Average Band Gap Research Articles

Perturbed multilayered structures of positive and negative index materials

In this work, we study the influence of disorder on the omnidirectional bandgap in a one dimensional stack of alternating positive and dispersive negative index materials. We achieve this through using the transfer matrix method to study wave propagation properties. In the case where the number of periods becomes infinitely large, the limit of the transmittance is derived from the trace of the matrix, and thus reducing the calculation complexity. The origin of the transmission resonances and their relation with the field localization for random systems are analyzed and compared with that of the periodic case. Our result shows that the zero average refractive index bandgap is not affected by small disorders in layer thickness or refractive index, and thus the multilayer stack is robust against fabrication. The finding is expected to achieve potential applications in optoelectronic sensor devices such as omnidirectional reflectors in airplane radomes. We also show that a random mixture of positive and dispersive negative index materials in an equal ratio always possesses a zero average refractive index bandgap.

Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water

Nanostructured zinc oxide thin films (ZnO) were prepared on conducting glass support (SnO2: F overlayer) via sol-gel starting from colloidal solution of zinc acetate 2-hydrate in ethanol and 2-methoxy ethanol. Films were obtained by spin coating at 1500 rpm under room conditions (temperature, 28–35°C) and were subsequently sintered in air at three different temperatures (400, 500 and 600°C). The evolution of oxide coatings under thermal treatment was studied by glancing incidence X-ray diffraction and scanning electron microscopy. Average particle size, resistivity and bandgap energy were also determined. Photoelectrochemical properties of thin films and their suitability for splitting of water were investigated. Study suggests that thin films of ZnO, sintered at 600°C are better for photoconversion than the films sintered at 400 or 500°C. Plausible explanations have been provided.

Unusually Large Franz-Keldysh Oscillations at Ultraviolet Wavelengths in Single-Walled Carbon Nanotubes

We report large electroabsorption susceptibilities in the ultraviolet region for single-walled carbon nanotubes (SWNT) supported on quartz that are approximately 10;{3} larger than the highest values reported to date for any system. The oscillatory behavior is described using a convolution of Airy functions in photon energy ascribing the effect to Franz-Keldysh oscillations. The metallic and semiconducting SWNT composition is varied, and it is shown that the confinement energy correlates with the average band gap for semiconducting SWNT in the film. The large susceptibilities arise from a subpercolated network of metallic SWNT that enhances the local electric field.

Hydrothermal Splitting of Titanate Fibers to Single-Crystalline TiO2 Nanostructures with Controllable Crystalline Phase, Morphology, Microstructure, and Photocatalytic Activity

A simple and efficient inorganic acid-assisted hydrothermal route has been developed for the synthesis of single-crystalline TiO2nanostructures with use of protonic tetratitanate hydrate fibers as a precursor. A variety of TiO2 nanostructures, including aligned nanorods, nanoporous nanostructures, nanocubes, and diamondshaped nanocrystals, have been prepared with this method. The morphology, crystalline phase, composition, average grain size, band gap, and microstructure of the nanostructures have been determined as a function of the nature and concentration of the inorganic acid (HCl, HNO3 ,o r H2SO4) used for the synthesis. The rutile phase is obtained by using HCl for a concentration range of 0.1-0.4 M and also for 6 M concentration, HNO3 for a concentration range of 0.1-0.2 M and >7 M, and H2SO4 for a narrow concentration range of 0.1-0.9 M. Likewise, the anatase phase is obtained by using both HCl and HNO3 for concentrations of around 0.9 M, and a broader concentration range of 2-7 M with H2SO4. An increase in the band gap of the TiO2 nanostructures from 2.97 to 3.35 eV is observed as the crystalline phase changes from rutile, to a mixture of rutile and anatase, and subsequently to pure anatase. Correspondingly, the photocatalytic activity of the synthesized nanostructured materials is found to be dependent on the crystalline phase, composition, and surface area.

