Dependence Of Band Gap Energy Research Articles

Asymmetric and symmetric interfaces in type II MWIR InAs/GaSb superlattices

We report on the comprehensive investigations of InAs/GaSb type-II superlattice (SL) with asymmetric and symmetric interfaces (IFs) formed using a migration-enhanced epitaxy (MEE) and an interrupted growth. The best structural and optical properties were observed for strain-balanced SL with asymmetric IFs grown using MEE at the bottom IF and continuous growth at the upper IF. The analysis of IF-related lines in Raman spectra of investigated SLs allowed for the evaluation of the quality of the InSb-like and GaAs-like interfaces. The thermal dependency of band gap energy was established from photoluminescence measurements according to the Varshni formula. The activation energies E1 and E2 for the dual-channel non-radiative recombination process were calculated. Linear characteristics of PL intensity vs. excitation power density were fitted using the relation IPL ∼ Eexck with different k. This allowed for the indication of dominant recombination mechanisms: radiative with a share of Auger process (k ≈ 0.8), pure radiative (k ≈ 1.0), and radiative with a slight share of Shockley–Read–Hall (k ≈ 1.1). Different behaviours of PL peak energy depending on the increase of excitation power density were observed: it was either blue-shifted, red-shifted, and/or a combination of both.

High-performance Ge/Si electro-absorption optical modulator up to 85°C and its highly efficient photodetector operation.

We studied a high-speed Ge/Si electro-absorption optical modulator (EAM) evanescently coupled with a Si waveguide of a lateral p-n junction for a high-bandwidth optical interconnect over a wide range of temperatures from 25 °C to 85 °C. We demonstrated 56 Gbps high-speed operation at temperatures up to 85 °C. From the photoluminescence spectra, we confirmed that the bandgap energy dependence on temperature is relatively small, which is consistent with the shift in the operation wavelengths with increasing temperature for a Ge/Si EAM. We also demonstrated that the same device operates as a high-speed and high-efficiency Ge photodetector with the Franz-Keldysh (F-K) and avalanche-multiplication effects. These results demonstrate that the Ge/Si stacked structure is promising for both high-performance optical modulators and photodetectors integrated on Si platforms.

Growth and characterization of PbMo0.75W0.25O4 single crystal: A promising material for optical applications

The present paper reports the structural and optical properties of PbMo0.75W0.25O4 single crystals grown by Czochralski method. XRD pattern of the crystal indicated well-defined two diffraction peaks associated with tetragonal crystalline structure. Raman and infrared spectra of the grown single crystals were presented to get information about the vibrational characteristics. Observed Raman modes were associated with modes of PbMoO4 and PbWO4. Eight bands were revealed in the infrared spectrum. The bands observed in the spectrum were attributed to multiphonon absorption processes. Transmission spectrum was measured in the 375–700 nm spectral region. The analyses of the spectrum resulted in direct band gap energy of 3.12 ± 0.03 eV. The compositional dependent band gap energy plot was drawn considering the reported band gap energies of PbMoO4, PbWO4 and revealed band gap of PbMo0.75W0.25O4 single crystal. An almost linear behavior of composition-band gap energy was seen for PbMo1-xWxO4 compounds. Urbach energy was also found from the absorption coefficient analysis as 0.082 ± 0.002 eV.

Temperature effects on optical characteristics of thermally evaporated CuSbSe2 thin films for solar cell applications

CuSbSe2 thin film was deposited by co-evaporation of binary CuSe and Sb2Se3 sources. The structural and morphological properties of the deposited thin film were investigated with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray analysis measurements. XRD pattern indicated that deposited thin film has an orthorhombic crystalline structure with the preferential orientation of (013) direction. SEM image presented that the thin film surface is almost uniform. The optical characteristics of the deposited CuSbSe2 thin film were investigated in detail by performing room temperature Raman, temperature-dependent transmittance spectroscopy, and photoluminescence techniques. Raman spectrum exhibited one mode at around 210 cm−1 associated with Ag vibrational mode. The derivative spectroscopy technique was used to obtain the band gap energy of the films. Temperature dependence of band gap energy was investigated by considering the Varshni model. The rate of change of band gap energy, absolute zero value of gap energy, and Debye temperature were determined as −1.3 × 10−4 eV/K, 1.21 eV, and 297 ± 51 K, respectively. The photoluminescence spectrum indicated the room temperature direct band gap energy as 1.30 eV.

