Absorption Spectra Of Impurities Research Articles

Impurity-induced enhancement of parity-forbidden optical intracenter transitions of shallow donors in silicon

Different defects in silicon crystals cause lattice distortion that may violate selection rules for certain intracenter optical transitions of hydrogen-like donor centers. Perturbations due to large concentration of substitutional donors enhance all parity-forbidden atomic transitions, but cause large concentration broadening of the transition lines, prohibiting observation of transitions at close energies. Substitutional residual carbon atoms in silicon induce, in contrast, a weak-to-moderate broadening of donor absorption lines enabling the resolution of some parity-forbidden intracenter transitions in impurity absorption spectra at moderate donor densities. Binding energies of several series of s- and d-type states were obtained by resolving of intracenter transitions terminating in these states, by low-temperature infrared absorption spectroscopy in carbon-rich and/or heavily doped silicon crystals.

Theoretical investigations of electronic structure, magnetic and optical properties of CdS-X (X= Sm, La, Ce) materials for optoelectronic applications

Current research is based on density functional theory investigations of structural, electronic, magnetic, and optical properties of X (X = Sm, La, Ce) doped CdS using Wien2k code. Spin-polarized calculations indicate non-magnetic character of pure and La doped CdS while magnetism is noticed upon Sm and Ce doping into CdS. Hubbard correction indicate shift of f-states toward the Fermi level and non-negative frequency modes in phonon spectra illustrate dynamical stability of proposed materials for synthesis. However, magnetic moment in Ce doped CdS (2.9188 μB) is larger than Sm doped CdS (2.7901 μB). Cd 4d-states, S 3p-states, La 5d-, Sm and Ce 4f-states show supreme role around the Fermi level which improve electronic properties. Reduction in band gap is noted which points good conduction. Optical absorption spectrum of impurities added CdS materials exhibit blueshift. Moreover, enhanced absorption and upgraded conductivity along with increasing refractive index of selected materials emphasize their potential uses in high energy UV photonics, sensors, spintronics, optoelectronics and energy harvesting devices.

Infrared absorption cross sections, and oscillator strengths of interstitial and substitutional double donors in silicon

Infrared absorption cross sections and corresponding oscillator strengths of several intracenter transitions of double donors in silicon, interstitial magnesium (Mg; group IIA) and substitutional chalcogens (Ch = S; Se; group VI), were determined for impurity densities in the ranges $1\ifmmode\times\else\texttimes\fi{}{10}^{14}--1.6\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^{3}$ for Mg and $2\ifmmode\times\else\texttimes\fi{}{10}^{13}--2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^{3}$ for chalcogens. The concentrations of electrically active atomic and diatomic donor centers were derived from the Hall effect measurements. The experimental integrated cross sections were obtained from low-temperature impurity absorption spectra. The oscillator strengths of related donor transitions were derived and compared with those for shallow single donors in silicon, both determined experimentally and predicted theoretically. The transitions of oscillator strengths of double donors follow the decreasing trend with decreasing radius of donor ground states and increasing an impurity binding energy.

Оптические сечения поглощения и силы осцилляторов двойного донора магния в кремнии

The optical properties of magnesium impurity in silicon, whose atoms at interstitial positions in the lattice are deep double donors with an ionization energy of 107.56 meV in the neutral state, were studied. For optical transitions from the ground state of a neutral center to the excited levels 2p0 and 2p, the absorption cross sections and oscillator strengths were determined. These parameters were calculated from the impurity absorption spectra that were measured at T  K in samples with different magnesium concentrations. The deep donor content in the samples was determined using Hall effect measurements in the temperature range 78–300 K. The obtained characteristics of intracenter transitions in magnesium were compared with the corresponding literature data for shallow Group V donors in silicon, which are substitutional impurities. It was found that the optical characteristics of the investigated transitions in magnesium are consistent with the dependences of the corresponding parameters on the ionization energy for shallow donors, extrapolated to the region of larger electron binding energies.

Fluorescence and absorption spectroscopy for warm dense matter studies and ICF plasma diagnostics

The burning core of an inertial confinement fusion (ICF) plasma produces bright x-rays at stagnation that can directly diagnose core conditions essential for comparison to simulations and understanding fusion yields. These x-rays also backlight the surrounding shell of warm, dense matter, whose properties are critical to understanding the efficacy of the inertial confinement and global morphology. We show that the absorption and fluorescence spectra of mid-Z impurities or dopants in the warm dense shell can reveal the optical depth, temperature, and density of the shell and help constrain models of warm, dense matter. This is illustrated by the example of a high-resolution spectrum collected from an ICF plasma with a beryllium shell containing native iron impurities. Analysis of the iron K-edge provides model-independent diagnostics of the shell density (2.3 × 1024 e/cm3) and temperature (10 eV), while a 12-eV red shift in Kβ and 5-eV blue shift in the K-edge discriminate among models of warm dense matter: Both shifts are well described by a self-consistent field model based on density functional theory but are not fully consistent with isolated-atom models using ad-hoc density effects.

