Gap Discontinuity Research Articles

Influence of Defect Density, Band Gap Discontinuity and Electron Mobility on the Performance of Perovskite Solar Cells

Influence of Defect Density, Band Gap Discontinuity and Electron Mobility on the Performance of Perovskite Solar Cells

(Invited) Ultrawide Bandgap Transistors for High Temperature and Radiation Hard Applications

Some of the best high-temperature commercial devices are now GaN field-effect transistors (FETs) on silicon substrates. However, these devices cannot meet requirements for space applications requiring high radiation hardness and for operations at temperatures as high as 600 oC. High temperature and radiation hard applications stimulated interest in developing transistors using ultrawideband gap materials including AlN, AlGaN with a high molecular fraction of aluminum, gallium oxide, diamond, boron nitride, and their heterojunctions. A wider bandgap and, therefore, larger energy required to produce an electron-hole pair and larger energy gap discontinuities in heterostructures formed by these materials make them both more tolerant to radiation and more capable of operating at higher temperatures. Insulated gate (Metal Insulator Heterostructure FET (MISHFET) structures with high K-materials implemented in the AlGaN materials system, the power FINFET configurations implemented in GaN and diamond, and gate edge and channel engineering approaches are key technologies for ultra-wide bandgap semiconductor applications. Using all AlGaN materials is now a proven approach to compete with GaN. Measured and predicted materials properties of BN and diamond promise an even better performance but the power device applications of these materials and their heterojunctions have not yet been sufficiently explored. I will review the material parameters of ultra-wideband gap semiconductors and specific device designs linking them to the expected radiation hardness and high-temperature performance and to improving the reliability and lifetime of ultra-wideband gap transistors.

An Engineering Model of Magnetic Flux Density and Electromagnetic Force Density at the Structural Discontinuity within Transformer Cores.

The structural discontinuities in the form of air gaps in transformer cores cause the concentration of electromagnetic force, which is an important source of transformer vibration and noise. In this paper, an engineering model of magnetic flux density and electromagnetic force density on transformer core discontinuities is analytically developed. Based on a reasonable structural simplification and assumptions, magnetic flux density and electromagnetic force density are deduced as explicit functions of the geometric, material, and electrical excitation characteristics of the gap region and the transformer core. The accuracy of the established model is validated by the finite element method (FEM) combined with a magnetic measurement experiment. According to this engineering model, the electromagnetic force density can be reduced by decreasing the gap ratio and increasing the gap thickness to a reasonable level. The outcome of this paper can help to understand the physical mechanism of the electromagnetic force generated by core air gap discontinuities, which is meaningful for noise control and the condition monitoring of transformers.

Kernel estimation of returns of retirement funds of employers based on monetary earnings (subscriptions and compensation) via regression discontinuity in Iraq

Regression Discontinuity (RD) means a study that exposes a definite group to the effect of a treatment. The uniqueness of this design lies in classifying the study population into two groups based on a specific threshold limit or regression point, and this point is determined in advance according to the terms of the study and its requirements. Thus , thinking was focused on finding a solution to the issue of workers retirement and trying to propose a scenario to attract the idea of granting an end-of-service reward to fill the gap ( discontinuity point) if it had not been granted. The regression discontinuity method has been used to study and to estimate the effect of the end -service reward on the cutoff of insured workers as well as the increase in revenues resulting from that. The research has showed that this reward has a clear effect on increasing revenues due to the regularity of workers in their work and their work continuity . It has also found that using Local Linear Smother (LLS) by using three models of bandwidth selection. Its results after the analysis in the Regression program have been as follows: The CCT (Calonico, Cattaneo & Titiunik) beamwidth gives the best performance followed by the local linear regression using the LK (Lembens and kalyanman) beamwidth. The real data has been used in sample size 71 represented in compensation as a variable of effectiveness (illustrative) X and the revenue as a result or an approved variable Y, while the results of the traditional OLS estimation method have not been good enough.