Impact of physicochemical properties on band gap energy and glass transition temperature of (GeS2)10(Sb2S3)90−x(AgI)x chalcohalide glasses for Near-IR applications

The dependence of band gap energy (Eg) and glass-transition temperature (Tg) on physico-chemical properties for the (GeS2)10(Sb2S3)90−x(AgI)x (10 ≤ x ≤ 50 at%) chalcohalide glasses were theoretically investigated. Chalcohalide glasses have wide technological applications owing to the aptitude of tuning their optical band gap through composition change. Therefore, Eg of the studied chalcohalide glasses was estimated using different methods, and it was turned out that it decreased as the AgI amount increases. This decrease was elucidated in terms of physico-chemical properties. In addition, we determined the positions of conduction band (ECB) and valence band (EVB) by using the computed values of Eg. The density and molar volume increased while the packing density decreased by adding AgI. The overall coordination number (CN), crosslinking density (CD), constraints number (Ns), cohesive energy (CE), heat of atomization (Hs) and overall mean bonds energy (〈E〉); computed via the chemical bond approach (CBA); decreased with the increment of AgI amount. In contrary, the floppy (F) and mean bonds energy (Es) increased. Finally, two estimations of Tg were presented based on 〈E〉 and CN. The two estimations showed a decreasing trend when adding AgI content.

Temperature-tuned band gap properties of MoS2 thin films

MoS2 is one of the fascinating members of transition metal dichalcogenides and has attracted great attention due to its various optoelectronic device applications and its characteristic as two-dimensional material. The present paper reports the structural and temperature tuned optical properties of MoS2 thin films grown by RF magnetron sputtering technique. It was observed that the atomic composition ratio of Mo:S was nearly equal to 1:2 and the deposited thin films have hexagonal crystalline structure exhibiting Raman peaks around 376 and 410 cm−1. The band gap energies were determined as 1.66 and 1.71 eV at 300 and 10 K, respectively and temperature dependency of band gap energy was analyzed by means of Varshni and O’Donnell-Chen models.

Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method

Cadmium zinc telluride is a tunable band gap II–VI compound semiconductor, which has received considerable attention due to its applications such as nuclear radiation detectors and industrial process monitoring. We have studied the optical properties of the ternary compound semiconductors Cd1−xZnxTe grown by vertical Bridgman-Stockbarger method in the whole range of zinc content 0 < x < 1. The crystal structure and material phase were characterized by X-ray diffraction (XRD) and Raman spectra. We measured the band gap energy for each of the samples by absorption and photoluminescence (PL) spectroscopy. From the experimental results we found that the band gap energy of Cd1−xZnxTe was located at around 1.457 eV with x = 0, and gradually shift to 2.174 eV for x equal to 1. The temperature-dependent absorption measurements to study the temperature dependence of band gap energy were also performed. Based on these analyses, the relationship between the band gap energies and the zinc content are verified and discussed. Furthermore, we will study the defect recombination mechanism by using temperature-dependent PL experiments.