ZnO thin films prepared by a modified water-based Pechini method

ZnO films were prepared by a modified water-based Pechini method where ethanol was added to avoid some disadvantages of solely water-based precursor solution. The ratio of ethanol to water (E/W) largely influences the quality of films, and the appropriate ratio is about 1.2. FT-IR spectra show that the addition of ethanol largely promotes the esterification reaction in the dried gel. XRD results show that all films are of wurtzite structure while the annealing temperature is not higher than 700 °C. The secondary phase Zn 2SiO 4 was found in the films annealed at 800 °C because of the reaction between SiO 2 substrate and ZnO film. SEM photographs show that all films consist of nanocrystalline grains with sizes in the range of 25–82 nm, and the addition of ethanol improves the growth of grains. Transmittance spectra reveal that all films have high average transmittance of about 90% in the range of 450–800 nm except those annealed at 800 °C. The average optical band gap of films is about 3.29 eV.

Bowing parameter of zincblende In xGa 1− xN

The zincblende In x Ga 1− x N alloys are studied by numerical analysis based on first-principles calculations. A 64-atom supercell is used to model these alloys. Our results indicate that the ternary In x Ga 1− x N alloys have an average band gap bowing parameter of 2.08 eV. The simulation results also suggest that the composition-dependent bowing parameter of the ternary In x Ga 1− x N alloys can be expressed by a third-order polynomial equation, b( x) = −4.7422 x 3 + 6.9592 x 2 − 2.3136 x + 2.1301 (eV).

Alloy composition fluctuation and band edge energy structure of In‐rich InxGa1–xN layers investigated by systematic spectroscopy

AbstractAfter the finding of the bandgap energy of InN as less than 1 eV, the re‐examination of the bandgap of InGaN alloys and its bowing parameter is an issue of interest. We study the absorption spectra of In‐rich InGaN layers by taking account of the Burstein‐Moss effect and bandgap renormalization with the analysis of photoluminescence and infrared reflectance spectra. We find the far larger broadening of optical transition and lattice vibration energies than those expected from the alloy composition fluctuation based on the x‐ray diffraction patterns. We estimate that local potential fluctuation by the alloy composition fluctuation give rise to the strong localization of carriers and relaxation of the momentum conservation rule in optical transition. The average bandgap energies of the films show the bowing parameter less than 1.4 eV; the value down to 0.7 eV is possible. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal lattice scattering mobility and carrier effective mass in intrinsicTl2InGaTe4 single crystals

Systematic structural, dark electrical resistivity and Hall coefficient measurements have been carried outon n-type Tl2InGaTe4 single crystals. The data from x-ray powder diffraction allowed determination of thetetragonal unit cell lattice parameters. Analysis of the electrical resistivity and carrierconcentration, which was recorded in the temperature range 210–350 K, reveals theintrinsic type of conduction with an average energy band gap of 0.85 eV. Thetemperature-dependent Hall mobility was observed to follow the law and was analysed assuming the domination of acousticphonons scattering. The experimental Hall mobility data forTl2InGaTe4 crystals agrees with the theoretical acoustic phonon scattering mobility data with anacoustic deformation potential of 7.6 eV.

Preparation and characterisation of sprayed tin sulphide films grown at different precursor concentrations

Tin sulphide films have been grown by spray pyrolysis technique at different precursor concentrations varied in the range, 0.01–0.2 M keeping other deposition parameters constant. The physical properties of the deposited films were systematically studied in relation to the precursor concentration. The studies indicated that the films grown in the precursor concentration range, 0.09–0.13 M were nearly stoichiometric with the Sn, S ratio of 1.06 and exhibited only SnS phase with a strong (1 1 1) preferred orientation that belongs to the orthorhombic crystal structure. These single-phase films showed an average electrical resistivity of 32.9 Ω cm, Hall mobility of 139 cm 2 V −1 s −1 and carrier density of ∼10 15 cm −3. These films had an average optical band gap of 1.32 eV with an absorption coefficient greater than 10 4 cm −1. These properties demonstrated that single-phase SnS films could be used as an absorber layer in the fabrication of heterojunction solar cells.