Integrated Actuation of Microfluidically Reconfigurable mm-Wave SPST Switches

This article demonstrates the design and performance of a single-pole single-throw (SPST) microfluidically reconfigurable switch. The switch is formed by placing a microfluidic channel over a microstrip line gap discontinuity. A selectively metallized plate located inside the microfluidic channel is repositioned with fluid flow to capacitively load the discontinuity to achieve the RF switching functionality. Different from prior works based on external micropumps, the SPST switch is integrated with a piezoelectric actuator. The switch layout is optimized to obtain a low actuation time. A switch prototype is characterized to exhibit low insertion loss (0.42 dB) at 30 GHz and wide bandwidth (18 GHz). It operates with rise and fall times down to 1.12 ms. It is actuated by three million switching cycles with no performance degradation. To the best of our knowledge, the presented switch exhibits the lowest reconfiguration time, highest operation frequency, and the highest reliability among the microfluidically reconfigurable devices reported in recent literature.

Millimeter-Wave Beam-Steering Focal Plane Arrays With Microfluidically Switched Feed Networks

Microfluidically switched feed networks are introduced to realize millimeter-wave beam-steering focal plane arrays (FPAs). Switching functionality is achieved by a selectively metallized plate (SMP) repositionable within a microfluidic channel that is brought in close proximity to the microstrip line feed network of an FPA. The feed network is strategically designed to exhibit gap discontinuities that are sequentially overlapped by the SMP metallizations. The capacitive couplings between the SMP metallizations and the gap discontinuities are designed to achieve wideband (~20 GHz) switching functionality with low insertion loss (IL $10\times $ lower than that of the prior work. This allows to achieve a faster beam-steering speed (0.27 s versus 5.25 s of previous designs). An integrated sensing approach for detecting the SMP position is also presented for the first time to facilitate the use of presented MFPAs with closed-loop position control.

Creating Zipper‐Like van der Waals Gap Discontinuity in Low‐Temperature‐Processed Nanostructured PbBi2nTe1+3n: Enhanced Phonon Scattering and Improved Thermoelectric Performance

AbstractNanoengineered materials can embody distinct atomic structures which deviate from that of the bulk‐grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi2nTe1+3n using a solution‐synthesized PbTe‐Bi2Te3 nano‐heterostructure as a precursor. The as‐obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper‐like van der Waals gap discontinuity and the random arrangement of septuple‐quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.

Creating Zipper-Like van der Waals Gap Discontinuity in Low-Temperature-Processed Nanostructured PbBi2n Te1+3n : Enhanced Phonon Scattering and Improved Thermoelectric Performance.

Nanoengineered materials can embody distinct atomic structures which deviate from that of the bulk-grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi2n Te1+3n using a solution-synthesized PbTe-Bi2 Te3 nano-heterostructure as a precursor. The as-obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper-like van der Waals gap discontinuity and the random arrangement of septuple-quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.

Nonlinearity in superconducting CPW open-end and gap discontinuities

In this paper, non-linear circuit models of the superconducting CPW (SCOW) open-ends and gaps are presented. The model can be used to create a fast and accurate analysis of the nonlinearity in SCOW applications such as end-coupled filters, which have such gap and open-end discontinuities. The non-linear circuit is solved using harmonic balance (HOB) method to compute the third-order intermodulation (IMD3) distortion and fundamental outputs. As an example, we analyze the nonlinearity of an end-coupled SCOW band pass filter (BP) to validate the model. The model can be used to minimize the nonlinearity in such superconducting structures via optimizing the resonators of superconducting filters. The main difference between the proposed model and the one in the literatures is that there is no clear discussion or method about nonlinearity in discontinuous SCOW structures because of absence of closed-form formula for some equivalent circuit model of discontinuities. As an example, for determining the value of the equivalent gap capacitance of a SCOW structure, we can’t find any easy-use and direct equation. But in this paper, we also propose a closed-form equation for calculating the such capacitances.

