Absorption Edge Characteristics Research Articles

On analysis of structural and optical absorption characterization of Bi35Sb5Se60 nanostructured thin films for photosensing application

In this extensive study, the primary focus is directed towards the intricate exploration of the optical absorption characteristics inherent in nanostructured Bi35Sb5Se60 thin films, prepared by thermal evaporation under a vacuum of 10−5 Torr, ensuring high purity and uniformity for optoelectronic applications. With a particular emphasis on their prospective application in photosensing devices. The research employs a comprehensive approach, integrating various spectrophotometric measurements that encompass both transmission and reflection analyses. The meticulous examination of the absorption edge characteristics reveals a direct energy gap of 1.82 eV, providing a foundational understanding of the material's electronic structure. The determination of the dispersion energy at 6.35 eV and the oscillator energy at 2.96 eV further enriches the understanding of the material's optical behavior, shedding light on its capacity to interact with incident light. A noteworthy highlight emerges with the identification of high photosensitivity exhibited by the heterojunction, particularly under lower reverse bias conditions, underscoring its potential utility in photosensing applications. The PL spectra reveal a highly broadened defect emission band with peaks at 475 nm, 490 nm, and 540 nm, providing insights into the recombination processes within the material. The identification of emission peaks related to the conduction band edge and acceptor levels further enhances the comprehension of the material's optical and electronic characteristics. underscores the potential of these nanostructured thin films for efficient photosensing devices. The findings presented in this research not only advance the understanding of the material's properties but also lay a robust foundation for further research and development in the design and optimization of photoresponsive devices within the realm of optoelectronic technologies.

Absorption edge characteristics of GaAs, GaSb, InAs, and InSb

The physical characteristics of the fundamental absorption edge of semi-insulating GaAs and unintentionally doped GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed to describe the key characteristics of the absorption edge. Among these parameters are the bandgap energy, the characteristic energy of the Urbach tail, and the absorption coefficient at the bandgap energy. The results indicate that the Coulomb interaction strongly influences the shape of the band edge with progressively less influence as the bandgap energy decreases. The energy dependence of the optical transition strength is observed to be nearly constant in narrow bandgap InSb.

Influence of substrate temperature on the chemical, microstructural and optical properties of spray deposited CH3NH3PbI3 perovskite thin films

Methylammonium lead iodide (CH3NH3PbI3) perovskite has been proposed as a key component of thin film solar cells. In this work, the influence of substrate temperature on the properties of spray deposited CH3NH3PbI3 thin films was investigated. The main purpose of this work was to determine the best conditions to deposit and crystallize CH3NH3PbI3 perovskite thin films in a single step. An automatic spray system was used for depositing a solution of lead iodide and methylammonium iodide onto heated substrates under ambient conditions for just one second. After deposition the samples were held at the same temperature for 30 s. The substrate temperature varied from 60 °C to 140 °C. Substrate temperatures in the low range (60 °C–80 °C) led to films with needle-like or dendritic grains, extra phases, and poor coverage due to the low solvent evaporation rate, while depositions above 120 °C led to thermal decomposition of the film. The best results were obtained for substrate temperatures of 100 °C and 120 °C, which produced dense perovskite films with large equiaxed grains and good coverage. The transmittance spectra of these films showed the perovskite characteristic absorption edge at 780 nm. In addition, the Fourier-transform infrared spectra did not show any peaks associated with the solvent, confirming its evaporation and indicating that a 31 s overall processing time is enough to achieve a full perovskite crystallization.

Structural, Optical and Magnetic Properties of Pristine, (Mn, Al) co-doped ZnO Nanocrystallites Synthesized via co-Precipitation Method

Undoped and (Mn, Al) co-doped Zinc Oxide nanoparticles are synthesized by chemical co-precipitation method at room temperature effectively via poly ethylene glycol (PEG) as stabilizing agent. XRD data reveals that all the concentrations acquire hexagonal wurtzite crystal structure with no secondary peaks concerning to Al or Mn, indicating successful dissolution of Al and Mn in to ZnO host lattice. The exact particle size is estimated using TEM illustrations, which is confirmed through the XRD results. EDS spectrum shows, no impurities are present in the samples other than manganese and aluminum. The absorption spectra of all the samples reveal characteristic absorption edge in the vicinity of 375 nm. PL spectra show that all the concentrations include defect associated peaks in the visible region. VSM measurements reveal the ferromagnetic nature for co-doped samples. Copyright © VBRI Press.

