The influence of defects and impurities in polycrystalline AlN films on the violet and blue photoluminescence

The influence of defects and impurities in polycrystalline aluminum nitride films on the violet and blue photoluminescence properties was investigated. The photoluminescence spectra show a broad emission band in the range from 380 nm to 550 nm, which consists of two components of the violet band centered at 400 nm and the blue band centered at 480 nm. When the native defects reduce and the crystal quality is improved by annealing in nitrogen atmosphere, the shoulder band around 480 nm declines. The center of the violet luminescence shifts from 400 nm to about 440 nm as the oxygen content increase from 2.9 at.% to 12.3 at.%. The intensity and the center of the violet emission vary mostly linearly with the oxygen content. Combining the results of X-ray diffraction and Auger Electron Spectroscopy, the violet emission around 400 nm (3.09 eV) can be attributed to the transition from the shallow donor to the deep acceptor related to the oxygen impurity, while the blue emission around 450 nm (2.58 eV) may originate from transition from the shallow donor to the deep acceptor related to the native defect.     

How defects in an adhesive layer influence the fatigue strength of bonded steel-sheet specimens

Fatigue specimens were manufactured with a number of artificial bond defects. The defects were different configurations of non-bonded areas within the nominally perfectly-bonded region. These defects included variants with a reduced degree of bond filling, bonds with a channel without adhesive, zig-zag-shaped bond edges, and finally bond areas with smooth corners. Fatigue testing was performed with the bond loaded in the peel mode. The stiffness was measured for the bonds and was found to vary significantly with defect type and size. The fatigue strength was also found to differ substantially for different bond defects. A finite element model combined with fracture mechanics was used to predict the stiffness drop during fatigue testing of the bonded specimens and to predict fatigue life. The effect of defects in the bond on the stiffness and life was predicted in a satisfactory way.     

Analysis of nitrogen defects in diamond with VUV photoluminescence

Various diamonds were analyzed with photoluminescence (PL) spectra excited with synchrotron radiation in the wavelength range 160-250 nm. The emission of type IaAB diamond begins near 300 nm and extends to 700 nm; two broad lines with maximums about 419 and 469 nm correspond to energies 2.96 and 2.64 eV, respectively. The spectral features observed in the PL excitation spectra show two vibrational progressions, A and B, related to nitrogen defects in diamond. Progression A has a spacing 1266 ± 20 cm(-1) and is associated with the N2 (or A) center of a nitrogen defect, whereas progression B has a spacing 1177 ± 20 cm(-1) related to the N4 (or B) center of a nitrogen defect. These vibrational progressions in PL excitation spectra of N2 and N4 centers in type IaAB diamond are here identified for the first time. We suggest the use of PL spectra excited in the region 170-240 nm to analyze and to identify the type of diamond.     

Surfactant-induced defects and photoluminescence of Co doped SnO2 nanorods

The microstructure and defect evolution of Co-doped SnO2 nanostructures have been mediated by the surfactant via a hydrothermal method. The microstructure is characterized by X-ray diffraction and Raman spectra in detail. It is found that the density of structural defects considerably increases with the increase of the surfactant amount used in the preparation process. Meanwhile, the orange emission enhances dramatically. The growth process and defect evolution have been investigated by the following time-dependent experiments. Due to the existence of hollow scrolls in the growth, the mesocrystal lamellar attachment and the rolling process are proposed for the possible growth mechanism. In the synthetic process, surfactant cations, which are incorporated into the network of the self-assembled structure, strongly interact with tin hydroxyl anions. After removing the surfactants by calcination, the vacancies create in the corresponding locations, which promote the orange emission. Our finding opens a new way to control SnO2 defects and strength the optical emission.     

Concentration Phase Separation of Substitution-Doped Atoms in TMDCs Monolayer

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.     

Blue-shift in photoluminescence of ion-milled HgCdTe films and relaxation of defects induced by the milling

Simultaneous measurements of electrical conductivity, the Hall coefficient, and photoluminescence (PL) spectra of ion-milled Hg_1 − xCd_xTe films (x ∼ 0.30 and 0.38) were performed during post-milling ageing of the films at 293 K. In the course of the PL study, a ‘relaxation’ of the blue-shift of the PL band of ion-milled Hg_0.70Cd_0.30Te was observed. The relaxation was caused by the decrease of the electron concentration due to gradual disintegration of defects induced by the milling. It is shown that while ion milling substantially changes the electrical properties of Hg_1 - xCd_xTe, its PL spectrum in the long-term is affected insignificantly.     

