Redistribution Of Point Defects Research Articles

The Modeling of Self-Consistent Electron–Deformation–Diffusion Effects in Thin Films with Lattice Parameter Mismatch

In our work, the model of self-consistent electron–deformation–diffusion effects in thin films grown on substrate with the mismatch of lattice parameters of the contacting materials is constructed. The proposed theory self-consistently takes into account the interaction of the elastic field (created by the mismatch of lattice parameters of the film and the substrate, and point defects) with the diffusion processes of point defects and the electron subsystem of semiconductor film. Within the framework of the developed model, the spatial distribution of deformation, concentration of defects, conduction electrons and electric field intensity is investigated, depending on the value of the mismatch, the type of defects, the average concentrations of point defects and conduction electrons. It is established that the coordinate dependence of deformation and the concentration profile of defects of the type of stretching (compression) centers, along the axis of growth of the strained film, have a non-monotonic character with minima (maxima), the positions of which are determined by the average concentration of point defects. It is shown that due to the electron–deformation interaction in film with a lattice parameter mismatch, the spatial redistribution of conduction electrons is observed and n-n+ transitions can occur. Information about the self-consistent spatial redistribution of point defects, electrons and deformation of the crystal lattice in semiconductor materials is necessary for understanding the problems of their stability and degradation of nano-optoelectronic devices operating under conditions of intense irradiation.

Interface effect of Fe and Fe2O3 on the distributions of ion induced defects

The stability of structural materials in extreme nuclear reactor environments—with high temperature, high radiation, and corrosive media—directly affects the lifespan of the reactor. In such extreme environments, an oxide layer on the metal surface acts as a passive layer protecting the metal underneath from corrosion. To predict the irradiation effect on the metal layer in these metal/oxide bilayers, nondestructive depth-resolved positron annihilation lifetime spectroscopy (PALS) and complementary transmission electron microscopy (TEM) were used to investigate small-scale defects created by ion irradiation in an epitaxially grown (100) Fe film capped with a 50 nm Fe2O3 oxide layer. In this study, the evolution of induced vacancies was monitored, from individual vacancy formation at low doses—10−5 dpa—to larger vacancy cluster formation at increasing doses, showing the sensitivity of positron annihilation spectroscopy technique. Furthermore, PALS measurements reveal how the presence of a metal–oxide interface modifies the distribution of point defects induced by irradiation. TEM measurements show that irradiation induced dislocations at the interface is the mechanism behind the redistribution of point defects causing their accumulation close to the interface. This work demonstrates that the passive oxide layers formed during corrosion impact the distribution and accumulation of radiation induced defects in the metal underneath and emphasizes that the synergistic impact of radiation and corrosion will differ from their individual impacts.

Structure-resistive property relationships in thin ferroelectric BaTiO_{3} films

A combined study of local structural, electric and ferroelectric properties of SrTiO_{3}/La_{0.7}Sr_{0.3}MnO_{3}/BaTiO_{3} heterostructures was performed by Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy in the temperature range 30–295 K. The direct correlation of film structure (epitaxial, nanocrystalline or polycrystalline) with local electric and ferroelectric properties was observed. For polycrystalline ferroelectric films the predominant polarization state is defined by the peculiarity of screening the built-in field by positively charged point defects. Based on Scanning Tunneling Spectroscopy results, it was found that a sequent voltage application provokes the modification of local resistive properties related to the redistribution of point defects in thin ferroelectric films. A qualitative analysis of acquired Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy images together with Scanning Tunneling Spectroscopy measurements enabled us to conclude that in the presence of structural defects the competing processes of electron injection, trap filling and the drift of positively charged point defects drives the change of resistive properties of thin films under applied electric field. In this paper, we propose a new approach based on Scanning Tunneling Microscopy/Spectroscopy under ultrahigh vacuum conditions to clarify the influence of point defects on local resistive properties of nanometer-thick ferroelectric films.

Capabilities of X-ray diffuse scattering method for study of microdefects in semiconductor crystals

