Fundamental Absorption Region Research Articles

Optical Properties of Undoped and Indium-doped Tin Oxide Thin Films

Thin films of Tin Oxide (SnO2), having thickness of 200 nm, were formed on to glass substrates by thermal evaporation of high-purity SnO2 powder in vacuum at various substrate temperatures (TS), ranging between 25 and 200°C. SnO2 films with varying thickness were also prepared for a fixed TS = 100°C. Further, doping of SnO2 films with Indium (In) was accomplished through solid state diffusion process by successive deposition of SnO2 and In films and subsequent annealing at 200°C for 10 minutes. Both undoped and doped films were characterized optically by UV-VIS-NIR spectrophotometry in the photon wavelength ranging from 300 to 2500 nm. In the visible photon wavelength range, the average optical transmittance (T%) of the films with varying TS was found to be 85%. The maximum value of T % was found to be 89 % around the wavelength of 700nm. The variation of absorption coefficient with photon energy in the fundamental absorption region is the steepest for TS = 100°C. The sub-band gap (SBG) absorption is also minimum for this Ts. A fluctuating behavior of the band gap energy (Eg) with Ts is observed attaining the highest value of 3.59 eV for Ts = 100°C. The band gap energy increases with thickness but T% in the visible range decreases. The T% in the visible range varies inversely with indium doping, being highest for undoped films. The Eg increases upto 2 wt% In doping and gradually decreases for enhanced doping. It seems reasonable to conclude that In doping does not bring favorable optical characteristics. Undoped SnO2 films having thickness of 200 nm and formed at substrate temperature of 100°C yield essential acceptable properties for photovoltaic applications.DOI: http://dx.doi.org/10.3329/jbas.v35i1.7975Journal of Bangladesh Academy of Sciences, Vol.35, No.1, 99-111, 2011

Effect of Ga incorporation on valence band splitting of OVC CuIn 3Se 5 thin films

Polycrystalline thin films of CuIn 2.95Ga 0.05Se 5 produced by the incorporation of Ga into the ordered vacancy compound CuIn 3Se 5 by a two-stage vacuum evaporation process were structurally, compositionally and optically characterized using X-ray diffraction, energy dispersive analysis of X-rays and optical absorbance measurements. From the X-ray diffraction data of the films, the structural parameters like lattice constants, tetragonal deformation, bond lengths and anion displacement were evaluated and their effect on the optical behavior of films was discussed. The Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation was used to elucidate the crystal field and spin orbital splitting parameters in the uppermost valence band of the compound, using the three energy gaps 1.649, 1.718 and 1.92 eV corresponding to the threefold absorption in the fundamental absorption region of the optical spectra of these films. The percentage contributions of Se p and Cu d orbitals to p–d hybridization in this compound were calculated using linear hybridization of orbitals model and the effects of p–d hybridization on the band gaps were studied.

Model representation of the transmission of high-power pulsed laser IR radiation in the fundamental absorption region in Cd x Hg1–x Te epitaxial layers

A model which describes the changes in the transmission of high-power laser pulses by Cd x Hg1–x Te in view of the variations of the optical parameters of the material with temperature is presented. The thermal fields, absorptance profiles, and transmittance of the semiconductor epitaxial structure have been calculated and compared with experimental data obtained for the wavelength of a carbon dioxide laser.

Features of Optical Properties and the Electronic Structure of Nanostructured Oxides of 3d-Metals

Features of the optical properties of nanostructured strongly correlated oxides CuO, Y3Fe5O12, and FeBO3 in a wide range of energies, which includes both the fundamental absorption region, low-energy electron excitations, and phonon modes have been investigated by methods of optical spectroscopy. Absorption and reflection spectra, Raman spectra, and ellipsometry measurements are carried out on the samples of oxide nanostructured ceramics. High-density nanoceramics were prepared by shock-wave loading and high pressure torsion methods from coarse-grain oxide powders. Strong smearing of the fundamental absorption edge is revealed for all nanoceramics. Anomalous red shift of the absorption edge Eg is found in CuO nanooxide. The features of the optical functions in nanooxides of 3d-metals may be attributed to high defectiveness of nanooxides, peculiarities in the electronic structure of strongly correlated oxides and their tendency to electronic phase separation.

