Effect of Interfacial Bonds on the Morphology of InAs QDs Grown on GaAs (311) B and (100) Substrates

The morphology and transition thickness (t _c) for InAs quantum dots (QDs) grown on GaAs (311) B and (100) substrates were investigated. The morphology varies with the composition of buffer layer and substrate orientation. And t _c decreased when the thin InGaAs was used as a buffer layer instead of the GaAs layer on (311) B substrates. For InAs/(In)GaAs QDs grown on high miller index surfaces, both the morphology and t _c can be influenced by the interfacial bonds configuration. This indicates that buffer layer design with appropriate interfacial bonds provides an approach to adjust the morphologies of QDs grown on high miller surfaces.     

Electrical characterisation of deep level defects in Be-doped AlGaAs grown on (100) and (311)A GaAs substrates by MBE

The growth of high mobility two-dimensional hole gases (2DHGs) using GaAs-GaAlAs heterostructures has been the subject of many investigations. However, despite many efforts hole mobilities in Be-doped structures grown on (100) GaAs substrate remained considerably lower than those obtained by growing on (311)A oriented surface using silicon as p -type dopant. In this study we will report on the properties of hole traps in a set of p -type Be-doped Al_0.29Ga0_.71As samples grown by molecular beam epitaxy on (100) and (311)A GaAs substrates using deep level transient spectroscopy (DLTS) technique. In addition, the effect of the level of Be-doping concentration on the hole deep traps is investigated. It was observed that with increasing the Be-doping concentration from 1 × 10¹⁶ to 1 × 10¹⁷ cm^-3 the number of detected electrically active defects decreases for samples grown on (311)A substrate, whereas, it increases for (100) orientated samples. The DLTS measurements also reveal that the activation energies of traps detected in (311)A are lower than those in (100). From these findings it is expected that mobilities of 2DHGs in Be-doped GaAs-GaAlAs devices grown on (311)A should be higher than those on (100).     

InGaAs Quantum Well Grown on High-Index Surfaces for Superluminescent Diode Applications

The morphological and optical properties of In_0.2Ga_0.8As/GaAs quantum wells grown on various substrates are investigated for possible application to superluminescent diodes. The In_0.2Ga_0.8As/GaAs quantum wells are grown by molecular beam epitaxy on GaAs (100), (210), (311), and (731) substrates. A broad photoluminescence emission peak (~950 nm) with a full width at half maximum (FWHM) of 48 nm is obtained from the sample grown on (210) substrate at room temperature, which is over four times wider than the quantum well simultaneously grown on (100) substrate. On the other hand, a very narrow photoluminescence spectrum is observed from the sample grown on (311) with FWHM = 7.8 nm. The results presented in this article demonstrate the potential of high-index GaAs substrates for superluminescent diode applications.     

Solid-Phase Synthesis of Non-metal (S,N)-Doped Tin Oxide Nanopowders at Room Temperature and its Photodegradation Properties for Wastewater of Biomass Treatment

Tin oxide and sulfur, nitrogen-doped tin oxide nano-powder catalysts were prepared by a solid phase reaction at room temperature, using the sodium p-toluene sulfonate (STS) surfactant as template. Theoretical calculation of the dehydration reaction energy of tin hydroxide was performed with the framework of DFT and their structures were characterized. And the UV-light degradation performance and mechanism used for the biomass wastewater were discussed, as well as, its COD and NH₃-N value. The results show that the large gap of the reaction energy between intramolecular dehydration (Er?=?2.81 eV) and intermolecular dehydration (Er?=?5.77 eV) for tin hydroxide causes the presence of amorphous SnO₂ and metastable tin hydroxide at 450 °C. The entry of S and N into the (110) crystal plane of SnO₂ reduces its energy band gap width, exhibiting the photocatalytic degradation rate (98.9%) of S?+?N-SnO₂-_STS sample for the rice straw powder treatment wastewater (RSPTW) irradiated by UV-light for 8 h. The excellent degradation capacity of RSPTW mainly comes from the hydroxyl radicals (·OH) and superoxygen radicals (·O₂^?) produced by the rich hydroxyl on the surface of S?+?N-SnO₂-_STS due to the regulatory effect of STS and lower calcined temperature. The sewage discharge of photodegraded RSPTW complies with Chinese National Level II Standards.

