Displacements Of Lattice Atoms Research Articles

Influence of inhomogeneous temperature and field distribution on sound generation and its effect on reflectivity of a thin film heated by a femtosecond pulse

One of the main methods for obtaining information about the generation of sound pulses in metals is to measure the reflection coefficient of a probe wave. Various theoretical models are used to interpret the results of measuring the contribution to reflection coefficient ΔR(t) due to sound-generated displacements of lattice atoms. The purpose of this paper is to establish the degree of accuracy of models used in the case of sound generation in thin films exposed to a femtosecond pulse. It is shown below that the assumption of uniform heating used for thin films is justified if the film thickness is less than the film heating depth and for thicker films at times greater than the film heating time over the entire thickness. For optically thick films, a relatively simple expression for the field can be used. If the film thickness is less than the skin layer depth of the pump field, then it is necessary to consider the field reflection from a substrate. In this case, depending on the optical properties of the metal and the substrate, taking into account reflection can lead to either an increase or a decrease in ΔR(t). It has been established that if the skin layer at the frequency of probe radiation is less than the film heating depth, then taking into account temperature gradients in the equation for the displacement of lattice atoms leads to small changes in ΔR(t). This makes it possible to significantly simplify calculations of the displacement of lattice atoms.

Understanding the fundamental driver of semiconductor radiation tolerance with experiment and theory

Space is the operating environment of a multitude of systems that our society is heavily reliant on today, however, maintaining operability there necessitates special consideration of the electronic systems' tolerance of space radiation. Electronic systems are critically dependent on the electronic properties of their semiconductor components, which are modified by space radiation with an adverse impact on the space system performance. What innate property allows some semiconductors to sustain little damage while others accumulate defects rapidly with dose is poorly understood, which limits the extent to which radiation tolerance can be implemented as a design criterion. To gain insight into what properties are drivers of semiconductor radiation tolerance, the first step is to generate a dataset of the relative radiation tolerance of a broad sampling of semiconductors. To accomplish this, Rutherford backscatter channeling experiments are used to compare the displaced lattice atom buildup in InAs, InP, GaP, GaN, ZnO, MgO, and Si as a function of stepwise alpha particle dose. With this experimental information on radiation-induced incorporation of interstitial defects in hand, hybrid density functional theory electron densities (and their derived quantities) are calculated and their gradient and Laplacian are evaluated to obtain key fundamental information about the interactions in each material. It is shown that simple, undifferentiated values (which are typically used to describe bond strength) are insufficient to predict radiation tolerance. Instead, the curvature of the electron density at bond critical points provides a measure of radiation tolerance consistent with the experimental results obtained. This curvature and associated forces surrounding bond critical points have the potential to disfavor the localization of displaced lattice atoms at these points, favoring their diffusion toward perfect lattice positions. With this criterion to predict radiation tolerance, simple density functional theory simulations can be conducted on potential new materials to gain insight into how they may operate in demanding high radiation environments.

Determination of neutron irradiation temperatures of SiC using electrical resistivity method

The irradiation temperature of silicon carbide (SiC) was determined post-irradiation by examination of the recovery of the electrical resistivity due to thermal annealing in a rapid heating/cooling optical furnace. High-purity, high-resistivity grade SiC is routinely used as a passive temperature monitor in neutron irradiation studies at the High Flux Isotope Reactor (HFIR), and this paper presents an alternative automated technique for determination of the irradiation temperature the SiC experienced. Neutron collisions with the atoms results in displaced lattice atoms (interstitials) that act as electron donors yielding a significant decrease in electrical resistivity. The irradiation defects become thermodynamically unstable and start to recombine, when annealed above the irradiation temperature, resulting in a recovery of the electrical resistivity. The resistivity is measured at a fixed elevated temperature above ambient, which is below the target irradiation temperature. When the resistivity is plotted as a function of annealing temperatures, a clear increase is observed due to the recovery of irradiation defects. We have demonstrated that this electrical resistivity measurement of SiC is effective to determine irradiation temperature of SiC. Energy levels of various defects in SiC were calculated from Arrhenius plots of electrical conductivity versus inverse temperature.