NANOSTRUCTURE AND PROPERTIES OF INDIUM TIN OXIDE (ITO) FILMS PRODUCED BY ELECTRON BEAM EVAPORATION

Nanocrystal indium tin oxide (ITO) thin films were grown by electron beam evaporation (e-beam). The ITO films were fabricated at substrate temperatures ranging from 100 to 400°C in O 2 partial pressure ranging from 0.10 to 100 mTorr. The surface morphology was monitored using atomic force microscopy (AFM). The charge in the surface morphology of ITO films was discussed in terms of grain size and crystallographic orientations. Grain size measurements of the solid dispersed and the AFM study for nanostructure showed that the oxides were in the nano range (20–30 nm). In general, the values of the optical bandgap for the films are consistently blue-shifted as compared with the crystal size. The average crystalline size determined from the shift of the optical gap were found to be in the range 20–30 nm, which is in excellent agreement with the data obtained from AFM. All ITO films average grain size was ~ 20 nm deposited by e-beam evaporation. The average optical transmittance was 90.50% in the visible range (400–700 nm) and the average bandgap was 3.98 eV. ESR spectrum of ITO film showed random oxygen vacancies which arise due to the changing crystal field effects.

Chemical Bond Properties and Isomer Shifts of Tl2Ba2Ca2Cu3O10

Tl2Ba2Ca2Cu3O10 was reported to be a superconductor with a highest transition temperature of 125 K among the homologous series of Tl2Ba2Can1CunO2n+4. The direct information on the Cu ion site at the atomic level is important for elucidating the superconductivity mechanism. The local bond properties of Tl2Ba2Ca2Cu3O10 were studied using the average band-gap model. The calculated results show that the covalency of Cu(1)O bond is 0.561, and the average covalency of Cu(2)O is 0.296. Mossbauer isomer shifts of 57Fe in Tl2Ba2Ca2Cu3O10 were calculated using the chemical surrounding factor, defined by covalency and electronic polarizability. It is verified that for lower doping, Fe substitute the Cu at the Cu (1) site in forms of Fe3+ and Fe4+; for higher doping, Fe3+ and Fe4+ ion occupies Cu(1) and Cu(2) site respectively. The studies show that the determination of the correspondence between spectrum components and actual copper sites occupied by Mossbauer nucleus was made easier with the aid of the calculation results of the chemical bond parameters.

Acoustic phonons scattering mobility and carrier effective mass in In 6S 7 crystals

Systematic dark electrical resistivity and Hall coefficient measurements have been carried out in the temperature range of 170–320 K on n-type In6S7 crystals. The analysis of the electrical resistivity and carrier concentration reveals the intrinsic type of conduction with an average energy band gap of ∼0.75 eV. The carrier effective masses of the conduction and valence bands were calculated from the intrinsic temperature-dependent carrier concentration data and were found to be 0.565m 0 and 2.020m 0, respectively. The temperature-dependent Hall mobility was observed to follow the μαT−3/2 law and was analyzed assuming the domination of acoustic phonons scattering. The acoustic phonons scattering mobility was calculated from the crystal's structural data with no assumptions. The experimental Hall mobility data of In6S7 crystals coincides with the theoretical acoustic phonons scattering mobility data with acoustic deformation potential of 6.4 eV.