     

Space‐charge limited conduction in polyaniline films

Polyaniline emeraldine base (PANI‐EB) powder was synthesized by oxidative polymerization of aniline. The PANI‐EB films were prepared by the solution‐casting technique. The temperature‐dependent dc conductivity measured in the range 173–303 K suggests that the PANI‐EB is a quasi‐one‐dimensional disordered conductor. The current‐voltage characteristics of the PANI‐EB films measured in the range 333–383 K showed the SCLC mechanism. The SCLC parameters such as free carrier density (p₀), trap density (p_t), the ratio between free carrier density to the total carrier density (θ), mobility (µ) and the effective hole mobility (µ_eff) were calculated. The activation energy (E_a = 0.32 eV) and the Fermi level (E_F = 0.42 eV) were estimated. As well as these, the trap parameters such as the trap filled limit voltage (V_TFL), the shallow trap density (N_t), the depth of the dominant trap level (E_t ? E_v), the density of states within the hole mobility edge (N_v) and the characteristic energy (E_c) were also calculated and presented. The exponential type of traps distribution with large number of traps was found to be due to the disorder and moisture in the polymer films. Copyright © 2004 Society of Chemical Industry     

Photovoltaic Properties of p-Doped GaAs Nanowire Arrays Grown on n-Type GaAs(111)B Substrate

We report on the molecular beam epitaxy growth of Au-assisted GaAs p-type-doped NW arrays on the n-type GaAs(111)B substrate and their photovoltaic properties. The samples are grown at different substrate temperature within the range from 520 to 580 °C. It is shown that the dependence of conversion efficiency on the substrate temperature has a maximum at the substrate temperature of 550 °C. For the best sample, the conversion efficiency of 1.65% and the fill factor of 25% are obtained.     

Low temperature reaction of point defects in ion irradiated 4H–SiC

The low temperature evolution of point defects induced in SiC by ion irradiation was investigated by deep level transient spectroscopy. The defects were introduced by irradiation with a 7.0 MeV beam of C⁺ ions at a fluence of 6 × 10⁹ cm^? 2. Annealing was then performed in the temperature range of 330–400 K in order to study the change in point defect structure with temperature. The low temperature annealing performed was observed to induce a change in the produced defects. The deep levels related to the S_x (E_C ? 0.6 eV) and S₂ defects (E_C ? 0.7 eV) recovered with annealing while, simultaneously, a new level, S₁ (E_C ? 0.4 eV), was formed. The activation energy of the S₁ defect is 0.94 eV, while the annealing of both the S_x and S₂ levels occurred with activation energy of 0.65 eV.     

Electrical properties of thick boron and nitrogen contained CVD diamond films

The acceptor and donor defects of thick (approx. 0.4 mm) free-standing boron and nitrogen containing microwave plasma CVD polycrystalline diamond films were investigated. Charge-based deep level transient spectroscopy (Q-DLTS) was applied to study impurity-induced defects, their density and energy distribution in the energy range of 0.01 eV≤E−E_v≤1.1 eV above the valence band. It was shown, that differential capacitance–voltage, and Hall effect measurements combined with DLTS data can be used to determine the degree of compensation, and the concentration of compensating donors (mostly the positively charged single-substitutional nitrogen (N⁺)) in p-type CVD polycrystalline diamond films. It was found, that incorporated boron atoms induce three levels of electrically active defects. Two of them with concentration (2–3)×10¹⁶ cm⁻³ each have activation energies of 0.36 and 0.25 eV with capture cross-sections of 1.3×10⁻¹³ and 4.5×10⁻¹⁹ cm², respectively. The third type of defect has an activation energy of 0.02 eV, capture cross-section 3×10⁻²⁰ cm² and concentration 10¹⁵ cm⁻³, this shallow trap being a probable general caterer of holes in low-doped films. The total concentration of electrically active uncompensated acceptors in all p-type diamond samples was approximately 2×10¹⁷ cm⁻³ with hole concentration of approximately 1.5×10¹⁴ cm⁻³ and hole mobility in the range of 30–40 cm² V⁻¹ s⁻¹ at room temperature. If assumed that compensating donors are mostly nitrogen, the films contained no less than 3×10¹⁶ cm⁻³ of N⁺.     