High-field impurity magneto-optics of Si:Se

Just as phosphorus in silicon produces a hydrogenic defect, the double donor selenium in silicon is an analog of helium. We have measured the impurity absorption spectrum at high magnetic field, and we show that the odd-parity excited states of Si:Se behave identically to those of Si:P. This fact allows us to isolate the electron-electron interactions (exchange and correlation) in the ground state from the quadratic Zeeman effect. The field tuning allows us to put upper limits on the strength of some of these interactions (e.g., at 30 T the electron-electron correlation interaction in the ground state of Se is less than about 40 \ensuremath{\mu}eV; at 30 T the quadratic Zeeman energy in the ground state of P is less than about 200 \ensuremath{\mu}eV).

Near-field effect in the absorption spectrum of impurities in crystals

The effect of the near-field interaction of shallow donors in GaAs on the intracenter absorption spectrum of impurities has been considered. It has been shown that the near-field interaction leads to broadening of the absorption line, which increases with the donor density. The absorption line shape is no longer Lorentzian: its low-frequency wing becomes more prolonged than the high-frequency wing.

Optical absorption spectra as a useful tool to find parameters of deep impurity centers in semiconductors.

We analyze physical models accounting for deep-level conduction band transitions to describe impurity absorption spectra in tetrahedral-structured semiconductors. The investigations were carried out for ZnSe crystals doped with transition metals (Ti, V, Cr, Mn, Fe, Co, Ni) from a vapor phase. It was shown that the impurities provide acceptor centers with ground state energy offset by 0.3-0.6 eV from the edge of the conduction band, forming long-wave bands in the absorption spectra of the materials studied.

Spectral dependences of the optical absorption in bismuth germanium oxide crystals annealed in vacuum

Results of experimental investigations into the spectral dependences of the light absorption coefficient of an undoped Bi12GeO20 crystal annealed in vacuum at a temperature of 650°С and subsequently annealed in the air atmosphere at temperatures in the range Т АА = 315–590°С are presented for the spectral range 400–1100 nm at room temperature. It is established that the optical absorption of the crystal after temperature annealing in vacuum increases in the entire examined spectral range. Subsequent annealing in air leads to gradual restoration of the initial impurity absorption spectrum in the range from 530 to 1100 nm. The wide band that can be caused by oxygen vacancies is observed in the spectrum of absorption changes induced by annealing in vacuum.

Absorption spectrum of atomic impurities in isotopic mixtures of liquid helium

We theoretically describe the absorption spectrum of atomic impurities in isotopic mixtures of liquid helium within a zero-temperature density functional approach. Two situations are considered. In the first one, the absorption spectrum of Na atoms attached to $^{4}\mathrm{He}$${}_{1000}$-$^{3}\mathrm{He}$${}_{{N}_{3}}$ droplets with ${N}_{3}$ values from 100 to 3000 is presented as a case study of an impurity that does not dissolve into helium droplets. In the second one, the absorption spectrum of Mg atoms in liquid $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixtures is presented as a case study of an impurity dissolved into liquid helium. We have found that the absorption spectrum of the impurity is rather insensitive to the isotopic composition because the line shift is mostly affected by the total He density around the impurity, not by its actual composition. For bulk liquid mixtures, results are presented as a function of pressure at selected values of the $^{3}\mathrm{He}$ concentration. The results for isotopically pure $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ liquids doped with Mg are compared with available experimental data.

Peculiarities of two-photon impurity absorption spectra in a quantum molecule with a tunnel-transparent potential barrier

The effect of dissipative tunneling parameters on the spectral dependence of the probability of two-photon impurity absorption in a quantum molecule is theoretically investigated. The effect of the tunnel barrier transparency on two-photon impurity absorption in a quantum molecule is shown to manifest itself as a change in the width of energy levels of the virtual and final states due to variations in such parameters of dissipative tunneling as temperature, photon-mode frequency, and the constant of interaction with a heat bath.

Absorption and transmission spectral measurement of fiber-optic components using supercontinuum radiation

We report on broadband absorption and transmission measurements using supercontinuum sources generated in microstructured fibers. The advantages of this measurement technique include high brightness and broad bandwidth. Furthermore, the high spatial coherence of the supercontinuum source allows for efficient coupling into the device under test compared to standard light bulbs. Employing this technique, we measure the impurity absorption spectrum of Erbium-doped fibers with both low and high dopant concentration. The transmission spectra of a photonic bandgap fiber and a high-finesse fiber cavity are also characterized.

