Extended Defects Research Articles

Diversity of Properties of Device Structures Based on Group-III Nitrides, Related to Modification of the Fractal-Percolation System

A fractal-percolation system that includes extended defects and random fluctuations in the alloy composition is formed during the growth of device structures based on Group-III nitrides. It is established that the specific features of this system are determined not only by the growth conditions. It is shown that the diversity of the electrical and optical properties of InGaN/GaN LEDs (light-emitting diodes) emitting at wavelengths of 450–460 and 519–530 nm, as well as that of the electrical properties of AlGaN/GaN HEMT (high-electron-mobility transistor) structures, is due to modification of the properties of the fractal-percolation system both during the growth process and under the action of the injection current and irradiation. The influence exerted by these specific features on the service life of light-emitting devices and on the reliability of AlGaN/GaN HEMT structures is discussed.

Investigation of deep level defects on Beryllium compensation doping of In0.53Ga0.47As/GaAs0.49Sb0.51 type-II superlattice photodiodes.

In this paper, deep level transient spectroscopy (DLTS) characterization was performed on Beryllium compensation doping of InGaAs/GaAsSb type-II superlattice photodiode. Three electron traps with the energy levels located at Ec-0.11 eV (E1), Ec-0.28 eV (E2), Ec-0.17 eV (E3), and a hole trap situated at Ev + 0.25 eV (H1) were revealed. The position distribution and depth concentration of these traps in SL absorption region was also explored. Furthermore, the bandlike states (E2) and localized states (E1 and H1) of extended defects were confirmed by DLTS measurements as a function of the filling-pulse time, these traps as generation-recombination centers are responsible for dominant dark current.

Influence of Dislocations in Transition Metal Oxides on Selected Physical and Chemical Properties

Studies on dislocations in prototypic binary and ternary oxides (here TiO2 and SrTiO3) using modern TEM and scanning probe microscopy (SPM) techniques, combined with classical etch pits methods, are reviewed. Our review focuses on the important role of dislocations in the insulator-to-metal transition and for redox processes, which can be preferentially induced along dislocations using chemical and electrical gradients. It is surprising that, independently of the growth techniques, the density of dislocations in the surface layers of both prototypical oxides is high (109/cm2 for epipolished surfaces and up to 1012/cm2 for the rough surface). The TEM and locally-conducting atomic force microscopy (LCAFM) measurements show that the dislocations create a network with the character of a hierarchical tree. The distribution of the dislocations in the plane of the surface is, in principle, inhomogeneous, namely a strong tendency for the bundling and creation of arrays or bands in the crystallographic <100> and <110> directions can be observed. The analysis of the core of dislocations using scanning transmission electron microscopy (STEM) techniques (such as EDX with atomic resolution, electron-energy loss spectroscopy (EELS)) shows unequivocally that the core of dislocations possesses a different crystallographic structure, electronic structure and chemical composition relative to the matrix. Because the Burgers vector of dislocations is per se invariant, the network of dislocations (with additional d1 electrons) causes an electrical short-circuit of the matrix. This behavior is confirmed by LCAFM measurements for the stoichiometric crystals, moreover a similar dominant role of dislocations in channeling of the current after thermal reduction of the crystals or during resistive switching can be observed. In our opinion, the easy transformation of the chemical composition of the surface layers of both model oxides should be associated with the high concentration of extended defects in this region. Another important insight for the analysis of the physical properties in real oxide crystals (matrix + dislocations) comes from the studies of the nucleation of dislocations via in situ STEM indentation, namely that the dislocations can be simply nucleated under mechanical stimulus and can be easily moved at room temperature.

