Showing papers in "Journal of Applied Physics in 2013"
TL;DR: Puurunen et al. as discussed by the authors summarized the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD.
Abstract: Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.
1,160 citations
TL;DR: In this paper, phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied and the coercivity values measured in both temperature intervals suggest a significant difference in the behavior of the two materials.
Abstract: Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...
940 citations
TL;DR: Nanofluids have seen enormous growth in popularity since they were proposed by Choi in 1995 as mentioned in this paper, and there were nearly 700 research articles where the term nanofluid was used in the title, showing rapid growth from 2006 (175) and 2001 (10).
Abstract: Nanofluids—a simple product of the emerging world of nanotechnology—are suspensions of nanoparticles (nominally 1–100 nm in size) in conventional base fluids such as water, oils, or glycols. Nanofluids have seen enormous growth in popularity since they were proposed by Choi in 1995. In the year 2011 alone, there were nearly 700 research articles where the term nanofluid was used in the title, showing rapid growth from 2006 (175) and 2001 (10). The first decade of nanofluid research was primarily focused on measuring and modeling fundamental thermophysical properties of nanofluids (thermal conductivity, density, viscosity, heat transfer coefficient). Recent research, however, explores the performance of nanofluids in a wide variety of other applications. Analyzing the available body of research to date, this article presents recent trends and future possibilities for nanofluids research and suggests which applications will see the most significant improvement from employing nanofluids.
679 citations
TL;DR: In this article, the authors classified and reviewed the physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation.
Abstract: Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies.
371 citations
TL;DR: In this paper, a theoretical model was developed based on the nonlocal empirical pseudopotential method to determine the electronic band structure of germanium tin (GeSn) alloys, and modifications to the virtual crystal potential accounting for disorder induced potential fluctuations were incorporated to reproduce the large direct band gap bowing observed in GeSn alloys.
Abstract: GeSn is predicted to exhibit an indirect to direct band gap transition at alloy Sn composition of 6.5% and biaxial strain effects are investigated in order to further optimize GeSn band structure for optoelectronics and high speed electronic devices. A theoretical model has been developed based on the nonlocal empirical pseudopotential method to determine the electronic band structure of germanium tin (GeSn) alloys. Modifications to the virtual crystal potential accounting for disorder induced potential fluctuations are incorporated to reproduce the large direct band gap bowing observed in GeSn alloys.
368 citations
TL;DR: In this paper, a parameterization of the Stillinger-Weber potential is presented to describe the interatomic interactions within single-layer MoS2 (SLMoS2) nanoribbons.
Abstract: We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2.
360 citations
TL;DR: In this paper, the authors employ ab-initio electronic structure calculations to study 60 LiMgPdSn-type quaternary Heusler compounds, including half-metals, spin-gapless semiconductors, and 9 semiconductor types.
Abstract: We employ ab-initio electronic structure calculations to study 60 LiMgPdSn-type (also known as LiMgPdSb-type) quaternary Heusler compounds. All compounds obey the Slater-Pauling rule with diverse electronic and magnetic properties. 41 compounds are found to be half-metals, 8 spin-gapless semiconductors, and 9 semiconductors. CoVTiAl and CrVTiAl compounds are identified as ferromagnetic and antiferromagnetic semiconductors, respectively, with large energy gaps in both spin directions. All magnetic compounds are expected to have high Curie temperatures making them suitable for spintronics/magnetoelectronics applications.
329 citations
TL;DR: In this article, an optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector can exceed the absorption in the TCO layer itself.
Abstract: Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers—or dielectric layers, in rear-passivated diffused-junction silicon solar cells—reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm2 silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm2 and a certified efficiency of over 22%.
286 citations
TL;DR: In this article, the authors developed a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell.
Abstract: The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by th...
276 citations
TL;DR: This review presents the state of this fast growing field of design and evolution in nature, and draws attention to newly opened directions for original research.
Abstract: This is a review of the theoretical and applied progress made based on the Constructal law of design and evolution in nature, with emphasis on the last decade. The Constructal law is the law of physics that accounts for the natural tendency of all flow systems (animate and inanimate) to change into configurations that offer progressively greater flow access over time. The progress made with the Constructal law covers the broadest range of science, from heat and fluid flow and geophysics, to animal design, technology evolution, and social organization (economics, government). This review presents the state of this fast growing field, and draws attention to newly opened directions for original research. The Constructal law places the concepts of life, design, and evolution in physics.