Optical Investigation of Sm doped ZrO2

A samarium doped polycrystalline ZrO2 bulk sample was investigated for its phase composition as well as optical properties within the temperature range 6–300 K. From micro-Raman measurements the existence of tetragonal as well as monoclinic phase was confirmed. The emission spectrum of Sm3+ reveals drastic changes under excitation near the region of fundamental absorption. Combined excitation-emission spectroscopy (CEES) measurements reveal three different sites for the dopant ions to be present in the sample. Nature and excitation scheme of the three sites are discussed.

Structure, Optical, and Room-Temperature Ferromagnetic Properties of Pure and Transition-Metal-(Cr, Mn, and Ni)-Doped ZnO Nanocrystalline Films Grown by the Sol−Gel Method

Transition-metal-(Cr, Mn, and Ni)-doped zinc oxide (ZnO) with the concentration of 2.5% and pure ZnO nanocrystalline (nc) films have been fabricated on quartz substrates by the sol−gel method. The X-ray diffraction analysis shows that the films are polycrystalline with the wurtzite phase. The E2high phonon mode shifts about 7 cm−1 with different transition metals, and the A1(LO) phonon mode redshifts 4 cm−1 with the ionic radius of doping elements. From the transmittance spectra, all films are highly transparent in the visible region and exhibit Urbach tail states in the ultraviolet range because of the crystalline defects and grain boundaries. The optical band gap of the films decreases with increasing the orbital occupation numbers of 3d electrons due to the orbital splitting of magnetic ions. It was found that a distinct and strong excitonic transition appears beyond the fundamental absorption region at room temperature. Ultraviolet and near-infrared electronic transitions can be observed and show the strong relationship with the transition metal doping. Moreover, orange, yellow and red luminescences strongly depend on the doping elements owing to different oxygen vacancy, oxygen interstitial, and surface morphology. Magnetic measurement results reveal that all the transition-metal-doped nc-ZnO films show the well-defined ferromagnetic features at room temperature. In particular, the nc-ZnO film doped with Mn shows the largest saturation magnetic moment, which agrees well with the theoretical predication. The ferromagnetic properties could be derived from the doping of magnetic ions, intrinsic defects, interfaces, and grain boundaries in the transition-metal-doped nc-ZnO materials.

Features of self-activated luminescence spectra of CdS:O in the context of band anticrossing theory

The interpretation of the nature of self-activated luminescence spectra of CdS:O is given on the basis of the band anticrossing theory, making it possible to take into account the effect of isoelectron oxygen impurity on the modification of the band structure. The bands of self-activated luminescence (SA and SAL) of CdS:O similar to those of ZnS-ZnSe:O are revealed. In the presence of dissolved oxygen OS, the presence of two additional H and L components is revealed in the composition of SA bands, which are owed to the transitions from E+ and E− subbands of the CdS:O split conduction band to the recombination level ESA. Their spectral dependence on the dissolved-oxygen concentration [OS] is determined. It is shown that the CdS green edge emission is similar to that of SAL; however, in contrast to ZnS-ZnSe:O, there is no H component in the fundamental-absorption region of the crystal. The analysis of the composition of the SA and SAL centers in the CdS crystals shows their equivalence to ZnS(ZnSe). From the experimental data, the position of the oxygen localized level E0, a decrease in the band gap Eg with [OS], and the band model of CdS:O are found. This study supplements similar investigations carried out previously for ZnS-ZnSe:O.

Reflection spectra of NaClO3, NaBrO3, and LiIO3 gyrotropic crystals in the vacuum UV region

The reflection spectra in the fundamental-absorption region, 5–25 eV (250–40 nm), of optically active crystals with cubic symmetry (NaClO3, NaBrO3) and uniaxial optically active crystal (LiIO3) have been investigated. It is shown that the reflection spectra of cubic crystals have a similar structure, which is determined by the electronic transitions in the XO3 group. The comparison of these spectra with the corresponding spectrum of lithium iodide made it possible to determine the type of transition in the spectra of cubic crystals. Using the projection operator method, it was shown that the sign of optical rotation of cubic crystals with symmetry T is independent of the screw axis sign. Possible reasons for the unprecedentedy large optical rotation of lithium iodide crystal in the optical axis direction are considered.

