Direct Transition Energy Research Articles

Optical characterization ofBi2Se3in a magnetic field: Infrared evidence for magnetoelectric coupling in a topological insulator material

We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor ${\text{Bi}}_{2}{\text{Se}}_{3}$. Far-infrared and midinfrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal $c$ axis of this layered material and supplemented with UV-visible ellipsometry to obtain the optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$. With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as an increase in the absorption edge due to direct electronic transitions. Magnetic fields $H\ensuremath{\parallel}c$ dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum free-carrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field.

Modifications of direct transition energies in β-FeSi2 epitaxial films grown by molecular beam epitaxy

Modifications of direct transition energies by crystal lattice deformations were confirmed in β-FeSi2 epitaxial films on Si(111) substrates. With an increasing of annealing temperature (Ta), lattice constants of a-axis expanded, and those of b- and c-axis shrank, resulting in the volume reduction in −0.2%. In photoreflectance measurements, the direct transition energy at Y point in the Brillouin zone of β-FeSi2 (∼0.92 eV) was shifted to lower photon energy with the increase in Ta. These results revealed that the band-gap energy was modulated systematically by the lattice deformation, which suggests a possibility of band-gap engineering by the lattice deformation in β-FeSi2 epitaxial films on Si(111) substrates.

Effect of annealing on optical properties of thin films with β-FeSi 2 quantum dots

Reactive deposition epitaxy was used to prepare β-FeSi 2 nanodots on a Si(001) surface. The influence of annealing on optical properties has been studied. Annealing increases the particle size and decreases absorption. The indirect optical transition at 0.84–0.89 eV dominates the absorption spectra below 1.3–1.5 eV. The film with smaller nanoparticles exhibits higher absorption with direct optical transition energies in the 1.3–1.5 eV range. It suggests that direct transition is dominant only in small (around 3 nm diameter) β-FeSi 2 nanodots. Larger particles are dominated by the indirect optical transition.

Photoreflectance study of β-FeSi2 epitaxial films grown by molecular beam epitaxy

Band structure of β-FeSi2 epitaxial and polycrystalline films was investigated by photoreflectance (PR) measurements. In the PR spectra, two critical points were obtained at Eg = 0.918 eV and Ei = 0.895 eV in the films. The Eg consistent with the direct transition energy at Y point in the Brillouin zone of β-FeSi2. The Ei was assigned as a transition-energy between conduction band and the acceptor-level originating from Si vacancies in β-FeSi2. The energy from top of valence band to the acceptor-level was determined to be 23 meV.

Renormalization of the band gap in highly photoexcited type-II ZnSe/BeTe structures

For the type-II ZnSe/BeTe heterostructures, a large (∼0.1 eV) red shift of the edge of interband recombination in the ZnSe layers is observed at high densities of spatially separated photoexcited electrons and holes (∼1013 cm−2). The observed magnitude of renormalization of the band gap exceeds the magnitudes predicted by the multiparticle theory for dense type-I electron-hole systems at the same concentrations of two-dimensional charge carriers. Numerical calculations show that macroscopic electric fields induced by separated charges have a profound effect on the energy of direct transitions in type-II structures, resulting in an additional decrease in the energy of the transitions. In wide structures, where the ZnSe layer thickness is ≳ 15 nm, the renormalization effect is less pronounced. This is attributed to incomplete spatial separation of photoexcited charge carriers in the case of profound band bending and, thus, to the less-pronounced effect of electric fields.

Phase composition and optical properties of thin films based on lanthanum and tungsten oxides

We have studied the phase composition and optical properties of 150-to 500-nm-thick films based on lanthanum and tungsten oxides, produced by magnetron sputtering followed by heat treatment at 773 K in vacuum and flowing oxygen. The oxide films have been found to have high transmission in the range 250 to 900 nm and an absorption band between 190 and 250 nm. Analysis of their absorption edge indicates that its complex structure is due to the presence of several phases. The energies of direct transitions evaluated from the spectra of the films coincide with the band gaps of the phases present in the films.

Investigation of oxidation process and properties of indium and tin-based thin-film heterostructures

Solid-phase processes in thin films (∼200 nm) based on tin and indium oxides (ITO structures) prepared by magnetron sputtering from a composite target (93 at % In and 7 at % Sn) and by layer-by-layer deposition of In/Sn/Si and Sn/In/Si structures from two magnetrons in a single vacuum cycle have been investigated in the work under their oxidation in an oxygen flow. Two ways of optically transparent semiconductor film formation have been compared using near-edge fundamental absorption spectroscopy, x-ray diffraction analysis, and electron microscopy and dynamics of the change in their optical and structure properties has been studied. In the case of oxidation of the layer-by-layer deposited structures, the heterogeneous phase composition of the film is confirmed both by the XRD data and by the optical results. Only wide-band-gap phases with an energy of direct transitions of 3.5–3.6 eV have been found in the films prepared by magnetron sputtering from a composite target after their oxidation. These wide-band-gap phases are associated with In2O3 oxide and a tin-doped indium oxide compound.