Characterictics of absorption edge of SiO 2 films

Silicon dioxide (SiO2) is one of the most widely used in various optical system as film material. The micro-structure and defects of SiO2 films are of great importance to the functions and performance of these optical systems. In this paper, the absorption edge characteristics of single layer SiO2 films prepared by electron beam evaporation, ion assisted deposition, and magnetron sputtering are investigated in detail via calculating their absorption edge spectrum, which is divided into three regions: the strong absorption, exponential absorption, and weak absorption regions. The bandgap, Urbach tail, and concentration of oxygen deficiency centers (ODC) are obtained by analyzing the measured absorption spectrum. By analyzing the bandgap, Urbach tail, and ODC data of SiO2 films prepared with different deposition techniques and annealed at different temperatures, the atomic arrangement as well as micro-defect information of SiO2 films are obtained and compared. Such information of SiO2 films are important to the preparation of high-performance optical coatings employing SiO2 as the low refractive index material.

Electronic States and Optical Transitions in an Asymmetric Quantum Dot Molecule

In the framework of adiabatic approximation the electronic states and direct interband absorption of light in the asymmetric double quantum dot molecule (QDM) having a shape of Cassini lemniscate revolution are discussed. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are derived. Nonmonotonic split and step-like behavior of the energy spectrum is revealed due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge characteristics depending on the QDM geometrical sizes are also revealed.

Synthesis, Surface Characterization and Photocatalytic Activity of TiO2 Supported on Almond Shell Activated Carbon

Three types of photocatalysts were synthesized by metal organic chemical vapor deposition and impregnation methods using the almond shell activated carbon as support. These photocatalysts denoted by (TiO2/ASAC (V), TiO2/ASAC (I1) and TiO2/ASAC (I2)) were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS) and nitrogen adsorption–desorption isotherms. SEM observation shows that TiO2 was deposited on activated carbon surface. XRD results confirm that TiO2 existed in a mixture of anatase and rutile phases. The DRS spectra show the characteristic absorption edge of TiO2 at approximate 380 nm corresponding to the optical band gap of 3.26 eV. Besides, FTIR spectrum indicated the presence of (Ti–O) groups. The specific surface area of photocatalysts decreased drastically in comparison with the original activated carbon. The catalysts were very efficient for the photodegradation of total organic carbon (TOC) from industrial phosphoric acid solution under UV irradiation. The kinetics of photocatalytic TOC degradation was found to follow a pseudo-first-order model. The prepared TiO2/ASAC showed high photoactivity for the photodegradation of TOC in the following order: TiO2/ASAC (V) > TiO2/ASAC (I1) > TiO2/ASAC (I2) > ASAC > TiO2 (P25).

Amorphous phase as possible origin of additional absorption bands in polycrystalline ZnO films

The optical absorption edge characteristics of undoped zinc oxide (ZnO) films with varying thickness (~100 to ~1000nm) prepared by means of polymeric precursor method (Pechini) on glass substrates have been investigated. Besides the normal absorption edge at about 3.3eV, related to the transition between the valence and the conduction bands, the films feature additional absorption bands in the energy region around 4eV. The experimental results show that this band is redshifted with increasing film thickness approaching a value of ~3.7eV for the thickest films (~1000nm). In contrast, increasing film thickness resulted in a significant blue shift of the bulk absorption edge stabilizing at ~3.3eV for the thickest films which corresponds to the single-crystal value. Although the appearance of additional absorption bands in ZnO has been attributed to excitonic resonances as those related to the exciton/longitudinal optical (LO) phonons, this argument is hardly sustainable for ZnO films with a high concentration of defects as those reported in the present investigation. The achieved results suggest that this phenomenon is rather linked to reduced crystallinity of the films. Certainly, the redshift of the additional optical band gap back towards the single‐crystal value with increasing film thickness was strong evidence that the additional absorption bands should be related to an amorphous phase. Particularly, it is possible that the amorphortization of the film increases the localized states in the conduction band. The improvement of the crystal quality with increasing film thickness was distinctly verified by X-ray diffraction analysis.