Optical spectra of nanoceramics of yttrium-iron garnet Y3Fe5O12 obtained by the method of intense plastic deformation

The features of the optical properties of nanostructured samples of iron-yttrium garnet Y3Fe5O12 (YIG) in the range of 0.5 to 4.7 eV, which includes both the fundamental-absorption region and low-energy electron excitations, have been investigated by spectroscopic ellipsometry. The results are discussed in comparison with the measurement data of a YIG single crystal. The dispersion of the optical functions in nanostructured samples is significantly different from that for the single crystal: the spectral density is redistributed from the energy region above the fundamental absorption edge to the region below the edge. It has been shown that the energy positions of the main electronic transitions in nanostructured samples are on the whole the same as in the single crystal; at the same time, the intensity of low-energy transitions increases. The possible causes of this increase and the resolution of the fine absorption structure in the bandgap of nanostructured Y3Fe5O12 are discussed.

10.1007/s11449-008-3009-2

Luminescence properties of a pure CdWO4 crystal and a CdWO4:Mo crystal doped with molybdenum in different concentrations have been investigated. The effect of molybdenum impurity on the intrinsic luminescence of CdWO4 has been found, and the role of the impurity in the formation of new luminescence centers has been investigated. The features of the formation of the intrinsic and impurity luminescence excitation spectra of CdWO4 and CdWO4:Mo crystals in the fundamental-absorption region have been considered. The reflection spectra of the CdWO4 crystal have been investigated taking into account the crystal structure anisotropy.

Luminescence study of the LuBO3 and LuPO4 doped with RE3+

Lutetium borates and phosphates doped with RE 3+ ions are perspective scintillators. In the paper, the results of the luminescence spectroscopy of LuPO 4 and LuBO 3 doped with Pr 3+ , Ce 3+ , Tb 3+ and Eu 3+ under synchrotron radiation excitation are presented. The processes of the energy transfer from the host lattice to the luminescence centers are considered. The creation of excitons at the edge of fundamental absorption region is shown. The bandgap values for lutetium borate and phosphate were estimated.

Injection-type photodetector arrays based on doped lead-tin tellurides: Possibilities and prospects

The photosensitivity of doped solid solutions of lead-tin tellurides is analyzed, taking into account charge-carrier injection from the contacts. The features of the photoresponse are considered both in the fundamental-absorption region and in the submillimeter spectral region. The experimental results of an investigation of the properties of photodetector devices based on the indicated compounds are presented, and the prospects of using them in the IR and submillimeter regions are considered.

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

Abstract This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce 3+ doped SrAlF 5 single crystals. Several Ce 3+ centers emitting near 4 eV due to 5d-4f transitions of Ce 3+ ions substituting for Sr 2+ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.

Dielectric functions of ferroelectric Pb0.5Sr0.5TiO3 film determined by transmittance spectroscopy

Pb0.5Sr0.5TiO3 (PST) films having a pure polycrystalline perovskite phase and well-grown grains were prepared by a chemical solution deposition method. Their optical response from 1.1 to 5.0 eV was investigated by transmittance spectroscopy, and the complex dielectric functions and optical parameters were evaluated in terms of the Tauc–Lorentz and Cauchy models. The direct optical band energy of the interband transition from the valence to conduction bands was determined to be 4.09 eV. Below this fundamental absorption region, the optical response in the energy range of 3–4 eV behaves as a local-states-related Urbach-band-tail absorption with the tail energy of about 243 meV. By fitting the refractive index using a single oscillator model, it is found that the optical dispersion of the PST films in the transparent region (1–3 eV) may originate mainly from the direct interband critical point transition between the valence and conduction bands.

First-Principles Study of Electronic, Absorption, and Thermodynamic Properties of Crystalline Styphnic Acid and Its Metal Salts

The electronic structure, absorption spectra, and thermodynamic properties of crystalline styphnic acid and its metal salts (potassium, barium, and lead styphnates) have been studied using density functional theory within the local density approximation. The results show that the metal states affect the electronic structure of styphnic acid by modifying the density of states of the O atoms of hydroxyls. The C-O bond fission may be favorable in the decomposition of styphnic acid and its metal salts. The absorption spectra of the four crystals display a few strong bands in the fundamental absorption region. Compared with styphnic acid, potassium, barium, and lead styphnates decrease its enthalpy, entropy, free energy, and heat capacity as the temperature increases. However, the differences of the thermodynamic functions between each metal salt are very small. As the temperature increases, the decomposition reactions of the four crystals are more and more favorable thermodynamically. It is also found that there is a relationship between the band gap and impact sensitivity for the four crystals.