Photoresponse of homostructure WSe2 rectifying diode

The WSe2 homostructure rectifying diode is demonstrated, p-type and n-type WSe2 regions were formed by adopting different layer WSe2 thicknesses. Typically, the common way to fabricate two dimensional material based diode is to use heterostructure by stacking two different carrier type and band gap energies materials. However, there is always lattice mismatch at the junction, leading to the degraded photoresponse of the diodes. In order to reduce the problems of heterostructure diode, layer thickness dependent band gap energy of WSe2 is reported and applied to fabricate homostructure diode. The ideality factor of the current-voltage characteristic is 1.64 and the current rectification ratio can reach to 103. The optical properties of the diode, including fill factor 45.7%, responsivity 16.02 mA/W and EQE 3.06% are reported. It is concluded that the WSe2 homojunction diode can be applied in photodetectors, switches, solar cells etc.

Influence of Spin State and Cation Distribution on Stability and Electronic Properties of Ternary Transition-Metal Oxides.

This work is a systematic ab initio study of the influence of spin state and cation distribution on the stability, dielectric constant, electronic band gap, and density of states of ternary transition-metal oxides. As an example, the chemical family of spinel ferrites MFe2O4, with M = Mg, Sc–Zn is chosen. Dielectric constant and band gap are calculated for various spin states and cation configurations via dielectric-dependent self-consistent hybrid functionals and compared to available experimental data. When choosing the most stable spin state and cation configuration, the calculated electronic properties are in reasonable agreement with measured values. The nature of the excitation is investigated through projected density of states. A pronounced dependence of band gap energy and dielectric constant on the spin state and cation configuration is observed, which is a possible explanation for the large variation of the experimental results, in particular, if several states are energetically close.

Tailoring luminescent colour and life persistence of undoped CdSiO3

In this work, undoped CdSiO3 samples with different luminescence spectra and light persistence were obtained by controlling the parameters of sol–gel synthesis. The process of monoclinic phase formation was monitored through XRD and TG-DTA measurements, combined to microstructural characterization using FTIR and SEM. The optical energy bandgap (Eg) of CdSiO3 was estimated from Tauc equation and indicates the dependency of bandgap energy on the starting pH used for synthesis. Three main peaks could be identified from PL and XEOL spectra, at 590nm, 490nm and 406nm. Their relative intensities yielded overall distinct luminescence colours, such as indigo or red. The luminescent mechanisms of CdSiO3 related to an intrinsic defect (VCd) were further investigated to show its key role in the charge trapping that causes long lasting luminescence.

A novel theoretical model for the temperature dependence of band gap energy in semiconductors

We report a novel theoretical model without any fitting parameters for the temperature dependence of band gap energy in semiconductors. This model relates the band gap energy at the elevated temperature to that at the arbitrary reference temperature. As examples, the band gap energies of Si, Ge, AlN, GaN, InP, InAs, ZnO, ZnS, ZnSe and GaAs at temperatures below 400 K are calculated and are in good agreement with the experimental results. Meanwhile, the band gap energies at high temperatures (T > 400 K) are predicted, which are greater than the experimental results, and the reasonable analysis is carried out as well. Under low temperatures, the effect of lattice expansion on the band gap energy is very small, but it has much influence on the band gap energy at high temperatures. Therefore, it is necessary to consider the effect of lattice expansion at high temperatures, and the method considering the effect of lattice expansion has also been given. The model has distinct advantages compared with the widely quoted Varshni’s semi-empirical equation from the aspect of modeling, physical meaning and application. The study provides a convenient method to determine the band gap energy under different temperatures.

Tuning optical properties of CdTe films with nanocolumnar morphology grown using OAD for improving light absorption in thin-film solar cells

CdTe films with transversal morphology in form of tilted nanocolumns were obtained by sublimation method using a rotating vapour source combined with the oblique angle deposition technique. The tilt angles of the nanocolumnar structures increases as the substrate inclination also increase. CdTe films exhibited cubic zinc blend lattice under compressive strain. Morphological and x-ray diffractometry analysis indicated that the nanocolumns are grains stacked in the nanocolumn preferential growth direction, except for the films with non-inclined substrate. We found an interesting dependence of band gap energy and the refractive index as functions of the microstrain distribution due to the nanocolumn tilt in the films from 0° to about 25°. These facts evidence the possibility of carried out film strain engineering for optimizing optoelectronics devices as we propose for the case of thin-film solar cells.

