Band Gap Region Research Articles

Electrical Analysis and Comparison of Ta2O5 Tunneling Oxide with SiO2 in Passivated c-Si Solar Cell

The Tantalum oxide material (Ta2O5) as tunnel oxide in comparison with the Silicon dioxide (SiO2) Tunnel oxide is proposed. The lower bandgap and high refractive index of Ta2O5 has several advantages over SiO2. The Ta2O5 is used as Anti-Reflecting coating earlier, but also has capability to behave as the tunneling oxide. In the ultrathin region, the Tantalum oxide (Ta2O5) potentially produces extremely high tunneling and high electric field. The greater electrical intensity increases both the transportation of carrier and passivation quality of designed c-Si structure. The carrier selectivity is also increased due to very high capacitive nature. The performance measurement is done in ASTM certified AM1.5G environment using Silvaco ATLAS 2D TCAD tool. The comparison has been made between Ta2O5 and SiO2, to show the advantage of Ta2O5 over SiO2. The lower bandgap of 4.8eV and higher affinity of 3.19 of Ta2O5 as compared to SiO2 proves to be better material as tunneling oxide. The bandgap and electric field region of both the materials have been studied. In first approach to use Ta2O5 as tunneling oxide, the power conversion 28.42% is obtained for 0.1nm thickness of the Ta2O5. External and internal quantum efficiencies are well above 80% and 90% respectively. The characteristics of the Ta2O5 tunneling based solar cell are compared with the previous reported solar cell.

InP and AlInP(001)(2 × 4) Surface Oxidation from Density Functional Theory.

The atomic structure and electronic properties of the InP and Al0.5In0.5P(001) surfaces at the initial stages of oxidation are investigated via density functional theory. Thereby, we focus on the mixed-dimer (2 × 4) surfaces stable for cation-rich preparation conditions. For InP, the top In–P dimer is the most favored adsorption site, while it is the second-layer Al–Al dimer for AlInP. The energetically favored adsorption sites yield group III–O bond-related states in the energy region of the bulk band gap, which may act as recombination centers. Consistently, the In p state density around the conduction edge is found to be reduced upon oxidation.

Waveguide bandgap engineering with an array of superconducting qubits

Waveguide quantum electrodynamics offers a wide range of possibilities to effectively engineer interactions between artificial atoms via a one-dimensional open waveguide. While these interactions have been experimentally studied in the few qubit limit, the collective properties of such systems for larger arrays of qubits in a metamaterial configuration has so far not been addressed. Here, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a waveguide. By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states, as well as the emergence of a polaritonic bandgap. Making use of the qubits quantum nonlinearity, we demonstrate control over the latter by inducing a transparency window in the bandgap region of the ensemble. The circuit of this work extends experiments with one and two qubits toward a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

Band-Gap Tuning in Two-Dimensional Spatiotemporal Phononic Crystals

We investigate the effect of small spatiotemporal modulations in subwavelength-dimensioned phononic crystals with large band gaps, on the frequency spectrum for elastic waves polarized in the plane of periodicity. When the radius of cylinders periodically placed inside a matrix of highly-contrasting elastic properties is time-varying, we find that due to the appearance of frequency harmonics throughout the spectrum, the notion of a band gap is destroyed in general, although with the appropriate tuning of parameters, in particular the modulation frequency, it is possible that some band-gap region is retained, making such systems possible candidates for tunable bandpass filters or phononic isolators, accordingly, and for sensor applications.

Spectral filtering of sub-bandgap radiation using all-dielectric gratings for thermophotovoltaic applications

Filtering of the sub-band spectral radiation is an attractive technique to overcome the lower efficiencies of direct conversion thermophotovoltaic technology. The poor performance of these systems is due to the relatively small portion of the incident energy being above the bandgap of photovoltaic cell. To effectively filter the majority of the sub-bandgap radiation and re-employ it as regenerative heat, a viable solution is to design an efficient spectrally selective filter ideally matched to the photovoltaic cell’s bandgap. Here, we have explored a high contrast amorphous silicon grating on a quartz substrate. Quartz due to its inherent nature inhibits transmission of sub-band gap radiation in the infrared (IR) region (>4.75 μm), whereas gratings further filter radiation above 1.8 μm suitable for GaSb photovoltaic cell. The optimized filter is fabricated using direct write laser lithography, and optical characterization result shows that 72% of incident radiation in unconvertible region (>1.80 μm) is filtered and recycled. Further, the thermal characterization results of IR filter carried out using a ceramic heater has shown the drop in effective temperature from 1074.9 to 813.2 K in above bandgap region. This suppressed radiation has contributed to an absolute increase in source body temperature by 16.0 K resulting in increase in the above bandgap radiation available for thermophotovoltaic conversion. The proposed spectral filtering design can be tailored to solar cells of any bandgap and is scalable for employment in various thermophotovoltaic applications.