Effect of phase separation on optical and electrical properties of Sn-Sb-Se glassy films

This paper reports the effect of phase separation on the structural, optical and electrical properties of thermally evaporated Snx Sb20Se70–x (x = 10, 11, 12.5) films. The X-ray diffraction studies of the devitrified films show the crystallization of SnSe2, Sn2Sb4Se8 and Sn4Sb4Se10 phases for x = 12.5 film, while Sb2Se3 is the other major phase in all the film samples. The optical constants are calculated by using Swanpole's standard envelop method. The dispersion of the refractive index is discussed in terms of the Cauchy's dispersion relation and the Wample–DiDomenico (WDD) single-oscillator model in the weak absorption region. The optical gap and the tailing parameter were calculated in the fundamental absorption region, while the Urbach energy was calculated by using the value of extinction coefficients. The dc conductivity measurements with temperature have also been performed and electrical conduction has been discussed in conjunction with the Meyer–Neldel rule for the present system. These results have been discussed taking into consideration the structural modifications induced by the phase separation in the present system. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Density functional theory study of high-pressure behavior of crystalline hexanitrostilbene

A detailed study of the structural, electronic, and absorption properties of crystalline hexanitrostilbene (HNS) under hydrostatic pressure of 0–80 GPa was performed with density functional theory. The results show that the structure is much stiffer in the b and c direction than along the a axis, showing that the compressibility of HNS crystal is anisotropic. As the pressure increases, the band gap gradually decreases. An analysis of density of states shows that the electronic delocalization in HNS gradually increases under the influence of pressure. An understanding of the stabilities of HNS under compression based on the electronic structure shows that an applied pressure increases the impact sensitivity of HNS to detonation initiation. As the pressure increases, HNS has relatively high optical activity. The absorption spectra of HNS at high-pressure display a few, strong bands in the fundamental absorption region.

Periodic DFT investigation on the structure and properties of TNAD crystal

AbstractDensity function theory calculations were performed at the GGA/PW91, GGA/PBE, and LDA/CA‐PZ levels to study the structures and properties of the crystalline TNAD (trans‐1,4,5,8‐tetranitrotetraazadecalin). The relaxed crystal structure compares well with the experimental data. Analysis on the band structures shows that the frontier energy bands are generally quite flat, and the energy gap between the highest occupied crystal orbital and the lowest unoccupied crystal orbital is about 3.4 eV, indicating that the crystal is an electrical insulator. All the atoms of TNAD make up both the lower and the higher energy bands. The projection of density of state demonstrates that the NNO2 bond is the most reactive region of the material. The lattice energy is predicted to be −155.13 kJ/mol at the LDA/CA‐PZ level, consistent with the previous studies, whereas it is underestimated by the GGA/PW91 (−70.41 kJ/mol) and GGA/PBE (−74.33 kJ/mol). The optical properties under ambient condition were investigated, including dielectric function, absorption coefficient, and reflectivity. The calculated absorption spectra show a number of absorption peaks in the fundamental absorption region, which are believed to be associated with different exciton states. And the reflectivity spectra are mainly composed of four peak structures, where the magnitude changes in the order of GGA/PBE < GGA/PW91 < LDA/CA‐PZ on the whole. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Electroluminescence and phototrigger effect in single crystals of GaS x Se1−x alloys

The effects of switching and electroluminescence as well as the interrelation between these effects in single crystals of GaS x Se1−x alloys are detected and studied. It is established that the threshold voltage for switching depends on temperature, resistivity, and composition of alloys, and also on the intensity and spectrum of photoactive light. As a result, a phototrigger effect is observed; this effect arises under irradiation with light from the fundamental-absorption region. Electroluminescence is observed in the subthreshold region of the current-voltage characteristic; the electroluminescence intensity decreases drastically to zero as the sample is switched from a high-resistivity state to a low-resistivity state. Experimental data indicating that the electroluminescence and the switching effect are based on the injection mechanism (as it takes place in other layered crystals of the III-V type) are reported.

Comparative DFT Study of Crystalline Ammonium Perchlorate and Ammonium Dinitramide

The electronic structure, vibrational properties, absorption spectra, and thermodynamic properties of crystalline ammonium perchlorate (AP) and ammonium dinitramide (ADN) have been comparatively studied using density functional theory in the local density approximation. The results shows that the p states for the two solids play a very important role in their chemical reaction. From the low frequency to high frequency region, ADN has more motion modes for the vibrational frequencies than AP. The absorption spectra of AP and ADN display a few, strong bands in the fundamental absorption region. The thermodynamic properties show that ADN is easier to decompose than AP as the temperature increases.