Value Of Dielectric Function Research Articles

Unveiling the thermoelectric, optical and mechanical performances of Hafnium and Zirconium based Beryllium oxide perovskite

The investigated research study includes ab-initio calculations for Beryllium based oxide perovskite namely XBeO3 (X= Zr and Hf). The study aims to comprehend their mechanical, optical, and thermoelectric properties. The density functional theory is used in conjunction with the full potential linearized augmented plane wave method via WIEN2K code to conduct these studies. ZrBeO3 and HfBeO3 both met the mechanical requirements for stability while having an anisotropic character. Mechanical characteristics demonstrate the brittleness of ZrBeO3 and the ductility of HfBeO3. The determined band profile of both materials shows semiconducting behavior with indirect bandgap. Optical properties are performed within photon energy range of 0–12 eV. ZrBeO3 and HfBeO3 are anticipated to have real dielectric function values of 5.45 and 3.75, respectively. The upward trend of absorption coefficient towards higher energies signifies attenuation in light transmittance through the materials making them strong candidate as UV-absorber. The thermoelectric parameters are finally assessed using the BoltzTrap code and their variances with temperature and chemical potential are also presented. ZrBeO3 and HfBeO3 exhibit intriguing thermoelectric characteristics, with TE efficiency values of 0.24 and 0.4, respectively. Additionally, thermoelectric study shows that ZrBeO3 and HfBeO3 are stable at room temperature and have potential applications in thermoelectric devices.

Transverse and Longitudinal Optical Phonon Analysis of Silicon dioxide (SiO2) from Rice Husk Charcoal

Abstract Rice husk is one of the biggest wastes from rice production. Rice husk can be used as an alternative in producing silicon dioxide (SiO2). Silicon dioxide is widely used in various industries, such as glass, ceramics, tires and electronics. In this paper, charcoal from rice husks is annealed at various temperatures of 800 °C, 850 °C and 900 °C and then characterized using FTIR. In previous studies, analysis using FTIR was able to show the functional groups of Silicon dioxide found in rice husk charcoal. FTIR data modified with the Kramers-Kronig (KK) equation can produce Longitudinal Optics (LO) and Transverse optics phonon (TO) values. The LO and TO values are the optical properties of the Silicon dioxide from rice husk charcoal. In addition, the value of the dielectric function can also be obtained. Based on the results, it shows that the LO values for each treatment were 907.47 cm−1, 958.62 cm−1, and 986.67 cm−1. The TO values for each treatment were 903.61 cm−1, 954.00 cm−1 and 964.41 cm−1. The dielectric function values of each treatment were 512.00 cm−1, 962.48 cm−1 and 1007.00 cm−1. By analyzing the optical-phonon parameters of Transverse Optics (TO) and Longitudinal Optics (LO), we succeeded in characterizing the SiO2 properties with the variations of annealing temperature. We determined that the TO and LO were not-monotonously affected by the variation in annealing temperature, which was determined by studying the optical-phonon wavelengths.

Exploring the physical properties of cubic CsGeBr3-nIn (n= 0, 1, 2, 3) compounds: Ab initio calculations of perovskites prospective for the application in solar cells

The cubic perovskites CsGeBr3-nIn (n = 0, 1, 2, 3) were investigated using the density functional theory (DFT) for their structural, electronic, and optical properties. The present DFT calculations are carried out using three models for exchange-correlation potential, namely PBE, mBJ, and YS-PBE0. The bandgap decreases in the above sequence of compounds except CsGeBrI2, which reveals the smallest bandgap. The mBJ approximation has a larger bandgap than the PBE and smaller than the YS-PBE0. Results of calculations within the YS-PBE0 approach for CsGeBr3 and CsGeI3 agree well with QSGW + SO results. The CsGeBr3-nIn compounds are direct bandgap semiconductors and CBM and VBM are positioned at the R point and determined mainly by Ge s-states and Br(I) p-states, respectively. Analysis of optical properties shows that the DFT calculations within the PBE model consistently produce the highest static dielectric function values, while the YS-PBE0 method gives the smallest values. The curves of optical coefficients shift toward lower energies when decreasing the Br atoms in CsGeBr3-nIn. The studied compounds are semitransparent in the infrared and visible regions and show promising potential for photovoltaic applications, including solar cells.

