Simulation of temperature-dependent material parameters and device performances for GaInAsSb thermophotovoltaic cell

Temperature-dependent material parameters and device performances of Ga_xIn_1−xAs_1−ySb_y TPV cells applied in low temperature (800–1200 °C) radiators are simulated using the PC-1D. As is well known, the optimum bandgap (E_g) decreases towards lower radiator temperatures. So far, the lowest achievable E_g of Ga_xIn_1−xAs_1−ySb_y at 300 K is 0.5 eV. We mainly considering the Ga_0.8In_0.2As_0.18Sb_0.82 (E_g = 0.5 eV) TPV cell. The effects of doping concentration and recombination mechanisms of the emitter layer on photovoltaic conversion efficiencies (η_cel) are analyzed in detail, and η_cel can be improved by optimizing doping concentration and suppressing carrier recombination. The effects of GaSb window layer on η_cel are also presented. It shows the type-II energy-band alignment GaSb(window)/GaInAsSb(emitter) heterostructure affect η_cel mainly through V_oc. For the first time, the effects of operating temperatures on device performances are analyzed based on temperature-dependent material parameters, and the temperature coefficients of the device performances are presented.     

Elastic constants and mechanical stability of InxAl1 − xAsySb1 − y lattice-matched to different substrates

Based on a pseudopotential approach under the virtual crystal approximation, the elastic modulus of In_xAl_1???xAs_ySb_1???y quaternaries lattice-matched to InP, GaSb and InAs substrates has been investigated. Our findings show a reasonably good accord with experiment. The dependence of the elastic features of interest on the indium concentration x shows a monotonic behaviour when In_xAl_1???xAs_ySb_1???y is lattice-matched to InP substrate. In that case, the elastic constants have larger values and the material system of interest becomes less harder and its rigidity becomes weaker. The mechanical stability criteria is verified in terms of elastic constants and shows that In_xAl_1???xAs_ySb_1???y is mechanically stable for each x and substrate being considered here. The change in indium content x and the substrate is found to have no much effect on both the Poisson ratio and machinability. The present study showed that a proper choice of the indium composition x and substrate may provide more diverse opportunities as regards the elastic modulus of In_xAl_1???xAs_ySb_1???y.     

A simple model for optimization design of high performance In1−x−y Ga y Al x As strained MQW DFB lasers

It is well known that complex rate equations and the couple wave equation have to be solved by the method of iteration in the simulation of multi-quantum well (MQW) distributed feedback Bragg (DFB) lasers, and a long CPU time is needed. In this paper, from the oscillation condition of lasers, we propose a simple and fast model for optimization of In_1–x–y Ga _y Al _x As strained MQW DFB lasers. The well number and the cavity length of 1.55 m wavelength In_1–x–y Ga _y Al _x As MQW DFB lasers are optimized using the model. As a result, the simple model gives almost the same results as the complex one, but 90% CPU time can be saved. In addition, a low threshold, high maximum operating temperature of 550–560 K, and high relaxation oscillation frequency of over 30 GHz MQW DFB laser is presented.     

The experimental and theoretical investigation of vibration spectra in ferroelectric semiconductor SbSBrxI1−x crystals

The vapor grown SbSBr_xI_1−x (x=0.1; 0.5; 0.9) crystals with clear mirror surfaces have been used for infrared reflection measurements with Fourier spectrometer. The vibration frequencies along c(z)-axis have been derived from Kramers–Kroning and optical parameters fitting analysis of the experimental reflectivity spectra at T=300 K. The theoretical vibration spectra of SbSBr_xS_1−x (x=0.1; 0.5; 0.9) crystals in paraelectric phase (T=300 K) along c(z)-axis have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. The theoretical vibration spectra of these crystals in a–b(x−y) plane have been determined in harmonic approximation. In this work we discuss the nature of anharmonism in SbSBr_xI_1−x crystals along the c(z)-axis.     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Structural and optical properties of Ba(Ti1−x,Nix)O3 thin films prepared by sol–gel process

