Photoluminescence in the band crossover region of AlxGa1−xAs

Transitions occurring In the band crossover region of Al_xGa_1−xAs were examined using photoluminescence. The transitions observed occur at higher energies and at lower intensities than expected. These results suggest that the accepted conduction band edge relationships do not hold true within the band crossover region, where the L, X and Γ conduction band minima approach the same energy values. It is proposed that an indirect transition is responsible for the observed photoluminescence response.     

Peculiarities of phase transitions in the iron-nanoclay surface system studied by Mössbauer spectroscopy

Results of an experimental investigation of the peculiarities of phase transitions in iron compounds localized in natural low-dimensional systems (nanoclays) are reported. Conditions for observation of the \((I_c \xrightarrow{{188 K}}I_h )\) phase transition by Mössbauer spectroscopy in thin films of ice adsorbed on the surface of typical clays are established for the first time. Data obtained by the proposed method for the geography of iron exchange complexes on aluminosilicate clay surface are presented.     

Blue shift in optical band gap of sol–gel derived Sn1−xZnxO2 polycrystalline thin films

Undoped and Zn doped SnO₂ thin films are deposited by sol–gel spin coating on glass substrate. XRD spectra with prominent peaks along (110) and (101) planes shows the polycrystalline nature of thin films. The particle size lies between 9.30 and 42.09 nm as estimated by Debye–Scherer method. SEM micrographs of the films contain pebble like structures spread throughout whose diameter decreases with increase in dopant concentration. Surface topology of the films is studied by atomic force microscopy. Transmission spectra show that all the films are highly transparent in the visible and IR spectral region (80–90 %) and a sharp absorption occurs near 300 nm. Approximately a change of 4 % is observed in the optical band gap by Zn doping. The optical band gap is tunable between 3.55 and 3.68 eV for 0 ≤ x ≤ 0.15 in nanocrystalline Sn_1?xZn_xO₂. Broad UV emission at 395 nm is observed in photoluminescence spectra of the films along with a blue emission. Emission intensity decreases as amount of Zn incorporated into SnO₂ increases.     

On the 3625 cm−1 OH stretching band in HNa-Y zeolites

The existence of a band at 3625 cm⁻¹ has been detected on a HNa-Y decationated zeolite, when it interacts with water at room temperature. The origin of this band is discussed and its evolution when different controlled amounts of water are introduced into the cell is presented.     

A study of the surface of carbon fiber by means of X-ray photoelectron spectroscopy—III

This paper reports a study of the surface composition of carbon fibers treated by various methods by means of X-ray photoelectron spectroscopy (SPS). C_1S X-ray photoelectron spectra showed that after the surface treatment of carbon fibers, the carbon atoms in the hydrocarbon were changed into

etc. oxygen-containing groups, that is, the results of surface oxidation and concentration increased with time but finally reached a constant level. Comparing experimental results for the treatments used, we found that all of these methods resulted in concentrations of oxygen groups on the surface in the order:

.Evidence was found for the formation of lactone groups

during treatment in an oxygen or nitrogen plasma, but not during treatment by nitric acid or anodic oxidation.     

Annealing behaviors of quench-deposited (CsCl)1−x(PbCl2)x films studied by optical absorption spectroscopy

All the films of the (CsCl)_1−x(PbCl₂)_x system prepared by quench deposition exhibit a similar fundamental absorption spectrum over the full mixing range. On annealing the films, drastic changes occur in the spectra showing three characteristic types of spectral features depending on the mixing ratio. Type 1 (x≥0.7) suggests the formation of mixed crystals of a PbCl₂ base. Type 2 (0.5≥x≥0.3) exhibits the spectrum of CsPbCl₃ crystal. Type 3 (x=0.1, 0.05), the most characteristic, is the spectrum of Cs₄PbCl₆ crystal, which was observed for the first time. The spectrum of the Cs₄PbCl₆ crystal exhibits novel structures. Despite the crystalline entity of the compound, it shows oscillator-like fundamental absorption and a wide (≈1 eV) window just above the first absorption peak, suggesting the peculiarity of the associated energy band structure. To a first approximation, the spectrum is explainable in terms of Pb²⁺-ion excitation of the Pb²⁺(Cl⁻)₆ quasi-complexes.     