Growth and characterization of molybdenum-doped rhenium diselenide

Single crystals of molybdenum-doped ReSe 2 have been grown by chemical vapor transport (CVT) process with Br 2 as a transporting agent. XRD has been used to confirm the triclinic-layered structure of the as-grown undoped and doped samples. The optical properties are studied by anisotropic photovoltage and electrolyte electroreflectance measurements at 300 K. Both indirect and direct transition energies of the doped samples all show different degrees of red shifts for molybdenum-doped samples. The broadening parameters of the excitonic transition features broadened significantly for Mo-doped ReSe 2 due to impurity scattering in layered structures. The electrical properties are investigated by the anisotropic conductivity and Hall measurements at 300 K. The influence of optical and electrical properties from the molybdenum dopant were compared and discussed.

Effect of annealing on properties of ZrSe 2 thin films

Thin films of ZrSe 2 have been prepared on stainless steel and fluorine-doped tin oxide-coated glass substrates using electrodeposition technique at potentiostatic mode. Double-distilled water containing precursors Zr and Se with ethylene diamine tetra-acetic acid disodium salt as a complexing agent was used to obtain good quality deposits by controlling the rate of reaction. The preparative parameters such as concentration of bath, deposition time, bath temperature, pH of the bath and annealing temperature have been optimized using photoelectrochemical (PEC) technique. The films deposited at optimum preparative parameters are annealed at different temperatures. The film annealed at 200 °C shows more photosensitivity. The as-deposited and annealed films at 200 °C have been characterized by X-ray diffraction (XRD), energy dispersive analysis by X-ray (EDAX), optical absorption and scanning electron microscopy (SEM). The XRD analysis of the as-deposited and annealed films showed the presence of polycrystalline nature with hexagonal crystal structure. EDAX study reveals that deposited films are almost stoichiometric. Optical absorption study shows the presence of direct transition and band gap energies are found to be 1.5 and 1.38 eV, respectively, for the as-deposited and annealed films. SEM study revels that the grains are uniformly distributed over the surface of substrate for the as-deposited as well as annealed film, which indicates formation of good and compact type of crystal structure.

Urea Coordinated Titanium Trichloride TiIII[OC(NH)2]6Cl3: A Single Molecular Precursor Yielding Highly Visible Light Responsive TiO2 Nanocrystallites

A coordination complex, Ti(III)[OC(NH2)2]6Cl3, was first synthesized via reacting hot alcoholic solutions of TiCl3 and urea, which was subsequently employed as a molecular precursor for nanocrystalline TiO2 via thermal decomposition. Fourier transform IR spectroscopy confirmed C=O-->Ti coordination bond formation, while Rietveld refinement revealed a hexagonal crystal structure (space group: Pc1) for the complex with a = b = 16.438(4) A, c = 15.423(3) A, alpha = beta = 90 degrees , gamma = 120 degrees , and V = 3608.9(13) A3. Thermal decomposition and phase evolution processes of the complex were investigated in air by combined means of elemental analysis, Fourier transform IR, differential thermal analysis/thermogravimetry, X-ray diffraction, and Raman spectroscopy. Characterizations of the resultant TiO2 powders were achieved by scanning electron microscopy, high-resolution transmission electron microscopy, the Brunauer-Emmett-Teller analysis, thermal desorption spectroscopy, and UV-vis spectroscopy. Simultaneous doping of C, N, and Cl was realized upon pyrolyzing the molecular precursor in air, leading to significantly lowered direct and indirect interband transition energies of the resultant TiO2. As a consequence, the anatase nanopowders obtained at 450 and 500 degrees C, with specific surface areas of 97.8 and 64.1 m2/g, respectively, exhibit significantly higher efficiency than Degussa P25 in the bleaching of methyl orange solution under visible light (mainly consisting two wavelengths of 405 and 436 nm at 81:100 intensity ratio) irradiation, either at a fixed weight of TiO2 loading or at a fixed surface area of the loaded TiO2 powder.