Structure, Optical Properties, and Magnetism of the Full Zn1–xCuxWO4 (0 ≤ x ≤ 1) Composition Range

Microcrystalline and submicrometer powders of Zn(1-x)Cu(x)WO(4) (0 ≤ x ≤ 1) have been prepared by a solid-state synthesis from stoichiometric quantities of the constituent d-block metal oxide and tungsten oxide as well as from a Pechini sol-gel synthesis starting from the d-block metal nitrate and ammonium metatungstate. The stoichiometry of the product is confirmed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. X-ray diffraction shows that for the entire range of compositions, a single-phase product crystallizes in the wolframite structure, with a symmetry-lowering transition from P2/c to P1[overline] at x = 0.20, concomitant with the first-order Jahn-Teller distortion of Cu(2+). Far-IR spectroscopy corroborates that symmetry lowering is directly related to the tetragonal distortion within the CuO(6) octahedra, with the Zn-O A(u) symmetry mode at 320 cm(-1) (x = 0) splitting into two stretches at 295 and 338 cm(-1) (x = 0.3). UV-vis-NIR spectroscopy shows an optical absorption edge characteristic of an indirect band gap that linearly decreases in energy from 3.0 eV (x = 0) to 2.25 eV (x = 1). SQUID magnetometry shows that Zn(1-x)Cu(x)WO(4) (0.1 ≤ x ≤ 1) has an effective moment of 2.30 ± 0.19 μ(B) per mol copper, typical of Cu(2+) in extended solids. For high concentrations of copper (x ≥ 0.8), two transitions are observed: one at high-temperature, 82 K (x = 1.0) that decreases to 59 K (x = 0.8), and the Néel temperature, 23.5 K (x = 1.0) that decreases to 5.5 K (x = 0.8). For x < 0.8, no long-range order is observed. A physical 1:1 mixture of both CuWO(4):ZnWO(4) shows magnetic ordering identical to that of CuWO(4).

The Optical Properties of Sputtered ZnO Films

The ZnO films were deposited on quartz glass substrate by RF magnetron sputtering. The ZnO films are highly c-axis oriented with the (002) plane parallel to the substrate. The grains size estimated by the XRD diffraction rises with increase in the Ar/O2 ratio. The optical absorption edge and the near absorption edge characteristics of ZnO films have also been investigated. The optical band gaps of the films have been evaluated from the absorption coefficient of ZnO films. It is found that the value of the optical band gap varys from 3.26 eV to 3.32 eV due to the energy gap enhancement induced by quantum confinement. This variation of band gap can be explained with the theory developed by Brus. For all the films, the absorption coefficient shows the Urbach exponential dependence in the near band edge regime. It is also observed that the value E0 decreases with the Ar/O2 ratio increasing.

Table-top reflectometer in the extreme ultraviolet for surface sensitive analysis

We suggest a reflectometer for thin film analysis based on a plasma-discharge source utilizing extreme ultraviolet (XUV) radiation in a wavelength region of 4–40 nm. In contrast to other laboratory based reflectometers, which are designed for the near normal incidence case to characterize XUV multilayer optics and maskblanks, in our approach we move to a selectable fixed grazing incidence angle that enables surface sensitive analysis of almost arbitrary ultrathin film systems providing high elemental contrast due to the characteristic absorption of XUV by matter. Most materials (e.g., Si, Al, Gd, and Ag) exhibit characteristic absorption edges allowing not only to determine layer thicknesses, surface or interlayer roughnesses in a stack, but also elemental composition and even analyzing the absorption fine structures. Together with our simulations we show that our polychromatic approach makes it possible to provide all these parameters in one measurement.

Resonant anomalous X-ray reflectivity as a probe of ion adsorption at solid–liquid interfaces

We discuss new opportunities to understand processes at the solid–liquid interface using resonant anomalous X-ray reflectivity (RAXR). This approach is illustrated by determination of element-specific density profiles at mica surfaces in aqueous electrolyte solutions containing Rb + and Sr 2+. The total interfacial electron density profile is determined by specular reflectivity (i.e., reflected intensity vs. momentum transfer, q, at an energy, E, far from any characteristic absorption edge). RAXR spectra (i.e., intensity vs. E at fixed q) reveal element-specific ion distributions. Key differences in the interaction of Rb + and Sr 2+ with mica are observed using resonant anomalous X-ray reflectivity: Rb + adsorbs in a partially hydrated state, but Sr 2+ adsorbs in both fully and partially hydrated states.