261 citations
TL;DR: In this article, an in-depth overview of the present status and novel developments in the field of plasma processing of low dielectric constant (low-k) materials developed for advanced interconnects in ULSI technology is presented.
Abstract: This paper presents an in-depth overview of the present status and novel developments in the field of plasma processing of low dielectric constant (low-k) materials developed for advanced interconnects in ULSI technology. The paper summarizes the major achievements accomplished during the last 10 years. It includes analysis of advanced experimental techniques that have been used, which are most appropriate for low-k patterning and resist strip, selection of chemistries, patterning strategies, masking materials, analytical techniques, and challenges appearing during the integration. Detailed discussions are devoted to the etch mechanisms of low-k materials and their degradation during the plasma processing. The problem of k-value degradation (plasma damage) is a key issue for the integration, and it is becoming more difficult and challenging as the dielectric constant of low-k materials scales down. Results obtained with new experimental methods, like the small gap technique and multi-beams systems with separated sources of ions, vacuum ultraviolet light, and radicals, are discussed in detail. The methods allowing reduction of plasma damage and restoration of dielectric properties of damaged low-k materials are also discussed.
TL;DR: In this article, a homogenization method to model a stack of second generation High Temperature Superconducting tapes under AC applied transport current or magnetic field has been obtained, where the idea is to find an anisotropic bulk equivalent for the stack such that the geometrical layout of the internal alternating structures of insulating, metallic, superconducting, and substrate layers is washed out while keeping the overall electromagnetic behavior of the original stack.
Abstract: A homogenization method to model a stack of second generation High Temperature Superconducting tapes under AC applied transport current or magnetic field has been obtained. The idea is to find an anisotropic bulk equivalent for the stack such that the geometrical layout of the internal alternating structures of insulating, metallic, superconducting, and substrate layers is “washed” out while keeping the overall electromagnetic behavior of the original stack. We disregard assumptions upon the shape of the critical region and use a power law E–J relationship allowing for overcritical current densities to be considered. The method presented here allows for a computational speedup factor of up to 2 orders of magnitude when compared to full 2-D simulations taking into account the actual dimensions of the stacks without compromising accuracy.
TL;DR: Wong et al. as mentioned in this paper used the FP-LAPW + lo method to compute the defect formation energy (DFE) of an individual ZnO NW and found that the green emission spectra of the NWs likely originate from neutral oxygen vacancies.
Abstract: mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW þ lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral VO DFEs from the DFT calculations along the ZnO [0001] and [10 � directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [10 � directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations. V C 2013 Author(s). All article
TL;DR: In this paper, X-ray structural analysis and high-temperature thermoelectric properties measurements are performed on polycrystalline samples of artificial mineral Cu12−xNixSb4S13 tetrahedrite.
Abstract: X-ray structural analysis and high-temperature thermoelectric properties measurements are performed on polycrystalline samples of artificial mineral Cu12−xNixSb4S13 tetrahedrite. Analysis of the atomic displacement parameter manifests low-energy vibration of Cu(2) out of CuS3 triangle plane. The vibration results in low lattice thermal conductivity of less than 0.5 W K−1 m−1. By tuning of the Ni composition x and decrease of electronic thermal conductivity, dimensionless thermoelectric figure of merit for x = 1.5 achieves 0.7 at 665 K, which is a considerably high value among p-type Pb-free sulfides. Because the tetrahedrite is an environmentally friendly material, it constitutes a good thermoelectric material for use in support of a sustainable society.
TL;DR: In this paper, the electron and phonon energy dispersion relations of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe2 were studied under linear response regime with different doping types, crystal orientations, and temperatures.
Abstract: Using ab-initio method and ballistic transport model, we study electron and phonon energy dispersion relations of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe2. Their electron and heat transports as well as their thermoelectric properties are also studied under linear response regime with different doping types, crystal orientations, and temperatures. Our results show that electron and phonon transports are not very sensitive to crystal orientations because the differences between group velocity and transmission of these carriers along different transport directions are not significant. Furthermore, as temperature increases, first peak values of thermoelectric figure of merit (ZT1st peak) increase linearly except for monolayer n-type WSe2/MoSe2 and p-type WS2, which have higher increasing rates when temperature is high due to the electron transport contribution from an additional valley. Among these various conditions, the results show that all monolayers have similar ZT1st peak a...