Electronic Properties and Stability of Artificial In-N Molecules

ABSTRACTIn the work reported here the Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) methods are used to predict electronic properties of several artificial molecules of InAsN and indium nitride whose structure and composition have been derived from those of the corresponding symmetry elements of the zincblende and wurtzite bulk lattices. Both quantum-confined and “vacuum” clusters (whose geometry has been optimized without any spatial constraints applied to the atomic positions) were studied focusing on the electronic energy level structure, direct optical transition energy (OTE), and charge and spin distributions. The obtained results indicate that inclusion of “impurity” atoms (such as As atoms) may enhance stability of both vacuum and confined pyramidal In-N molecules and provide for manipulations of the OTE in a wide range of its values. The CI/CASSCF/MCSCF OTEs of the studied wurtzite-based clusters may also vary in a wide range, from 1.7440 eV for the smallest pre-designed prismatic molecule In6N6 to 6.9780 eV for its almost perfect prismatic “vacuum” counterpart. These evaluations closely correlate with experimental data available in literature.

Virtual Synthesis and Optoelectronic Properties of Prismatic Artificial Molecules of In-N and Zn-O: Comparative Studies

ABSTRACTThe Hartree-Fock (HF), restricted open shell HF (ROHF), configuration interaction (CI), complete active space (ICASCF), and multiconfiguration self-consistent field (MCSCF) methods provide sophisticated fundamental theory-based, computational tools to study structure, composition,chemistry and electronic properties of small artificial molecules composed of semiconductor compound atoms. These tools are used to synthesize virtually several prismatic In-N and Zn-O artificial molecules whose structure is derived from that of the symmetry elements of the respective wurtzite bulk lattices. Applications of spatial constraints to the atomic coordinates allow modeling molecular synthesis in quantum confinement, to obtain pre-designed molecules with tunable electronic properties. Relaxation of these constraints, or optimization, leads to the corresponding molecules synthesized in “vacuum”. The development of computational templates of the studied artificial molecules synthesized in confinement reflects effects of quantum confinement on the electronic level structure, bonding, the direct optical transition energy, and charge and spin density distributions of the molecules. Comparison of the structure and properties of these molecules to those of their vacuum counterparts leads to a conclusion that a small changes in atomic positions in otherwise structurally similar molecules cause a significant change in their electronic properties. Thus, the electronic properties of artificial molecules can be tuned by changing their synthesis conditions that are defined by atomistic details of quantum confinement where the molecules are synthesized.

Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules

ABSTRACTThe Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) approximations are used to study computationally the electronic properties of zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of zinc oxide. Such molecules may provide realistic models for small ZnO quantum dots (QDs) synthesized in “vacuum” or quantum confinement (such as that of well-defined nanopore arrays of silica and alumina membranes) using variety of methods in particular, supercritical fluid deposition. The computational direct optical transition energy (OTE) of the confined molecule appears to be several times smaller than that of the corresponding vacuum cluster. The charge and spin density distributions of these molecules (CDDs and SDDs, respectively) differ significantly, revealing dramatic effects of quantum confinement on electronic properties of Zn-O clusters. The obtained results suggest that manipulations with the electronic properties of the confined clusters by sophisticated design of their quantum confinement may provide means for synthesis of Zn-O – based electronic materials that combine a wide, tunable band gap with large, tunable exciton binding energy.

Studies on electrodeposited Cd 1− xFe xS thin films

Thin films of Cd 1− x Fe x S have been prepared on stainless steel and fluorine doped tin oxide (FTO) coated glass substrates using electrodeposition technique. Double distilled water containing precursors of Cd, Fe and S are used with ethylene diamine tetra-acetic acid (EDTA) disodium salt as a complexing agent to obtain good quality deposits by controlling the rate of reactions. The different preparative parameters like concentration of bath, deposition time, pH of the bath and Fe content in the bath have been optimized by photoelectrochemical (PEC) technique in order to get good quality thin films. Different techniques have been used to characterize electrodeposited Cd 1− x Fe x S thin films. The X-ray diffraction (XRD) analysis reveals that the films Cd 1− x Fe x S are polycrystalline in nature with crystallite size 282 Å for the films deposited with optimized preparative parameters. Scanning electron microscopy (SEM) study for the sample deposited at optimized preparative parameters reveals that all grains uniformly distributed over the surface of stainless steel substrate indicates well defined growth of polycrystalline Cd–Fe–S material. Optical absorption shows the presence of direct transition and band gap energy decreases from 2.43 to 0.81 eV with the increase of Fe content from 0 to 1. PEC study shows the films of Cd 1− x Fe x S with x = 0.2 are more photosensitive than other compositions.

Crystal-chemical aspect of formation of CdAs2-ZnAs2 solid solutions

—Thin AgIn5S8 films are grown on glass substrates by pulsed laser deposition using bulk crystals as targets, and their structure, chemical composition, surface morphology, and optical properties (transmission and reflection spectra in the range 0.5–2.5 µm) are investigated. The transmission and reflection data are used to evaluate the absorption coefficient of the films and the energies of direct and indirect interband transitions. The results obtained for laser-deposited AgIn5S8 films agree well with those for bulk crystals.