Behaviour of impurity elements during the weathering of ilmenite

AbstractX-ray spectromicroscopy has been applied to the characterization of weathered ilmenite sand samples from Australian localities. X-ray absorption near edge spectroscopy (Xanes) studies were performed at the K-edges of the major elements Fe and Ti and minor impurity elements Mn and Cr. An extended suite of reference samples with crystallite sizes ranging from 1 nm to μm size were measured to establish if the absorption edge characteristics were influenced by crystal size effects. No changes were detected for oxides of Cr3+, Fe3+ or Ti4+, but the mixed Fe2+/Fe3+ oxide, Fe3O4, showed an edge shift to higher energies (by 1.5 eV) in a nanocrystalline sample. The Xanes study of a composite ilmenite grain with an unweathered primary ilmenite core and a highly weathered rim showed that Fe was present as Fe2+ in the core and Fe3+ in the rim whereas Mn was present as Mn2+ in both core and rim. Chromium, which is incorporated into the grains during weathering, is present predominantly as Cr3+, although minor (~15%) Cr6+ also occurs in highly weathered grains. The absorption K-edges of Fe3+ and Mn2+ are shifted markedly (by 2–3 eV) to higher energies in titanate phases relative to the binary oxides Fe2O3 and MnO.

Effect of lithium impurity on the opto-electrical properties of zinc oxide films

Lithium-doped zinc oxide (ZnO) films were prepared on C-plane sapphire substrates by the e-beam deposition technique in vacuum. The dependences of the optical absorption edge characteristics, structural, and photoelectrical properties on Li content (from 0 to 10 at.%) were investigated. It is found that the variations of the energy bandgap Eg, the spread in the tail of the band edge (parameter E0), the electrical resistivity ρ, and the lattice constant c with impurity concentration do not have monotonic character. Abrupt jumps of the optical and electrical characteristics were observed as the impurity concentration changed near 0.8 at.% Li. The significant enhancement of photoconductivity response for 0.8 at.% Li-doped ZnO films also correlates with the enhanced electrical resistivity up to 2 × 106 Ω cm and the appearance of a photoinjection current. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasmadispersion of multisubband electron systems over liquid helium

Density-density response functions are evaluated for nondegeneratemultisubband electron systems in the random-phase approximation forarbitrary wavenumber and subband index. We consider bothquasi-two-dimensional and quasi-one-dimensional systems forelectrons confined to the surface of liquid helium. The dispersionrelations of longitudinal intrasubband and transverse intersubbandmodes are calculated at low temperatures and for long wavelengths.We discuss the effects of screening and two-subband occupancy on theplasmon spectrum. The characteristic absorption edge of theintersubband modes is shifted relatively to the single-particleintersubband separation and the depolarization shift correction canbe significant at high electron densities.

Correlation between titania film structure and near ultraviolet optical absorption

Titania is a material with structural flexibility, and as a result, readily forms both crystalline polymorphs and an amorphous structure in thin films grown near room temperature. The goal of this study is to correlate fundamental optical absorption edge characteristics with the phase constituency of titania films. To that end, films with coexistent rutile, anatase, and amorphous constituents were sputter deposited onto fused silica and 〈111〉-Si substrates. The films were then subjected to cyclic annealing in air at moderate temperature (700 and 1000 °C) to affect phase changes. Bragg–Brentano x-ray diffraction was used for phase identification and near ultraviolet-visible transmission and reflection spectrophotometry was used to determine the optical absorption coefficient at the onset of interband transitions. The optical absorption coefficient was modeled within the framework of the coherent potential approximation (CPA), with Gaussian site disorder introduced into the valence and conduction bands of a perfect virtual crystal. Two parameters of the disordered crystal were defined: the optical band gap, Ex, and the slope of absorption edge, Eo. The results are discussed in terms of two extreme cases: (1) film states containing a large rutile volume fraction (0.70–1) share a rutile virtual crystal, with Eg=3.22 eV. Data for these states was combined with single crystal data to develop an expression interrelating Eg, Ex, and Eo. This expression is applicable to any structure with a rutile virtual crystal. The relationship between structural disorder (i.e., the volume fraction of amorphous material) and electronic disorder (i.e., Eo), is quantitatively consistent with the CPA model. (2) Film states containing a small rutile volume fraction (0.02–0.17), and hence a large anatase+amorphous component, share a nonrutile virtual crystal, with Eg=3.41 eV. The effect of increasing the structural disorder (i.e., the rutile volume fraction), in these states is to shift Ex to lower values, which is qualitatively consistent with the CPA model. Furthermore, anatase and amorphous components can be modeled using the same nonrutile virtual crystal, indicating these structures have a common short-range order in the sputter deposited films of this study.