TL;DR: In this article, the intrinsic carrier mobility of silicene is calculated using first-principles methods incorporating density functional theory, Boltzman transport equation, and the deformation potential theory.
Abstract: The intrinsic carrier mobility of silicene is calculated using first-principles methods incorporating density functional theory, Boltzman transport equation, and the deformation potential theory. The electron mobility is 2.57×105 cm2V-1s-1 and the hole mobility is 2.22×105 cm2V-1s-1 at room temperature, which is smaller than that of graphene but still very high.
TL;DR: In this paper, a detailed explanation of the fiber formation process is presented using high speed photography to capture the jet initiation process at the orifice and to track the trajectories of the resulting jets.
Abstract: A newly developed method of producing nanofibers, called forcespinning, has proven to be a viable alternative to mass produce nanofibers. Unlike electrospinning, the most common method currently being employed (which draws fibers through the use of electrostatic force), forcespinning utilizes centrifugal forces which allow for a host of new materials to be processed into nanofibers (given that electric fields are not required) while also providing a significant increase in yield and ease of production. This work presents a detailed explanation of the fiber formation process. The study is conducted using high speed photography to capture the jet initiation process at the orifice and to track the trajectories of the resulting jets. The effects that influential controllable parameters have on the fiber trajectories and final fiber diameters are presented. The forcespinning controllable parameters include the spinneret angular velocity and aspect ratio, orifice radius and orientation, fluid viscoelasticity and surface tension, fluid fill level, solvent evaporation rate, temperature, and distance of spinneret orifice to collector.
TL;DR: In this article, an ultrathin and broadband absorber is investigated, which is composed of a periodic array of loop-dielectric multilayered structure, and the authors show that the absorption at normal incidence is above 90% in the frequency range of 8.37-21 GHz.
Abstract: An ultrathin and broadband absorber is investigated in this paper. The metamaterial absorber is composed of a periodic array of loop-dielectric multilayered structure. By tuning the scale factor of the loop and the height of every layer, a desirable refractive index dispersion spectrum is realized, which is the reason to realize a successive anti-reflection in a wide frequency range. The interference mechanism and resonance absorption are identified through analytical derivation and numerical simulations. Numerical results show that the absorption at normal incidence is above 90% in the frequency range of 8.37–21 GHz. Moreover, the structure has a thickness of 3.65 mm (only 0.10λ to 0.26λ at the lowest and highest frequencies, respectively). The explanation to the physical mechanism of the metamaterial absorber is presented and verified.
TL;DR: Graphene (G)-Fe3O4 nanohybrids were fabricated by first depositing β-FeOOH crystals with diameter of 3-5nm on the surface of the graphene sheets as mentioned in this paper.
Abstract: Graphene (G)–Fe3O4 nanohybrids were fabricated by first depositing β-FeOOH crystals with diameter of 3–5 nm on the surface of the graphene sheets. After annealing under Ar flow, β-FeOOH nanocrystals were reduced to Fe3O4 nanoparticles by the graphene sheets, and thus G–Fe3O4 nanohybrids were obtained. The Fe3O4 nanoparticles with a diameter of about 25 nm were uniformly dispersed over the surface of the graphene sheets. Moreover, compared with other magnetic materials and the graphene, the nanohybrids exhibited significantly increased electromagnetic absorption properties owing to high surface areas, interfacial polarizations, and good separation of magnetic nanoparticles. The maximum reflection loss was up to −40.36 dB for G–Fe3O4 nanohybrids with a thickness of 5.0 mm. The nanohybrids are very promising for lightweight and strong electromagnetic attenuation materials.
TL;DR: In this paper, stable and well dispersed functionalized functionalized graphene-ethylene glycol (EG) + distilled water nanofluids having graphene nano-sheets (GnS) volume concentration between 0.041 and 0.395 vol.