Ba(Ti_1−x,Ni_x)O₃ thin films were prepared on fused quartz substrates by a sol–gel process. X-ray diffraction and Raman scattering measurements showed that the films are of pseudo-cubic perovskite structure with random orientation and the change of lattice constant caused by Ni-doping with different concentrations is very small. Optical transmittance spectra indicated that Ni-doping has an obvious effect on the energy band structure. The energy gap of Ba(Ti_1−x,Ni_x)O₃ decreased linearly with the increase of Ni concentration. It indicates that the adjusting of band gap can be achieved by controlling the Ni-doping content accurately in Ba(Ti_1−x,Ni_x)O₃ thin films. This has potential application in devices based on ferroelectric thin films.     

Comparison of the band alignment of strained and strain-compensated GaInNAs QWs on GaAs and InP substrates

We present a comparison of the band alignment of the Ga_1−xIn_xN_yAs_1−y active layers on GaAs and InP substrates in the case of conventionally strained and strain-compensated quantum wells. Our calculated results present that the band alignment of the tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells on InP substrates is better than than that of the compressively strained Ga_1−xIn_xN_yAs_1−y quantum wells on GaAs substrates and both substrates provide deeper conduction wells. Therefore, tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells with In concentrations of x0.53 on InP substrates can be used safely from the band alignment point of view when TM polarisation is required. Our calculated results also confirm that strain compensation can be used to balance the strain in the well material and it improves especially the band alignment of dilute nitride Ga_1−xIn_xN_yAs_1−y active layers on GaAs substrates. Our calculations enlighten the intrinsic superiority of N-based lasers and offer the conventionally strained and strain-compensated Ga_1−xIn_xN_yAs_1−y laser system on GaAs and InP substrates as ideal candidates for high temperature operation.     

Synthesis, characterization and melt processing of Y1−x(Yb0.9Nd0.1)xBa2Cu3Oz superconductors

Effects of a combined substitute of Yb and Nd on Y site on the superconducting properties of YBa₂Cu₃O_y have been studied. We synthesized Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z compound with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Here, the ratio of Yb–Nd was fixed to be 9:1 for obtaining 123 phase without secondary phases. The melt processing thermal profiles for Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z with x = 0.2 and 0.4 and the addition of 40 mol% {Y_1−x(Yb_0.9Nd_0.1)_x}₂BaCuO₅ and 0.5 wt% Pt in air were determined on the basis of the thermal analysis results. All samples showed a low grain growth rate, particularly for high x values, which may be partially ascribed to un-optimized thermal schedules. Although almost all the samples exhibited low J_c values, the sample with x = 0.2 exhibited T_c of 88.8 K and a relatively high J_c value of 16,000 A/cm² at 77 K for H//c-axis.     

Thickness study of Al:ZnO film for application as a window layer in Cu(In1−xGax)Se2 thin film solar cell

Structural, electrical and optical properties of Al doped ZnO (Al:ZnO) thin film of various thicknesses, grown by radio-frequency magnetron sputtering system were studied in relation to the application as a window layer in Cu(In_1−xGa_x)Se₂ (CIGS) thin film solar cell. It was found that the electrical and structural properties of Al:ZnO film improved with increasing its thickness, however, the optical properties degraded. The short circuit current density, J_sc of the fabricated CIGS based solar cells was significantly influenced by the variation of the Al:ZnO window layer thickness. Best efficiency was obtained when CIGS solar cell was fabricated with electrically and optically optimized Al:ZnO window layer.     

Growth and physical properties of sol–gel derived Co doped ZnO thin film

Zn_1−xCo_xO films were grown on glass by sol–gel spin coating process. The Zn_1−xCo_xO thin films with 10 at.% Co were highly c-axis oriented. The electrical resistivity of the films at 10 at.% Co had the lowest value due to the highest c-axis orientation. XPS and AGM analyses indicated that Co metal clusters weren’t formed, and the ferromagnetism was appeared at room temperature. The characteristics of the electrical resistivity and room temperature ferromagnetism of sol–gel derived Zn_1−xCo_xO films suggest a potential application to dilute magnetic semiconductor devices.     