Band Offset Reduction at Defect-Rich p/i Interface Through a Wide Bandgap a-SiO:H Buffer Layer

In single junction p-i-n solar cells, the optical losses can be mitigated by inserting the wide band gap amorphous silicon oxide layer at the defect-rich p/i interface. In this paper, a simulation and experimental study on the performance of p-i-n solar cells by inserting the intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) buffer layer at the p/i interface is reported. The i-a-SiO:H film has been deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) (13.6 MHz) at a low substrate temperature of approximately 230°C. The p/i interface is crucial to solar cell performance because the first few nanometers of the intrinsic layer are a defect-rich layer, having the band gap discontinuity, resulting in band offset. Thus, the carrier recombination probability increases in the vicinity of p/i interface because of high defect density and short carrier lifetime. Aided by optically calibrated simulations and with the support of experimental results, this study shows that a wide band gap thin undoped a-SiO:H buffer layer with higher photoconductivity reduces the band-gap offset and minimizes the recombination of photogenerated charge carriers at the defect-rich p/i interface. It has also been found that the a-SiO:H buffer layer augmented the electric field inside the device. As a result, the overall performance of the a-Si:H-based single junction solar cell has significantly improved. By employing a ∼5 nm thick a-SiO:H buffer layer, the blue response of the cell has been improved, resulting in 7.34% and 18.62% enhancement in fill factor (FF) and power conversion efficiency ( η ), respectively, as compared to the buffer-less cell.

Use of mitochondrial COI gene for the identification of family Salticidae and Lycosidae of spiders

In recent years, DNA barcoding has become quite popular for molecular identification of species because it is simple, quick and an affordable method. Present study was conducted to identify spiders of most abundant families, i.e. Salticidae and Lycosidae from citrus orchards in Sargodha district using DNA barcoding. A total of 160 specimens were subjected to DNA barcoding but, sequences up to 600 bp were recovered for 156 specimens. This molecular approach proved helpful to assign the exact taxon to those specimens which were misidentified through morphological characters in the study. We were succeeded to discriminate six species of Lycosidae and nine species of Salticidae through DNA barcoding. Results revealed the presence of clear barcode gap (discontinuity in intra- and inter-specific divergences) for members of both families. Furthermore, the maximum intra-specific divergence was less than NN (nearest neighbour) distance for all species. This suggested the reliability of DNA barcoding for spider’s identification up to species level. We got 98% success in our study. It is concluded from present study that DNA barcoding is more reliable tool especially for immature spiders, when morphological characters are ambiguous.

Theory of valley-dependent transport in graphene-based lateral quantum structures

Modulation of electronic states in two-dimensional (2D) materials can be achieved by using in-plane variations of the band gap or the average potential in lateral quantum structures. In the atomic configurations with hexagonal symmetry, this approach makes it possible to tailor the valleytronic properties for potential device applications. In this work, we present a multi-band theory to calculate the valley-dependent electron transport in graphene-based lateral quantum structures. As an example, we consider the structures with a single interface that exhibits an energy gap or potential discontinuity. The theoretical formalism proceeds within the tight-binding description, by first deriving the local bulk complex band structures in the regions of a constant gap or potential and, next, joining the local wave functions across the interface via a cell-averaged current operator to ensure the current continuity. The theory is applied to the study of electron reflection off and transmission through an interface. Both reflection and transmission are found to exhibit valley-contrast behavior that can be used to generate valley-polarized electron sources. The results vary with the type of interfaces, as well as between monolayer and bilayer graphene based structures. In the monolayer case, the valley contrast originates from the band warping and only becomes sizable for incident carriers of high energy; whereas in AB-stacked bilayer graphene, the vertical interlayer coupling emerges as an additional important cause for valley contrast, and the favorable carrier energy is also found to be drastically lower. Our numerical results clearly demonstrate the propitious valleytronic properties of bilayer graphene structures.

