Optical absorption spectra of magnesium-implanted GaP

Optical absorption spectra of magnesium-implanted GaP single crystals are measured in the photon energy range below the fundamental edge. It is found that implantation results in the formation of exponential absorption tails with tailing energies up to about 0.8 eV and absorption coefficients up to about 10⁵ cm⁻¹ at 2.0 eV. The results are compared with optical measurements on GaP single crystals implanted with other ions and with optical absorption data of amorphous GaP.     

Mechanism of epitaxial ferrite-spinel layer formation on magnesium oxide substrate

The initial stage of small gap heteroepitaxial growth of magnesium-manganese ferrite films during chemical gas transportation reactions on orientation (100) magnesium oxide substrate was studied. Film growth would begin with origination of parallely oriented three-dimensional nuclei, their shapes at lower (1170 K) and at appear (1420 K) film synthesis temperature limits being different. After little islands had grown together and continuous layer formed the secondary nuclei would appear, their shapes resembling cut-up pyramids. Continuous layer minimum thickness was 0.15 μm. The growing together of little islands results in film block structure. Block sizes could be as big as 300–400 μm if films grow at the speed of 2 μm/min, the density of islands being 10⁶ cm⁻². Linear density of unconformity dislocations at film-substrate boundary would be (3.7 ÷ 7.4) 10³ cm⁻¹. It would depend on magnesium content for a film. Elastic stresses in films would come to 7 · 10⁸ ÷ 3 · 10⁹ dyne/cm². X-ray topography pictures of block structures and of accumulations of admixtures at block boundaries are presented.     

Optical absorption spectra and electron diffraction patterns of rare-earth oxycarbonates

The 4.2 K optical absorption spectra of Ia-(NdO)₂CO₃ and II-(NdO)₂CO₃ are reported. Type Ia exhibits, apparently, a single site for neodymium, and type II, at least two sites. Electron-diffraction patterns obtained from flakes suggest a large orthorhombic unit cell for type Ia and confirm the hexagonal symmetry of the unit cell of type II.     

Optical spectra of systems with nearly unstable excited states: modulated lambda-shaped spectra

Absorption spectra of faded small molecules in superfluid droplets of ⁴He display an oscillating structure of the zero-phonon lines. To understand the origin of these oscillations, we have examined the theory of optical spectra of impurity centres in condensed matter. The attention was paid to the case of the optical transition taking place in the vicinity of a nearly stable or an unstable state. In this case the ratio of the mean vibration frequencies in the final and initial states is small. We have found that in this case a strong enhancement of the low-energy phonon sideband appears. As a result, the spectrum (or its envelope) has the shape of an asymmetrical lambda letter. It may also reveal Airy-type oscillations, characteristic of a nearly free motion in the final state. Such oscillations have been observed in the hot luminescence of self-trapped excitons in Xe crystals. In our opinion, of an analogous origin is the oscillating structure of the zero-phonon lines in superfluid droplets of ⁴He.     

Effect of an applied electric current in epitaxial growth of GaAs layer on patterned GaAs substrate

Selective epitaxial growth of a GaAs layer on SiN_x masked Si-doped semi-insulating (1 0 0) GaAs substrate was performed by current-controlled liquid-phase epitaxy (CCLPE) in the conventional liquid-phase epitaxy. Experiments were carried out with and without the application of electric current. Surface morphology of (1 0 0) facet of the grown layer and the vertical and lateral growth rates were significantly improved under applied electric current. A thick layer of about 330 μm was achieved at relatively low growth time of 6 h with a current density of 20 Acm⁻². The epitaxial growth is realized by both electromigration of the solute and supercooling under a constant rate of furnace cooling. The dislocation density of the grown layer was significantly reduced, compared with that of the substrate (4×10⁴ cm²).     

Infrared optical characterization of epitaxial layer – substrate systems (II). Experimental results for AlxGa1−xAs/GaAS

Infrared reflectance and transmittance spectra and Raman scattering spectra of the epitaxial layer-substrate system Al_xGa_1−xAs/GaAs with compositions in the range x = = 0.08–0.49 are measured in the wavenumber range from 40 to 4000 cm⁻¹. In analysing the spectra in terms of the respective theoretical relations for an optical two-layer system the thickness of the layers, the optical mode characteristics and the free carrier parameters are determined. From a comparison with existing literature data for Al_xGa_1−xAs it is concluded that infrared optical measurements on epitaxial layer-substrate systems can be successfully employed to evaluate the material parameters of epitaxial layers with thicknesses down to a few micrometers.     

Optical absorption spectra of a-Si:H MIS structure measured by transient photocapacitance spectroscopy

The transient photocapacitance spectroscopy (TPC) was applied to measure the optical absorption spectra of hydrogenated amorphous silicon (a-Si:H) in metal-insulator–semiconductor (MIS) structure. The measured spectra show the features of optical transitions from both D⁰ and D⁻ defect bands to conduction band. The stability of measurement was checked against the change of probe frequency. The availability of TPC in detecting the defect density changes was also tested by checking the well-known defect density increase in a-Si:H after light illumination.     

Optical spectra of glasses

This presentation focuses on several ways in which structural and dynamical information can be obtained from optical spectroscopy by examining results of infrared and Raman studies of selected oxide glasses. Though these studies represent only a small fraction of the work on most of the glasses mentioned, and many other important forms of optical spectroscopy are not treated here, they illustrate useful approaches to the study of amorphous materials by optical methods.     