1.4 eV photoluminescence in chlorine-doped polycrystalline CdTe with a high density of defects

The 1.4 eV photoluminescence (PL) band in a polycrystalline CdTe : Cl with a high density of defects was studied as a function of the chlorine dopant concentration and temperature. For a material with a high density of defects this band has a smooth, non-symmetrical shape without any apparent phonon structure. The energy of the intensity maximum of the 1.4 eV band depends on the concentration of chlorine and varies from 1.389 to 1.408 eV. The temperature quenching of the PL intensity for all samples was measured and the activation energy (ET) was found. Minimum and maximum values of the ET were 0.10 and 0.20 eV, respectively. A configurational – coordinate model is proposed for the 1.4 eV PL band in which the excited state of the recombination centre lies within the conduction band. In this model the temperature quenching of the PL is pictured as being essentially due to an electronic transition from the excited state directly to the ground state (the internal mechanism). This revised version was published online in November 2006 with corrections to the Cover Date.     

Quantitative investigation in the influence of oxalic impurities on photoluminescence properties of porous AAOs

Anodic alumina oxides (AAOs) prepared in oxalic acid (H₂C₂O₄) solution at a constant voltage of 40 V were investigated by the techniques of temperature programmed desorption combined mass spectrograph (TPD-MS), thermogravimetric analysis (TGA), differential thermal analysis (DTA) and photoluminescence (PL). The results indicate that the desorption of H₂O, CO and CO₂ and the decomposition of H₂C₂O₄ which led to the weight loss of AAOs samples occurred below 400 °C, while the water desorption was the main reason for weight loss. In the temperature range of 400–900 °C, a small quantity of aluminum oxalate (Al₂(C₂O₄)₃) decomposed and gave birth to CO and CO₂, during this process CO became the major product. The drastic decompositions of aluminum carbonate (Al₂(CO₃)₃) and Al₂(C₂O₄)₃ took place near 930 °C, which produced a large amount of CO₂. The amount of (C₂O₄)²⁻ embedded in bulk AAOs was calculated versus the total weight of AAOs film, and the PL intensity of AAOs was correlated to the doping of oxalic acid radical. The above results may have great significance in the better application of AAOs as matrix of photonic crystals.     

A photoluminescence study of defects in non-stoichiometric gallium arsenide using concurrent electrical and structural characterization

Photoluminescence has been used to identify centres associated with point defects and impurities in single crystal gallium arsenide, and this has been correlated with electrical and structural characterization of the same samples. Five different types of defect centre have been identified by this technique in material grown under different conditions of stoichiometry. These studies show systematic differences between the photoluminescence data obtained from stoichiometric and non-stoichiometric gallium arsenide. Precision lattice parameter and density measurements on the same gallium arsenide specimens indicate a deviation from stoichiometry on the arsenic-rich side of the gallium arsenide phase and it is suggested that defects primarily responsible for this stoichiometric deviation are arsenic interstitials in concentrations up to 3×10¹⁸ cm^?3. Photoluminescence measurements associate these defects in arsenic-rich gallium arsenide with an energy level of 0.036 eV.     

The influence of orientation on the photoluminescence behavior of ZnO thin films obtained by chemical solution deposition

A new type of ZnO thin films synthesized from chemical solution deposition at low temperature has been presented. X-ray powder diffraction and field emission scanning electron microscopy investigation reveal that the novel structured ZnO film is uniform and its [0001] direction is parallel to the substrate. The photoluminescence spectrum of this film shows strong ultraviolet band-gap emission and weak defect-related visible emission comparing to that of [0001]-oriented film, indicating high crystal quality of the non-[0001]-oriented ZnO film.     

A photoluminescence study of vanadium-related defects in n-type, semi-insulating and p-type Cd(Zn)Te

A photoluminescence (PL) study of vanadium-related defects in semi-insulating and co-doped p-type and n-type CdTe:V crystals gives evidence of the presence of the V²⁺–Zn complex. In addition to the ³T₂(F)→³A₂(F) emission of V³⁺ near 0.5 eV and the ⁴T₂(F)→⁴T₁(F) transition of V²⁺ near 0.45 eV, two further luminescence bands are detected at higher energies. The first emission band (I), peaking around 0.8 eV, is correlated to the V²⁺–Zn complex and the second one (II), peaking around 0.6 eV, is attributed to the acceptor level introduced by the cadmium vacancies. Varying the zinc concentration in CdTe, we analyse the behaviour of the vanadium impurity charge state. We show that the V²⁺ internal transition decreases with zinc alloying due to the formation of the V²⁺–Zn complex. The emission bands related to isolated V_Cd are present with high intensity only in the p-type crystals, in which all the vanadium content is in the V³⁺ oxidation state, whereas, in the semi-insulating and n-type crystals, the PL spectrum is dominated by Emission I related to the V²⁺–Zn complex. The presence of this complex in the semi-insulating crystals used in photorefractive (PR) applications and the dominance of this complex over the optical properties of Cd(Zn)Te:V imply the contribution of this complex to the PR processes.     

