Performance study of InAs/GaAs quantum dot covered by graded InxGa1 − xAs layer

InAs/GaAs quantum dots (QDs) with graded In_xGa_1 − xAs strained-reducing layer (SRL) are grown by metal-organic chemical vapor deposition, the effects of Indium (In) composition and thickness in In_xGa_1 − xAs on QD morphological characteristics and optical properties are investigated. Compared with In_xGa_1 − xAs SRL with fixed In content, gradient In_xGa_1−xAs SRL can further improve the growth quality of InAs QDs, enhance luminescence intensity and extend emission spectrum toward longer wavelength.     

Principal band gaps and bond lengths of the alloy (AlxGa1−x)1−zInzPyAs1−y lattice matched to GaAs

The principal band gaps (E(Γ),E(L), and E(X)) and bond lengths (d(x,y,z))of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y (where, 0 < x + z < 1, and 0 < y < 1) are calculated over the entire composition range based on the first order correlated function expansion (CFE) scheme. Defining the lattice strain parameter as , it is found that a good quality of alloy (defining ? < ∼ 0.5%) can be obtained in the composition region : 0 < x < ∼ 0.3, 0 < y < ∼ 0.2 and 0 < z < ∼ 0.1. The first order CFE lattice matching relations and corresponding band gaps for the alloy on the GaAs substrate are also determined. It is found that the principal band gaps of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y lattice matched to GaAs covers band gap ranges: 1.45 eV E < (Γ) 2.69 eV, 1.80 eV < E(L) < 2.38 eV, and 1.97 eV < E(X) < 2.20 eV, while the direct band gap covers from 1.45 eV to 2.05 eV. Our theoretical prediction was compared with the existing experimental data.     

Effect of GaAs substrate misorientation on InxGa1 − xAs crystalline quality and photovoltaic performance

This paper presents the quality of In_xGa_1 − xAs (0 < x < 0.2) layers grown on GaAs substrate with different miscut angle (2° and 15°) by metal organic chemical vapor deposition. The crystalline quality of In_xGa_1 − xAs layers was found to strongly depend on indium content and substrate misorientation. The In_0.16Ga_0.84As solar cells with PN structure were grown on a 2°- and 15°-off GaAs substrates. It was found that the photovoltaic performance of In_0.16Ga_0.84As solar cell grown on 2°-off GaAs substrate was better than that of In_0.16Ga_0.84As grown on a 15°-off GaAs substrate, because the In_xGa_1 − xAs films grown on 15°-off GaAs substrate shows a highly strain relaxation in active layer of solar cell, which causes the high dislocation density at the initial active layer/In_xGa_1 − xAs graded layer interface.     

Quantum-confinement effect on recombination dynamics and carrier localization in cubic InN and InxGa1 − xN quantum boxes

In this study, the quantum confinement effect on recombination dynamics and carrier localization in cubic InN (c-InN) and cubic In_xGa_1 − xN (c-In_xGa_1 − xN) low dimensional structures are theoretically examined. The small InN and In-rich In_xGa_1 − xN low dimensional structures show a strong quantum confinement effect, which results in ground states away from the band edge and discrete eigen-states. Depending on composition, temperature, and size of the InN and In_xGa_1 − xN low dimensional structures, quantum confinement effect can affect the exciton dimensions (D). In InN quantum cubes, the strong quantum confinement effect leads to temperature-dependent radiative lifetimes showing a large size variation. The nearly-temperature-independent and shorter radiative lifetimes in small InN and In-rich In_xGa_1 − xN low dimensional structures suggest that the strong quantum confinement leads to 0 D carrier confinement, stronger carrier localization, and high recombination efficiency. Reported radiative lifetimes were found to match very well with our simulation results of In-rich quantum cubes, which indicates that spontaneous emissions come from the radiative recombination of localized excitons in In-rich In_xGa_1 − xN clusters. The simulation results could provide important information for the designs and interpretations of c-InN and c-In_xGa_1 − xN devices.     

Band offsets of InXGa1−XN/GaN quantum wells reestimated

Recently In_XGa_1−XN/GaN heterostructures and quantum wells (QWs) have gained immense importance in the application of III-V nitride materials. Reported values of the ratios of conduction band offset to valence band offset for In_XGa_1−XN/GaN QW structures, ΔE_c:ΔE_v, vary widely from 38:62 to 83:17. While trying to explain the unusual shifts in the photoluminescence (PL) spectra, obtained from In_XGa_1−XN/GaN QW structures, it has been found that a band offset ratio, ΔE_c:ΔE_v = 55:45, explains all the experimental data precisely. In this paper detailed theories, procedures, results and discussions to establish the newly estimated band offsets will be presented.     