Structural and optical properties of AgIn5S8 films grown by pulsed laser deposition

Thin AgIn 5 S 8 films are grown on glass substrates by pulsed laser deposition using bulk crystals as targets, and their structure, chemical composition, surface morphology, and optical properties (transmission and reflection spectra in the range 0.5-2.5 μm) are investigated. The transmission and reflection data are used to evaluate the absorption coefficient of the films and the energies of direct and indirect interband transitions. The results obtained for laser-deposited AgIn 5 S 8 films agree well with those for bulk crystals.

Infrared and ultra violet-visible spectroscopic investigation of plasma polymerized N,N,3,5-tetramethylaniline thin films

Plasma polymerized N,N,3,5-tetramethylaniline (PPTMA) thin films were deposited on to glass substrates at room temperature by a capacitively coupled parallel plate reactor. The as-grown and heat-treated thin films of about 400 nm thick were characterized by infrared (IR) and ultraviolet–visible (UV–Vis) spectrometry. The structural analyses reveal that PPTMA thin films are formed with certain amount of conjugation, which modifies on heat treatment. From the UV–Vis absorption spectra, allowed direct transition (Eqd) and indirect transition (Eqi) energy gaps are determined to be 2.80 and 1.56 eV, respectively. While Eqd increases a little, Eqi decreases, on heat treatment of PPTMA. The calculated values of B for all the samples indicate an increase in structural order/conjugation in PPTMA thin films on heat treatment.

On the optical properties of bismuth oxide thin films prepared by pulsed laser deposition

The optical properties of bismuth oxide films prepared by pulsed laser deposition (PLD), absorption in the photon energy range 2.50–4.30 eV and optical functions ( n, k, ɛ 1, and ɛ 2) in the domain 3.20–6.50 eV, have been investigated. As-prepared films ( d=0.05–1.50 μm) are characterized by a mixture of polycrystalline and amorphous phases. The fundamental absorption edge is described by direct optical band-to-band transitions with energies 2.90 and 3.83 eV. The dispersion of the optical functions provided values of 4.40–6.25 eV for electron energies of respective direct transitions. In the spectral range 400–1000 nm, bismuth oxide films show a normal dispersion, which can be interpreted in the frame of a single oscillator model.

Photocurrent spectroscopy of solution-grown CdS films annealed in CdCl2 vapour

Thin cadmium sulfide (CdS) films (100–130 nm) are deposited on glass from an alkaline solution. The films are annealed in air, in the vicinity of a CdCl2 source, at different temperatures (280–400°C) and for different time periods (below 75 min). The change in some of the optoelectronic parameters (resistivity, direct and indirect transition energies) as a result of annealing is studied using modulated photocurrent spectroscopy. The experimental results reveal that annealing in air, in the absence of a CdCl2 source, reduces the bandgap energy of CdS by 0.12 eV due to the formation of valence-band states. Annealing in CdCl2 vapour removes the tail states and establishes the bulk bandgap (2.42 eV). It also increases the film's photoconductivity by a factor of 10 under white light illumination of intensity 100 mW cm−2, yielding a dark-to-light resistivity ratio of 3 × 104. The film's optoelectronic properties are controlled by two defect energy levels that are located at 0.14 and 1.30 eV below the conduction band edge. The increase in photoconductivity by annealing is due to the improvement of excess carrier lifetime as a result of changes in the 1.30 eV energy level.

Growth and characterization of tungsten and molybdenum-doped ReSe2 single crystals

Single crystals of W- and Mo-doped ReSe2 have been grown by chemical vapor transport (CVT) process with bromine as a transporting agent. The nominal doping concentration for tungsten is 0.5% while EDX measurement indicates a 4% doping concentration for molybdenum. Single crystalline platelets up to 5mm×5mm surface area and 100μm in thickness were obtained. From the X-ray diffraction patterns, the doped crystals are found to crystallize in the triclinic-layered structure. The doping effects of the materials are characterized by temperature-dependent conductivity, optical absorption and piezoreflectance measurements. The activation energies for the impurity carriers decrease with doping. The indirect energy gaps of the doped samples show different red shifts for W- and Mo-doped samples. The direct band edge excitonic transition energies remain unchanged for W-doped samples, while for Mo-doped ReSe2 the transition energies shift toward lower energy locations. The broadening parameters of the excitonic transition features show small increase for W-doped ReSe2 due to impurity scattering, whereas for Mo-doped samples the transition features broadened significantly. The influence of the varying concentrations of the two dopants will be compared and discussed.