X-ray topographic study of GaSb substrates for epitaxial layer growth

The depth of the layer damaged by silicing of GaSb ingots has been determined by X-ray topography. The changes in the defect structure of substrates after successive etching to different depths by Br₂/CH₃OH and 1 HF:3 NNO₃:1 CH₃COOH have been studied. The effect of heat treatment carried out under conditions similar to the epitaxial process of substrates with completely removed damaged layer has been investigated.     

Electronic absorption spectra of lead iodide

Optical absorption studies on gel grown single crystals and vacuum evaporated thin film of lead iodide have been carried out at room temperature and liquid nitrogen temperature. The exciton peak in the case of gel grown crystal was observed at 4940 Å whereas for the film at 4900 Å. The presence of lead colloids in the film was revealed through a broad featureless band around 5500 Å. The absorption tails in the temperature range from 80 to 300 K are found to obey Urbach rule. Examining the various power law dependences of the observed optical density on photon energy, it has been established that the transition is of direct type resulting in a band gap of 2.53 ev.     

Optical absorption spectra and related parameters of ammonium zinc chloride crystal in the antiferroelectric and commensurate phases

Optical absorption along the three crystallographic axes of (NH₄)₂ZnCl₄ single crystals is reported over a temperature range from 276 K to 350 K. Our results clearly confirm the antiferroelectric to commensurate phase transition at 319 K. Analysis reveals that at the absorption edge the type of transition is the indirect allowed one. The indirect band gaps at various temperatures are determined and their temperature dependence is estimated in the antiferroelectric and commensurate phases. Optical parameters related to the temperature dependence of the energy gap are evaluated. The single‐effective oscillator model was used to describe the imaginary part of the dielectric constant, which facilitate calculation of the dipole moment parameters. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap and the result is compared with that calculated graphically. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach characteristic parameters are determined and their temperature dependence is investigated. Correlation between different parameters is considered. Most of the results confirm the optical isotropic nature of AZC crystals. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical spectra of sodium phosphate glasses

The visible, infrared, and ultraviolet spectra of seven glasses in the Na₂OP₂O₅ binary system were obtained. Changes in spectra were related to structural changes occurring the glass. Dopant ions Co²⁺, Cu²⁺, and Pb²⁺ were used to infer structural changes occurring as the metaphosphate composition was crossed going from pyrophosphate compositions into ultraphosphate compositions. The presence of chemically bound water was found to cause the overall modifier-to-former ratio to remain constant for glasses containing greater than 65 mol.% P₂O₅.     

Optical absorption behaviour of magnesium-implanted GaAs

Optical absorption spectra of magnesium implanted GaAs single crystals are measured in the photon energy range below the fundamental absorption edge. It is found that the absorption coefficient increases with increasing fluence up to a saturation value of about 5 · 104 ⁴ cm⁻¹ at 1.36 eV. The results are compared with optical measurements on GaAs single crystals implanted with other ions.     

Suppression of epitaxial silicon layer doping with boron in the presence of hydrogen chloride

The present paper informs about thermodynamic calculations of the B Cl H system, carried out irrespective of data, already published by other authors. It further deals with recent experimental results on silicon doping with boron in the presence of hydrogen chloride, the layer growth rate being varied. Finally the possibility of quantitatively interpreting the suppressing influence of hydrogen chloride on the doping of silicon with boron is discussed.     

Optical absorption and EPR spectral studies of vauquelinite

Vauquelinite ‐ a mineral ‐ was investigated by EPR, NIR and optical spectroscopic techniques. EPR studies on powder sample confirm the presence of Cu(II) and Fe(III) impurity in the mineral. Optical absorption spectrum also indicates that Cu(II) impurity is present in rhombically distorted octahedral structure and Fe(III) in octahedral structure. NIR results are due to water fundamentals and phosphate combination tone. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetic approach to the decomposition of radiation-induced absorption spectra

We propose a kinetic approach to the decomposition of the radiation-induced absorption spectra into individual Gaussian absorption bands. It is based on the correlated analysis of a series of induced absorption spectra obtained in the course of multi-step irradiation and during room temperature long-term post-radiation annealing. The methodology is demonstrated using data on Co⁶⁰ gamma irradiated samples of commercial boro-silicate glass N-BK7 from SCHOTT. We show that physically meaningful spectroscopic parameters of absorption bands can be obtained without a priori knowledge of the band peak position and bandwidth.     

Optical and structural properties of epitaxial GaN films grown by pulsed laser deposition

Epitaxial GaN films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The properties of GaN films were improved by increasing the growth temperature to 800°C and the nitrogen pressure during growth to 10 mTorr. Room-temperature photoluminescence exhibits a strong band-edge emission at 3.4 eV. From transmission electron microscopy (TEM), the predominant defects in PLD-GaN observed are stacking faults parallel to the interface and screw dislocations along c-axis, the latter differing from the previously published results where most of the threading dislocation in GaN grown by other techniques are of edge type with Burgers vector of .     

Optical spectra of erbium and thulium in germanate glass

Oscillation strengths, emission and excitation spectra and decay times of Tm³⁺ and Er³⁺ in germanite glasses were measured. The results were compared with the data obtained previously in borate and phosphate glasses. The role of glass phonons on the transition probabilities, half-width of emission bands and the Stokes shift is discussed. Also, the influence of the cavity and the site symmetry in which the rare earth is situated on transition probabilities is discussed. Bands in the emission spectrum of Er³⁺, absent in the borate and phosphate, are detected in the germanite glasses. Probabilities of relaxation from ${}^1\text{D}{}_2\text{→}{}^1\text{G}{}_4$ levels of Tm³⁺ in various glass hosts are calculated.