The capabilities of X-ray diffuse scattering (XRDS) method for the study of microdefects in semiconductor crystals have been overviewed. Analysis of the results has shown that the XRDS method is a highly sensitive and information valuable tool for studying early stages of solid solution decomposition in semiconductors. A review of the results relating to the methodological aspect has shown that the most consistent approach is a combination of XRDS with precision lattice parameter measurements. It allows one to detect decomposition stages that cannot be visualized using transmission electron microscopy (TEM) and moreover to draw conclusions as to microdefect formation mechanisms. TEM-invisible defects that are coherent with the matrix and have smeared boundaries with low displacement field gradients may form due to transmutation doping as a result of neutron irradiation and relaxation of disordered regions accompanied by redistribution of point defects and annihilation of interstitial defects and vacancies. For GaP and InP examples, a structural microdefect formation mechanism has been revealed associated with the interaction of defects forming during the decomposition and residual intrinsic defects. Analysis of XRDS intensity distribution around the reciprocal lattice site and the related evolution of lattice constant allows detecting different decomposition stages: first, the formation of a solution of Frenkel pairs in which concentration fluctuations develop, then the formation of matrix-coherent microdefects and finally coherency violation and the formation of defects with sharp boundaries. Fundamentally, the latter defects can be precipitating particles. Study of the evolution of diffuse scattering iso-intensity curves in GaP, GaAs(Si) and Si(O) has allowed tracing the evolution of microdefects from matrix-coherent ones to microdefects with smeared coherency resulting from microdefect growth during the decomposition of non-stoichiometric solid solutions heavily supersaturated with intrinsic (or impurity) components.

Origin of n-type conductivity in ZnO crystal and formation of Zn and ZnO nanoparticles by laser radiation

Electrical and optical properties of hydrothermally grown ZnO crystal, as well as structural changes at its surface have been investigated before and after irradiation by pulsed Nd:YAG laser. The spreading resistance measurements have shown a monotonous increase of conductivity by three orders of magnitude when the laser intensity I has been varied from zero to 290 MW/cm2. The PL spectra have revealed an increase of concentration of Zn interstitials at the surface after irradiation by I = 3.5 MW/cm2. Formation of Zn nanoparticles on the crystal surface has been observed at I > 290 MW/cm2. The study of surface structure at I = 315 MW/cm2 has shown that these Zn nanoparticles tend to transform into ZnO nanoparticles after an irradiation by more than 2 laser pulses. A theoretical model of thermal generation and redistribution of point defects has been elaborated to explain the origin of experimentally observed n-type conductivity. According to this model and experimental facts, the n-type conductivity originates from Zn interstitials, which are moved to the crystal surface by large temperature gradient during the laser processing. As a result, Zn-rich surface layer is formed and Zn nanoparticles grow, which are later oxidized into ZnO nanoparticles. We have shown a possibility to control the size distribution of these nanoparticles by choosing appropriate intensity and number of laser pulses.

Spatial-temporal redistribution of point defects in three-layer stressed nanoheterosystems within the limits of self-assembled deformation-diffusion model

The model of spatial-temporal distribution of point defects in a three-layer stressed nanoheterosystem GaAs/In$_x$Ga$_{1 - x}$As/GaAs considering the self-assembled deformation-diffusion interaction is constructed. Within the framework of this model, the profile of spatial-temporal distribution of vacancies (interstitial atoms) in the stressed nanoheterosystem GaAs/In$_x$Ga$_{1 - x}$As/GaAs is calculated. It is shown that in the case of a stationary state ($t > 5\tau _d^{(2)}$), the concentration of vacancies in the inhomogeneously compressed interlayer is smaller relative to the initial average value $N_{d0}^{(2)}$ by 16%

Electric-field-induced point defect redistribution in single-crystal TiO2–x and effects on electrical transport

The spatial redistribution of non-stoichiometric point defects in rutile TiO2 is studied as a function of voltage and time. Single crystals are equilibrated initially to a well-defined stoichiometry with n-type conductivity and a carrier concentration on the order of 1018cm−3. The crystals are subsequently electroded with Pt contacts that exhibit Schottky behavior. When subjected to an applied voltage of 15V, a time-dependent increase and saturation in the leakage current is observed, which is associated with an accumulation of point defects and an attendant decrease in stoichiometry at the cathode electrode. This local change in stoichiometry degrades the Schottky barrier, leading to asymmetric electrodes and thus macroscopic rectifying behavior. Cathodoluminescence spectroscopy shows that Ti interstitials dominate the point defect redistribution process. Under larger applied voltages, of around 30V, qualitatively different behavior is observed in which the resistivity increases as a function of time. This behavior is associated with condensation of point defects into a region of extended defects and Magnéli phases near the cathode, sufficient to increase the bulk stoichiometry and resistivity. These experiments demonstrate that a one-dimensional drift-diffusion process, as opposed to filamentary growth, dominates in these experimental conditions and that the Pt–TiO2–Pt system remains closed, with no significant oxygen transport across the Pt–TiO2 interfaces. We believe this is the first observation of a second higher-voltage regime in which the bulk stoichiometry and thus resistivity is increased as large concentrations of defects condense into metallic Magnéli phases in the near-electrode regions.