Nature and energy structure of impurity and intrinsic defects in V-doped Cd1−xHgxTe

The nature and the energy structure of impurity and intrinsic defects, including the formation of anisotropic impurity centres, are determined in V-doped semi-insulating Cd1−xHgxTe (x ⩽ 0.037) crystals which were grown by the Bridgman technique for the first time. From detailed analysis of the spectral dependence of the photodiffusion current data obtained for the different directions of light propagation in crystals as well as the impurity absorption spectra, we established that for the investigated crystals anisotropic V2+ and V3+ centres are formed. Two mechanisms of the electron photogeneration from the ground impurity state to the conduction band (direct photoionization and auto-ionization from the excited state V2+ ions) are established. It is shown that the efficacy of the auto-ionization of electrons depends on the position of the excited state relative to the conduction band bottom. The photoinduced impurity centres are formed as a result of the illumination of the crystals with the light with energy about 1.50 eV. The nature and photoionization energy of these photoinduced centres are determined. The scheme of the impurity and intrinsic defects energy levels in the energy gap of the investigated crystals is presented.

Time-domain analysis of electronic spectra in superfluid 4He

Electronic absorption spectra of impurities in superfluid helium is developed in time domain, using time-dependent density functional theory to describe liquid 4He and time-dependent perturbation theory to describe the electronic degrees of freedom of the impurity. Angularly isotropic potentials are used to describe the molecule–helium interactions in the ground and excited electronic states. The calculations rationalize experimentally observed phonon side-bands in 4He droplets and in bulk helium, and allow assignments of spectral features to specific motions of the liquid.

Optical fractionation of an ensemble of impurity molecules with respect to the magnitude of the piezochromic effect

A method of hole burning in the 0-0 band of the impurity-absorption spectra of polymers and glasses is suggested. This method is selective not only with respect to the transition energy, but also with respect to the magnitude of the piezochromic effect. It is shown that, in combination with the modulation of the wavelength of probe radiation, this method makes it possible to determine the contribution of the impurity molecules with a specified magnitude of the piezochromic effect to the total absorption and luminescence of a sample. This circumstance allows one to study experimentally the correlation of the piezochromic effect with other characteristics of impurity molecules.

Electric dichroism of a multiwell spectral hole as an indicator of anisotropy in the distribution of stark moments of impurity molecules

Burning of multiwell holes in impurity-absorption spectra of glasses and polymers in an alternating electric field was theoretically studied. A case of circularly polarized burning radiation with a spectrum consisting of a series of close equidistant lines that shift synchronously with the variation of the electric field across the sample was considered. It was shown that an electric field applied to the sample after hole burning changes the optical density in the hole center, and that this change depends on the orientation of the plane of polarization of the probing radiation. Centrosymmetric impurity molecules, whose Stark moments (with randomly distributed magnitudes and directions) are governed only by their interaction with matrix molecules, and molecules with small intrinsic permanent Stark moments were considered. It was shown that the measurements of electric dichroism of a multiwell spectral hole make it possible to study both the anisotropy of matrix-induced moments and additional moments that arise, for example, when a metal atom leaves the plane of a metalloporphyrin molecule. It was also shown that the sensitivity and selectivity of the suggested method exceed those of known methods.

Composition dependence of the four A bands for thin-solid films of KBr 1– xI x:Tl +

Thin crystalline solid films of KBr 1– x I x :Tl + were prepared from a melt by a cell method and impurity absorption spectra were investigated at 11 K for the whole composition range. In the middle composition range no spectral traces of phase separation were observed, indicating that the films were in a state equivalent to that of a supersaturated solid solution. The A band of KBr:Tl + and that of KI:Tl + were persistent with each other and two additional bands attributable to the A bands of the mixed crystals were apparent in the range of 0.01≤ x≤0.5. According to the results of Gaussian fitting of the bands, it was evident that two of them have threshold compositions with increasing I − ion composition x. In addition, the I − ion concentration around the impurities was found to be greater than the I − ion composition of the host materials. These findings are discussed in terms of a model, according to which the bands are associated with various I − ion configurations around the impurities.

Determination of the distribution function of Stark dipole moments induced by a matrix in amorphous impurity systems

The burning kinetics of closely spaced equidistant holes in impurity absorption spectra of glasses and polymers is theoretically studied in an alternating electric field (alternating in the magnitude, sign, and direction). The height of barriers between wells of the burnt multiwell spectral hole depends on the shape of the distribution of impurity molecules over the Stark dipole moments. Centrally symmetric impurity molecules are considered whose Stark moment is determined exclusively by the interaction with matrix molecules. Model calculations performed for different hypothetical distribution functions of Stark moments yield substantially different kinetics. Therefore, the method described allows one to verify experimentally various hypotheses about the nature of matrix-induced Stark moments.

Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge1−xSix heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.

Compensation dependence of Ga impurity absorption spectra in highly compensated Ge: Ga, As

Compensation dependence of Ga impurity absorption spectra in highly compensated Ge: Ga, As