Vortex pinning properties of –Ba–Cu–O and –Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y_1-xLa_x)–Ba–Cu–O and (Y_1-xPr_x)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density J_c at 77 K is improved in magnetic fields parallel to the c-axis above 2–4.5 T and the corresponding irreversibility field, H_irr, shifts to the higher value in both bulks.     

Conversion of egg shell membrane to inorganic porous CexZr1−xO2 fibrous network

The binary system CeO₂–ZrO₂ is thermally stable and has superior reduction–oxidation properties. It has been commonly used in the three-way catalytic converters for automobiles. In this work, an inorganic biomorphic porous Ce_xZr_1−xO₂ fibrous network was successfully synthesized by using the egg shell membrane (ESM) as templates, and its morphology was a perfect replica of the original ESM. The synthesis involved a simple infiltration and calcination process. A fresh ESM was peeled from a chicken egg shell. It was soaked in a Ce(NO₃)₃ and Zr(NO₃)₄ mixture before it was calcined at 700 °C in ambient environment. The fibers in the biomorphic network had diameter ranged from 1 to 4 μm, and they were composed of Ce_xZr_1−xO₂ nanocrystallites having an average grain size of 10 nm.     

Energy band gaps for the GaxIn1−xAsyP1−y alloys lattice matched to different substrates

A pseudopotential formalism within the virtual crystal approximation in which the effects of composition disorder are involved is applied to the Ga_xIn_1−xAs_yP_1−y quaternary alloys in conditions of lattice matching to GaAs, InP and ZnSe substrates so as to predict their energy band gaps. Very good agreement is obtained between the calculated values and the available experimental data for the alloy lattice matched to InP and GaAs. The alloy is found to be a direct-gap semiconductor for all y compositions whatever the lattice matching to the substrates of interest. The (Γ→Γ) band-gap ranges and the ionicity character are found to depend considerably on the particular lattice-matched substrates suggesting therefore that, for an appropriate choice of y and the substrate, Ga_xIn_1−xAs_yP_1−y could provide more diverse opportunities to obtain desired band gaps, which opens up the possibility of discovering new electronic devices with special features and properties.     

Transport phenomena of SmSe1−x As x

Results are presented on the measurements of unit cell parameter and electrical resistivity under pressure on SmSe_1−x As _x forx=0.1, 0.2, 0.3 and 0.4. The electrical resistivity values are found to be decreasing with increase of pressure and also with increase of arsenic concentration. The semiconductor to metallic transition is induced by chemical alloying of SmSe with SmAs similar to that observed under pressure. The electrical resistivity values are also calculated which are in good agreement with the experimental values. In this calculation, the carrier mobility is of negative sign and so the sample SmSe_1−x As _x is found to ben-type semiconductor.     

Growth and properties of air-processed GdBa2Cu3O7−δ superconductors with fined Gd2BaCuO5 and Gd2Ba4CuFeOy additions

We have succeeded in synthesizing a powder form of Gd₂Ba₄CuFeO_y (Gd2411) in air. GdBa₂Cu₃O_7−δ (Gd123)/Gd₂BaCuO₅ (Gd211) precursor powders added with different amounts of Gd₂Ba₄CuFeO_y (x = 0, 0.002, 0.004, 0.02) in molar ratio to Gd123 have been fabricated successfully into the form of large, single grains by the top seeded melt growth (TSMG) process. The relation between the additions amounts of Gd2411/Gd211 and critical current density (J_C) was analyzed. We found Gd2411 particles stably exist in the Gd123 matrix without degradation of superconducting properties owing to the existence of the Fe magnetic ion. The trapped field was observed to increase significantly compared with the bulk without Gd2411 additions.     