Multi-axial and multi-energy channeling study of disorder evolution in ion-irradiated nickel

To better understand defect structure and the evolution of irradiation-induced damage in single crystal Ni, in situ Rutherford backscattering spectroscopy in channeling geometry (RBS/C) is performed along the <100>, <110>, and <211> axes with different probing beam energies. The RBS/C data reveal that damage evolution occurs in three steps. The first step at low doses (up to 0.2 dpa), characterized by a linear increase with dose, is related to the formation of point defects and small clusters. The second step in the intermediate dose range (0.2–1.0 dpa) shows a sublinear increase in disorder to saturation. This sublinear increase is due to the growth of defect clusters resulting from the interaction of irradiation-induced defects with already existing damage from previous ion impacts. The third step at high doses (1.0–32.4 dpa) exhibits a surprising decrease in the disorder level, which may be attributed to defect evolution from black spot defects to large dislocation loops that leads to strain relaxation. In addition, the damage extends much deeper than the predicted depths and is attributed to long-range defect migration effects confirmed by transmission electron microscopy (TEM) observations. Although similar damage evolution trends have been observed along all channeling directions, the disorder accumulation is largest along the <100> and <211> axes than observed along <110> axis. This “preferential” disordering process along <100> and <211> suggests that more defects are shielded by the <110> atomic rows than the two other axes. The co-existence of both uncorrelated displaced lattice atoms and dislocation loops in Ni irradiated at 1 ion/nm2 is revealed by the energy-dependent RBS/C studies along all three axes. In contrast, dislocation loops and stacking fault tetrahedral are simultaneously present in the crystal structure of Ni irradiated at 100 ions/nm2 that is consistent with previous molecular dynamics simulations and TEM observations.

High-level damage saturation below amorphisation in ion implanted β-Ga2O3

Ion implantation induced effects were studied in single crystalline 〈010〉 oriented bulk β-Ga2O3 at room temperature using P, Ar and Sn ions with ion fluences ranging from 1×1011 up to 2×1015cm−2. Rutherford backscattering spectrometry in channelling configuration (RBS) using He ions of various ion energies was applied for damage analysis. Clear damage peaks are visible in the RBS spectra. The concentration of displaced lattice atoms in the maximum of the distribution (as deduced from the channelling spectra) increases with increasing ion fluence up to a saturation value of about 90%. Once this level is reached, further implantation only leads to a broadening of the distribution, while the concentration remains at 90%. The ion fluence dependence of maximum damage concentration is represented by a common model assuming two types of defects: point defects (which can recombine with those already existing from previous ion impacts) and non-recombinable damage clusters. The damage produced dominantly consists of randomly displaced lattice atoms, which indicates point defects and point defect complexes. For higher damage levels also a contribution of correlated displaced lattice atoms can be identified. This suggests that the damage clusters are not amorphous. A possible explanation of the observed results could be the formation of another phase of Ga2O3.

Low-temperature damage formation in ion-implanted SiC and its correlation with primary energy deposition

4H-SiC was implanted with He+, B+ and Mg+ ions over a wide range of ion fluences at 15 K. Damage analysis was performed with Rutherford backscattering spectrometry in channelling configuration. Cross-sections of direct-impact-damage formation and stimulated growth of damage were extracted by fitting model curves to the maximum damage concentration versus the ion fluence. These cross-sections are related to quantities representing the primary energies deposited in the displacement of lattice atoms and in electronic interactions by empirical formulas. The empirical dependences found are in agreement with the empirical model previously proposed for III–V compounds ion implanted at 15 K. This suggests that at this temperature thermal effects can be widely excluded and universal dependences seem to exist.

Low-temperature damage formation in ion implanted InP

Damage formation in ion implanted InP is studied by quasi–in situ Rutherford backscattering spectrometry (RBS) in channelling configuration. Subsequent implantation steps are performed at 15K each followed by immediate RBS analysis without changing the environment or the temperature of the sample. 30keV He, 150keV N and 350keV Ca ions were applied. The depth distribution of damage is in good agreement with that calculated with the SRIM code. The evolution of damage at the maximum of the distribution as a function of the ion fluence is described assuming damage formation within single ion impacts and stimulated growth of damage when the collision cascades start to overlap with cross sections σd and σg, respectively. These cross sections are found to depend on the primary energies deposited in the displacement of lattice atoms and in electronic interactions calculated with the SRIM code. The obtained empirical formulas are capable to represent the experimental results for different III–V compounds implanted at 15K with various ion species.