Abstract: Stable and well dispersed functionalized graphene–ethylene glycol (EG) + distilled water nanofluids having graphene nano-sheets (GnS) volume concentration between 0.041 and 0.395 vol. % are prepared without any surfactant. Graphene nano-sheets are prepared from high purity graphite powder by Hummers method followed by exfoliation and reduction by hydrogen gas. Thus, obtained hydrogen exfoliated graphene (HEG) is then functionalized using acid. The graphene nano-sheets are characterized using XRD, TEM, Raman spectroscopy, and FTIR spectroscopy. Thermal conductivity and viscosity measurements are performed both as a function of graphene loading and temperature between 10 and 70 °C. Thermal conductivity enhancement of ∼15% for a loading of 0.395 vol. % f-HEG is observed at room temperature. The measured nanofluid's thermal conductivity is explained well in terms of the expression derived by Nan et al. (J. Appl. Phys. 81, 6692 (1997)), which considers matrix-additive interface contact resistance of mis-oriented ellipsoidal particles. The viscosity of the prepared f-HEG nanofluids and the base fluid (EG + distilled water) displays non-Newtonian behaviour with the appearance of shear thinning and nearly 100% enhancement compared to the base fluid (EG + DI water) with f-HEG loading of 0.395 vol. %. Known theoretical models for nanofluid's viscosity fail to explain the observed f-HEG volume concentration dependence of the nanofluid's viscosity. Temperature dependence of the studied nanofluid between 10 and 70 °C is explained well by the correlations proposed earlier for nanofluids with spherical nanoparticles. Electrical conductivity of the f-HEG nanofluids shows significant enhancement of ∼8620% for 0.395 vol. % loading of f-HEG in a base fluid of 70:30 mixture of EG and distilled water.
TL;DR: In this paper, a triple-band polarization-independent metamaterial absorber using square-shaped closed ring resonators over wide angle of incidence was proposed for airborne and surveillance radar signal absorption applications.
Abstract: In this paper, we propose a triple band polarization-independent metamaterial absorber using square-shaped closed ring resonators over wide angle of incidence. The unit cell consisting of various square loops is designed by using the parametric analysis so that it exhibits a triple band absorption response with two bands lying in C-band and one in X-band for airborne and surveillance radar signal absorption applications. Furthermore, in X-band, the absorber exhibits a broadband response with full width at half maxima bandwidth of 940 MHz (9.43%). The structure exhibits bandwidth enhanced properties for any angle of polarization under normal incidence. It also shows high absorption for wide angle of incidence up to 60°. The proposed structure is fabricated and experimental results show proper matching with the simulated responses.
TL;DR: High throughput (combinatorial) materials science methodology is a relatively new research paradigm that offers the promise of rapid and efficient materials screening, optimization, and discovery as mentioned in this paper, which is characterized by synthesis of a "library" sample that contains the materials variation of interest (typically composition).
Abstract: High throughput (combinatorial) materials science methodology is a relatively new research paradigm that offers the promise of rapid and efficient materials screening, optimization, and discovery. The paradigm started in the pharmaceutical industry but was rapidly adopted to accelerate materials research in a wide variety of areas. High throughput experiments are characterized by synthesis of a “library” sample that contains the materials variation of interest (typically composition), and rapid and localized measurement schemes that result in massive data sets. Because the data are collected at the same time on the same “library” sample, they can be highly uniform with respect to fixed processing parameters. This article critically reviews the literature pertaining to applications of combinatorial materials science for electronic, magnetic, optical, and energy-related materials. It is expected that high throughput methodologies will facilitate commercialization of novel materials for these critically important applications. Despite the overwhelming evidence presented in this paper that high throughput studies can effectively inform commercial practice, in our perception, it remains an underutilized research and development tool. Part of this perception may be due to the inaccessibility of proprietary industrial research and development practices, but clearly the initial cost and availability of high throughput laboratory equipment plays a role. Combinatorial materials science has traditionally been focused on materials discovery, screening, and optimization to combat the extremely high cost and long development times for new materials and their introduction into commerce. Going forward, combinatorial materials science will also be driven by other needs such as materials substitution and experimental verification of materials properties predicted by modeling and simulation, which have recently received much attention with the advent of the Materials Genome Initiative. Thus, the challenge for combinatorial methodology will be the effective coupling of synthesis, characterization and theory, and the ability to rapidly manage large amounts of data in a variety of formats.
TL;DR: In this paper, it is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3.
Abstract: expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8eV to 1.0eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics. V C 2013 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4799126]
TL;DR: In this article, homogeneous ceramics-polymer nanocomposites consisting of surface treated BaTiO3 (BT) particles as fillers and poly(vinylidene fluoride) polymer as matrix have been prepared using a solution casting process.