Ab-initio insights into electronic structures, optical and photocatalytic properties of Janus WXY (X/Y = O, S, Se and Te)

Two-dimensional materials attracted widespread attention due to their striking performances compared with bulk materials. In this presentation, we theoretically investigate the electronic structures, optical and photocatalytic properties of Janus WXY (X/Y is O, S, Se and Te) through first-principles calculations. Our results indicate the recombination rate of photogenerated electron-hole pairs could be distinctly reduced because of the internal electric field of Janus WXY (X/Y is O, S, Se and Te) nanostructures. Moreover, Janus WXY (X/Y is S, Se and Te) own the decent band gaps and band-edge positions with water redox potential, i.e. the reduction and oxidation potentials are both contained by the corresponding band gap regions, demonstrating the significent potential as efficient photocatalysts for water splitting. The acid environment (pH = 0) can be used as the most suitable condition to forecast the significant potentical applcation for Janus WXY (X/Y is S, Se and Te) materials as distinguished photocatalysts for water splitting reactions. Addtionally, the response of the band gaps (EG) of Janus WXY (X/Y is S, Se and Te) systems to tensile strain strains is more sensitive than compressive strain. These findings are intended to provide appealing significance for the fascinating applications of Janus WXY (X/Y is S, Se and Te) in the field of water splitting and offer a reliable guidelines for experiments.

Accurate Characterization of Electromagnetic Band-Gap Structures

In this paper, electromagnetic band-gap (EBG) structures are accurately characterized using a free-space method. The field transmission and reflection parameters are exploited to determine the effective material parameters both inside and outside the band-gap regions of the EBG structures. The electromagnetic (EM) properties of the effective refractive index, effective wave impedance, effective permittivity, and effective permeability at multiple band-gap regions are derived from a single scattering computation. The derived phase and attenuation constants correctly define the boundaries of the band-gap region and the level of attenuation within the band gap. Deviations in the incidence angle from the normal direction alter the EM properties; hence, the operating bandwidth of the band-gap region is as expected due to the inherent structure anisotropy of the unit cells.

Surface modification by high-energy heavy-ion irradiation in various crystalline ZnO facets.

Self-assembled surface nanoscale structures on various ZnO facets are excellent templates for the deposition of semiconductor quantum dots and manipulation with surface optical transparency. In this work, we have modified the surface of c-, m- and a-plane ZnO single-crystals by high-energy W-ion irradiation with an energy of 27 MeV to observe the aspects of surface morphology on the optical properties. We kept ion fluences in the range from 5 × 109 cm-2 to 5 × 1011 cm-2 using the mode of single-ion implantation and the overlapping impact mode to see the effect of various regimes on surface modification. Rutherford backscattering spectroscopy in the channeling mode (RBS-C) and Raman spectroscopy have identified a slightly growing Zn-sublattice disorder in the irradiated samples with a more significant enhancement for the highest irradiation fluence. Simultaneously, the strong suppression of the main Raman modes and the propagation of the modes corresponding to polar Zn-O vibrations indicate disorder mainly in the O-sublattice in non-polar facets. The surface morphology, analysed by atomic force microscopy (AFM), shows significant changes after ion irradiation. The c- and a-plane ZnO exhibit the formation of small grains on the surface. The m-plane ZnO forms a sponge-like surface for lower fluences and grains for the highest fluence. The surface roughness itself increases with the irradiation fluence as shown by AFM measurement as well as spectroscopic ellipsometry (SE) analysis. The damage caused by high-energy irradiation leads to non-radiative processes and suppression of the near-band-edge peak as well as the deep-level emission peak in the photoluminescence spectra. Furthermore, the refraction index n and the extinction coefficient k of irradiated samples, determined by SE, have features corresponding to the particular exciton states blurred and are slightly lower in the optical bandgap region especially for the polar c-plane ZnO facet.