Tuning of optical performance of CrS2 monolayer using strain engineering

In the present manuscript, the compressive and tensile strain in both uniaxial and biaxial directions are used to investigate the electronic and optical properties of CrS2 monolayer. It is revealed that both types of strain have a significant impact on the bandgap. For tensile strain, the value of Eg keeps decreasing but for compressive strain, the bandgap value first rises to 1.196 eV at 8% of uniaxial strain and then subsequently falls to 1.132 eV at 12% strain. The value of Eg in the biaxial case reaches to 1.75 eV at 6% compressive strain, and subsequently decreases to 0.60 eV at 12% compressive strain. The value of Eg for biaxial tensile strain continues to decrease and reached to 0.11 eV at 12% strain. The value of dielectric function i.e., ε2(ω) changes from 2.97 to 1.82 and then to 1.95 under 8% and 12% uniaxial compressive strain. ε2(ω) also changes from 2.97 to 2.52 and then to 3.30 under 4% and 12% biaxial compressive strain, respectively. A blue as well as a red shift in the absorption edge under compressive and tensile strains, respectively are observed. Our results imply that strain can be a useful tool for altering the optoelectronic properties of CrS2 monolayer.

Carrier mobility and high-field velocity in 2D transition metal dichalcogenides: degeneracy and screening

The effect of degeneracy and the impact of free-carrier screening on a low-field mobility and a high-field drift velocity in MoS2 and WS2 are explored using an in-house ensemble Monte Carlo simulator. Electron low field mobility increases to for MoS2 and to for WS2 when temperature decreases to and carrier concentration is around . In the case of holes, best mobility values were and , reached at similar temperature and carrier concentration conditions while at room temperature these fall to and for MoS2 and WS2, respectively. The carrier screening effect plays a major role at low fields, and low and intermediate temperatures, where a combination of large occupancy of primary valleys and carrier–phonon interactions dominated by relatively low energy exchange processes results in an enhanced screening of intrinsic scattering. For electrons, degeneracy yields to transport in secondary valleys, which plays an important role in the decrease of the low field mobility at high concentrations and/or at room temperature. The high-field drift velocity is not much affected by carrier screening because of an increased carrier scattering with surface optical polar phonons, favouring larger phonon wavevector interactions with small dielectric function values.

Hydrostatic pressure-tuning of photoelectric properties of perovskite Cs2SeI6 through first-principles investigation

Pressure engineering can produce a inner change and atoms redistribution for perovskite Cs2SeI6. As a result, the photoelectric properties of perovskite Cs2SeI6 would be varied under different pressure conditions. Base on the density functional theory (DFT), the photoelectric properties of perovskite Cs2SeI6 under different hydrostatic pressure have been investigated by using the first-principles. The lattice constant, unit cell volume and bond length of perovskite Cs2SeI6 are significantly reduced with the increase hydrostatic pressure. Moreover, perovskite Cs2SeI6 has the optimal optical bandgap value of 1.34 eV under a pressure of 1.5 GPa. Therefore, we only study the photoelectric properties of Cs2SeI6 when the hydrostatic pressure is 0 and 1.5 GPa in this paper. The calculated results show that Cs2SeI6 is stable under the pressure of 0 GPa and 1.5 GPa according to the calculations of Born-Huang stability criterion and phonon dispersion spectrum. It can be found that perovskite Cs2SeI6 are all soft, ductile and anisotropic before and after pressure. In addition, when the hydrostatic pressure is 1.5 GPa, perovskite Cs2SeI6 has larger values of dielectric functions, absorption coefficient and optical conductivity. This study indicates that perovskite Cs2SeI6 is an excellent photovoltaic material after applied hydrostatic pressure, which has great potential in application of perovskite solar cells.