Effect of I/N interface on the performance of superstrate hydrogenated microcrystalline silicon solar cells

A study on the effect of the n-type hydrogenated amorphous silicon (a-Si:H) buffer layers at the I/N interfaces on the overall performance of superstrate (pin-type) hydrogenated microcrystalline silicon (μc-Si:H) single-junction solar cells with wide band gap silicon oxide doped supporting layers is presented. By integrating the n-type a-Si:H materials with proper band gap and thickness values at the I/N interface, the band gap discontinuity between the μc-Si:H intrinsic and n-type nc-SiOx:H layers can be successfully compensated for, thereby improving the carrier collection, reducing the accumulation and recombination of photo-generated carriers near the I/N interface, and improving the overall current and power output of the μc-Si:H cells. Further adoption of an optimized hydrogen profiling technique based on the optimized n-type a-Si:H buffer layers yields an initial conversion efficiency of 10.05% for μc-Si:H single-junction solar cells with a thickness of 2μm.

Advanced Functional Materials: Intrinsic and Doped Silicon Oxide

ABSTRACTThe unique properties of silicon oxide materials, no matter intrinsic or doped, utilized in thin film solar cells (TFSCs) in the area of photovoltaic (PV) are making TFSCs one of the most attractive photovoltaic technologies for the development of high-performing electricity production units to be integrated in everyday life. In comparison to other silicon materials, the particular diphasic structure of silicon oxide materials, in which hydrogenated microcrystalline silicon (μc-Si:H) crystallites are surrounded by an oxygen-rich hydrogenated amorphous silicon (a-Si:H) phase, causes them present excellent photoelectrical material properties, such as a low-parasitic absorption in the broadband spectral range, independent controllability of longitudinal and lateral conductivity, refractive indices (3.5-2.0), band gap (2.0-2.6 eV) and conductivity tenability (with orders of 1-10-9S/cm) with oxygen doping, and so on. Various types of silicon oxide materials, including intrinsic, p- or n- type, further applied in TFSCs have also played significant roles in improving the efficiency of various types of single-, dual-, and triple-junction thin-film solar cells from both the optical and electrical points of view. In this paper, we present our latest progress in studying the performance improvement role of intrinsic or doped silicon oxide materials in pin-type a-Si:H, a-SiGe:H, and μc-Si:H single-junction solar cells. By effectively tuning the band gap values of intrinsic a-SiOx:H materials with oxygen doping and adopting the layers with a suitable band gap (1.86 eV) as the P/I buffer layers of a-Si:H solar cells fabricated on metal organic chemical vapor deposition (MOCVD) boron-doped zinc oxide (ZnO:B) substrates, a significant Vocincreases up to 909 mV and an excellent external quantum efficiency (EQE) response of 75% at the 400 nm typical wavelength can be achieved by matching the band gap discontinuity between the p-type nc-SiOx:H window and a-Si:H intrinsic layers. The serious leakage current characteristics of pin-type narrow-gap (Eg<1.5 eV) a-SiGe:H single-junction solar cells can also be finely tuned by integrating an n-type μc-SiOx:H layer with a small oxygen content in addition to improving the long-wavelength response, an effective approach gives rise to the highest FF of 70.62% for pin-type a-SiGe:H single-junction solar cells with an average band gap of 1.48 eV. In addition, our studies proved that the application of p-type μc-SiOx:H window layers in μc-Si:H single-junction solar cells can effectively improve the short-wavelength light coupling by suppressing the parasitic absorption and promoting the anti-reflectivity with a graded refractive index profile. On the basis of the optimum single-junction solar cells with omnipotent silicon oxide materials, an initial efficiency of 16.07% has been achieved for pin-type a-Si:H/a-SiGe:H/μc-Si:H triple-junction solar cells with an active area of 0.25 cm2. The omnipotent properties of silicon oxide layers in TFSCs, including effective optical coupling and trapping, suitability in compensating for the band gap discontinuity, the shunt-quenching capacity, and so on, make them likely to be extended to other types of solar cells such as polycrystalline chalcopyrite Cu(In,Ga)Se2(CIGS) and perovskite-sensitized solar cells, opening up new opportunities for acquiring solar cells with higher performance.

