Spatial Distribution Of Defects Research Articles

The Spatial Distribution of Defects and Its Dependence on Seed Polarity and Off-Orientation during Growth of 4H-SiC Single Crystals

The defect distribution in 4H-SiC single crystals in dependence on the seed polarity and its off-orientation was investigated by KOH-etching, optical microscopy and X-ray topography. Micropipe density, stacking fault density and dislocation density were determined for 2” crystals grown in <000-1> direction 0 - 7° off towards <11-20> and for crystals up to 1” in diameter grown in <11-20> or a- and <1-100> or m-directions and using repeated a-face growth. For the growth in polar directions the micropipe density and dislocation density decrease with increasing offorientation of the seed. A similar behavior was found for the stacking fault density and dislocation density in non-polar directions with off-orientation to c-direction. Nevertheless, while the dislocation density could be reduced up to three orders of magnitude for the growth along non-polar directions, the stacking fault density was continuously increasing. Additionally, the defect distribution after repeated a-face growth will be discussed in terms of growth related and kinetic models.

Positron Annihilation Studies of Microstructure of Ultra Fine Grained Metals Prepared by Severe Plastic Deformation

In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.

Dendritic Flux Avalanches and Nonlocal Electrodynamics in Thin Superconducting Films

We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

Resonant tunneling through defects in an insulator: Modeling and solar cell applications

In this paper, a model for electron tunneling through defects in an insulator is presented. The three-dimensional results for the electron transmission coefficient can be obtained by characterizing the tunneling process in terms of a defect density and capture cross section. Fitting the model parameters by comparison with the results of a full three-dimensional tunneling-through-defect simulation, this model can be used to calculate and predict the electron transmission for various spatial distributions of defects without performing the complex three-dimensional calculations. Energy selective contacts using the resonant tunneling for carrier extraction have been proposed as a means to achieve a higher efficiency in future generations of photovoltaic devices. Resonant tunneling through defects in an insulator, where the defects may be atoms or quantum dots, may provide a possible implementation for such energy selective contacts. With the present model, the influences of the tunneling effective mass, insulator thickness, and defect distribution on the electron transmission coefficient have been investigated. The results suggest that the introduced defects should lie in the middle of a reasonably thick insulator to improve the carrier energy selectivity.

Bridgman growth of paratellurite single crystals

The growth of paratellurite single crystals by the vertical-gradient freezing technique is reported for the first time. Boules of 120mm long and 25mm in diameter were obtained under a temperature gradient of 10°Ccm−1 and translation rates lower than 0.6mmh−1. The spatial distribution of defects along the growth axis reveals a continuous evolution of the free convective fluid-flow regime as growth proceeds. Gas bubbles and dark inclusions rejected to the periphery in the upper part of the crystal are observed to lay preferentially in (100), (001), (11¯0) and (112) crystallographic planes. Among them, SEM and microprobe analyses evidenced the presence of metallic platinum, while micro-Raman experiments allowed to assess oxygen as being actually the gas content of occluded bubbles, originated at the crucible wall through Pt-induced dissociation of Te–O bonds.

Cathodoluminescence microscopy and spectroscopy of n-type 4H-SiC epilayers

Cathodoluminescence (CL) microscopy and spectroscopy have been used to investigate the nature and spatial distribution of defects and impurities in n-type epitaxial 4H-SiC. CL microscopy reveals the existence of 6H-SiC polytype inclusions, while CL spectra recorded at different excitation conditions show luminescence emission related to deep levels in the SiC epilayer. Deconvolution of the mentioned spectra indicates the complex character of the deep-level CL emission, that is actually composed of four bands centred near 2.72, 2.56, 2.42 and 2.00 eV at 88 K. The origin of these bands is discussed considering previous deep level transient spectroscopy measurements carried out in the same material investigated in the present work.

Investigate the dynamics of accumulation and recombination of radiation defects in natural and synthetic halite samples has been investigated by means of electron spin resonance

The dynamics of accumulation and recombination of radiation defects in natural and synthetic halite samples has been investigated by means of electron spin resonance. The F-centers accumulation essentially depends on their distribution in crystal that caused by its microstructure properties. It was found out that in natural halite γ-irradiation produced the heterogeneous spatial distribution of defects. In places of preferable accumulation of defect the recombination occurs at smaller temperatures in comparison with their uniform arrangement. The realized investigations are important from the view of safe use of salt formations for radioactive waste isolation.