A comprehensive study of ultrafast carrier dynamics of LT-GaAs: Above and below bandgap regions

We report the study of ultrafast carrier dynamics of low temperature grown GaAs over wide spectral range (2.75 eV–0.88 eV) with above and below bandgap (Eg) optical excitations using ultrafast transient absorption spectroscopy. The presence of defects and their subsequent effect on the pathway and lifetime of carrier relaxation with probing at different energy levels is presented. The carrier relaxation pathways and their corresponding models are predicted. It is observed that for pumping (2.58 eV) and probing above Eg, the transient behavior is dominated by band filling while renormalization is in prominence when probed near to Eg. For below Eg pumping (0.88 eV) the optical behavior is mostly governed by defects. The calculated trap time is in the range of ~128 fs to ~807 fs while overall recombination time shows a wide variation from ~1.38 ps to ~1.25 ns depending on the position of traps/defects in the forbidden band.

Enhanced transmission induced by embedded graphene in periodic, quasiperiodic, and random photonic crystals

The study of photonic crystals, artificial materials whose dielectric properties can be tailored according to the stacking of their constituents, remains an attractive research field, and it is the basis of photonic devices based on the generation, processing, and storage of photons. In this paper, we employ a transfer-matrix treatment to study the propagation of light waves in periodic, quasiperiodic (Fibonacci, Octonacci, and Dodecanacci), and random dielectric multilayers with graphene embedded. The structures considered here are composed of two building blocks, silicon dioxide (building block A = S i O 2 ) and titanium dioxide (building block B = T i O 2 ). We calculate their transmission spectra as a function of incident angle θ and reduced frequency Ω . Our main goal is to investigate the enhancement of the transmission due to the presence of graphene. In particular, we show that bandgap regions become passband regions when graphene is embedded in the optical multilayers. More specifically, for a range of the incident angle θ and reduced frequency Ω , the PCs with graphene embedded display an unexpected property: the electromagnetic radiation is transmitted mainly through the multilayers and not reflected or absorbed as expected for large structures.

Wideband Microstrip Patch Antennas with Transverse Electric Modes

A wideband microstrip patch antenna, exciting the fundamental transverse electric (TE) mode, is investigated. The excitation of the TE mode is facilitated through replacing both of the patch and ground plane of a conventional microstrip antenna with artificial magnetic conductors (AMC), consisting of unipolar compact photonic bandgap (UC-PBG) unit cells. The AMC patch and the ground plane of this antenna behave as magnetic conductors within the bandgap region of the unit cells. Similar to conventional patch antennas, it is shown that by cutting a U-shaped slot in the AMC patch, wideband characteristics are realized. The antenna shows a 40% impedance bandwidth and operates at the TE10 mode. Moreover, the width of the patch is 1.75 times smaller than its length, reducing the overall size of the antenna by about 60%, compared with the conventional U-slot PEC antenna supporting the transverse magnetic (TM) mode.

Correlating facet orientation, defect-level density and dipole layer formation at the surface of polycrystalline CuInSe2 thin films

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined analytical approach based on scanning tunneling spectroscopy (STS), scanning electron microscopy, and electron back scatter diffraction (EBSD), where we perform these experiments on identical surface areas as small as 2 × 2 µm2 with a lateral resolution well below 50 nm. The topography of the absorber surfaces indicates two main morphological features: micro-faceted, long basalt-like columns and their short nano-faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typical for the presence of pronounced oxidation-induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano-faceted terminations of the basalt-like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt-like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non-polar planes. Ab-initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O-induced defects, in comparison to non-polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p-n junction formation in chalcopyrite thin-film solar cells.

Stability, electronic structures, and band alignment of two-dimensional IIA -IV- N2 materials

Structural and phonon properties, formation and cohesive energies, and electronic structures of ${\mathrm{II}}_{A}$-IV-${\mathrm{N}}_{2}$ (where ${\mathrm{II}}_{\mathrm{A}}=\mathrm{Be}$ or Mg; $\mathrm{IV}=\mathrm{Si}$, Ge, or Sn) monolayers with a graphenelike planar structure are systematically studied. Stability and property evolution with the variation of constituent group ${\mathrm{II}}_{A}$ and group IV elements are revealed. Dynamical and elastic stability of ${\mathrm{II}}_{A}$-IV-${\mathrm{N}}_{2}$ monolayers is justified by phonon and elasticity calculations, respectively. Their wide band gaps ranging from 3.32 to 5.61 eV are predicted by the $GW$ method on top of density functional calculations. The fat-band analysis and charge-density calculation of selected eigenstates unveil the effect of chemical environment and elemental substitution on electronic states near the band-gap region. The quasi-free-electron state and the parabolic dispersion of the lowest conduction band are advantageous for electron transport in electronic applications. A close examination of the band alignment and structural similarity between monolayers of ${\mathrm{II}}_{A}$-IV-${\mathrm{N}}_{2}$ compounds and III nitrides indicates that it is possible to form type-I or type-II heterostructures and alloy systems composed of monolayers of ${\mathrm{II}}_{A}$-IV-${\mathrm{N}}_{2}$ compounds and III nitrides. The findings in this work will promote research aiming at the synthesis, characterization, and application of novel two-dimensional materials, alloys, and heterostructures.