Physical properties of rhodium retrieved from modeling its dielectric function by a simulated annealing approach

Optical, charge carriers transport, quantum mechanics, magnetic, thermal, and plasmonic properties of the transition metal rhodium are considered. An extended Drude-Lorentz (DL) model is applied to describe the dielectric function (DF) of rhodium in a spectral range going from the mid-infrared (12.4 μm) to the vacuum ultraviolet (32 nm). The Drude term of the DF includes, as optimization parameters, the inverse of the high frequency dielectric constant, the volume plasma frequency and scattering frequency of the electrons, the scattering frequency of holes relative to that of electrons, the ratio between the effective masses of electrons and holes, the number of holes per atom relative to that of electrons, and the renormalized times between grain boundary scattering events for electrons and holes. The Lorentz contribution to the DF includes the number of conduction electrons per atom, the oscillator strengths, the resonance energies, and the Lorentzian widths. Values of the parameters involved in the DF are optimized by an acceptance-probability-controlled simulated annealing method that minimizes spectral differences between the real and imaginary parts of the DF values obtained from the literature and those evaluated from the DL parametric formulation, accounting for the presence of electrons and holes as charge carriers. Once an optimized spectral description of the DF of rhodium is obtained, a large set of charge-transport, magnetic, thermal, plasmonic, and quantum mechanics derived quantities are evaluated: mobilities, relaxation times, Fermi velocities, effective masses, electrical and thermal conductivities, heat capacity coefficients, Hall coefficient, diamagnetic and paramagnetic susceptibilities, effective number of Bohr magnetons, Fermi energies and corresponding densities of states, energy loss functions, effective number of charge carriers participating in conduction, and effective number of electrons involved in inter-band transitions.

Probing the local dielectric function of WS2 on an Au substrate by near field optical microscopy operating in the visible spectral range

The optoelectronic properties of nanoscale systems such as carbon nanotubes (CNTs), graphene nanoribbons and transition metal dichalcogenides (TMDCs) are determined by their dielectric function. This complex, frequency dependent function is affected by excitonic resonances, charge transfer effects, doping, sample stress and strain, and surface roughness. Knowledge of the dielectric function grants access to a material’s transmissive and absorptive characteristics. Here we use the dual scanning near field optical microscope (dual s-SNOM) for imaging local dielectric variations and extracting dielectric function values using a pre-established mathematical inversion method. To demonstrate our approach, we studied a monolayer of WS2 on bulk Au and identified two areas with differing levels of charge transfer. The experiments highlight a further advantage of the technique: the dielectric function of contaminated samples can be measured, as dirty areas can be easily identified and excluded for the calculation, being important especially for exfoliated 2D materials (Rodriguez et al., 2021). Our measurements are corroborated by atomic force microscopy (AFM), Kelvin force probe microscopy (KPFM), photoluminescence (PL) intensity mapping, and tip enhanced photoluminescence (TEPL). We extracted local dielectric variations from s-SNOM images and confirmed the reliability of the obtained values with spectroscopic imaging ellipsometry (SIE) measurements.

Insight into electronic and optical properties of Eu+2-doped CaTiO3 from GGA+U calculations

We have carried an analytical review of electronic and optical properties of bulk and slab CaTiO3:Eu+2 by employing the first-principle calculations that are linked with Density Functional Theory (DFT). The properties of bulk and slab have been explored using the generalized gradient approximation with the Hubbard parameter (GGA ​+ ​U) approach. Electronic properties, such as band structure, total and partial density of states (TDOS and PDOS) have been calculated for bulk and slab CaTiO3:Eu+2. Based on the obtained results, the direct band gaps are examined for both bulk and slab with bandgap values 2.839eV and 0.289eV respectively. Besides, the transitions are also observed which are mainly due to O-p, Eu-f, Ti-d, and Eu-d for both bulk and slab. The role of band structure calculation, regarding optical properties, is discussed. Plasmon-energy is the main reason for peaks of the energy loss function,12.5 ​eV, for both bulk and slab. The static values of dielectric functions, absorption coefficient, and refraction are also calculated. Our results revealed that slab CaTiO3:Eu+2 material is more suitable for optical devices due to its direct bandgap nature and small bandgap value (0.289eV).