Universal Method for Large-Scale Synthesis of Layered Transition Metal Dichalcogenides.

The layered transition metal dichalcogenides are currently amongst the most intensively investigated materials. These compounds constitute a broad family of materials, with characteristic layered structures, covering both semiconductors and metallic materials. The great attention arises from the possibility to exfoliate these materials down to single layers with many unique properties, such as thickness dependent band-gap energy, and the possibility of tuning transport properties by phase transitions. The research in the field of transition metal dichalcogenides is also motivated by their high electrocatalytic activity towards several industrially important reactions, such as the hydrogen evolution reaction, as well as many other applications in nano- and optoelectronics. Although these materials are studied intensively, their availability is extremely limited and only disulfides of molybdenum and tungsten are broadly commercially available. Here an optimized procedure for simple direct synthesis of transition metal dichalcogenides using powder metals and elemental chalcogens is reported. The optimized thermal treatment allowed the synthesis scaling of the sulfides, selenides and tellurides of 4th, 5th, 6th, and 7th group of layered-structure dichalcogenides. The synthesized transition metal dichalcogenides were single phase. The phase purity, structure, and morphology were investigated in detail by electron microscopy and EDS, X-ray diffraction, and Raman spectroscopy.

Composition-tuned band gap energy and refractive index in GaSxSe1−x layered mixed crystals

Transmission and reflection measurements on GaSxSe1−x mixed crystals (0 ≤ x ≤ 1) were carried out in the 400–1000 nm spectral range. Band gap energies of the studied crystals were obtained using the derivative spectra of transmittance and reflectance. The compositional dependence of band gap energy revealed that as sulfur (selenium) composition is increased (decreased) in the mixed crystals, band gap energy increases quadratically from 1.99 eV (GaSe) to 2.55 eV (GaS). Spectral dependencies of refractive indices of the mixed crystals were plotted using the reflectance spectra. It was observed that refractive index decreases nearly in a linear behavior with increasing band gap energy for GaSxSe1−x mixed crystals. Moreover, the composition ratio of the mixed crystals was obtained from the energy dispersive spectroscopy measurements. The atomic compositions of the studied crystals are well-matched with composition x increasing from 0 to 1 by intervals of 0.25.

Influence of interparticle electronic coupling on the temperature and size dependent optical properties of lead sulfide quantum dot thin films

We report on the quantum dot (QD) size, temperature, and inter-dot coupling dependence on the optical absorption and emission for PbS QD thin films. Inter-dot coupling is induced by ligand exchange from oleic acid to 1,2-ethanedithiol, and the expected band gap red-shift observed for coupled QD thin films is accompanied by a modification to the temperature-dependence of the band gap energy. The amplitude and temperature dependence of the photoluminescence (PL) Stokes shift support recombination via a mid-gap state and also indicate that the application of band gap-specific models to fit the temperature dependence PL peak energy is inadequate. Electronically coupled QD thin films show PL quenching with decreasing temperature, following a Boltzmann model which is consistent with thermally activated carrier transport. Enhancing the inter-dot coupling results in the dynamic PL decay signal changing from single- to bi-exponential behavior, reveals a size-dependent transport activation energy, and yields a negative temperature dependent band gap energy for the smallest QD diameters.