Study of induced structural, optical and electrochemical properties of Poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) and their blend as an effect of graphene doping

Composites of graphene with Poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) and their blends have been prepared via a facile chemical mixing approach. Graphene has been synthesized using a liquid phase chemical exfoliation method and characterized using imaging, electronic and spectroscopic techniques. Morphological analysis depicts the multi-layered structure of prepared graphene having approximately 9 layers, as estimated from Raman spectroscopy and AFM measurements. The value of energy gap between HOMO and LUMO of the prepared graphene has been determined using UV–Visible absorption spectroscopy and CV measurement, and found to be in close proximity. Further, UV–Visible and Raman spectroscopy suggest the formation of some localized trapping sites in between the band gap region of P3HT/PCBM and P3HT-PCBM blends as an effect of graphene doping leading to change in refractive index and bandgap values. Moreover, CV results also favour decrease in energy gap in composites after graphene loading Thus present study supports the well-dispersion and non-agglomeration of graphene within these polymers proper charge transfer between graphene layer and polymer chain. The sort of transfer of charge carriers from P3HT to graphene to PCBM is also conferring through the Raman measurements in blended films. Such interaction may help in designing of optoelectronic device with optimized photovoltaic performance.

A first principle based study on the mechanical and thermal properties of UO2: Effect of La and Dy fission product concentrations

The amount of fission products substitution in the uranium dioxide (UO2) fuel matrix and subsequently their interaction with the host is important in the perspective of safe and efficient operation of nuclear reactors as fission products can change the structural, mechanical and thermo-physical properties of the host fuel significantly. Therefore, structural, elastic, mechanical and thermo-physical properties of lanthanide (Ln) fission products doped UO2 with different doping concentrations of Ln atoms are studied by employing density functional theory with Hubbard U correction. In this study, two Ln atoms (Ln = La and Dy) and different doping concentrations in UO2e.g. 6.25%, 12.5%, 25%, and 50% are considered. The mechanical property such as bulk modulus of Ln doped UO2 are directly related to the Ln doping concentration. Expressions are generated to extract various properties at very low and high Ln doping concentrations. The thermo-physical properties like heat capacity and coefficient of thermal expansion are also evaluated based on quasi-harmonic approximation to the phonons frequencies which are obtained from density functional perturbation theory. The UO2 lattices with higher Ln atom concentrations exhibit new electronic energy bands near the band gap region, which are dominated by Ln atomic orbitals. From the present study, we derived correlations between La and Dy doping concentration and various thermo-physical properties of UO2. Moreover, the properties obtained from our first principle calculations show a good agreement with the available experimental and theoretical values. Thus, our systematic study with varying fission product concentrations will provide insight to the UO2 fuel properties under reactor conditions.

Effect of the band gap and the defect states present within band gap on the non-linear optical absorption behaviour of yttrium aluminium iron garnets

Abstract The effect of variation of band gap and electronic defect energy levels within the band gap on non-linear optical absorption of 532 nm (green) laser light by coral-like yttrium aluminium-iron garnet (YAIG, Y3Al5-xFexO12) ceramics prepared by solution combustion route is investigated using Z-scan technique. Our yttrium aluminium garnet (YAG) sample with band gap of ≥ ~ 6.5 eV, is transparent to visible light. The band gap of YAG was tuned by substituting Fe at the aluminium site and consequently, the surface defects in YAG were also modified. The defects in the YAIG samples introduce electronic defect states within the band gap. We observed that the intensity of the non-linear absorption (NLA) signal increases with increase in the amount of iron content. We assigned this enhancement of NLA signal to: (1) the decrease in the band gap from ≥ ~ 6.5 eV for YAG to ~ 2.6 eV for yttrium iron garnet (YIG) facilitating two photon absorption (2 PA) process rather than three photon absorption (3 PA), and (2) the increase in excited state absorption (ESA) due to the spread of the defect energy states into the band gap region. As the band gap gets closer to the photon energy of the laser, the non-linear optical absorption is enhanced due to ESA, in combination with the occurrence of the relatively stronger 2 PA process rather than the very weak 3 PA process. Our results demonstrate a method to tune the NLA performance of YAG through iron substitution at the Al sites and provide a better understanding of the non-linear absorption behaviour of YAIG ceramics for their application in optical limiting devices such as laser shields.