Interfacial State Density and Conductance-Transient Three-Dimensional Profiling of Disordered-Induced Gap States on Metal Insulator Semiconductor Capacitors Fabricated from Electron-Cyclotron Resonance Plasma-Enhanced Chemical Vapor Deposited SiOxNyHz Films

An electrical characterization of Al/SiOxNyHz/Si metal–insulator–semiconductor (MIS) structures has been carried out. SiOxNyHz films of different compositions have been obtained from these structures by varying gas flow in the electron-cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) system. The presence of nitrogen in the films increases the dielectric constant value and degrades the interface quality, as our measurements demonstrate. The effect of thermal annealing has also been determined. Capacitance–voltage (C–V) results show that unannealed samples exhibit positive flat-band voltages, whereas annealed ones exhibit negative values. On the other hand, from deep-level transient spectroscopy (DLTS) measurements we can conclude that interfacial state density diminishes when thermal treatments are applied. Moreover, conductance transient analysis provides the energetic and spatial distribution of defects in the films and demonstrates that thermal improvement affects not only the interface, but also the insulator bulk.

Spatial distribution profiles of defects in mercury cadmium telluride after ion implantation

The spatial distribution of defects in Hg 1− x Cd x Te are determined after ion implantation by hydrogen and argon ions at constant low current densities and after implantation by copper, tungsten and aluminium ions in the case of pulsed bombardment at high ion current densities. Secondary ion mass spectrometry, electron–positron annihilation, Rutherford back scattering and differential Hall measurements are used to obtain distribution profiles of impurity atoms, vacancies and electrically active defects. The results show that the electron concentration in the implanted layer saturates with increasing ion dose and that the peak depth is also dose dependent. The results show a clear difference between the low and high current cases. For a constant low current density beam, the implanted ions are localised around the mean projected range. When a strong pulsed beam is used the range is shorter than the theoretical estimate. An explanation for this behaviour is given.

Capacitance study of defects induced in heavily damaged region formed in hydrogen implanted Si

Capacitance–voltage ( C– V) characteristics at high frequency (1 MHz) as a function of temperature ( T) were measured to study defects created by hydrogen implantation followed by high temperature annealing. The initial material used in this study was p-type Czochralski Si (Cz-Si) implanted with 130 keV H 2 + at a high dose of 4×10 16 cm −2. The post-implantation annealing was carried out in argon ambient at 450 °C per 10 h, i.e. at the temperature of thermal donor creating in Si. The presence of the deep-level defects manifest themselves through several unusual features in C– V( T) characteristics. A flat region, sudden jump of capacitance and hysteresis effect in C– V data were observed during sweeping bias voltage. Our experiments by filling and emptying deep-level defects with carriers before the voltage sweep also revealed unusual hysteresis effects in C– V curves. The results obtained can be readily understood if we assume that deep-level defects in the depletion region are present in very high concentration, the defect spatial distribution is inhomogeneous as usual in implanted samples, and the defects control the electrical characteristics. We also conclude that a relatively thick p-type layer is formed during implantation at the surface of the n-type Si that is formed as a result of annealing.

Investigation of 4H-SiC Schottky diodes by ion and X-ray micro beam induced charge collection techniques

Silicon carbide has recently emerged as an attractive material for ionisation radiation detection. The high band gap and high radiation damage resistance should allow the fabrication of detectors capable to operate at high temperature and in high radiation fields. The development of SiC radiation detectors in the field of spectroscopy imposes severe constraints in the electronic quality and homogeneity of the material. In this work we present an investigation of the charge collection properties of ‘detector grade’ 4H-SiC Schottky diodes performed by means of the X-ray and ion beam induced charge collection (XBICC and IBIC) techniques. Such techniques allow the minority carrier diffusion length of the material to be evaluated and mapping of the transport properties to be performed with a spatial resolution of the order of 1 μm. The investigated detectors are formed by Schottky contact (Au) on the epitaxial layer and an ohmic contact on the back side of 4H-SiC substrates. IBIC measurements were performed using protons of energy 0.7–1.7 MeV. The IBIC spectra show a complete charge collection generated by ionisation in the depletion region. Similar analysis was also performed in steady state conditions using data from photocurrent measurements carried out at European Synchrotron Radiation Facility using 3 keV photons. IBIC and XBICC maps were obtained by recording the mean pulse height and the mean photocurrent as a function of the photon or ion impact co-ordinates. The analysis of such maps allowed us to individuate the spatial distribution of defects and contact imperfections.