In situ hydrostatic pressure induced improvement of critical current density and suppression of magnetic relaxation in Y(Dy0.5)Ba2Cu3O7−δ coated conductors

We report on the effect of in situ hydrostatic pressure on the enhancement of the in-magnetic-field critical current density parallel to the crystallographic c-axis and vortex pinning in epitaxial Y(Dy0.5)Ba2Cu3O7−δ coated conductors prepared by metal organic deposition. Our results show that in situ hydrostatic pressure greatly enhances the critical current density at high fields and high temperatures. At 80 K and 5 T we observe a ten-fold increase in the critical current density under the pressure of 1.2 GPa, and the irreversibility line is shifted to higher fields without changing the critical temperature. The normalized magnetic relaxation rate shows that vortex creep rates are strongly suppressed due to applied pressure, and the pinning energy is significantly increased based on the collective creep theory. After releasing the pressure, we recover the original superconducting properties. Therefore, we speculate that the in situ hydrostatic pressure exerted on the coated conductor enhances the pinning of existing extended defects. This is totally different from what has been observed in REBa2Cu3O7−δ melt-textured crystals, where the effect of pressure generates point-like defects.

Self-Diffusion in Amorphous Silicon by Local Bond Rearrangements.

Experiments on self-diffusion in amorphous silicon (Si) were performed at temperatures between 460 to 600° C. The amorphous structure was prepared by Si ion implantation of single crystalline Si isotope multilayers epitaxially grown on a silicon-on-insulator wafer. The Si isotope profiles before and after annealing were determined by means of secondary ion mass spectrometry. Isothermal diffusion experiments reveal that structural relaxation does not cause any significant intermixing of the isotope interfaces whereas self-diffusion is significant before the structure recrystallizes. The temperature dependence of self-diffusion is described by an Arrhenius law with an activation enthalpy Q=(2.70±0.11) eV and preexponential factor D_{0}=(5.5_{-3.7}^{+11.1})×10^{-2} cm^{2} s^{-1}. Remarkably, Q equals the activation enthalpy of hydrogen diffusion in amorphous Si, the migration of bond defects determining boron diffusion, and the activation enthalpy of solid phase epitaxial recrystallization reported in the literature. This close agreement provides strong evidence that self-diffusion is mediated by local bond rearrangements rather than by the migration of extended defects as suggested by Strauß etal. (Phys. Rev. Lett. 116, 025901 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.025901).

Defects at grain boundaries: A coarse-grained, three-dimensional description by the amplitude expansion of the phase-field crystal model

We address a three-dimensional, coarse-grained description of dislocation networks at grain boundaries between rotated crystals. The so-called amplitude expansion of the phase-field crystal model is exploited with the aid of finite element method calculations. This approach allows for the description of microscopic features, such as dislocations, while simultaneously being able to describe length scales that are orders of magnitude larger than the lattice spacing. Moreover, it allows for the direct description of extended defects by means of a scalar order parameter. The versatility of this framework is shown by considering both fcc and bcc lattice symmetries and different rotation axes. First, the specific case of planar, twist grain boundaries is illustrated. The details of the method are reported and the consistency of the results with literature is discussed. Then, the dislocation networks forming at the interface between a spherical, rotated crystal embedded in an unrotated crystalline structure, are shown. Although explicitly accounting for dislocations which lead to an anisotropic shrinkage of the rotated grain, the extension of the spherical grain boundary is found to decrease linearly over time in agreement with the classical theory of grain growth and recent atomistic investigations. It is shown that the results obtained for a system with bcc symmetry agree very well with existing results, validating the methodology. Furthermore, fully original results are shown for fcc lattice symmetry, revealing the generality of the reported observations.

Heterogeneous photo-catalysis from the perspective of the adsorbed state. The photo-oxidation of propan-2-ol to propan-2-one on rutile titanium dioxide

O2-titrations accompanying ESR, adsorption isotherm and TPD studies of well-crystallised powdered rutile provide insights into the nature of the reduced surface following adsorption of propan-2-ol at 300 K. Estimates have been made of the mean dimensions of the octa-decahedral micro-crystallites, and of the manner in which UV-A radiation penetrates into a randomised particle-bed. The reduced surface contains a small fractional coverage (0.003monolayer) of paramagnetic Ti3+A sites (1.1 × 1016 m−2) that appear to be associated mainly with the crystallite edges, and a large majority (>98%) of diamagnetically-coupled Ti3+A-B spin-pairs arranged in the form of extended defects on the crystallite faces. UV-A is found to activate only a thin layer of particles at the bed exterior (0.83%) on which a large fraction (>30%) of the Ti4+oct sites become reduced to paramagnetic Ti3+A at 78 K, and mainly to spin-pairs at 300 K, with implications for their subsequent reactivity with oxidising gases.