Kinetic model for electric-field induced point defect redistribution near semiconductor surfaces

The spatial distribution of point defects near semiconductor surfaces affects the efficiency of devices. Near-surface band bending generates electric fields that influence the spatial redistribution of charged mobile defects that exchange infrequently with the lattice, as recently demonstrated for pile-up of isotopic oxygen near rutile TiO2 (110). The present work derives a mathematical model to describe such redistribution and establishes its temporal dependence on defect injection rate and band bending. The model shows that band bending of only a few meV induces significant redistribution, and that the direction of the electric field governs formation of either a valley or a pile-up.

Effect of annealing in liquid cadmium upon photoluminescence in polycrystalline cadmium telluride grown under nonequilibrium conditions

The effect of annealing in liquid Cd at a temperature of 600°C on the photoluminescence spectra of polycrystalline CdTe produced under conditions of rapid crystallization is demonstrated. It is shown that the annealing-induced redistribution of point defects completely suppresses the emission attributed to nonstandard acceptors with activation energies of ∼48, ∼98, and ∼120 meV and observed in the luminescence spectra of as-prepared crystals and radically modifies the emission structure in the range 1.2–1.35 eV, which corresponds to extended defects caused by twinning. In the photoluminescence spectra of the annealed poly-crystals, the emission related to exciton-impurity complexes involving hydrogen-like donors and CuCd acceptors is dominant. A correlation between the concentration of extended defects and the intensity of long-wavelength emission (in the range 0.8–1.2 eV) is established.

Homo- and Hetero-Structure Formation in Semiconductors by Laser Radiation: First Stage of Quantum Cones Formation

A possibility of formingquantum cones (QC) by Nd:YAG laser radiation on the surface of semiconductorssuch as Si and Ge crystals, and SiGe and CdZnTe solid solutions has been shown.A two-stage mechanism of quantum cone formation has been proposed. The first stage is generation and redistribution of point defects (impurity atoms and intrinsic point defects – vacancies and self-interstitials) in a temperature gradient field, the so-called thermogradient effect. As a result a new phase is formed on the irradiated surface, for example a Ge phase forms on the surface of a SiGe solid solution. The second stage is characterized by mechanical plastic deformation of the strained top layer leading to the formation of quantum cones, due to selective laser radiation absorption of the top layer. The first stage is more difficult for understanding of the physical processes which takeplace during of growth of QC, especially in pure intrinsic elementary semiconductors (Ge, Si) and compounds (CdTe, GaAs). Therefore, this research is focused on the investigation of the first stage of QC formation by laser irradiation. As a result of the investigation, a new mechanism for p-n junction formation in the elementary semiconductors and heterojunction in solid solutions by laser radiation as a first stage of QC formation is proposed.

Electric Field-Induced Point Defect Redistribution in TiO2

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Фізична кінетика перерозподілу точкових дефектів у опромінюваних кристалах

Фізична кінетика перерозподілу точкових дефектів у опромінюваних кристалах

Epitaxial growth of Nb-doped SrTiO3 films by pulsed laser deposition

Nb-doped SrTiO3 thin films are potential candidates for transparent conducting oxide electrodes and other applications in oxide electronics and optoelectronics. In this work we grew epitaxial films of Nb-doped SrTiO3 on (001)-oriented SrTiO3 single crystal substrates and examined their crystal structure, defects and strain state by means of high-resolution X-ray diffraction (mapping in reciprocal space) and complementary methods. The films were deposited by pulsed laser deposition (PLD) under oxygen flow (60mTorr) or in vacuum (5×10−3mTorr). The substrate temperature was 700°C and the substrate-to-target distance was 55 or 70mm, respectively. Strained epitaxial films with homogeneous concentration of point defects and dopants were obtained in both cases as long as the film thickness did not exceed 200nm. Films deposited in oxygen displayed considerably higher concentration of point defects (presumably cation vacancies) than their vacuum-deposited counterparts. Films of intermediate thickness (between 200 and 1000nm) exhibited significant structural inhomogeneity accompanied by strain relaxation via redistribution of point defects across the film. Thicker films (>1000nm) grown in oxygen exhibited changes in the film growth mode and the resultant microstructure, providing a complementary mechanism for strain relaxation.

