Fabrication and characterization of Pb1−xYbxTe-based alloy thin-film thermoelectric generators grown by thermal evaporation technique

In this study p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te powders synthesized by solid-state microwave technique were used to fabricate thermally evaporated thin films. The nanostructure and composition of the films were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). Electrical characterizations of the as-deposited films in terms of the Seebeck coefficient and electrical conductivity and power factor were conducted at a range of 298 K to 523 K. The microthermoelectric devices were composed of 20-pair and 10-pair p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te thin films on glass substrates. The dimensions of the thin-film thermoelectric generators, which consisted of 20-pair and 10-pair legs connected by aluminum electrodes, were 23 mm×20 mm and 12 mm×10 mm, respectively. The 20-pair p–n thermocouples in series generated a maximum open-circuit voltage output (V_oc) of 0.581 V and a maximum output power of 25.87×10^?8 W at a temperature difference ΔT=164 K, whereas the 10-pair p–n thermocouples generated 0.311 V and 13.71×10^?8 W maximum V_oc and maximum output power, respectively, at ΔT=164 K.     

Chalcogen-based thermoelectric power generation device using p-type Bi0.4Sb1.6Se2.4Te0.6 and n-type Bi2Se0.6Te2.4 prepared by solid-state microwave synthesis

This study reports on the fabrication of a chalcogen-based thermoelectric power generation (TEG) device using p-type Bi_0.4Sb_1.6Se_2.4Te_0.6 and n-type Bi₂Se_0.6Te_2.4 legs. Electrical power generation characteristics were monitored by changing both the temperature conditions and the number of p–n couples required to generate maximum power. The significance of the resistances including the internal resistance and contact resistance between legs and electrodes, are discussed. The maximum output power obtained with the 18 p–n couples device was 273.2 mW under the thermal condition of T_H=523 K hot-side temperature and ΔT=184 K temperature difference.     

Investigation on the thermal behavior of 0.15 μm gate-length In0.4Al0.6As/In0.4Ga0.6As MHEMT

In this study, we have successfully investigated the electrical performances of In_0.4Al_0.6As/In_0.4Ga_0.6As metamorphic high-electron-mobility transistor (MHEMT) at temperatures range from 275 K to 500 K comprehensively. By extracting the device S-parameters, the temperature dependent small signal model has been established. At room temperature, 0.15 μm T-gate device with double δ-doping design exhibits f_T and f_MAX values of 103 GHz and 204 GHz at V_ds = 1 V, an extrinsic transconductance of 678 mS/mm, and a current density of 578 mA/mm associated with a high breakdown voltage of ?13 V. Power measurements were evaluated at 40 GHz and the measured output power, linear power gain, and maximum power-added efficiency, were 7.12 dBm, 10.15 dB, and 23.1%, respectively. The activation energy (E_a) extracted from Arrhenius plots is = 0.34 eV at 150 ≦ T ≦ 350 K. The proposed device is promisingly suitable for millimeter-wave power application.     

Effect of magnetic field on galvanomagnetic properties of mica/Bi/EuS heterostructures

The dependences of the Hall coefficient R_H and magnetoresistance Δρ/ρ on magnetic field (B=0.01?1.0 T) were obtained in the temperature range 77–300 K for thin Bi films with thicknesses d=40–250 nm, grown on mica substrates and covered by a EuS layer. It was established that in the entire temperature range for all Bi films the criterion of weak field was fulfilled at magnetic fields up to 1 T: R_H remained practically constant in the entire range of magnetic field and Δρ/ρ for all investigated samples changed with changing magnetic field according to a parabolic law.     

Strain sensitivity of gate leakage in strained-SOI nMOSFETs: A benefit for the performance trade-off and a novel way to extract the strain-induced band offset

The impact of biaxial stress on gate leakage is investigated on fully-depleted silicon-on-insulator (FD-SOI) nMOS transistors, integrating either a standard gate stack or an advanced high-κ/metal gate stack. It is demonstrated that strained devices exhibit significantly reduced leakage currents (up to ?90% at E_ox = 11 MV/cm for σ_tensile = 2.5 GPa). This specific effect is used to extract the conduction band offset ΔEc induced by strain and is shown to be accurate enough to monitor stress in MOSFETs. This new technique is much less sensitive to gate oxide defects than the method based on the threshold voltage shift ΔV_T. This accurate experimental extraction allowed us to pick out realistic values for the deformation potentials in silicon (Ξ_u = 8.5 eV and Ξ_d = ?5.2 eV), among the published values.     

