-wave infrared InAs/InAsSb superlattices (SLs) was investigated. Unintentially doped (n-type) and various concentrations of Be-doped (p-type) SLs were characterized using variable-field Hall and photoconductive decay techniques. Minority carrier lifetimes in p-type InAs/InAsSb SLs are observed to decrease with increasing carrier concentration, with the longest lifetime at 77 K determined to be 437 ns, corresponding to a measured carrier concentration of p ₀ = 4.1 × 10¹⁵ cm^?3. Variable-field Hall technique enabled the extraction of in-plane hole, electron, and surface electron transport properties as a function of temperature. In-plane hole mobility is not observed to change with doping level and increases with reducing temperature, reaching a maximum at the lowest temperature measured of 30 K. An activation energy of the Be-dopant is determined to be 3.5 meV from Arrhenius analysis of hole concentration. Minority carrier electrons populations are suppressed at the highest Be-doping levels, but mobility and concentration values are resolved in lower-doped samples. An average surface electron conductivity of 3.54 × 10^?4 S at 30 K is determined from the analysis of p-type samples. Effects of passivation treatments on surface conductivity will be presented.     

On the Thermoelectric Properties of Zintl Compounds Mg3Bi2−x Pn x (Pn = P and Sb)

A series of Zintl compounds Mg₃Bi_2-x Pn _x (Pn = P and Sb) have been synthesized by the solid-state reaction method. While Sb can be substituted to a level as high as x = 1.0, P can be substituted only up to x = 0.5. The thermoelectric potential of these compounds has been evaluated by measuring resistivity (ρ), Seebeck (α) and Hall coefficients, and thermal conductivity between 80 K and 850 K. The measured resistivity and Seebeck coefficient values are consistent with those expected for small-bandgap semiconductors. Hall measurements suggest that the carriers are p type with concentration (p) increasing from ~10¹⁹ cm^?3 to ~10²⁰ cm^?3 as the Bi content is increased. The Hall mobility decreases with increasing temperature (T) and reaches a more or less similar value (~45 cm²/V s) for all substituted compositions at room temperature. Due to mass defect scattering, the lattice thermal conductivity (κ _L) is decreased to a minimum of ~1.2 W/m K in Mg₃BiSb. The power factor (α ²/ρ) is found to be rather low and falls in the range 0.38 mW/m K² to 0.66 mW/m K². As expected, at a high temperature of 825 K, the total thermal conductivity (κ) of Mg₃BiSb reaches an impressive value of ~1.0 W/m K. The highest dimensionless figure of merit (ZT) is realized for Mg₃BiSb and is ~0.4 at 825 K.     

Microstructures and Thermoelectric Properties of Sintered Misfit-Layered Cobalt Oxide

Misfit-layered cobalt oxide Ca₃Co₄O₉ is considered to be a prospective material for thermoelectric conversion. The thermoelectric properties are anisotropic owing to its anisotropic crystal structure. The crystal has preferred thermoelectric properties along the a–b plane. Therefore, the thermoelectric properties are improved and controlled by the degree of orientation of the sintered sample. In the present work, Sr-doped misfit cobalt oxide Ca_2.7Sr_0.3Co₄O₉ was prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K. The Seebeck coefficient α, electrical resistivity ρ, and dimensionless figure of merit ZT were measured as a function of the compression pressure applied in the uniaxial molding. α, ρ, and ZT as functions of the degree of orientation and the relative density are experimentally clarified and explained by calculations using the compound model.     

Thermoelectric Properties of Bi0.5Sb1.5Te3 Prepared by Liquid-Phase Growth Using a Sliding Boat

A liquid-phase growth process using a graphite sliding boat was applied for synthesis of p-type Bi_0.5Sb_1.5Te₃. The process lasted only 60 min, including rapid heating for melting, boat-sliding, and cooling. Thick sheets and bars of 1 mm and 2 mm in thickness having preferable crystal orientation for thermoelectric conversion were successfully prepared by the process. Control of carrier concentration was attempted through addition of excess tellurium (1 mass% to 10 mass%) to optimize the thermoelectric properties of the material. The Hall carrier concentration was found to be decreased by addition of excess tellurium. The electrical resistivity and Seebeck coefficient varied depending on the carrier concentration. As a result, the maximum observed power factor near 300 K was 4.4 × 10^?3 W/K²m, with corresponding Hall carrier concentration of 4.6 × 10²⁵ m^?3. Thus, thermoelectric properties were controllable by addition of excess tellurium, and a large power factor was thus obtained through a simple and short process.     