Meyer–Neldel parameter as a figure of merit for quality of thin-film-transistor active layer?

The temperature dependence of the subthreshold current is investigated for both amorphous and polycrystalline silicon thin-film-transistors (poly-Si TFTs). The poly-Si devices are issued from various fabrication processes and present, therefore, different active layer crystalline quality. They belong to three categories: (i) furnace thermal annealed non-hydrogenated; (ii) thermal annealed hydrogenated; and (iii) excimer laser annealed polysilicon active layer TFTs. Temperature measurements of the subthreshold current are performed in the linear mode for temperatures varying from 20 to 120 °C. This current is thermally activated. Moreover, except for laser crystallised active layer devices, TFTs exhibit a Meyer–Neldel behaviour related to the quality of the active layer. For amorphous devices the Meyer–Neldel characteristic energy ( E MN) value equals 0.051 eV. For polysilicon structures E MN increases from 0.041 to 0.063 eV when the deposition pressure increases from 10 to 90 Pa, and is reduced after hydrogen passivation (0.038 against 0.064 eV). Our experimental study suggests that the Meyer–Neldel behaviour is related both to the spatial distribution of defects in the active layer and to the density of bandgap states, in particular to the band tail state concentration.

Spatial distribution of defects in ultra-fine grained copper prepared by high-pressure torsion

A study of ultra-fine grained copper produced by high-pressure torsion is presented. Positron annihilation spectroscopies (slow positron implantation, lifetime and Doppler broadening spectroscopies), transmission electron microscopy, X-ray diffraction spectroscopy, and microhardness measurements have been employed in order to identify defects and their spatial distribution, i.e. lateral distribution and depth profile, in the specimens. Two types of defects could be identified: dislocations in the distorted regions along grain boundaries and microvoids of size of 3–4 vacancies inside the grains. No change in grain size as a function of the distance from the center of the sample disk was observed. The mean coherent domain size close to the surface was found to be 80 ± 20 nm and it slightly increases with depth. The concentration of microvoids has been found to decrease with depth and it slightly varies with the distance from the center of the disk, being lowest in the center. No position dependence of the concentration of dislocations was observed.

Investigation of spatial distribution of defects in ultra-fine grained copper

Ultra-fine grained copper prepared by high pressure torsion has been studied by means of slow positron implantation spectroscopy with Doppler broadening measurement. In addition, conventional positron lifetime and Doppler broadening spectroscopy have been utilised. Defects present in the specimens were identified, their spatial distribution and depth profile have been determined. The results are discussed in correlation with those obtained by XRD and TEM.

Thermal stability of ultrafine grained copper

Thermal stability of ultrafine grained (mean grain size 150 nm) copper prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy correlated with transmission electron microscopy. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high number of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Calculations of the lifetime of a positron trapped at a microvoid as a function of its size were performed to obtain information about sizes of the microvoids. Abnormal grain growth, when isolated recrystallized grains grow inside the deformed matrix, takes place from $160\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ From 280 to $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ recrystallization occurs. Strongly inhomogeneous spatial distribution of defects does not allow application of the simple trapping model. Therefore a model of positron behavior in the ultrafine grained materials was developed in the present work. The model takes into account inhomogeneous spatial distribution of defects and allows for determination of dislocation density, concentration of microvoids, linear size of coherent domains, and volume fraction of the distorted regions. Moreover using this model it was possible to determine the activation energy of the recrystallization.

Fractal characteristics of composite pipes defects in torsion testing

The epoxy-glass composite pipes of l=330, 800 and 1000 mm in length and φ=30, 35, 41 mm in diameter were subject to torsion till failure. The pipes were formed by filament winding on the mandrel at the angle α=22°,27°,32° for successive diameters. Torsional angle, torsional moment and acoustic emission (AE) were recorded during the test. This paper represents an attempt to use fractal analysis software to process data obtained from AE testing. It shows the dependence of fractal measure and fractal dimension on the amplitude of acoustic events along the axis of the tested pipe in their relation to the spatial distribution of defects in the material.

Interaction of hydrogen with radiation defects in p-Si crystals

Interaction of hydrogen with radiation defects in p-Si crystals was studied by deep-level transient spectroscopy. Hydrogen was introduced into the electron-irradiated crystals using wet chemical etching in a solution of nitric and hydrofluoric acids at room temperature with subsequent annealing at 380 K under reverse bias applied to the formed Schottky diodes. It is found that the passivation of radiation defects is accompanied with the formation of new electrically active centers with the concentration profile dependent on the hydrogen concentration. It is shown for the first time that hydrogen passivates the electrical activity of the CsCi centers. Based on the data about the spatial distribution of defects and the kinetics of passivation, the plausible origin of the newly formed centers is analyzed. The radii of hydrogen capture by divacancies, the K centers, the CsCi complexes, and new centers were determined.