CaMnxZr(1-x)O3: A novel NTC thermo-sensitive ceramic for applications in a wide temperature range

The perovskite CaMn_xZr_(1-x)O₃ (x = 0, 0.05, 0.10, 0.15) (CMZO) NTC (Negative Thermal Coefficient) thermo-sensitive ceramic were prepared by the solid phase method that can be used as high temperature thermistor materials in the range of 200–1200 °C.The XRD pattern showed that CMZO is a finite solid solution and that a second phase, CaMnO₃, will be formed after exceeding the solid solution limit (i.e., at x = 0.1). The thermal constant B value was found to decrease from 19521 K to 9217–9535 K, with a corresponding decrease in activation energy, E_a, from 1.69 eV to 0.82 eV. The Nyquist plot indicates that the grain boundary resistance determines the overall complex impedance. Moreover, the aging drift rate (ΔR/R₀) was found to be lower than 4.3%, which is an indication of an excellent high temperature stability of the CMZO ceramic material.     

Phase,domain, and microstructures in Sr2+ substituted low-temperature sintering PZT-based relaxor ferroelectrics

The Ag-Pd internal electrode of multilayer piezoelectric ceramics needs to be sintered below 1000°C, and lead wires and components need to be welded with lead-free solder at 260°C. PNN–PMW–PZT–xSr piezoelectric ceramics with high Curie temperature (T_c > 260°C) were synthesized at a low sintering temperature (960°C) to meet the requirements of multilayer piezoelectric devices. The relationship between structures (phase, domain, and microstructures) and electrical properties (piezo/ferroelectric properties, and dielectric relaxation) in the Sr²⁺ substituted ceramics was investigated. Rietveld refinement and Raman spectra show that Sr²⁺ substitution can cause the phase change and increase the force constant of [BO₆] octahedron. The piezoelectric response increases with increasing the content of the tetragonal phase (CTP) in the rhombohedral-tetragonal (R-T) coexisted ceramics. The ceramics with 0.6 mol% Sr²⁺ substitution have minimum activation energy for domain wall movement (E_a) of 0.0362 eV which favors the formation of nanometer-sized domains, and possess excellent electrical properties (d₃₃ = 623 pC/N, d₃₃^* =783 pm/V, T_c =295°C). The higher the CTP, the lower the E_a. The lower E_a favors the rotation of polarization direction and extension, and is beneficial to the generation of the nanometer-size domains, resulting in high piezoelectric properties.     

Raman scattering of InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces

ABSTRACT: We present a comparative analysis of Raman scattering by acoustic and optical phonons in InAs/AlAs quantum dot superlattices grown on (001) and (311)B GaAs surfaces. Doublets of folded longitudinal acoustic phonons up to the fifth order were observed in the Raman spectra of (001)- and (311)B-oriented quantum dot superlattices measured in the polarized scattering geometries. The energy positions of the folded acoustic phonons are well described by the elastic continuum model. Besides the acoustic phonons, the spectra display features related to confined transverse and longitudinal optical as well as interface phonons in quantum dots and spacer layers. Their frequency positions are discussed in terms of phonon confinement, elastic stress, and atomic intermixing.     

Optical band gap shift in thin LiNbO3 films grown by radio-frequency magnetron sputtering

Thin polycrystalline LiNbO₃ films were deposited by the radio-frequency magnetron sputtering (RFMS) method and ion-beam sputtering (IBS) method under different conditions. Study of the adsorption band edge of fabricated films reveals direct and indirect optical transition. Depending on the particular technological sputtering RFMS regime, the direct energy gap varies from 3.8 to 4.4 eV. Band tails induced by the defects formation due to the reactive plasma effect on the film structure are responsible for indirect optical transitions in the studied films. Thermal annealing has a prominent effect on trap concentration and strain in as-grown films leading to rise in direct band energy up to 4.4 eV which is close to the value for bulk LiNbO₃.     

Surface Morphology Evolution of GaAs by Low Energy Ion Sputtering

Low energy Ar⁺ ion sputtering, typically below 1,200 eV, of GaAs at normal beam incident angle is investigated. Surface morphology development with respect to varying energy is analyzed and discussed. Dot-like patterns in the nanometer scale are obtained above 600 eV. As the energy approaches upper eV range regular dots have evolved. The energy dependent dot evolution is evaluated based on solutions of the isotropic Kuramoto-Sivashinsky equation. The results are in agreement with the theoretical model which describes a power law dependency of the characteristic wavelength on ion energy in the ion-induced diffusion regime.     