The Surface Kinetics of MBE-Grown CdTe (211)B During In Situ Cyclic Annealing

The surface kinetics of CdTe (211)B grown by molecular beam epitaxy (MBE) is investigated using spectroscopic ellipsometry (SE) during in situ cyclic annealing. A method of measuring sublimation rates from high-index surfaces without use of reflection high-energy electron diffraction is presented. The effect of Te₂ overpressure on the activation energy of sublimation for the CdTe (211)B surface is reported. The sensitivity of SE to surface temperature and film thickness was leveraged to monitor sublimation rates of CdTe stabilized by a Te₂ overpressure. The sublimation activation energy was found to increase from 0.45 eV to 2.94 eV under the Te₂ beam pressure regime investigated.     

Comparative Study of Shallow Acceptor Levels in Unintentionally Doped p-Type GaSe Crystals Prepared by the Bridgman and Liquid Phase Solution Growth Methods

GaSe crystals were grown by liquid phase solution growth, using the temperature difference method under controlled vapor pressure. Temperature-dependent Hall effect measurements were used to show the different thermal activation energies of the acceptor levels of the commercially available Bridgman-grown GaSe crystals and our liquid phase solution-grown crystals. Unintentionally doped GaSe crystals showed p-type conduction; also, the thermal activation energies of the acceptor levels of the Bridgman-grown crystals were 30 meV + E _v and 50 meV + E _v while that of liquid phase-grown crystals was 15 meV + E _v (where E _v is the energy of the valence band). We conclude that low temperature growth and stoichiometry control can effectively inhibit native point defect formation.     

Variable-Field Hall Measurement and Transport in LW Single-Layer n-Type MBE Hg1−x Cd x Te

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg_1?x Cd _x Te (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 10⁵ cm² V^?1 s^?1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.     

Properties of ZnO Thin Films Codoped with Lithium and Phosphorus

The properties of ZnO thin films codoped with lithium and phosphorus have been characterized. The films were deposited from high-purity ZnO and Li₃PO₄ solid targets onto c-plane sapphire substrates by radiofrequency (RF) magnetron sputtering. A substrate temperature of 900°C was determined as optimum for depositing undoped ZnO films with background electron concentration of 9.9 × 10¹⁵ cm^?3 as the buffer layer on the sapphire substrate. Postdeposition annealing was carried out using rapid thermal processing in O₂ at temperatures ranging from 500°C to 1000°C for 3 min. Analyses performed using low-temperature photoluminescence spectroscopy measurements revealed luminescence peaks at 3.356 eV, 3.307 eV, 3.248 eV, and 3.203 eV at 12 K for the codoped samples. X-ray diffraction 2θ-scans showed a single peak at about 34.4° with full-width at half-maximum of about 0.09°. Hall-effect measurements revealed initial p-type conductivities, but these were unstable and toggled between p-type and n-type over time with Hall concentrations that varied between 2.05 × 10¹³ cm^?3 and 2.89 × 10¹⁵ cm^?3. The fluctuation in the carrier type could be due to lateral inhomogeneity in the hole concentration caused by stacking faults in the films. An additional cause could be the small Hall voltages in the measurements, which could be significantly impacted by even small spikes in signal noise inherent in the measurements.     