Abstract: In this work, homogeneous ceramics-polymer nanocomposites consisting of surface treated BaTiO3 (BT) particles as fillers and poly(vinylidene fluoride) polymer as matrix have been prepared using a solution casting process. The nanocomposites exhibit enhanced dielectric permittivity and reduced loss tangent. The frequency and temperature dependencies of the dielectric permittivity and loss tangent of the nanocomposites suggest that the introduced BT phase and interface areas contribute to the improvement of the dielectric responses. Meanwhile, the X-ray diffraction patterns and Differential Scanning Calorimetry (DSC) curves indicate that the incorporation of ceramic particles contributes to the decrease of the crystallite size, the increase of the crystallinity, and the shift of the crystallization temperature of the polymer matrix. Furthermore, the dielectric displacement and energy density of the nanocomposites are significantly enhanced and an energy density of 3.54 J/cm3 was obtained under an electric field of 200 MV/m with the BT concentration of 20 vol. %. The results indicate that the introduced ceramic fillers and interface areas have positive influences on the structure of the polymer matrix and contribute to the enhancement of the dielectric responses and energy storage properties of the nanocomposites.
TL;DR: In this paper, the authors reported the analysis of the Urbach effect in the absorption spectra of the undoped ZnO thin films and found the phonon energy to be ℏωp=76´±´4´meV.
Abstract: We report the analysis of the Urbach effect in the absorption spectra of the undoped ZnO thin films. The absorption coefficients of the ZnO thin films show the exponential rise, also known as the Urbach tails, just below the free exciton peak. Fitting of the steepness parameter of the Urbach tails yields the phonon energy to be ℏωp=76 ± 4 meV, consistent with ℏωp=72 meV measured from the photoluminescence spectra of ZnO. The temperature dependence of the Urbach energy, the steepness parameter, and the energy gap strongly suggests that the observed Urbach effect is a result of the cumulative effect of impurities, structural disorders, and electron-phonon interaction in the absorption processes.
TL;DR: In this article, the predicted band parameters such as band gap, spin-orbit splitting energy (ΔSO), band offsets and strain of GaAsBiN on GaAs versus N and Bi compositions based on recent experimental data are presented.
Abstract: GaAsBiN is a potentially interesting alloy which may be exploited in near- and mid-infrared photonic devices. Here we present the predicted band parameters such as band gap (Eg), the spin-orbit splitting energy (ΔSO), band offsets and strain of GaAsBiN on GaAs versus N and Bi compositions based on recent experimental data. We also show how bismuth may be used to form alloys whereby ΔSO > Eg thereby providing a means of suppressing non-radiative CHSH (hot-hole producing) Auger recombination and inter-valence band absorption. We determine the optimum conditions where ΔSO > Eg, which is expected to improve the high-temperature performance and thermal stability of light emitting devices. It is also shown that preferential band offsets are achievable with GaAsBiN, which makes this material system promising for photonic devices operating in the near- and mid-infrared.
TL;DR: In this paper, the authors performed hybrid functional calculations of native point defects and dangling bonds (DBs) in α-Al2O3 to aid in the identification of charge-trap and fixed-charge centers in Al 2O3/III-V metaloxide-semiconductor structures.
Abstract: We performed hybrid functional calculations of native point defects and dangling bonds (DBs) in α-Al2O3 to aid in the identification of charge-trap and fixed-charge centers in Al2O3/III-V metal-oxide-semiconductor structures. We find that Al vacancies (VAl) are deep acceptors with transition levels less than 2.6 eV above the valence band, whereas Al interstitials (Ali) are deep donors with transition levels within ∼2 eV of the conduction band. Oxygen vacancies (VO) introduce donor levels near midgap and an acceptor level at ∼1 eV below the conduction band, while oxygen interstitials (Oi) are deep acceptors, with a transition level near the mid gap. Taking into account the band offset between α-Al2O3 and III-V semiconductors, our results indicate that VO and Al DBs act as charge traps (possibly causing carrier leakage), while VAl, Ali, Oi, and O DBs act as fixed-charge centers in α-Al2O3/III-V metal-oxide-semiconductor structures.
TL;DR: In this paper, all electron fullpotential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (101¯0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models.
Abstract: In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (101¯0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction, and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termi...
Journal Article•