High quality CVD diamond for detection applications: structural characterization

High quality synthetic diamonds have been grown on single-crystal silicon by microwave plasma enhanced chemical vapour deposition, using a CH 4–H 2 gas mixture at variable methane concentrations in the range 0.6–2.2%. Film surface morphology and preferential orientation have been investigated by scanning electron microscopy and X-ray diffraction. Raman spectroscopy and photoluminescence (PL) have been utilized to monitor the crystalline quality and to study the spatial distribution of defects in the deposited films, in view of their application as active materials for particle detectors. In particular, a detailed micro-Raman study shows that the diamond crystals on the growth surface have intrinsic quality comparable with that of the best natural diamonds, as evidenced by Raman peaks at 1332 cm −1 narrower than 2.4 cm −1, by the complete absence of any non-diamond carbon feature at approximately 1500 cm −1 and by a very weak luminescence background in the 100–6500 cm −1 spectral range. A broad PL band at approximately 2.05 eV, almost completely absent at the growth surface, is detected moving towards the film-substrate interface. A worsening of the global quality of the films is found with increasing methane concentration in the gas mixture, as witnessed by the increased broadband PL in the films grown at higher methane concentrations. The excellent quality of the films was independently assessed through their high detection-performance. The alpha-particle spectra, measured by using a 5.5 MeV 241Am source, showed a decrease in the detection efficiency with increasing methane concentration. This worsening in the detection performance is correlated with the lower global quality of the films grown at higher CH 4 concentrations, clearly evidenced by the structural characterization.

Mechanical property degradation in irradiated materials: A multiscale modeling approach

High-energy particle irradiation of low stacking fault energy, face centered cubic (fcc) metals produces significant degradation of mechanical properties, as evidenced in tensile tests performed at or near room temperature. Post-irradiation microstructural examination reveals that approximately 90% of the radiation-induced defects in copper are stacking fault tetrahedra (SFT). Radiation damage is an inherently multiscale phenomenon involving processes spanning a wide range of length and time scales. Here we present a multiscale modeling methodology to study the formation and evolution of defect microstructure and the corresponding mechanical property changes under irradiation. At the atomic scale, molecular dynamics (MD) simulations have been used to study the evolution of high energy displacement cascades, SFT formation from vacancy rich regions of displacement cascades, and the interaction of SFTs with moving dislocations. Defect accumulation under irradiation is modeled over diffusional length and time scales by kinetic Monte Carlo (KMC), utilizing a database of displacement cascades generated by MD. The mechanical property changes of the irradiated material are modeled using three-dimensional dislocation dynamics (DD). Key input into the DD includes the spatial distribution of defects produced under irradiation, obtained from KMC, and the fate of dislocation interactions with SFTs, obtained from MD.

High quality CVD diamond: a Raman scattering and photoluminescence study

High quality synthetic diamonds were grown on single-crystal silicon by microwave plasma enhanced chemical vapour deposition (CVD). A careful optimisation of both the experimental setup and the growth parameters was necessary before that the achievement of the best results was made possible. The films were deposited using a CH4-H2 gas mixture at methane concentrations variable in the range 0.6-2.2%, while the substrate temperature was fixed at 750 °C. Raman spectroscopy and photoluminescence (PL) were utilised to monitor the quality of the deposited films and to study the spatial distribution of defects, respectively. Micro-Raman analysis shows that linewidths of the diamond peak lower than 2.4 cm-1 can be easily measured at the growth surface, indicating that the crystalline quality of individual grains is comparable to that of the best natural diamonds. The excellent phase purity of the diamond microcrystals at the growth surface is witnessed by the complete absence of any non-diamond carbon feature and by a very weak luminescence background in the 1.6-2.4 eV spectral range. A worsening of the quality of the diamond particles is found moving from the growth surface towards the film-substrate interface. A photoluminescence feature at about 1.68 eV, commonly associated to Si impurities, is distinctly observed as the exciting laser beam is focused close to the interface. A progressive degradation of the global quality of the films is found with increasing methane concentration in the gas mixture, as witnessed by an increased PL background in the films grown at higher methane concentrations.