Effect of pressure on the properties of DH diode lasers in AlxGa1-xAsySb1-y/GaSb system

Abstract

A helium pressure appparatus for diode laser studies up to 1.4 GPa at 77–300 K has been developed. DH lasers with Al_xGa_1-xAs_ySb_1-y active layers (x=0-0.05) lattice-matched to GaSb substrates have been investigated. It has been shown that in lasers with x,y=0 pressure dependences of the threshold current density (J_th) and the average electron lifetime at the threshold (τ) measured at 80 K depend strongly on the quadratic recombination of L^c ₆ electrons, the characteristic coefficient being 1.5×10^?11 cm³s^?1. The pressure-composition equivalence coefficient dx/dP=2.2×10^?10 Pa^?1 has been obtained for the lowest temperatures used.     

Momentum dependence of pseudogap and superconducting gap in variation theory

To consider the origin of a pseudogap and a superconducting (SC) gap found in the high-T_c cuprates, we evaluated the momentum dependence of the singlet gap corresponding to the pseudogap and the SC gap in the t–J model, using an optimization variational Monte Carlo (VMC) method. In the underdoped regime, the singlet gap is significantly modified from the simple d_x²-y²(d)-wave gap (∝ cos k_x − cos k_y) by the contribution of long-range pairings. Its angular dependence at the quasi Fermi surface is qualitatively consistent with those experimentally observed in both hole and electron-doped cuprates. On the other hand, a SC gap is almost unchanged, preserving the original simple d-wave form. Thus, it seems that the incoherent part of the singlet gap mainly influences the forms of observed gaps.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Effect of material parameters on the open-circuit voltage in a GaInAsSb thermophotovoltaic cell

In this paper, a numerical simulation on the open-circuit voltage (V_OC) of the P-GaSb window/P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter/N-Ga_0.8In_0.2As_0.18Sb_0.82 base/N-GaSb structure thermophotovoltaic (TPV) cell is performed and an analysis of the effects of device parameters on V_OC is presented. The simulations are carried out with the fixed spectral control filter and for the radiator temperature of T_rad = 950 °C, cell temperature of T_dio = 27 °C, the radiation photons are injected from the front P-region. The thick P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter with the longer minority carrier diffusion length is the main optical absorption region. The simulated results are compared with the available experimental data, and a good agreement is obtained. The effects of the layer thickness, carrier concentration, injection level and main recombination mechanisms (e.g. the radiative, Auger, bulk Shockley–Read–Hall (SRH) and surface recombination) of the P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter and N-Ga_0.8In_0.2As_0.18Sb_0.82 base on V_OC are analyzed. It indicates that the parameters of the emitter region have stronger effect than that of the base region on V_OC. Dependence of V_OC on the material parameters of P-GaSb window layer is also analyzed, both the carrier concentration and thickness of P-GaSb window layer have effect on V_OC. Moreover, adding a back surface reflector (BSR) to the TPV cell can increase V_OC.     

Effects on structure and superconductivity for La and Ca codoping in YBa2Cu3Oy cuprates

The nominal composition of Y_0.8Ca_0.2Ba_2−xLa_xCu₃O_y (YBLCO) cuprates with x≤0.50 has been synthesized by the standard solid state reaction technique. X-ray diffraction and the resistivity measurements are used to characterize the structure and the superconductivity of YBLCO cuprates. There is no structural phase transition in the whole doping range. The dependencies of the lattice constants and some other structural parameters on the content of La for the samples YBLCO with x≤0.20 are different than those for the samples with x≥0.25. The zero resistance temperature T_c0 increases with the increase of the content of La in YBLCO as x≤0.20, and decreases as x≥0.25. We compared these results with those of Nd-doped Y_0.8Ca_0.2Ba_2−zNd_zCu₃O_y cuprates. It seems that T_c0 is related to the structural parameters due to Ca and La codoping in YBLCO.