Mapping quantum dot-in-well structures on the nanoscale using the plasmon peak in electron energy loss spectra

Plasmon energy losses experienced by fast electrons passing through a thin specimen are readily measured using electron energy loss spectroscopy. We describe a simple model of the shape and energy of the single scattering distribution plasmon peak, which relates the energy loss spectrum to the material parameters of average band gap ${\overline{E}}_{g}$, lattice parameter $a$, core ion dielectric constant ${\ensuremath{\epsilon}}_{c}$, and plasmon lifetime $\ensuremath{\tau}$. This model is used to interpret maps of plasmon energy and full width at half maximum taken from an $\mathrm{In}\mathrm{As}∕{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dot-in-well structure. We find that the bulk plasmon peak shifts are mainly determined by the strain in the structure, which may provide a new means of mapping strain on the nanoscale. The regions of maximum plasmon peak width appear to correspond to regions of high strain gradient. It is possible to separate the effects of plasmon lifetime from the other factors, leaving a parameter which is a combination of ${\overline{E}}_{g}$, $a$, and ${\ensuremath{\epsilon}}_{c}$. We have been able to map this parameter with a spatial resolution of a few nm.

Bandgap mapping for III-V quantum well by electron spectroscopy imaging

The concept of 'bandgap mapping' was proposed originally to map the inhomogeneity of band energy in III-V semiconductors with a spatial resolution of a few nanometres in a scanning transmission electron microscope with the focus beam mode. In this paper, several techniques were developed to demonstrate the possibility to map the distribution of bandgap energies for GaN/AlN quantum-well structures using electron spectroscopy imaging (ESI). The phase correlation function was used to register different energy-loss images among ESI series with an accuracy of 1 pixel. The energy dispersion of ESI series was improved by a fast Fourier transform interpolation method. An iterative multivariable least square algorithm was derived to refine the fitting of the single scattering distribution to an analytic form of the density of states function a(E - E(g))(0.5). The inhomogeneity of the band energy of the quantum well can be revealed from the band-energy map. A threshold filter method is applied to estimate the average value and SD of the bandgap energy from barrier and well regions in the energy map. The average bandgap energy of AlN and GaN is determined to be 5.62 +/- 0.35 and 3.87 +/- 0.36 eV, respectively. The effect of delocalization on the accuracy of band-energy determination is discussed. The 2sigma(E(g)) accuracy of this analysis is comparable to half of the energy resolution of the ESI experiment.

A Novel Approach for the Evaluation of Band Gap Energy in Semiconductors

On the basis of absorption nature of semiconductors, we present a novel and simple method to determine the band gap energies of semiconductors directly from their absorption spectra at any temperatures, without any fitting processes and restrictions of sample thickness. The key point of the approach is the different dependence of the absorption coefficient derivative on the photon energy at different absorption regions in semiconductors. We first demonstrate and verify the approach by detailed temperature-dependent absorption measurements, combined with photoluminescence measurements and empirical band gap equations for the direct band gap of uniform InAs films, and then extend successfully to the indirect band gap of elemental Ge and to the ternary HgCdTe alloys with composition gradient. Furthermore, we have also shown that our approach can not only evaluate the average band gap energy for ternary semiconductor alloys, but also estimate their composition uniformity to monitor the material quality.

High Quality Amorphous Silicon Germanium Alloy Solar Cells Made by Hot-wire CVD at 10 Å/s

ABSTRACTHigh quality amorphous silicon germanium (a-SiGe:H) alloys have been obtained using the hot wire chemical vapor deposition (HWCVD) from a gas mixture of SiH4, GeH4, and H2 at a deposition rate of ∼10 Å/s. Solar cells in a SS/n-i-p/ITO configuration are evaluated in which the n- and i-layers are deposited by HWCVD at NREL and the microcrystalline p-layer by conventional RF glow discharge in a separate reactor by United Solar. Effects of hydrogen dilution and step-wise bandgap profile have been studied and optimized. The best cell has an average optical bandgap of 1.6 eV and incorporates multi-bandgap steps where the narrow-most bandgap is near the p-i interface. J-V characteristics are measured under AM 1.5 illumination with a λ>530 nm filter. The best initial power output obtained exceeds 4 mW/cm2, which is usually used as an indicator for a good quality middle-gap cell. Double-junction cells are made on textured Ag/ZnO back reflectors. The bottom cell uses the optimized a-SiGe:H alloy cell by HWCVD, and the top cell uses an optimized a-Si:H cell near the amorphous-to-microcrystalline transition by PECVD at ∼1 Å/s. The best double-junction cell made to date exhibits an initial AM 1.5 active-area efficiency of 11.7%, and a stable efficiency after 1000 hours of one sun light soaking of 9.6%.