Modulation of electronic and optical properties of ZnO by inserting an ultrathin ZnX (X = S, Se and Te) layer to form short-period (ZnO)5/(ZnX)1 superlattice

Abstract We demonstrate that the electronic and optical properties of ZnO can be improved by inserting into it with an ultrathin ZnX (X = S, Se and Te) layer to form short-period (ZnO)5/(ZnX)1 superlattices (SLs), which is a very promising substitution for p-type doping of ZnO. Results show that the SLs have some superior properties compared with bulk ZnO due to the strong modulation of the ZnX layer. The valence band arrangement of heavy hole (HH), light hole (LH) and crystal-field split-off hole is in the decreasing order from the valence band maximum (VBM) of (ZnO)5/(ZnX)1. The important band edge states are mainly determined by Zn and X atoms in the ZnX layer. The HH and LH have a very large effective mass along the c-axis of (ZnO)5/(ZnX)1. The peak value of dielectric function and absorption coefficient along the x/y direction at fundamental absorption edge is larger than that along the z direction, which indicates a transverse electric polarized light emission from c-plane.

Temperature dependence of band gap ratio and Q-factor defect mode in a semiconductor quaternary alloy hexagonal photonic-crystal hole slab

We present numerical predictions for the photonic TE-like band gap ratio and the quality factors of symmetric localized defect as a function of the thickness slab and temperature by the use of plane wave expansion and the finite-difference time-domain methods. The photonic-crystal hole slab is composed of a 2D hexagonal array with identical air holes and a circular cross section, embedded in a non-dispersive III–V semiconductor quaternary alloy slab, which has a high value of dielectric function in the near-infrared region, and the symmetric defect is formed by increasing the radius of a single hole in the 2D hexagonal lattice. We show that the band gap ratio depends linearly on the temperature in the range 150–400 K. Our results show a strong temperature dependence of the quality factor Q, the maximum (\(Q = 7000\)) is reached at \(T = 350\,\hbox {K},\) but if the temperature continues to increase, the efficiency drops sharply. Furthermore, we present numerical predictions for the electromagnetic field distribution at \(T = 350\,\hbox {K}.\)

Effect of reactor pressure on optical and electrical properties of InN films grown by high‐pressure chemical vapor deposition

AbstractThe influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In‐N and N‐In bonding states as well as small concentrations of In‐O and N‐O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm‐3 and the room‐temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Free-carrier absorption and Burstein–Moss shift effect on quantum efficiency in heterojunction silicon solar cells

Effects of the carrier concentration on the dielectric function of indium tin oxide (ITO) films were investigated by spectroscopic ellipsometry using Tauc–Lorentz and Drude oscillator terms. The real and imaginary parts of dielectric function values increased with increasing carrier concentration. Samples with lower refractive indices had (440) preferable crystallographic orientations. A more significant Burstein–Moss (B–M) effect leads to a lower extinction coefficient (k) value in the short-wavelength regions, while more Free-carrier absorption (FCA) leads to a higher k value in the near-infrared region. The electrical properties obtained from optical studies, such as the carrier concentration and the carrier mobility, were compared with those obtained by van der Pauw measurements. The observed discrepancies between optically and electrically obtained values were due to carrier transport and grain boundaries. ITO films with a widened band gap energy due to the B–M shift are key to improve quantum efficiency (QE) at short-wavelengths in heterojunction silicon (HJ) solar cells. The reduced QE at long-wavelengths is attributed to optical scattering in the ITO films due to FCA. Despite widening the band gap, the short-circuit current density (Jsc) decreased from 36.27 mA/cm2 to 34.17 mA/cm2 with increasing carrier concentration from 6.15 × 1020 cm−3 to 9.84 × 1020 cm−3. This result demonstrates that the FCA effect may affect Jsc more in HJ solar cells. The ITO film, prepared at the lowest carrier concentration, was used as an antireflection layer to fabricate HJ solar cells via optimization of the passivated layer. It resulted in a cell efficiency of 19.12% and an open-circuit voltage of 702 mV.