Properties of nanocones formed on a surface of semiconductors by laser radiation: quantum confinement effect of electrons, phonons, and excitons

On the basis of the analysis of experimental results, a two-stage mechanism of nanocones formation on the irradiated surface of semiconductors by Nd:YAG laser is proposed for elementary semiconductors and solid solutions, such as Si, Ge, SiGe, and CdZnTe. Properties observed are explained in the frame of quantum confinement effect. The first stage of the mechanism is characterized by the formation of a thin strained top layer, due to redistribution of point defects in temperature-gradient field induced by laser radiation. The second stage is characterized by mechanical plastic deformation of the stained top layer leading to arising of nanocones, due to selective laser absorption of the top layer. The nanocones formed on the irradiated surface of semiconductors by Nd:YAG laser possessing the properties of 1D graded bandgap have been found for Si, Ge, and SiGe as well, however QD structure in CdTe was observed. The model is confirmed by "blue shift" of bands in photoluminescence spectrum, "red shift" of longitudinal optical line in Raman back scattering spectrum of Ge crystal, appearance of Ge phase in SiGe solid solution after irradiation by the laser at intensity 20 MW/cm2, and non-monotonous dependence of Si crystal micro-hardness as function of the laser intensity.

Real-time in situ nanoclustering during initial stages of artificial aging of Al–Cu alloys

We report an experimental demonstration of real-time in situ x-ray diffraction investigations of clustering and dynamic strain in early stages of nanoparticle growth in Al–Cu alloys. Simulations involving a simplified model of local strain are well correlated with the x-ray diffraction data, suggesting a redistribution of point defects and the formation of nanoscale clusters in the bulk material. A modal, representative nanoparticle size is determined subsequent to the final stage of artificial aging. Such investigations are imperative for the understanding, and ultimately the control, of nanoparticle nucleation and growth in this technologically important alloy.

Massive point defect redistribution near semiconductor surfaces and interfaces and its impact on Schottky barrier formation

Abstract Nanoscale depth-resolved cathodoluminescence spectroscopy calibrated with deep level transient spectroscopy of native point defects and capacitance–voltage measurements of free carrier densities, all at the same metal–semiconductor interface, demonstrate that native point defects can (i) increase by order-of-magnitude in densities with tens of nanometers of the semiconductor surface, (ii) alter free carrier concentrations and band profiles with the surface space charge regions, and (iii) dominate the Schottky barrier formation for metal contacts to ZnO and many other single crystal compound semiconductors. The spatial redistribution of electrically active defects within the surface space charge can be understood in terms of temperature-dependent atomic diffusion enabled by low formation energies and driven by strain and electric fields as well as metal-specific chemical reactions near room temperature, consistent with first-principles calculations of interfacial segregation and migration barriers. These results underscore the importance of native point defects in charge transport and barrier formation at semiconductor interfaces.

Analysis of the relief of the GaAs surface formed upon exposure to diffraction-modulated laser radiation

A relief of the GaAs surface irradiated by a laser through diffraction masks having different profiles is investigated. The incident beam energy density does not exceed the threshold value. Clusters of point defects are formed in the crystal surface layer upon exposure to diffraction-modulated laser radiation. With the help of theoretical calculations, it is established that clusterization in the surface layer results from redistribution of point defects in the periodic field of thermoelastic stresses.

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.

Calculation of radiation-induced deformation of SUS 316 in the ITER-relevant condition

SUS 316 stainless steel will be used in ITER as a major structural material. However, accumulation of radiation-induced deformation data in the anticipated operation temperature range, centered at about 300 °C, is not adequate. Although it will be used mainly in annealed condition, cold-working and consequent introduction of network dislocations is inevitable at various parts of the structure. Computer simulation was performed to examine the radiation-induced deformation of SUS 316 steel, with emphasis on the effect of network dislocation density, for the ITER-relevant condition. Irradiation creep rates and radiation-induced stress relaxation behaviors were examined at 300 °C for various displacement rates, applied stresses and network dislocation densities. Rather complicated dependence on dislocation density has been predicted in both creep deformation and stress relaxation. This complexity results from a competitive redistribution of point defects to the operating deformation mechanisms and from the predominance among them that depends on the network dislocation density.

Non-thermodynamic approach to including bombardment-induced post-cascade redistribution of point defects in dynamic Monte Carlo code

The redistribution of the elements as a result of atomic relocations produced by the ions and the recoils due to the ballistic and transport processes is investigated by making use of a dynamic Monte Carlo code. Phenomena, such as radiation-enhanced diffusion (RED) and bombardment-induced segregation (BIS) triggered by the ion bombardment may also contribute to the migration of atoms within the target. In order to include both RED and BIS in the code, we suggest an approach which is considered as an extension of the binary collision approximation, i.e. it takes place “simultaneously” with the cascade and acts as a correction to the particle redistribution for low energies. Both RED and BIS models are based on the common approach to treat the transport processes as a result of a random migration of point defects (vacancies and interstitials) according to a probability given by a pre-defined Gaussian. The models are tested and the influence of the diffusion and segregation is illustrated in the cases of 12 keV 121Sb + implantation at low fluence in SiO 2/Si substrate and of self-sputtering of Ga + ions during profiling of SiO 2/Si interfaces.