GaSb and GaSb/AlSb Superlattice Buffer Layers for High-Quality Photodiodes Grown on Commercial GaAs and Si Substrates

The objective of this work is the integration of InGaAs/GaSb/GaAs heterostructures, with high indium content, on GaAs and Si commercial wafers. The design of an interfacial misfit dislocation array, either on GaAs or Si substrates, allowed growth of strain-free devices. The growth of purposely designed superlattices with their active region free of extended defects on both GaAs and Si substrates is demonstrated. Transmission electron microscopy technique is used for the structural characterization and plastic relaxation study. In the first case, on GaAs substrates, the presence of dopants was demonstrated to reduce several times the threading dislocation density through a strain-hardening mechanism avoiding dislocation interactions, while in the second case, on Si substrates, similar reduction of dislocation interactions is obtained using an AlSb/GaSb superlattice. The latter is shown to redistribute spatially the interfacial misfit dislocation array to reduce dislocation interactions.

Formation of dislocations in LiF irradiated with 3He and 4He ions

Influence of the irradiation with 13.5 MeV 3He and 5 MeV 4He ions on the micro-structure and mechanical properties of LiF single crystals was studied. The depth profiles of nanoindentation, dislocation mobility, selective chemical etching and photoluminescence served for the characterization of damage. Strong ion-induced increase of hardness and decrease in dislocation mobility at the stage of track overlapping due to accumulation of dislocations and other extended defects was observed. At high fluences (1015 ions/cm2) the hardness saturates at about 3.5 GPa (twofold increase in comparison to a virgin crystal) thus confirming high efficiency of light projectiles in modifications of structure and properties. The effects of ion-induced increase of hardness and decrease of dislocation mobility are observed also beyond the ion range and possible mechanisms of such damage are discussed.

Influence of Ar Implantation on the Precipitation in Au Ion Irradiated AISI 316L Solution Annealed Alloy

200 μm thick solution annealed AISI 316L stainless steel foils were implanted with Ar ions to produce a 0.25 at. % concentration-depth plateau extending from the near surface to a depth of ≈ 250 nm, and then annealed at 550°C for 2 hours to form small Ar bubbles and Ar-vacancy clusters. Distinct sets of samples (including control ones without Ar) were irradiated at the temperature of 550 °C with Au ions accelerated at 5 MeV to produce an average damage content about ≈36 dpa at the region containing the Ar plateau. These samples were investigated by transmission electron microscopy using plan-view specimens prepared by ion milling. In contrast with the control samples where the irradiation causes the formation of a high concentration of extended defects and large cavities, carbonite precipitation of 1:1 metal-carbon (MC) content with a cubic structure occurs only in the samples containing the Ar bubbles. This precipitation phenomenon is not commonly observed in the literature. The results are interpreted considering that the precipitate growth process requires the emission of vacancies which are synergistically absorbed by the growth of the Ar bubbles.

(Invited) Localized Covalent Defects on Carbon Nanotube Devices for Sensor Applications

Functionalization of carbon nanotube devices is essential to tailor their affinity with other molecules, which is especially critical for applications as sensors. Among functionalization approaches, covalent chemistry produces stable moieties on nanotubes sidewalls, but the chemistry is invasive and alters the electronic properties of the material. In this presentation, I will describe recent efforts to control covalent diazonium chemistry on carbon nanotubes and understand its impact on electrical conductance. In particular, I will contrast experiments between extended and localized defect coverage of adducts on the electrical response of devices, and present applications in single-molecule sensors.