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

Optical transitions in GaAs1−xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k·p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%.

Moore Curve Fractal‐Shaped Miniaturized Complementary Spiral Resonator

ABSTRACTIn this article, a novel design of complementary spiral resonators as miniaturized metamaterial particles based on Moore curve fractal shape is presented. The proposed design of metamaterial unit cell is investigated using a microstrip line loaded with series gap discontinuities and implementing fractal shape complementary spiral resonator in ground plane that behaves as a composite right left hand transmission line. It is observed that Moore curve fractal‐shaped spiral resonator provides 49% size reduction as compared to conventional split ring resonators. An equivalent lumped element circuit model of the proposed composite right left handed transmission line is developed to analyze the proposed topology. A close agreement between electromagnetic simulation, circuit simulation, and measurement is obtained. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1950–1954, 2013

DESIGN OF DEFECTED GROUND BAND PASS FILTERS USING STEPPED IMPEDANCE RESONATORS

In this paper, band-pass fllters based on the microstrip conflguration and utilizing coupled defected ground structures are presented. The 50› microstrip line has a gap discontinuity in the middle and characterized by stepped impedance resonators on either half. The four fractal geometries tested for defected ground are the flrst and second iterations of the modifled Moore curve, the closed staircase curve and a dual concentric closed staircase curve. The fllters have a compact size and planar geometry. The modifled Moore flrst iteration has a measured pass band from 2.27GHz to 11.86GHz, whereas the second iteration has a measured pass band from 1.85GHz to 6.71GHz. The measured pass band of the closed staircase fllter is from 2.38GHz to 7.21GHz. The dual-concentric closed-staircase DGS fllter ofiers a dual-band response with the measured pass band being from 2.41GHz to 5.01GHz and from 5.81GHz to 8.35GHz. All the fllters can have applications in the UWB region. A parametric study showing the efiect of the various controlling parameters is also presented.

Effect of a-Si：H interface buffer layer on the performance of hydrogenated amorphous silicon germanium thin film solar cell

In the light of the open circuit voltage and fill factor reduction resulting from band gap discontinuities and high defect densities at interfaces when more germanium is mixed into the intrinsic layer of hydrogenated amorphous silicon germanium solar cell, the insertion of a-Si:H buffer layer with proper band gap into PI interface not only mitigates band gap discontinuities and interface recombination, but also improves the electric field distribution by reducing the defect densities at PI interface, thus the collection efficiency of a-SiGe:H solar cell is enhanced. By inserting a-Si:H buffer layer into IN interface and designing band gap profile along the a-SiGe:H intrinsic layer further, the 8.72% conversion efficiency of single junction a-SiGe:H solar cell is achieved when only Al back reflector is added as back contact.

Gap Discontinuity in Microstrip Lines: An Accurate Semianalytical Formulation

A semianalytical full-wave formulation is used to analyze a narrow gap in a microstrip line. The analysis assumes that the length of the gap is small compared to the strip width, but allows for an arbitrarily high frequency. The formulation accounts for radiation from the gap into space and into surface waves. Based on this formulation, the scattering parameters of the gap are obtained along with the complete equivalent circuit of the gap. The percentage of power lost due to gap radiation from an incident mode on the microstrip line is found, and the physics of the gap radiation are examined.

Ternary alloys Cd y Zn1 − y O and Mg x Zn1 − x O as materials for optoelectronics

Thin films of Cd y Zn1 − y O and Mg x Zn1 − x O (y = 0−0.35, x = 0−0.45) ternary alloys have been grown by pulsed laser deposition onto sapphire substrates. The record solubility limits of Cd (y = 0.3) and Mg (x = 0.35) have been achieved in hexagonal zinc oxide. The mismatch of the lattice parameters a of Cd0.2Zn0.8O and Mg0.35Zn0.65O does not exceed 1%; in this case, the band gap discontinuity is 1.3 eV. The surface roughness of the films does not exceed 2.5 nm at x = 0−0.27 and y = 0−0.20.