Hall Effect Studies of AlGaAs Grown by Liquid-Phase Epitaxy for Tandem Solar Cell Applications

We report results from Hall effect studies on Al _x Ga_1?x As (x = 0.23–0.24) with bandgap energies of 1.76 ± 0.01 eV grown by liquid-phase epitaxy (LPE). Room-temperature Hall measurements on unintentionally doped AlGaAs revealed p-type background doping for concentrations in the range 3.7–5.2 × 10¹⁶ cm^?3. Sn, Te, Ge, and Zn-doped AlGaAs were also characterized to study the relationship between doping concentrations and the atomic fractions of the dopants in the melt. Temperature-dependent Hall measurements were performed to determine the activation energies of the four dopants. Deep donor levels (DX centers) were dominant for Sn-doped Al_0.24Ga_0.76As, but not for Te-doped Al_0.24Ga_0.76As. Comparison of the temperature-dependent Hall effect results for unintentionally and intentionally doped Al_0.24Ga_0.76As indicated that the impurity contributing to the p-type background doping had the same activation energy as Mg. We thus suggest a Te-doped emitter and an undoped or Ge-doped base to maximize the efficiency of Al _x Ga_1?x As (x ～ 0.23) solar cells grown by LPE.     

Charge carrier mobility in n-CdxHg1−x Te crystals subjected to dynamic ultrasonic stressing

The Hall mobility was studied in the n-Cd_xHg_1?x Te crystals subjected to dynamic ultrasonic stressing (W _US≤10⁴ W/m², f=5–7 MHz). It was found that, in field of the ultrasonic deformation, an increase in the carrier mobility in the impurity conduction region (T<120 K) and a decrease in the intrinsic conduction region (T>120 K) occurred in all tested samples. In this case, the magnitude of the sonic-stimulated variation in μ_H increases with decreasing structural perfection of a crystal. Different mechanisms of ultrasonic influence on μ_H with regard to scattering by optical phonons, ionized impurities, and alloy potential are analyzed, with the current flow conditions in the crystal taken into account. It is shown that, in the impurity conduction region, the main cause of the sonic-stimulated increase of the Hall mobility is the smoothing of the macroscopic intracrystalline potential that results from the inhomogeneity of the crystals. In the intrinsic conduction region, a decrease in mobility is caused by an increase in the intensity of scattering by the optical phonons.     

The mechanisms of hole scattering in p-Hg0.8Cd0.2Te crystals at low temperatures

The resistivity and Hall effect were investigated in p-Hg_0.8Cd_0.2Te crystals that contained from 1.5×10¹⁵ to 1.7×10¹⁸ cm^?3 Cu atoms. The measurements were carried out in the temperature range of 4.2–100 K. It is demonstrated that, in order to correctly determine the Hall mobility of holes at low temperatures, one should exclude the contribution of hopping charge transfer. It was found that heavy holes are scattered at 77 K by each other, by impurity ions, by composition fluctuations, and by lattice vibrations. In compensated crystals, holes are scattered only by lattice vibrations at low temperatures. For uncompensated crystals, when calculating the mobility, it is necessary to make allowance for the hole scattering by positively charged centers formed due to trapping of excess holes by acceptors.     

Phase Content Influence on Thermoelectric Properties of Manganese Silicide-Based Materials for Middle-High Temperatures

The higher manganese silicides (HMS), represented by MnSi _x (x = 1.71 to 1.75), are promising p-type leg candidates for thermoelectric energy harvesting systems in the middle-high temperature range. They are very attractive as they could replace lead-based compounds due to their nontoxicity, low-cost starting materials, and high thermal and chemical stability. Dense pellets were obtained through direct reaction between Mn and Si powders during the spark plasma sintering process. The tetragonal HMS and cubic MnSi phase amounts and the functional properties of the material such as the Seebeck coefficient and electrical and thermal conductivity were evaluated as a function of the SPS processing conditions. The morphology, composition, and crystal structure of the samples were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction analyses, respectively. Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal stability of the final sintered material. A ZT value of 0.34 was obtained at 600°C for the sample sintered at 900°C and 90 MPa with 5 min holding time.     

Lower Thermal Conductivity and Higher Thermoelectric Performance of Fe-Substituted and Ce, Yb Double-Filled p-Type Skutterudites

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce _x Yb _y Fe _z Co_4?z Sb₁₂ (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce_0.5Yb_0.5Fe₃CoSb₁₂. A high power factor (27 μW/K² cm) combined with a rather low thermal conductivity for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂ (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.     