Characterizations of solid-state microwave-synthesized Sb2Te3-based alloys with various compositions of bismuth in Bi2xSb2(1−x)Te3

In this article, thermoelectric (TE) materials based on p-type Sb₂Te₃ samples and dispersed with x amounts of Bi (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_2xSb_2(1−x)Te₃ were synthesized via a standard solid-state microwave route. The microstructure of the ingots was characterized by field emission scanning electron microscopy. As-synthesized ingots were formed by the assembly of micro-sheet grains. The phase composition of the powders was characterized by X-ray diffraction, revealing a rhombohedral structure. The influence of variations in Bi content (x) on the TE properties of the resulting alloy was studied in the temperature range of 303 K to 523 K. Increases in x caused a decrease in hole concentration and electrical conductivity and an increase in Seebeck coefficient. A maximum power factor of 4.96 mW/mK² was obtained at about 373 K for a Bi_2xSb_2(1−x)Te₃ sample with x=0.2.     

Temperature-dependent ambipolar electrical characteristics of pentacene-based thin-film transistors: The impact of opposite-sign charge carriers

The temperature-dependent electrical and charge transport characteristics of pentacene-based ambipolar thin-film transistors (TFTs) were investigated at temperatures ranging from 77 K to 300 K. At room temperature (RT), the pentacene-based TFTs exhibit balanced and high charge mobility with electron (μ_e) and hole (μ_h) mobilities, both at about 1.6 cm²/V s. However, at lower temperatures, higher switch-on voltage of n-channel operations, almost absent n-channel characteristics, and strong temperature dependence of μ_e indicated that electrons were more difficult to release from opposite-signed carriers than that of holes. We observed that μ_e and μ_h both followed an Arrhenius-type temperature dependence and exhibited two regimes with a transition temperature at approximately 210–230 K. At high temperatures, data were explained by a model in which charge transport was limited by a dual-carrier release and recombination process, which is an electric field-assisted thermal-activated procedure. At T < 210 K, the observed activation energy is in agreement with unipolar pentacene-based TFTs, suggesting a common multiple trapping and release process-dominated mechanism. Different temperature-induced characteristics between n- and p-channel operations are outlined, thereby providing important insights into the complexity of observing efficient electron transport in comparison with the hole of ambipolar TFTs.     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.     

Temperature dependence of the electrical characteristics up to 370 K of amorphous In-Ga-ZnO thin film transistors

The temperature dependence in the typical temperature operating range from 300 K up to 370 K of the electrical characteristics of IGZO TFTs fabricated at temperatures not exceeding 200 °C is presented and modeled.It is seen that up to T = 330 K, the transfer curves show a parallel shift toward more negative voltages. In both subthreshold and above threshold regimes, the drain current shows Arrhenius-type dependence. In the latter case, for low temperatures, the activation energy is around 0.35 eV for V_GS = 10 V, reducing as V_GS is increased. The observed behavior is consistent with having the VRH transport mechanism as the predominant one in conduction.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

Threshold voltage instabilities in p-channel power VDMOSFETs under pulsed NBT stress

Threshold voltage instabilities induced in p-channel power VDMOSFETs by pulsed negative bias temperature stressing are presented and compared with corresponding instabilities found after the static NBT stress. Degradation observed under the pulsed stress conditions depends on the frequency and duty cycle of stress voltage pulses, and is generally lower than the one found after the static NBT stress. Optimal frequency and duty cycle ranges for application of investigated devices are proposed as well. By selecting an appropriate combination of frequency range (1 kHz < f < 5 kHz) and duty cycle (about 25%), the pulsed stress-induced ΔV_T can be reduced to a quarter of ΔV_T found after the static NBT stress.     

Physical investigations on Sb2S3 sprayed thin film for optoelectronic applications

Sb₂S₃ thin films have been obtained at 250 °C on glass substrates using the spray pyrolysis techniques. The structural study by means of XRD analysis shows that Sb₂S₃ thin film crystallized in the orthorhombic phase. The discussion of some structural constants has been made by means of both XRD and AFM investigations. Moreover, the optical analysis via the transmittance and the reflectance measurements reveals that Sb₂S₃ sprayed thin film has a direct transition with the band gap energy E_g equal to 1.72 eV. The analysis in 300–2500 nm domain of the refractive index data through Wemple–DiDomenico model leads to the single oscillator energy (E₀=2.32 eV), and the dispersion energy (E_d=10.03 eV). The electrical study leads to the dc activation energy is of the order of 0.72 eV and the maximum barrier high is W_M=0.87 eV. From the power exponent variation in terms of the heated temperature, it is found that the mechanism of conduction matches well the correlated barrier hopping CBH model.     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