Surface elastic waves whispering gallery modes based subwavelength tunable waveguide and cavity modes of the phononic crystals

We studied the characteristics of a phononic crystal which consists of hollow pillars mounted on the surface of semi-infinite substrate. In this study, the existence of surface waves whispering gallery modes referred as localized cavity modes of the hollow pillars at the subwavelength frequency spectrum is reported. The interaction of the hollow pillars with the surface waves induce localized cavity modes and by tuning the inner radius of the hollow pillars these confined modes can be adjusted/tuned inside the bandgap region, thus introducing new applications for the surface waves based phononic crystals and acoustic metamaterial devices. These subwavelength confined cavity modes possess high-q narrow passband characteristics induced by the interaction of cavity modes with the surface waves propagating at the surface of semi-infinite substrate. We demonstrate the subwavelength waveguiding and other functionalities of these localized cavity modes by tuning the inner radius of the hollow pillars and allowing the specific wavelength frequencies to pass through the monochannel, multichannel, and linear cavity based compact waveguides. To make the study more general, we demonstrate the wave multiplexing phenomena for the identical frequencies (diameters of the hollow pillars are kept constant) for all types of waveguides. However, by varying the inner radii of the hollow pillars, the high-q localized frequencies can be tuned. All the physical phenomena presented shows excellent agreement and it indicates promising applications for the manipulation of surface waves in the surface acoustic waves devices, surface waves sensor and actuator technology, and other photonic/phononic and metamaterial applications.

Oxidation/reduction cycles and their reversible effect on the dipole formation at CuInSe2 surfaces

The defect-electronic properties of {112} microfaceted surfaces of epitaxially grown ${\mathrm{CuInSe}}_{2}$ thin films are investigated by scanning tunneling spectroscopy and photoelectron spectroscopy techniques after various surface treatments. The intrinsic ${\mathrm{CuInSe}}_{2}$ surface is found to be largely passivated in terms of electronic defect levels in the band-gap region. However, surface oxidation leads to an overall high density of defect levels in conjunction with a considerable net surface dipole, which persists even after oxide removal. Yet, a subsequent annealing under vacuum restores the initial condition. Such oxidation/reduction cycles are reversible for many times providing robust control of the surface and interface properties in these materials. Based on ab initio simulations, a mechanism where oxygen dissociatively adsorbs and subsequently diffuses to a subsurface site is proposed as the initial step of the observed dipole formation. Our results emphasize the relevance of oxidation-induced dipole effects at the thin film surface and provide a comprehensive understanding toward passivation strategies of these surfaces.

On the excitation mechanism of visible responsible Er-TiO2 system proved by experimental and theoretical investigations for boosting photocatalytic activity

One of the challenges in the field of heterogeneous photocatalysis is the development of new type of photoactive materials activated by low-power and low-cost irradiation sources, and to understand the mechanism of photocatalytic reaction in the presence of those materials. Photocatalysis based on Er-TiO2 has become an attractive process to remove contaminants from aquatic environments. Unfortunately, there is no available literature presenting the systematic experimental and theoretical study of the effects of erbium ions on the photocatalytic activity of TiO2 and visible light excitation mechanism of this system. Accordingly, to obtain solar-responsive photocatalysts, nanostructured Er-TiO2 was prepared via hydrothermal method. The physicochemical characterization of samples indicate that modified photocatalysts contain both doped erbium and the surface adsorbed erbium species (e.g., Er2O3, Er(OH)x). Predicted electronic structures of Er-doped TiO2 indicate that the formation of new Er-4f states within the band gap region of TiO2 might be crucial for observed photocatalytic effects towards Vis response. However, the up-conversion process is not responsible for Vis activity and efficient O2− generation. Moreover, Er modification of TiO2 results in an inhibition of the growth of titania crystallites, and thus an increase in the specific surface area, which are also beneficial for an efficient degradation of pollutants.