From In Situ Characterization to Process Control of Quantum Dot Systems

Quantum confined semiconductor nanoparticles (quantum dots, QDs) are promising candidates for various applications in emerging fields like electronics, solar cells, sensors and diagnostics. However, a larger scale production of QDs at high product quality is still missing. One of the key requirements to address this issue in the near future was identified to be a fast and in situ applicable characterization method. Suitable characterization requires knowledge on the full shape of the particle size distributions (PSDs) under investigation. Thus, determination of a mean particle size together with the width of the PSD is not sufficient. In the following, a method will be presented that allows the derivation of arbitrary shaped PSDs for QDs with direct band gap based on their optical absorbance spectra. After validation of the technique by means of ZnO nanoparticles the transfer of the concept to other QD materials like PbS and PbSe will be proven. Therefore we will extend our methodology and show how our approach can be used to derive spectral properties like the size dependent band gap energy. This is realized by proper calibration of the calculation results against PSDs determined by an independent analysis technique like transmission electron microscopy (TEM).

The local crystal structure and electronic band gap of β-sialons

The electronic properties of β-Si3N4 and β-sialons (β-Si6−z Al z O z N8−z ) solid solutions were characterized using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). The electronic structure measurements reveal a single bonding environment for both the Si and Al atoms, which corresponds to a specific nonrandom structural arrangement of the Al–O solute atoms into nanotube-like clusters or channels, running parallel to the c-axis of the β-Si3N4 host structure. Compared to an arrangement of alternating Si–N and Al–O slabs (“Dupree model”), lower total energy and overall better agreement to the experimentally observed electronic features confirm this “Al–O nanotube” model for β-sialon originally proposed by Okatov to be closer to the true chemical topology of the β-Si6−z Al z O z N8−z solid solution series. The β-sialons are shown to be wide band gap semiconductors with the band gap reduction arising from the O p-states moving toward the Fermi level. This band gap reduction provides the ability for direct band gap transitions, which is very important for practical applications. In contrast to the previous observations, both measurement and theory indicate a linear dependence of band gap energy with composition z. The experimental (theoretical) electronic band gaps of β-Si6−z Al z O z N8−z with z = 0.0, 2.0, and 4.0 as determined by XAS/XES (DFT) are 7.2 ± 0.2 (5.88), 6.2 ± 0.2 (3.45), and 5.0 ± 0.2 (2.39) eV, respectively. The considerable discrepancy between experimental and theoretical values is attributable to the shortcomings of DFT, which often underestimates the electronic band gap energy.

Reanalyzing the basic bandgap reference voltage circuit considering thermal dependence of bandgap energy

The basic bandgap reference voltage generator, BGR, is thoroughly analyzed and relations are reconstructed considering dependency of bandgap energy, Eg, to absolute temperature. The previous works ...

Compositional effect of zirconium on structural, electronical, elastic and thermodynamic properties of PbTi(1−x)ZrxO3

The structural, electronic, elastic properties and the Debye temperature of PbTi(1−x)ZrxO3 compounds are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). The lattice constants of the quaternary compounds PbTi(1−x)ZrxO3 increase with the increasing of Zr composition. The calculated lattice constants are found to agree with the available experimental data. The band structures show that those compounds have indirect band gap except x=0.65 and 0.75, and the composition dependence of band gap energy exhibits increasing trend. The calculated elastic constants indicate that all of the PbTi(1−x)ZrxO3 compounds are mechanically stable. The shear modulus, Young’s modulus Poisson’s ratio σ and the ratio B/G are also calculated. Finally, The averaged sound velocity (vm), the longitudinal sound velocity (vl), transverse sound velocity (vt) and the Debye temperature (θD) are obtained.

A modified simplified coherent potential approximation model of band gap energy of III–V ternary alloys

Based on the modification of the simplified coherent potential approximation (SCPA), a model is developed to calculate the composition dependence of the band gap energy of III-V ternary alloys with the same anion. The derived equation is used to fit the experimental band gap energy of InxAl1−xN, InxGa1−xN and AlxGa1−xN with x from 0 to 1. It is found that the fitting results are better than those done by using SCPA. The fitting results are also better than those obtained by using the formula with a small bowing coefficient, especially for InxAl1−xN. In addition, our model can also be used to describe the composition dependence of band gap energy of other III-V ternary alloys.