Concentration measurement of NO using self-absorption spectroscopy of the γ band system in a pulsed corona discharge

Nitric oxide (NO) concentrations were measured using the γ band system spectrum based on the strong self-absorption effect of NO in pulsed corona discharges. The radiative transitional intensities of the NO γ band were simulated based on the theory of molecular spectroscopy. The intensities of some bands, especially γ(0,0) and γ(1,0), are weakened by the self-absorption. The correlations between the spectral self-absorption intensities and NO concentration were validated using a modified Beer-Lambert law with a combined factor K relating the branching ratio and the NO concentration, and a nonlinear index α that is applicable to the broadband system. Optical emissive spectra in pulsed corona discharges in NO and N2/He mixtures were used to evaluate the two parameters for various conditions. Good agreement between the experimental and theoretical results verifies the self-absorption behavior seen in the UV spectra of the NO γ bands.     

Study on the temperature dependence of the OH− absorption band in Hf-doped LiNbO3 crystals

We study in this paper the temperature dependence of the OH^? absorption band in LiNbO₃ crystals with different Hf-doping concentrations. It is found that the shoulder at 3500 cm^?1 appears in the doping level of 2.6 mol%, becomes more evident at 3 and 4 mol%, and then gradually faints when the doping concentration increases. This result is explained by the non-monotonic change of Nb vacancy density in the three stages of the defect evolution involved in the Hf incorporation of LN lattice. Moreover, the absorption difference analysis of the OH^? bands is consistent with this explanation and reveals that the mobility of H⁺ ions may be reduced by the strong trapping of the highly electronegative Nb vacancies.     

Preparation and properties of ferroelectric Pb1 − xCaxTiO3 thin films produced by the polymeric precursor method

Pb_{1 – x} Ca _x TiO₃ thin films with x = 0.24 composition were prepared by the polymeric precursor method on Pt/Ti/SiO₂/Si substrates. The surface morphology and crystal structure, and the ferroelectric and dielectric properties of the films were investigated. X-ray diffraction patterns of the films revealed their polycrystalline nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness. The multilayer Pb_{1 – x} Ca _x TO₃ thin films were granular in structure with a grain size of approximately 60–70 nm. The dielectric constant and dissipation factor were, respectively, 174 and 0.04 at a 1 kHz frequency. The 600-nm thick film showed a current density leakage in the order of 10^–7 A/cm² in an electric field of about 51 kV/cm. The C-V characteristics of perovskite thin films showed normal ferroelectric behavior. The remanent polarization and coercive field for the deposited films were 15 C/cm² and 150 kV/cm, respectively.     

Optical spectroscopy of Ce3+ ions in Gd3(AlxGa1−x)5O12 epitaxial films

Epitaxial films of Ce-doped Gd₃(Al_xGa_1−x)₅O₁₂ with x = 0.00, 0.22, 0.31, 0.38 formula units were grown using liquid-phase epitaxy method, and their optical properties were studied. The emission of Ce³⁺ ions can be observed only when Al³⁺ ions are incorporated into the garnet structure, resulting in a shift of the 5d Ce³⁺ states from the conduction band to the bandgap. It is shown that the shift is caused by the cumulative effect of gradual low-energy shift of the lowest 5d level of Ce³⁺ and the raise of the garnet bandgap energy with increasing Al³⁺ concentration.     

Preparation and characterization of in1−xGaxAsyP1−y epilayers by liquid-phase epitaxy

In_1–x Ga _x As _y P_1–y epilayers with three different solid compositions of ln_0.73Ga_0.27As_0.60P_1.40, In_0.59Ga_0.41As_0.87P_0.13 and ln_0.53Ga_0.47As were grown on (1 0 0) InP substrate at 623° C by the step cooling technique of liquid-phase epitaxy. From the optical transmission measurements, the corresponding wavelengths of the InGaAsP epilayers were 1.30, 1.55 and 1.69 m, respectively, which are in good agreement with those obtained from the calculations using Vegard's law. The full widths at half maximum of the photoluminescent spectra at 14 K of these layers were as low as 18.6, 22.5 and 7.9meV, respectively. The electron mobility of the InGaAsP epilayers is a function of the solid composition with the ln_0.53Ga_0.47As epilayer having the highest electron mobility. The mobility and concentration of this layer are 8,873cm²V^–1 sec^–1, 9.7×10¹⁵cm^–3 and 22,900 cm²V^–1 sec^–1, 8.5×10¹⁵cm^–3 at 300 and 77 K, respectively. The compensation ratio is between 2 and 5.     