The influence of capping thioalkyl acid on the growth and photoluminescence efficiency of CdTe and CdSe quantum dots

The influence of thioalkyl acid ligand was evaluated during aqueous synthesis at 100?°C and under hydrothermal conditions (150?°C) of CdTe and CdSe quantum dots (QDs). Experiments performed with 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) demonstrated that the use of MHA and MUA allowed for the preparation of very small nanoparticles (0.6-2.5?nm) in carrying out the reaction under atmospheric pressure or in an autoclave and that the photophysical properties of QDs were dependent on the ligand and on the synthesis conditions. The influence of various experimental conditions, including the Te-to-Cd ratio, temperature, and precursor concentration, on the growth rate of CdTe or CdSe QDs has been systematically investigated. The fluorescence intensities of CdTe QDs capped with MPA, MHA, or MUA versus pH were also found to be related to the surface coverage of the nanoparticles.     

How clayey fines in aggregates influence the properties of lime mortars

Despite the fact that the maximum content of fines in aggregates is restricted in national and international standards, the use of unwashed sands in restoration mortars is often demanded by restorers due to their colouring properties. The colour of these aggregates may be caused by clay minerals in the fine fraction below 63 μm. Hence, this study aims to determine the influence of loam and clay contents in a quartz aggregate on the properties of fresh and hardened lime- and lime-cement-mortars. The experimental results revealed that the main effect of clay fines in aggregates is an increase of the water demand for a constant mortar consistency. As a consequence, the higher water/binder ratio causes a strong decrease of the mortar quality with respect to mechanical, hygral and durability properties.     

The influence of surface defects on high-cycle fatigue of engineering materials

An investigation was performed on the influence of surface defects, such as indentor's imprint, etching pits, and nonmetallic inclusions, upon high-cycle fatigue of alloyed steel 20Kh13, carbon steel 20, and titanium alloy VT3-1 in torsion and rotational bending. Defects were initiated on the surface after its polishing, grinding, and surface plastic deformation. The analysis of the results obtained involved the use of the theoretical stress concentration factor values and the magnitudes of residual stresses in the surface layer after different types of surface treatment, as well as residual stresses generated by the introduced defects. A method has been substantiated for predicting the influence of defects on fatigue limit taking into account the defect area.Published in Problemy Prochnosti, No. 1, pp. 5–16, January, 1996.     

Damage mechanism of composite cemented backfill based on complex defects influence

In order to reduce mining waste discharge and realize efficient application of tailing material in underground backfilling, the characteristics and mechanism of backfill damage is analyzed, and also the composed behavior and failure characteristic of cemented backfill. Five types of backfills were tested under uniaxial compression loading in SHT4206 electro‐hydraulic servo testing system. Based on the experimental results, the damage constitutive equations of cemented backfills with five cement?tailing ratios were proposed on the basis of damage mechanics and then validated. Research shows that the damage growth rate of backfills with lower cement?tailing is lower before peak value of stress, but it grows more rapidly after that. In addition, four backfill combination schemes were designed for mechanical test. Combining with research results on fracture characteristic of different backfills, a defects coupled constitutive model based on continuum damage mechanics was established. Research shows that the piecewise nonlinear model can well embodies effect of complex defects in backfills on the stress‐strain curve. Accuracy of the model is well verified by measured curve of backfill composite members. The results of this work provide a scientific basis for failure process prediction of backfills and reasonable matching design of framed filling.     

Fatigue life calculation for titanium alloys considering the influence of microstructure and manufacturing defects

A model is presented for calculation of the fatigue life to crack initiation in specimens with surface stress concentrators. The model is tested for the results of fatigue testing specimens of Ti–6Al–4V titanium alloy condensate prepared by electron-beam physical vapor deposition method (EB PVD-method). The specimens had manufacturing defects, such as column defects of different diameters reaching the specimen surface. Comparison between results of calculation and experiment showed a good agreement. In order to fill the model, it is sufficient to have the results from monotonic tensile testing and characteristics of microstructure and texture of the starting material.     

Development of plastic strains in the vicinity of fissurelike defects under the influence of residual stresses

Strength of Materials -