Application of the Weibull distribution function for modeling the kinetics of isothermal dehydration of equilibrium swollen poly (acrylic acid) hydrogel

A theoretical procedure for evaluating the density distribution function (p(E_a)) of apparent activation energies (E_a), based on modeling of the experimental isothermal conversion curves by Weibull probability function of reacting times was established. This procedure was applied on isothermal dehydration of poly (acrylic acid) hydrogel (PAAH) at four different operating temperatures (306, 324, 345 and 361 K). It was established that: (a) the experimental conversion curves at all temperatures can be described by the Weibull probability distribution function of reacting times in a wide range of the degree of conversion (0.14 ? α ? 0.95); (b) the Weibull distribution parameters (β and η) show functional dependences on the operating temperature, and (c) the apparent activation energy (E_a) varies with the degree of conversion (α). It was shown that the density distribution function of activation energies is independent on the temperature, and because of this fact, the considered function is an Eigen characteristic of the dehydration studied. The calculated p(E_a) function is in good agreement with the experimental p(E_a) function (p(E_a)_exp), obtained by the Miura procedure.     

A graphene flake under external electric fields reconstructed from field-perturbed kernels

The energy, dipole moment, and polarizability of a finite hydrogen terminated zigzag graphene flake (C₄₆H₂₀, in 2 × 7 rings) are calculated in the absence of and in the presence of external electric fields reaching 5 × 10⁹ Vm⁻¹ [=0.01 atomic units (a.u.)]. The field intensities studied are typically found in nanoelectronics and in the tip-sample gap of a scanning tunneling microscope (STM). The change in the total energy ΔE(eV) can be closely fitted to a quadratic function of the field {1.549 × 10⁴ [E (a.u.)]²} while the dipole moment μ(debye) to a linear function [2.7 × E (a.u.)]. The results obtained with three different chemical models [MP2, density functional theory (B3LYP), and Hartree–Fock calculations, all with a 6-311G(2d,2p) basis set] are consistent in both trends and in absolute values. These results obtained from direct calculations are reproduced with a remarkable accuracy from a linear scaling fragmentation scheme called the kernel energy method (KEM). The KEM reproduces all studied field-free and response properties of this graphene flake model, in relative and absolute terms, independently of the underlying chemical model. An observation consistent with the known stiffness of graphene is that geometry optimization under a field as strong as 0.01 a.u. insignificantly alters the total energy and the geometry of a (smaller) zigzag C₂₈H₁₄ flake, the difference in the field-induced stabilization energy (ΔΔE) being only 0.006 eV (less than 1% of ΔE) and the average field-induced displacement of nuclear positions ∼0.0046 Å [B3LYP/6-31G(d,p)].     

Experimental and computational studies of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO)-oligomers

4H-Cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO), CPDTO ketal (CPDTO-k) and their oligomers have been synthesized and their optical absorption, fluorescent, electrochemical properties are characterized. Structure optimization and time-dependent energy calculation have been carried out for (CPDTO-k)_1,2,3,6 and (CPDTO)_1,2,3,6 using the B3LYP functional and standard split valence plus polarization basis set 6-31G(d,p) in the DFT formalism. The pi conjugation between carbonyl oxygen and thiophene, which is orthogonal to the backbone conjugation, is evident from the DFT calculation and analysis, and is responsible for the observed slower narrowing of bandgaps (E_g) of CPDTO oligomers with increasing number of repeat units. The orthogonal conjugation also leads to different distribution of LUMO from that of HOMO and makes CPDTO oligomers weak in the lowest energy absorption. Enhanced intensity and red-shifted peak wavelength of the lowest energy UV–vis absorption of CPDTO film suggest the presence of strong intermolecular interactions among CPDTO molecules in the solid state, which may explain why the bandgap (1.1–1.2 eV) reported for the electrochemically deposited CPDTO homopolymer film is significantly lower than the bandgap (1.5–1.7 eV) estimated in this study for PCPDTO chloroform solution from the E_gs of CPDTO oligomers.     