Dielectric function representation by B‐splines

AbstractAccurate dielectric function values are essential for spectroscopic ellipsometry data analysis by traditional optical model‐based analysis techniques. In this paper, we show that B‐spline basis functions offer many advantages for param‐ eterizing dielectric functions. A Kramers–Kronig consistent B‐spline formulation, based on the standard B‐spline recursion relation, is derived. B‐spline representations of typical semiconductor and metal dielectric functions are also presented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Progress in the room-temperature optical functions of semiconductors

Optical functions of a specific semiconductor in a particular wavelength region are often needed in optics and optoelectronics research. Basic optical properties of some materials are available in handbooks, but recent advances in the experimental techniques for growth, sample preparation and determination of optical functions, as well as theoretical advances in modeling the optical functions, prompted demand for a review of the state of the art of the optical functions of some important semiconductor materials. This paper gives an overview of the experimental methods for the determination of optical functions in the 1–6 eV spectral range and the available models for calculating the optical functions. Also, the paper summarizes the progress in the study of optical functions of several important semiconductors such as GaP, InP, InAs, GaSb, InSb, AlSb, AlxGa1−xAs, HgxCd1−xTe, SixGe1−x, GaN, InN, AlN, 6H–SiC, and ZnO. Besides discussing the available data and expected positions of optical transitions, the paper focuses on the model of the dielectric function which allows reproducing accurately the experimental dielectric function values for all the materials considered here.

Microstructural information related to thin film optical measurements

The optical properties of a material in the infrared spectral range are predominantly determined by composition and structure. Methods are presented which are used to evaluate the information contained in optical properties of microscopically heterogeneous systems and to obtain parameter values. These methods are based on effective-medium theory and utilize accurate bulk dielectric function values of constituents. Infrared ellipsometric techniques may be used to provide optical data, which can be related to the microstructural information. Theories that establish limits on allowed values of the dielectric response of two-component systems, regardless of their microstructure, provide an indication of the realiability of these parameters. Results are presented for vanadium oxide films.

Phonon density of states for MgF 2

Effects of the carrier concentration on the dielectric function of indium tin oxide (ITO) films were investigated by spectroscopic ellipsometry using Tauc–Lorentz and Drude oscillator terms. The real and imaginary parts of dielectric function values increased with increasing carrier concentration. Samples with lower refractive indices had (440) preferable crystallographic orientations. A more significant Burstein–Moss (B–M) effect leads to a lower extinction coefficient (k) value in the short-wavelength regions, while more Free-carrier absorption (FCA) leads to a higher k value in the near-infrared region. The electrical properties obtained from optical studies, such as the carrier concentration and the carrier mobility, were compared with those obtained by van der Pauw measurements. The observed discrepancies between optically and electrically obtained values were due to carrier transport and grain boundaries. ITO films with a widened band gap energy due to the B–M shift are key to improve quantum efficiency (QE) at short-wavelengths in heterojunction silicon (HJ) solar cells. The reduced QE at long-wavelengths is attributed to optical scattering in the ITO films due to FCA. Despite widening the band gap, the short-circuit current density (Jsc) decreased from 36.27 mA/cm2 to 34.17 mA/cm2 with increasing carrier concentration from 6.15 × 1020 cm−3 to 9.84 × 1020 cm−3. This result demonstrates that the FCA effect may affect Jsc more in HJ solar cells. The ITO film, prepared at the lowest carrier concentration, was used as an antireflection layer to fabricate HJ solar cells via optimization of the passivated layer. It resulted in a cell efficiency of 19.12% and an open-circuit voltage of 702 mV.