Inhomogeneous lattice distortions in the Zn0.99Cd0.01Se crystal

The structural state of a Zn0.99Cd0.01Se crystal at temperatures of 78 and 300 K has been investigated using thermal neutron diffraction. It has been found that the diffraction patterns of the crystal contain diffuse scattering regions in the vicinity of the basic structure reflections, which are independent of temperature. It has been shown that the effects of diffuse scattering are caused by transverse displacements of lattice atoms, which are induced by impurities of the Cd2+ ions whose sizes substantially exceed the sizes of the Zn2+ ions. The obtained results have been used to construct a picture of the influence of cadmium ions on the structural state of the crystal under study.

Empirical modeling of the cross section of damage formation in ion implanted III-V semiconductors

In this letter, the cross section of damage formation per individual ion is measured for III-V compound semiconductors ion implanted at 15 K, applying Rutherford backscattering spectrometry. An empirical model is proposed that explains the measured cross sections in terms of quantities representing the primary energies deposited in the displacement of lattice atoms and in electronic interactions. The resulting formula allows the prediction of damage formation for low temperatures and low ion fluences in these materials and can be taken as a starting point for further quantitative modeling of damage formation including secondary effects such as temperature and ion flux.

A new class of spontaneously polarized materials

aterials have been a defining feature of the evolution of human culture and the electrical properties of materials are now at the forefront of this evolution. Advances occur both through the discovery of new electrical characteristics and through new devices based upon these discoveries. Here we describe a hitherto unsuspected electrical property of materials. We have recently found that solid thin films of many common or garden molecules harbour very large electric fields [1]. ese fields are a property of the film and appear spontaneously when films of species such as nitrous oxide or isoprene are laid down on a substrate. e fields point in a direction perpendicular to the surface and may exceed 108 V/m. e films can be as thin as ten monolayers for large electric fields to be established. is new class of ferroelectric-like materials are of the order-disorder type, as opposed to displacive ferroelectrics such as the class of perovskites [2], in which fields are generated through small displacements of lattice atoms from their positions in the non-ferroelectric phase.We are hesitant to name the present films simply ‘ferroelectric’ since, while they satisfy the requirement that they harbour spontaneous macroscopic electric fields, we have not shown that these fields can be reversed by application of an external field. Note also that the films which we describe are not ‘electrets’, which possess a fixed charge or ordered dipole orientation induced by charge injection or an externally imposed electrostatic field.

Ion-beam-induced damage formation in CdTe at a temperature of 15 K

Damage formation in 〈1 1 1〉-oriented CdTe single crystals irradiated with 270 keV Ar ions at fluences ranging from 1.0 × 10 11 to 6.0 × 10 16 cm −2 was investigated in-situ using Rutherford backscattering spectrometry (RBS) in channeling configuration. Irradiation and subsequent analysis were performed at 15 K without changing the target temperature. CdTe is not rendered amorphous even after irradiation with several 10 16 cm −2. Defect profiles calculated from the RBS channeling spectra using the computer code DICADA show the formation of a broad defect distribution which extends much deeper into the crystal than the projected range of the implanted ions. Energy-dependent RBS channeling analysis was used to identify the defects as predominantly uncorrelated displaced lattice atoms.

Radiation damage in ZnO ion implanted at 15 K

Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10 11 to 7 × 10 16 cm −2. The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms. Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10 13 cm −2 the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E d(Zn) = 65 eV and E d(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10 15 cm −2 a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10 16 cm −2 the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.

Radiation stability of fluorite-type nuclear oxides

Oxides with the fluorite-type structure are radiation tolerant materials. They are widely used or envisaged in hostile nuclear environments, such as nuclear fuels or inert transmutation matrices for actinide burning. Study of the radiation stability of this class of solids in various radiative fields is of major importance. Two issues which may affect the stability of materials are considered in this work: the production of radiation damage (ballistic contribution); the modification of the matrix composition by doping (chemical contribution). Both contributions may drastically affect the solid stability. Urania and zirconia single crystals were chosen as fluorite-type canonical systems. They were implanted with low-energy inert gases (He or Xe). The damage in-growth, due to both ballistic and chemical contributions, was investigated by in situ RBS/C experiments in the channelling mode and TEM. Two main steps in the disordering kinetics were observed for both inert gases. Relevant key parameters were found to be: the number of displaced lattice atoms created by the slowing-down of energetic ions during the implantation process; the concentration of noble gas atoms in the solid which cause the formation of large stress fields surrounding gas aggregates.