Combined Chemical Microanalysis Using SAM and X-PEEM

Both, PEEM (Photoemission Microscopy) and SAM (Scanning Auger Microprobe) are surface sensitive techniques providing spatially resolved chemical analysis. We compare these techniques and present results on thin High Temperature Superconductor films. The X-PEEM makes use of tuneable monochromatised soft X-rays from a synchrotron radiation source to excite electrons from core levels. A higher secondary electron yield is observed when tuning the source above one of the characteristic absorption edges. These true secondaries are detected with the electron microscope in parallel imaging mode. Therefore, PEEM gives besides the imaging additional information on the chemical bonding and its local order making use of NEXAFS and EXAFS spectroscopies.

Sputter deposition of tungsten trioxide for gas sensing applications

Tungsten trioxide thin films were grown by reactively sputtering a circular WO3 target in different Ar–O2 atmospheres. After deposition, data on the structural, optical and electrical properties were obtained by using transmission electron microscopy (TEM) to examine the structure and the morphology of the films, UV-VIS spectrophotometry to determine optical absorption edge characteristics, and Hall effect measurements to determine the change carrier mobility and the film resistivity. In addition, the film resistance variations in controlled atmospheres were examined and the gas sensing properties of films grown under different conditions were compared. The aim of the study was to examine the effect of O2 concentration in the sputtering atmosphere on structural, optical, electrical and sensing properties. © 1998 Kluwer Academic Publishers

Growth and structure of internal Cu/Al 2O 3 and Cu/Ti/Al 2O 3 interfaces

Thin Cu films and Cu/Ti bilayers were grown on (0001) α-Al 2O 3 by molecular beam epitaxy (MBE) at substrate temperatures ranging from 373 K to 473 K. Results on growth behaviour, low-energy interface orientation relationships and the nature of bonding at the interfaces are reported. Transmission electron microscopy (TEM) and in-situ reflection high-energy electron diffraction (RHEED) studies show that Cu/Ti bilayers nucleate and grow epitaxially on the Al 2O 3 substrates with an (111) Cu〈110〉 Cu∥(0001) Ti〈21̄1̄0〉 Ti∥(0001) Al 2 O 3 〈101̄0〉 Al 2 O 3 orientation relationship. In contrast, Cu films on (0001) α-Al 2O 3 grow in two stages. Initially, the Cu nuclei have random orientation but they realign with increasing film thickness into an heteroepitaxial (111) Cu〈110〉 Cu∥(0001) Al 2O 3 〈101̄0〉 Al 2O 3 orientation relationship. In high-resolution transmission electron microscopy (HRTEM) no reaction phases are observed at the atomically sharp Cu/Al 2O 3 and Ti/Al 2O 3 interfaces. Electron energy-loss spectroscopy (EELS) of the interfaces reveals bonding between copper or titanium and the oxygen sublattice of Al 2O 3, The interfacial widths of copper and titanium atoms involved in the bonding are estimated using the characteristic absorption edge data. In the case of Ti/Al 2O 3 the interfacial width is estimated to be 0.5 nm±0.1 nm. The lower affinity of Cu to oxygen results in a smaller interfacial width of 0.34 nm±0.06 nm. These values correspond to the apparent fraction of the metal layers that participate in the bonding with the oxygen sublattice of (0001) α-Al 2O 3.

Sputter deposition of boron nitride using neon–nitrogen discharges

Boron nitride films were grown by sputtering a hexagonal boron nitride (BN) pressed powder target in Ne/N2 discharges at two levels of substrate bias. Optical emission spectrometry was used to monitor the N2+ ion population in the discharge. Postdeposition, information about B–N bonding in the films was obtained using near ultraviolet-visible spectrophotometry to determine optical absorption edge characteristics, and infrared transmission spectrometry to determine vibrational frequencies of B–N groups. The results are compared to films grown in Ar/N2 discharges at the same excitation conditions. The goal was to examine the effect of an enhanced N2+ ion population (characteristic of Ne/N2 discharges) on B–N bonding. The results show that N2+ enhancement alone, without substrate bias, cannot produce sp3 bonding. Biasing results in sp3-bonded BN with wurtzite short-range order. Without exception, films on grounded substrates have sp2 bonding. A simple model is proposed to relate optical edge disorder in sp2-bonded BN (associated with B–N bond length randomness) to N2+ adsorption at the growth interface.