Comparison of the quality of single-crystal diamonds grown on two types of seed substrates by MPCVD

Microwave plasma chemical vapor deposition (MPCVD) was used to grow single-crystal diamonds on two types of single-crystal diamond seed substrates prepared by high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) methods. The quality of diamonds grown on the different seed substrates was compared. Fluorescence characteristics showed that the sectors of the HPHT seed substrates were obviously partitioned. Raman and absorption spectra showed that the CVD seed substrate produced higher-quality crystals with fewer nitrogen impurities. X-ray topography showed that the HPHT seed substrate had obvious growth sector boundaries, inclusions, dislocations, and stacking faults. The polarization characteristics of HPHT seed substrate were obvious, and the stress distribution was not uniform. When etching HPHT and CVD seed substrates using the same parameters, the etching morphology and extent of different growth sectors of the two substrates differed. Although extended defects were inevitably formed at the interface and propagated in the CVD layer, the dislocation density of a 1 mm-thick CVD layer grown on a CVD seed substrate was only half that of a 1 mm-thick CVD layer grown on an HPHT seed substrate. Therefore, the use of CVD seed substrate enabled the growth of a relatively higher-quality CVD single-crystal diamond.

Radiation hardness of Kr+ ion implanted BaWO4 at room temperature

Single crystalline 〈100〉 oriented BaWO4 was implanted at room temperature with 370 keV Kr ions and ion fluences ranging from 1 × 1012 to 4 × 1016 ions/cm2. The damage built-up during implantation was investigated by quasi in situ Rutherford backscattering spectrometry in channelling configuration (RBS/C) using a two-beam chamber. Up to the highest ion fluence applied no amorphization is observed indicating that BaWO4 has a high resistance to amorphization by ion irradiation. The shape of the RBS/C spectra indicates the existence of extended defects. Damage formation in the bulk proceeds in two steps, which can be attributed to formation and recombination of point defects and formation and growth of defect clusters including extended defects. Additionally a significant damage accumulation at the surface is observed. Some damage recovery occurs after annealing up to 600 °C. Although after annealing at 700 °C the amount of damage drastically decreased, complete damage recovery could not be obtained.

Understanding arsenic incorporation in CdTe with atom probe tomography

Overcoming the open circuit voltage deficiency in Cadmium Telluride (CdTe) photovoltaics may be achieved by increasing p-type doping while maintaining or increasing minority carrier lifetimes. Here, routes to higher doping efficiency using arsenic are explored through an atomic scale understanding of dopant incorporation limits and activation in molecular beam epitaxy grown CdTe layers. Atom probe tomography reveals spatial segregation into nanometer scale clusters containing > 60 at% As for samples with arsenic incorporation levels greater than 7–8 × 1017 cm−3. The presence of arsenic clusters was accompanied by crystal quality degradation, particularly the introduction of arsenic-enriched extended defects. Post-growth annealing treatments are shown to increase the size of the As precipitates and the amount of As within the precipitates.

Band-Like Behavior of Localized States of Metal Silicide Precipitate in Silicon

Deep-level transient spectroscopy (DLTS) investigations of energy levels of charge-carrier traps associated with precipitates of metal silicide often show that they behave not like localized monoenergetic traps but as a continuous density of allowed states in the bandgap with fast carrier exchange between these states, so-called band-like behavior. This kind of behavior was ascribed to the dislocation loop bounding the platelet, which in addition exhibits an attractive potential caused by long-range elastic strain. In previous works, the presence of the dislocation-related deformation potential in combination with the external electric field of the Schottky diode was included to obtain a reasonable fit of the proposed model to experimental data. Another well-known particular property of extended defects—the presence of their own strong electric field in their vicinity that is manifested in the logarithmic kinetics of electron capture—was not taken into account. We derive herein a theoretical model that takes into account both the external electric field and the intrinsic electric field of dislocation self-charge as well as its deformation potential, which leads to strong temporal variation of the activation energy during charge-carrier emission. We performed numerical simulations of the DLTS spectra based on such a model for a monoenergetic trap, finding excellent agreement with available experimental data.