Inhomogeneous lattice distortions in a Zn1 − x CrxSe crystal

The structure of a Zn1−xCrxSe semiconductor crystal with x = 0.0029 was studied using thermal neutron diffraction. It was detected that the diffraction patterns of the crystal contain regions of diffuse scattering concentrated in the vicinity of the strong Bragg reflections. It was shown that the diffuse scattering was caused by transverse displacements of lattice atoms induced by chromium impurities, which undergo static tetragonal Jahn-Teller distortions in ZnSe.

Analysis of microstress in neutron irradiated polyester fibre by X-ray diffraction technique

Microstresses developed in the crystallites of polymeric material due to irradiation of high-energy particle causes peak broadening and shifting of X-ray diffraction lines to lower angle. Neutron irradiation significantly changes the material properties by displacement of lattice atoms and the generation of helium and hydrogen by nuclear transmutation. Another important aspect of neutron irradiation is that the fast neutron can produce dense ionization at deep levels in the materials. The polyethylene terephthalate (PET) fibre of raw denier value, 78.2, were irradiated by fast neutron of energy, 4.44 MeV, at different fluences ranging from 1×109 n/cm2 to 1 × 1012 n/cm2. In the present work, the radiation heating microstresses developed in PET micro-crystallites was investigated applying X’Pert-MPD Philips Analytical X-ray diffractometer and the effects of microstresses in tensile strength of fibre measured by Instron have also been reported. The shift of 0.45 cm−1 in the Raman peak position of 1614.65 cm{−1} to a higher value confirmed the development of microstresses due to neutron irradiation using micro-Raman technique. The defects due to irradiation were observed by SEM micrographs of single fibre for virgin and all irradiated samples.

Effect of ?-Irradiation on Superconductivity in Polycrystalline YBa2Cu3O7-?

A bulk polycrystalline sample of YBa_{2}Cu_{3}O_{7-\delta} (\delta \approx 0.1) has been irradiated by \gamma-rays with ^{60}Co source. Non-monotonic behavior of T_c (defined as the temperature at which normal resistance is halved) with increasing irradiation dose \Phi (up to about 220 MR) is observed: T_c decreases at low doses (\Phi \leq 50 MR) from initial value (\approx 93 K) by about 2 K and then rises, forming minimum. At highest doses (\Phi \geq 120 MR) T_c goes down again. The temperature width, \delta T_c, of resistive transition increases rather sharp with dose below 75 MR and somewhat drops at higher dose. We believe that this effect is revealed for the first time at \gamma-irradiation of high-T_c superconductor. The cross section for the displacement of lattice atoms in YBCO by \gamma-rays due to the Compton process were calculated, and possible dpa values were estimated. The results obtained are discussed taking into account that the sample is granular superconductor and, hence, the observed variations of superconducting properties should be connected primarily with the influence of \gamma-rays on intergrain Josephson coupling.

Implanted p– n junctions in GaN

29 Si + ion implantation into GaN(Mg), followed by rapid thermal annealing at 1100°C was used to create n +/ p junctions. The junction ideality factor was ∼2, indicative of a high density of generation–recombination centers, and the breakdown voltage was 13 V at 5.1×10 −4 A cm −2. Transmission electron microscopy revealed a high density (>10 10 cm −2) of implantation damage-related dislocations in the material, due to incomplete annealing of displaced lattice atoms. Higher annealing temperatures and improved junction passivation are needed for improved stand-off voltages.

Defects in silicon afterB+implantation: A study using a positron-beam technique, Rutherford backscattering, secondary neutral mass spectroscopy, and infrared absorption spectroscopy