Syntheses and optical characterization of poly[1,8–octanedioxy–1,4–phenylene–1,2–ethylene–1,4–phenylene] and its copolymer derivatives

We have investigated the optical properties of a series of poly( p-phenylene vinylene) (PPV) copolymers. Three-wide bandgap PPV copolymers were synthesized and characterized. The new PPV copolymers showed a large blue shift in both absorption and emission that their average bandgap is over 3.1 eV. A significant solvatochromic effect was observed in the photoluminescence (PL) spectra of copolymers. The dimethoxy-substituted copolymer showed the highest quantum efficiency and processability among three copolymers.

Atomic and electronic structures of nanometer sized silica particles

The Austin Model 1 semi-empirical method combined with geometry optimization procedure was used to study atomic and electronic structures of nanoscale silica particles containing up to 192 atoms (64 Si and 128 O). Twofold Si–O rings were found to dominate in nanometer sized particles. The fraction of larger rings increases with increasing particle sizes. Bond length and bond angle distributions were calculated. The shortest Si–O, O–O and Si–Si interatomic distances are approximately 1.7, 3.0 and 3.3 Å, respectively. Average O–Si–O and Si–O–Si bond angles are approximately 110 ∘ and 130 ∘. Average electronic energy band gap is 6.9 eV.

Sellmeier coefficients and dispersion of thermo-optic coefficients for some optical glasses

The refractive index and its variation with temperature, the thermo-optic coefficient (d n/dT), are analyzed with two separate physically meaningful models for more than a dozen of some important Schott and Ohara optical glasses to find the refractive index at any operating temperature for any wavelength throughout the transmission region. The room-temperature catalog values of refractive indices are fitted with a two-pole Sellmeier equation. Both the average electronic absorption band gap and the lattice absorption frequency, lying in the vacuum UV and IR regions, respectively, contribute to the refractive indices and their dispersion. The estimated absorption band gaps are at 8.5-11.9 eV, and these values agree with the measured values at 8.8-11.6 eV satisfactorily for normal optical glasses. The higher-index glasses have electronic absorption in the region of 5.6-6.3 eV, and the estimated band gap of SF6 glass is 6.0 eV. The dispersion of thermo-optic coefficients is accounted for satisfactorily with a model, based on three physical parameters, the thermal expansion coefficient and excitonic and isentropic optical band gaps that are in the vacuum UV region. These optical constants are used to compute refractive indices at any operating temperature and wavelength. The Abbé number and the chromatic dispersion characteristics of these glasses are evaluated from the computed optical constants; the values of the chromatic dispersions are evaluated particularly at the three optical windows of the optical fiber communication systems and femtosecond technology.

Sellmeier Coefficients and Chromatic Dispersions for Some Tellurite Glasses

Sellmeier coefficients are necessary to optimize the design parameters of optical devices. These coefficients are computed for binary tellurite glasses to find the chromatic dispersion behavior at any wavelength, particularly at the optical windows of optical fiber communication systems and femtosecond technology. A single‐oscillator DiDomenico and Wemple's dispersion equation is not sufficient to represent accurately the measured refractive indexes throughout the transmission range. As a minimum, a two‐pole Sellmeier equation is necessary to represent the data more accurately. Both the average electronic absorption band gap and the lattice absorption frequency, lying in the UV and IR region, respectively, affect the refractive indexes and their dispersion. Various other optical properties are discussed. A single‐oscillator dispersion equation is unable to verify the existence of zero‐dispersion wavelength.