Charging effect at grain boundaries of MoS2

Grain boundaries (GBs) are inherent extended defects in chemical vapor deposited (CVD) transition metal dichalcogenide (TMD) films. Characterization of the atomic structure and electronic properties of these GBs is crucial for understanding and controlling the properties of TMDs via defect engineering. Here, we report the atomic and electronic structure of GBs in CVD grown MoS2 on epitaxial graphene/SiC(0001). Using scanning tunneling microscopy/spectroscopy, we find that GBs mostly consist of arrays of dislocation cores, where the presence of mid-gap states shifts both conduction and valence band edges by up to 1 eV. Our findings demonstrate the first charging effect near GBs in CVD grown MoS2, providing insights into the significant impact GBs can have on materials properties.

Defect stabilization and reverse annealing in ZnO implanted with nitrogen at room and cryogenic temperature

Despite the fact that nitrogen is a potential acceptor dopant and one of the most studied elements in ZnO, lacking understanding of associated defects and their thermal evolution limits realization of reliable p-type doping of ZnO. Here, we use ion implantation to introduce N at room temperature (RT) and 15 K in ZnO samples with/without a pre-existing buried disorder layer formed by Ag ion bombardment aligned along the [0001] direction. The buried layer contains a high concentration of extended defects, which act as traps for migrating point defects. Channeling analysis shows that reverse annealing occurs in all the N implanted samples during post-implant heat treatment above 600 °C with strong non-linear additive damage accumulation in the co-implanted samples. The reverse annealing effect is less stable in the RT co-implanted sample and the data suggest that a high local concentration of intrinsic point defects, like Zn interstitials, promotes the stability of the N-defect clusters responsible for the reverse annealing. This suggestion is also corroborated by enhanced and defect-mediated Ag outdiffusion at 1100 °C in the RT co-implanted samples.

Helium and deuterium irradiation effects in tungsten-based materials with titanium

Pure Tungsten (W) will be used as plasma facing component in fusion devices due to its high melting point, good thermal conductivity and low sputtering yield. However, its structural application as plasma facing component (PFC) is still restricted by its low fracture toughness associated with the high ductile to brittle transition temperature (DBTT).In the present study tungsten‑titanium (W-Ti) samples were produced by Ti implantation at room temperature and 500 °C with a constant fluence of 2 × 1021 at/m2 and an energy of 100 keV. In order to understand the fundamental mechanisms which govern the behavior of defect dynamics in tungsten under reactor conditions, W-Ti materials were implanted at room temperature with 10 keV of He+ with a constant fluence of 5 × 1021 at/m2 and 5 keV of D+ with fluences in the range of 0.1 × 1021–5 × 1021 at/m2. Surface structure and morphology changes were investigated by scanning electron microscopy and X-ray diffraction. Rutherford backscattering spectrometry, nuclear reaction analysis and thermal desorption spectroscopy methods were used to provide information about the distribution of Ti, He and D on W.No changes in the microstructure were observed after Ti implantation in the W plates. NRA analysis showed that D retention in the W-Ti samples is higher after sequential He and D implantation when compared with single D implantation. The diffractogram of W-Ti samples implanted with He evidence a broadening of the W peaks. This effect is believed to be associated with the high volume fraction of the bubbles that may cause internal stress fields inducing extended defects like dislocations which distort the crystal lattice.

How to apply the phase field method to model radiation damage

The phase field (PF) method provides a valuable means of predicting radiation induced microstructure evolution of domains ranging roughly from 100 nm to 100 μm in size and for lengths of time ranging from microseconds to years, depending on the rate of diffusion. In this work we summarize how to apply the PF method to modeling radiation damage. We begin by summarizing the PF method, and discuss the numerical solution of the PF equations. We then summarize approaches for representing the free energy of defects within a material. Next, three PF approaches for representing extended defects are discussed: the Wheeler, Boettinger, and McFadden model, the Kim, Kim, Suzuki model, and the Grand Potential model. We use a simple 1D void growth problem to compare the predictions of these models, as well as their computational expense. We end by describing how defect generation, recombination and annihilation at sinks can be described by the PF method, as well as discussing modifications required to model nucleation.

Electron spectra of graphene with local and extended defects

We show theoretically a possibility to increase the superconducting transition temperature Tc of graphene by introducing defects. Eventually, the peak of local electron densities shifts to the Fermi level with Tc above ambient temperature in line with recent observations.