Highly (00l)-textured BiFeO3 thick films integrated on stainless steel foils with an optimized piezoelectric performance

Piezoelectric BiFeO₃ films were successfully prepared on 304 stainless steel (SS) foils via RF-magnetron sputtering at 450 °C. By adopting a LaNiO₃/Pt/Ti tri-layer bottom electrode, a predominant (100)-orientation and a better crystalline morphology were achieved in the BiFeO₃ film, resulting in better ferroelectric and dielectric properties targeted for piezoelectric applications than those directly grown on SS or LaNiO₃/SS. These properties include a large remnant polarization (P_r~68 μC/cm²), a large coercive field (E_c~241 kV/cm) and a sizable self-bias (E_bi~ 158 kV/cm), as well as a low dielectric constant and a reduced dielectric loss (ε_r≤200 and tan δ ≤ 0.026 in 1 kHz-1 MHz). Based on this optimal BiFeO₃-SS heterostructure, large converse and direct transverse piezoelectric coefficients |e_31,f| ~ 1.7 C/m² and 1.07 C/m² were achieved in prototype piezoelectric actuator and energy harvester structures, respectively. These results suggest that BiFeO₃-SS heterostructures have a great potential for applications in lead-free, metal-based piezoelectric micro-electro- mechanical systems (piezo-MEMS).     

Free-exciton luminescence spectrum broadening due to excitonic complex in diamond

The excitation density evolution of a band-edge luminescence spectrum of diamond in the photon energy region 5.14 to 5.40 eV was investigated at 190, 240 and 290 K with the fifth harmonic (5.82 eV) of a pulsed YAG laser in the excitation density range 2.2 to 272 mJ/cm². A broadening of a free-exciton luminescence (FE) spectrum with the peak energy (E_peak) of 5.28 eV due to the growth of the low-energy tail was observed with increasing excitation density. From a detailed spectrum analysis, it is found that the broadening originates in excitonic complex (EC) luminescence (E_peak = 5.26 eV) in addition to electron-hole plasma (EHP) luminescence (E_peak = 5.23 eV). The EC luminescence intensity was observed to be proportional to the 1.5 power of the FE luminescence intensity, which is the same manner as in the case of the EC luminescence in crystalline silicon. The excitation density dependence of the EHP luminescence intensity is discussed with percolation theory.     

The anisotropic conductivity of ferroelectric La2Ti2O7 ceramics

Ferroelectric ceramics with perovskite-like layered structure (PLS) have good potential for high temperature piezoelectric applications due to their high Curie point (T_c). The electrical conduction behaviour of these materials is a critical parameter to consider for practical applications. In this study, we prepared textured ceramics of the typical PLS ferroelectric La₂Ti₂O₇ using spark plasma sintering and investigated the electrical properties using impedance spectroscopy and Seebeck measurements. The results reveal that the bulk resistivity along the parallel direction is much higher than that along the perpendicular direction. The activation energy for conduction (E_a) along the parallel direction is 1.45 eV, which is close to half of the optical band gap and much higher than the 0.67 eV along the perpendicular direction. Electrical conduction along both the directions is dominated by p-type hole conduction. No appreciable contribution of oxide ion conduction to the measured conductivity is observed.     

Physical insights into the grain size effect on the electrical properties of nanocrystalline La2Zr2O7 pyrochlores

The role of grain size on the electrical properties of nanocrystalline La₂Zr₂O₇ (LZO) ceramics prepared by chemical co-precipitation is reported. Grain size from 10 to 70 nm was obtained. The XRD and Raman scattering showed a pyrochlore structure of La₂Zr₂O₇. Morphology and chemical analysis were done by SEM and XPS, respectively. Electrical conductivity studies using impedance spectroscopy resolved the conductivity borne to grain and grain boundary with a non-Debye nature of conduction. The activation energy (E_a) for electrical conduction through grain was almost the same for all the grain sizes and equal to 1.20 ± 0.03 eV. However, the E_a of the grain boundary had decreased from 1.17 eV to 0.78 eV with an increase in grain size, and the conduction is due to oxygen ion migration. The conduction mechanism was governed by the non-overlapping small polaron tunneling model. Additionally, ion-ion correlation in conduction is increased with increasing grain size.