Study of lattice deformation and atomic bond length for AlxGa1−xN epi-layers with synchrotron radiation X-ray absorption spectroscopy

The lattice deformation in wurtzite Al_xGa_1?xN epi-layers (0.19 ≤ x ≤ 0.58) grown on sapphire substrates by metal organic chemical vapor deposition has been studied with synchrotron radiation X-ray absorption spectroscopy and X-ray diffraction. The result reveals that Al substitution for Ga in AlGaN epi-layers induces a significant contraction in crystal lattice and c/a ratio (c and a are the lattice constants for AlGaN epi-layer, respectively), while the substitution only results in a slight expansion in internal parameter u, which is used to describe the lattice deformation of wurtzite structure. This fact suggests that u is not sensitive to Al substitution for Ga. A detailed analysis of extended X-ray absorption fine structure of Ga K-edge spectra show that the Ga–N bond length has only a weak composition dependence, while the Ga–Ga (Al) bond lengths are strongly dependent on the Al composition.     

Determination of da/dN-ΔK 1 curves for small cracks in alumina in alternating bending tests

Crack-growth relations under cyclic fatigue conditions are mostly determined for long cracks. In order to determine da/dN-K curves for small cracks from lifetimes under cyclic load a procedure has been derived which is based on a method usually applied to subcritical crack growth. To prove the cyclic effect and to demonstrate the procedure in detail, measurements were carried out on an Al₂O₃-ceramic in bending with anR-ratio ofR=–1 and two types of relatively small cracks, namely natural cracks and Knoop-cracks. It was found that both crack types exhibit the same da/dN-K relation. The exponent of the Paris law for fatigue crack growth is significantly different from the exponent of the power law for subcritical crack growth.     

Cd1−xMnxTe thin films deposited by close-spaced sublimation for application in solar cells

High-quality Cd_1−xMn_xTe polycrystalline films with (1 1 1) preferred orientation were deposited by close-spaced sublimation (CSS) method. The XRD and optical absorption analysis indicated that the band gap of the film was about 1.6 eV. The as-grown Cd_1−xMn_xTe films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. The effect of various post-deposition treatments with vapors of chlorine-containing materials (CdCl₂ and/or MnCl₂), in Ar or H₂/Ar ambient, on the properties of Cd_1−xMn_xTe cells was studied.     

On the optical constants of amorphous GexSe1−x thin films of non-uniform thickness prepared by plasma-enhanced chemical vapour deposition

The preparation of layers of amorphous Ge_xSe_1−x (with Ge atomic concentrations x=0, 0.17, 0.25 and 0.34) by plasma-enhanced chemical vapour deposition (PECVD) using the hydrides, GeH₄ and H₂Se, as precursor gases is described in detail. Information concerning the structure of the films was obtained from Raman spectroscopy. The optical transmission was measured over the 300 to 2500 nm spectral region in order to derive the refractive index and extinction coefficient of these PECVD films. The expressions proposed by Swanepoel, enabling the calculation of the optical constants of a thin film with non-uniform thickness, have successfully been applied. The refractive-index dispersion data were analysed using the Wemple–DiDomenico single-oscillator fit. The optical-absorption edges have been all of them described using the `non-direct transition' model proposed by Tauc. The optical gaps were calculated using Tauc's extrapolation, resulting in values ranging from 1.93 eV for a-Se to 2.26 eV for a-GeSe₂.     

Effect of chemical substitution on the morphology and optical properties of Bi1−xCaxFeO3 films grown by pulsed-laser deposition

The morphological characteristics as well the optical properties of Ca-doped BiFeO₃ films grown by pulsed-laser deposition technique have been investigated. AFM images revealed that calcium has a radical effect on the surface features of BiFeO₃ films. By utilizing spectrophotometer, transmission behaviour of the films was investigated. Local IV characteristics of the films disclosed about three orders of magnitude enhancement concerning electrical conductivity through Ca doping. X-ray photoelectron spectroscopy results revealed that Ca can reduce the valence state of iron in the compound.