The distribution of defects in Si (100), (110), and (111) after boron implantation and annealing processes was measured by means of different methods. Boron implantation was carried out at 300 K with three energies (50, 150, and 300 keV or 30, 90, and 180 keV) in multiple mode to obtain a homogeneously damaged layer. Ion fluences ranged from ${10}^{14}$ to ${10}^{16}{\mathrm{B}}^{+}{\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ The profile of vacancy-type defects was detected by variable-energy positron annihilation spectroscopy (VEPAS). The defect concentration increases proportionally to $\sqrt{\ensuremath{\Phi}},$ where \ensuremath{\Phi} is the ion fluence. It was found that the line-shape parameter $S$ of the positron-electron annihilation peak in the implanted layer increases with \ensuremath{\Phi}. The divacancy $(2v)$ concentration observed by infrared absorption spectroscopy (IRAS) was nearly constant in all samples (about $1.8\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). It can be concluded that divacancies are not the main vacancy-type defect and the increasing $S$ parameter must be attributed to additional defects of larger open volume. A value ${S}_{\mathrm{defect}}{/S}_{\mathrm{bulk}}=1.048$ was fitted for the dominating defect, where ${S}_{2v}{/S}_{\mathrm{bulk}}=1.04.$ Rutherford backscattering (RBS) measurements were carried out to detect the distribution of displaced lattice atoms. The defect-production rate was proportional to $\sqrt{\ensuremath{\Phi}}$ again. The concentration profiles of implanted ions were measured with sputtered neutral mass spectrometry (SNMS). In addition, Monte Carlo calculations were done with the TRIM code. The nearly homogenous defect distributions up to a depth of 1 \ensuremath{\mu}m found by VEPAS, TRIM, and RBS are in very good accordance. The samples were annealed up to 1150 K. It was found that the annealing behavior of vacancylike defects depends on the implantation dose and on the sample material under investigation. The divacancies are annealed at 470 K as measured by IRAS. An annealing stage of vacancy clusters at 725 K was observed in all samples by VEPAS. In Czochralski material, a decrease of the $S$ parameter below the value of defect-free Si was observed after annealing at about 750 K. This can only be explained by the appearance of a different defect type, most likely an oxygen-vacancy complex. At high ion fluences ${(10}^{16}{\mathrm{B}}^{+}{\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ an increase of the $S$ parameter above the defect value at room temperature was observed after annealing at 700 K in a region 100 nm below the surface. This high $S$ parameter is caused by the creation of larger vacancy clusters. These defects remain stable after annealing at 850 K. Correlated RBS and SNMS measurements were done at identically implanted samples for all annealing stages.

Application of ion-beam-analysis techniques to the study of irradiation damage in zirconium alloys

Ion-beam-analysis techniques are being used to provide an understanding of the nature of collision cascades, irradiation-induced phase changes, lattice location of solute atoms and defect-solute atom interactions in various zirconium alloys. In zirconium intermetallic compounds, such as Zr 3Fe, Zr 2Fe, ZrFe 2, and Zr 3(Fe x ,Ni 1 − x ), electron and ion irradiations have been used to obtain detailed information on the crystalline-to-amorphous transformation occurring during the irradiation. Transmission-electron-microscopy (TEM) observations have provided information on the nature of the damage produced in individual cascades, the critical dose required for amorphization, and the critical temperature for amorphization. In a study on the electron-energy dependence of amorphization in Zr 3Fe, Zr 2Fe and ZrCr 2 in situ high-voltage-electron-microscope investigations were combined with high-energy forward-elastic-recoil measurements to yield information on the threshold displacement energies for Zr and Fe or Cr in these lattices, as well as the role of secondary displacements of lattice atoms by recoil impurities (C,O) at low electron energies. In Zr implanted with 56Fe ions and subsequently bombarded with 40Ar ions at 723 K, subsequent secondary-ion-mass-spectrometry (SIMS) analyses were used to monitor the effect of irradiation on the migration of Fe in the Zr lattice. In addition, ion-channeling investigations have been used to determine the lattice sites of solute atoms in Zr as well as the details of the interaction between the solute atoms and the irradiation-produced defects.

Determination of lattice displacements in 2 MeV Se + implanted GaAs by RBS and PIXE channeling experiments

By combining temperature dependent RBS measurements with RBS and PIXE angular scan analysis an attempt is made to study whether in weakly damaged ion implanted GaAs layers a difference in the damaging of the Ga and As sublattice occurs. It is found that the half width of the RBS angular scan curves at zero-depth reflects the dose dependence of the concentration and the displacement distance of displaced lattice atoms which were determined by means of temperature dependent RBS measurements. The analysis of the measured PIXE angular scan curves and the comparison with those calculated from RBS angular scans indicates that the As sublattice seems to be slightly more damaged than the Ga sublattice.