Low-Temperature Physical and Thermoelectric Properties of Ba8Ni5Ge41

Resistivity, Hall resistivity, thermopower, thermal conductivity, and magnetization are reported for polycrystalline Ba₈Ni₅Ge₄₁. Ba₈Ni₅Ge₄₁ is diamagnetic with susceptibility χ _dia = (?2.4 to ?2.82) × 10^?7 emu/g. Semiconductor-like behavior was observed for the resistivity. The thermopower shows positive values for a wide temperature range. The Hall resistivity indicates the dominance of electrons, suggesting the existence of multiband conductance. At room temperature, the thermal conductivity is 1.78(5) W/K m. The highest ZT of Ba₈Ni₅Ge₄₁ is 0.0016 at about 278 K.     

Development of 3.7 % Efficient Cu2ZnSnSe4 Solar Cells by Selenizing Cu-Zn-Sn Films Deposited by DC Sputtering on TiN-Protected Mo/Glass Substrates

Cu₂ZnSnSe₄ (CZTSe) films for solar cell devices were fabricated by sputtering of a Cu-Zn-Sn target followed by post-selenization at 500–600 °C for 1 h in the presence of single or double compensation discs to supply Se vapor. The optimized selenization conditions avoided the Se deficiency and enhanced the grain growth of CZTSe films. The 600 °C-selenized CZTSe films adjacent with double discs obtained the large grains of 2–5 μm and had a [Cu]/([Zn]+[Sn]) ratio of 0.94 and a [Zn]/[Sn] ratio of 1.34. In order to fabricate the device on Mo-coated glass substrates, a TiN reaction barrier layer was coated before the Cu-Zn-Sn sputtering coating. The TiN-CZTSe device had 3.7 % efficiency (η), as compared to 0.58 % for the TiN-free one. The efficient device had the CZTSe layer with hole concentration (n _p) of 3.4 × 10¹⁷ cm^?3, Hall mobility (μ) of 54 cm² V^?1 s^?1, and electrical conductivity (σ) of 2.9 Ω^?1 cm^?1.     

Thermally Stable BaTiO3-Bi(Mg0.75W0.25)O3 Solid Solutions: Sintering Characteristics,Phase Evolution,Raman Spectra,and Dielectric Properties

(1 ? x)BaTiO₃-xBi(Mg_0.75W_0.25)O₃ [(1 ? x)BT-xBMW, 0.02 ≤ x ≤ 0.24] ceramics were synthesized by a two-step solid-state reaction technique. X-ray diffraction (XRD) patterns show that a systematic structure evolution from tetragonal to pseudocubic phase was observed at x = 0.07. Raman spectra analysis illustrates that a change in average structure was observed with increasing x, and the local crystal symmetry which deviated from the idealized cubic perovskite structure appeared as x ≥ 0.07. Temperature dependence of dielectric properties indicates that the phase transition temperature (T _c) decreased with increasing x. Moreover, (1 ? x)BT-xBMW (0.07 ≤ x ≤ 0.24) ceramics show good dielectric thermal stability over a wide temperature range, which indicates that these ceramics are candidates for thermal stability devices.     

Electromigration and Thermomechanical Fatigue Behavior of Sn0.3Ag0.7Cu Solder Joints

The anisotropy of Sn crystal structures greatly affects the electromigration (EM) and thermomechanical fatigue (TMF) of solder joints. The size of solder joint shrinkage in electronic systems further makes EM and TMF an inseparably coupled issue. To obtain a better understanding of failure under combined moderately high (2000 A/cm²) current density and 10–150°C/1 h thermal cycling, analysis of separate, sequential, and concurrent EM and thermal cycling (TC) was imposed on single shear lap joints, and the microstructure and crystal orientations were incrementally characterized using electron backscatter diffraction (EBSD) mapping. First, it was determined that EM did not significantly change the crystal orientation, but the formation of Cu₆Sn₅ depended on the crystal orientation, and this degraded subsequent TMF behavior. Secondly, TC causes changes in crystal orientation. Concurrent EM and TC led to significant changes in crystal orientation by discontinuous recrystallization, which is facilitated by Cu₆Sn₅ particle formation. The newly formed Cu₆Sn₅ often showed its c-axis close to the direction of electron flow.