Optical properties of nanocrystalline Ga1−xInxSb/SiO2 films

Ga_1−xIn_xSb (x=0.19, 0.38, 0.63) nanoparticles embedded in a SiO₂ matrix were grown on the glass substrates by radio-frequency magnetron co-sputtering. X-ray diffraction patterns strongly support the existence of nanocrystalline Ga_1−xIn_xSb in the SiO₂ matrix. The changes in binding energies with Ga_1−xIn_xSb nanocrystals deposition have been directly observed by X-ray photoemission spectroscopy, and these show the existence of Ga_1−xIn_xSb nanocrystals in the SiO₂ matrix. Room-temperature Raman spectra show that the Raman peaks of the Ga_1−xIn_xSb-SiO₂ composite film have a larger red-shift of about 95.3 cm⁻¹ (longitudinal-optical mode) and 120.1 cm⁻¹ (transverse-optical mode) than that of bulk GaSb, suggesting the existence of phonon confinement and tensile stress effects. Additionally, the room-temperature optical transmission data exhibit a large blue-shift with respect to that of the bulk semiconductor due to the strong quantum confinement effect.     

Unusual changes observed in the photoluminescence of annealed InxGa1 − xN/GaN quantum wells explained

In_xGa_1 − xN/GaN heterostructures and quantum wells (QWs) are particularly important in the application of III-V nitride materials for light emitting diodes and laser diodes. The photoluminescence (PL) emissions from In_xGa_1 − xN/GaN QW structures have been reported, where, for successive annealing operations, the PL peak suffers a primary red shift, followed by a blue shift. The observed phenomenon remains unexplained because of its complexity. This paper is intended towards a proper explanation of the observed experimental results through suitable quantum mechanical models and computations, whether the band gap of InN is 1.95 eV or 0.7 eV.     

Characterization of AlxGa1−xAs layers grown on (100) GaAs by metallic-arsenic-based-MOCVD

We present the electrical and structural characterization of Al_xGa_1−xAs layers grown in a metallic-arsenic-based-MOCVD system. The gallium and aluminium precursors were the metal-organic compounds trimethylgallium (TMGa) and trimethylaluminium (TMAl), respectively. Al_xGa_1−xAs layers that were grown at temperatures less than 750 °C present a high electrical resistivity. Independent of the used III/V ratio the samples that were grown at temperatures greater that 750 °C were n-type with an electron concentration of around 10¹⁷ cm⁻³ and a carrier mobility of 2200 cm²/V-s. Chemical composition studies by SIMS exhibit the presence of silicon, carbon and oxygen as the main residual impurities. Silicon concentration of around of 10¹⁷ cm⁻³ is very close to the free carrier concentration determined by the Hall-van der Pauw measurements. Composition homogeneity and structural quality are demonstrated by Raman measurements. As the growth temperature is increased the layers compensation decreases but the Raman spectra show that the crystalline quality of the layers diminishes.     

Tartarate precursors to CoxZn1−xGa2O4 spinel oxides

The Co_xZn_1−xGa₂O₄ spinels (x = 0.25, 0.5, 0.75, 1) were synthesized by a wet chemistry method–the precursor route via tartarate decomposition. The complex precursors have been characterized by IR, UV–Vis, magnetic measurements and luminescence spectroscopy. XRD, SEM, IR, UV–Vis and luminescence spectroscopy were used for the structural and morphological investigation of Co_xZn_1−xGa₂O₄ spinels; magnetic susceptibilities of the spinel were also measured. The X-ray diffraction patterns confirmed the formation of gallate spinel phase, Co_xZn_1−xGa₂O₄.     

Band offset calculations of ZnSxSe1−x/ZnSySe1−y heterostructures

In order to design devices based on II-VI materials, it is necessary to know the potential across the interface between two materials. Following our recent calculations which prove that the band gap energy of ZnS_xSe_1−x alloy has a nonlinear behaviour versus the sulphur composition x, it appears that an accurate knowledge of band offsets for ZnS_xSe_1−x/ZnS_ySe_1−y structures will be useful to model devices based on this heterostructure. On the basis of a model-solid theory, we report in this work the band offset calculations for zinc blende pseudomorphically strained ZnS_xSe_1−x/ZnS_ySe_1−y interface. From the results obtained, we have calculated the band gap energies of ZnS_xSe_1−x layers pseudomorphically strained on ZnS_ySe_1−y substrate as a function of compositions x and y in the whole range 0 ≤ x,y ≤ 1. Also, the band gaps of bulk ZnS_xSe_1−x deposed on ZnS_ySe_1−y for several values of y have been calculated versus the sulphur content x. Analytical formulas fitting these bands have been obtained. In view of the lack of theoretical calculations, our results seem likely to be useful especially in the design of ZnS_xSe_1−x structures for optoelectronic devices applications.     

Competitive growth mechanisms of InAs quantum dots on InxGa1 − xAs layer during post growth interruption

We investigated the effect of the post growth interruption (GI) on InAs quantum dots (QDs) grown on In_xGa_1 − xAs strained buffer layers (SBL). When QDs were grown on the 5 and 10% In content SBLs by using post GI, the size of QDs increased as its density decreased. Based on the 50 meV red-shift of PL in these cases, the transport of materials between QDs leads to the increase of QD size with maintaining its composition during the post GI. On the other hand, when using SBLs with the 15 and 20% In contents, the size of QDs increased, but its density was a little reduced. In addition, PL results were observed blue-shifted by about 20 meV and 2 meV, respectively. Considering the interruption of source gases during the post GI, these observations are strong evidence of the Ga incorporation from 15 and 20% In content SBLs. Therefore, these results imply that the dominant mechanism which increases the size of QDs during the post GI depends on the growth condition of SBL.     

Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.     

Impurity induced band-gap narrowing in p-type CuIn1 − xGax(S,Se)2

Green's functions modelling of the impurity induced effects in p-type CuIn_1 − xGa_xS₂ and CuIn_1 − xGa_xSe₂ (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at N_c ≈ 10¹⁷-10¹⁸ cm^− 3 for impurities with ionization energy of E_A ≈ 30-60 meV. (ii) For acceptor concentrations N_A > N_c, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E_g = E_g⁰ − ΔE_g. The energy shift of the valence-band maximum ΔE_v1 is roughly twice as large as the shift of the conduction-band minimum ΔE_c1. (iii) ΔE_v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N_A = 10²⁰ cm^− 3 implies a band-gap narrowing in the order of ΔE_g ≈ 0.2 eV, thus E_g = E_g⁰ − 0.2 eV, and an optical band gap of E_g^opt ≈ E_g⁰ − 0.1 eV.     

Effect of temperature and pressure on the electronic structure of GaxIn1−xAsyP1−y alloys lattice matched to GaAs substrate

Based on the empirical pseudo-potential within the virtual crystal approximation including the effective disorder potential, calculations of the electronic structure of the zinc blende quaternary Ga_xIn_1−xAs_yP_1−y alloys have been studied. The direct and indirect energy gaps of the considered alloys have been determined over the entire composition x and y in condition of being lattice matched to GaAs substrate. In addition, the effect of temperature and hydrostatic pressure of these energy gaps have been studied. Comparison of the calculated results with the experimental and published data showed good agreement.     

High-frequency magneto-electrical properties of Zn1 − x − yAlxCoyO thin films

The Al doping effects on high-frequency magneto-electric properties of Zn_{1 − x − y}Al_xCo_yO (x = 0-10.65 at.%) thin films were systematically studied. In the current work, the Zn_{1 − x − y}Al_xCo_yO thin films were deposited by magnetron co-sputtering onto quartz substrates. The magneto-impedance spectra of the thin films were measured by an impedance analyzer. Among all the doped films studied, the thin film with 6.03 at.% Al-doping showed the highest ac conductivity and relaxation frequency. To characterize the relaxation mechanism underlying the magneto-electric properties, a Cole-Cole impedance model was applied to analyze the impedance spectra. The analyzed result showed that the magneto-impedance of the Zn_{1 − x − y}Al_xCo_yO is contributed by multiple processes of magnetization dynamics and dielectric relaxation. The results imply that Zn_{1 − x − y}Al_xCo_yO may be applicable for high-frequency magneto-electric devices.     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Optical and electrical properties of vapour phase grown Zn1 − xCrxTe crystals

The optical and electrical properties of vapour phase grown crystals of diluted magnetic semiconductor Zn_1 − xCr_xTe were investigated for 0 ≤ x ≤ 0.005. The diffuse reflectance spectra exhibited an increase in the fundamental absorption edge (E₀) with composition x and were also dominated by a broad absorption band around 5200 cm^− 1 arising from ⁵T₂ → ⁵E transition. The ⁵T₂ and ⁵E levels originate from the crystal field splitting of the ⁵D free ion in the ground state. The electrical resistivity measurements revealed semiconducting behaviour of the samples with p-type conductivity in the temperature range of 200-450 K.     

Electronic properties of GaxIn1−xP from pseudopotential calculations

Based on a pseudopotential scheme within the virtual crystal approximation, we present a theoretical investigation of the electronic properties of Ga_xIn_1−xP. The effect of alloy disorder on energy band-gaps has been examined and found to be not negligible. The composition dependence of energy band-gaps and electron effective mass is shown to be non-linear. In agreement with experiment, a direct-to-indirect band-gap crossover is found to occur close to x = 0.7. Besides, the electron valence and conduction charge densities for Ga_0.50In_0.50P derived from pseudopotential band-structure calculations are reported and trends in bonding and ionicity are discussed.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

High-Q X-ray scattering study of InxGa1 − xN/GaN multi-quantum wells

We present a high momentum transfer (Q) X-ray scattering method to determine the thickness and indium composition of an In_xGa_1 − xN well in InGaN/GaN multiple quantum wells. At high-Q, it is demonstrated that the scattering signal from InGaN well layers is separated from that of GaN barrier layers. The structure factor of the well layers is determined from the envelope of the superlattice reflections. The thickness and the indium composition of the well layer are obtained directly from the structure factor. We discuss the high-Q analysis in comparison with the analysis of low-Q data where the scattering from InGaN well and GaN barrier interferes strongly.