Phase transformation kinetics and optical properties of Ga–Se–Sb phase-change thin films

Phase transformation kinetics in Ga₂₅Se_75?xSb_x glasses have been determined by non-isothermal differential scanning calorimetric measurements at heating rates of 5, 10, 15, 20 and 25 K/min. The values of glass transition (T_g) and crystallization temperature (T_c) are found to be composition and heating rate dependent. The activation energy of crystallization and glass transition have been determined from the dependence of T_c and T_g on the heating rate. Thin films of Ga₂₅Se_75?xSb_x glasses have been prepared by vacuum evaporation technique with thickness 400 nm. These thin films were crystallized by thermal annealing and laser-irradiation. The phase change phenomena have been studied by measuring optical absorption of as-prepared and crystallized thin films in the wave length region 400–900 nm. The optical absorption data indicate that the absorption mechanism is non-direct transition. Optical band gap values decrease with increase in Sb contents in Ga–Se as well as with increase in annealing temperature and laser-irradiation time. The optical band gap is shifted due to crystallization by annealing/laser-irradiation. As the phase of the films changes from amorphous to crystalline, a non sharp change of the optical band gap is observed. This gradual decrease in optical band gap was explained to be a result of an amorphous–crystalline phase transformation.     

The optimum fin width in p-MuGFETs with the consideration of NBTI and hot carrier degradation

Generally it is known that NBTI degradation increases with decrease of a channel width in p-MOSFETs but hot carrier degradation decreases. In this work, a guideline for the optimum fin width in p-MuGFETs is suggested with consideration of NBTI and hot carrier degradation. Using the device lifetime defined as the stress time necessary to reach ΔV_TH = 10 mV, the optimum fin widths have been extracted for different stress voltages and temperatures. When a fin width is narrower than the optimum fin width, the device lifetime is governed by the NBTI degradation. However, when fin width is wider than the optimum fin width, the device lifetime is dominantly governed by hot carrier degradation. The optimum fin width decreases with the increase of the stress voltage but it increases with the increase of the stress temperature.     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3– xCa0.6La0.8/3TiO3 ceramics with V2O5 addition

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.6La_0.8/3TiO₃ has dielectric properties of dielectric constant ε_r ∼ 109, Q × f value ∼ 17,600 GHz and a large positive τ_f value ∼ 213 ppm/°C. (Mg_0.95Co_0.05)TiO₃ ceramics possesses high dielectric constant (ε_r ∼ 16.8), high quality factor (Q × f value ∼ 230,000 GHz), and negative τ_f value (−54 ppm/°C). As the x value varies from 0.1 to 0.8, (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system has the dielectric properties as follows: 21.55 < ε_r < 75.44, 21,000 < Q × f < 90,000 and −10 < τ_f < 140. By appropriately adjusting the x value in the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system, zero τ_f value can be achieved. With x = 0.15, a dielectric constant ε_r ∼ 25.78, a Q × f value ∼ 84,000 GHz (at 9 GHz), and a τ_f value ∼ 2 ppm/°C were obtained for 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics sintered at 1400 °C for 4 h. For practical application in communication systems, it is desirable to be able to sinter at lower temperatures. Therefore, V₂O₅ was as a sintering aid for lowering the sintering temperature of0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics. At the same time, the 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramic system with 0.5 wt% V₂O₅ can be obtained good properties at the microwave frequencies for 1200 °C.     

A comparative study of electrical and optical properties of InN and In0.48Ga0.52N

We present results of our studies concerning electrical and optical properties of In_0.48Ga_0.52N and InN. Hall measurement were carried out at temperatures between T=77 and 300 K. Photoluminescence (PL) spectrum in InN and In_0.48Ga_0.52N. InN has a single peak at 0.77 eV at 300 K. However, the PL in In_0.48Ga_0.52N has two peaks; a prominent peak at 1.16 eV and a smaller peak at 1.55 eV. These two peaks are attributed to Indium segregation corresponding to a high Indium concentration of 48% and a low concentration of 36%. High electric field measurements indicate that drift velocity that tends to saturate at around V_d=1.0×10⁷ cm/s at 77 K in InN at an electric field of F=12 kV/cm. However, in In_0.48Ga_0.52N the I–V curve is almost linear up to an electric field of F=45 kV/cm, where the drift velocity is V_d=1.39×10⁶ cm/s. At applied electric fields above this value a S-type negative differential resistance (NDR) is observed leading to an instability in the current and to the irreversible destruction of the sample.