Depth Profiles Of Defects Research Articles

Nonlinear effects in defect production by atomic and molecular ion implantation

This report deals with studies concerning vacancy related defects created in silicon due to implantation of 200 keV per atom aluminium and its molecular ions up to a plurality of 4. The depth profiles of vacancy defects in samples in their as implanted condition are carried out by Doppler broadening spectroscopy using low energy positron beams. In contrast to studies in the literature reporting a progressive increase in damage with plurality, implantation of aluminium atomic and molecular ions up to Al3, resulted in production of similar concentration of vacancy defects. However, a drastic increase in vacancy defects is observed due to Al4 implantation. The observed behavioural trend with respect to plurality has even translated to the number of vacancies locked in vacancy clusters, as determined through gold labelling experiments. The impact of aluminium atomic and molecular ions simulated using MD showed a monotonic increase in production of vacancy defects for cluster sizes up to 4. The trend in damage production with plurality has been explained on the basis of a defect evolution scheme in which for medium defect concentrations, there is a saturation of the as-implanted damage and an increase for higher defect concentrations.

Second-harmonic generation studies of implantation defects depth profile in hydrogen implanted garnet film

We demonstrate that complete depth profile of structural defects induced by ion implantation, with spatially separated lattice strain and implant ion contributions can be obtained with the use of second-harmonic generation (SHG) effect. The SHG effect was measured in transmission geometry for (111)-oriented garnet films implanted with H2+ ions of 60 keV energy and 1.5 × 1016 cm−2 dose, and etched to different depths along implantation range. Extremely strong sensitivity of the measured SHG intensity patterns in implanted samples on angle of incidence was found. The integral d¯ and differential d¯′ depth profiles of nonlinear optical susceptibility tensor were obtained from the experiment with the use of a phenomenological model developed for crystals of C3v symmetry. Direct correlation between perpendicular strain-related part of d¯′ profile and the induced effective magnetic anisotropy field is shown. Prominent peak in d¯′ located much deeper than the strain profile range was found, and its origin as related to the effect induced by hydrogen presence in the garnet crystal structure was interpreted.

Depth profiling and morphological characterization of AlN thin films deposited on Si substrates using a reactive sputter magnetron

It is well-known that the characteristics of aluminum nitride thin films mainly depend on their morphologies, the quality of the film-substrate interfaces and the open volume defects. A study of the depth profiling and morphological characterization of AlN thin films deposited on two types of Si substrates is presented. Thin films of thicknesses between 200 and 400 nm were deposited during two deposition times using a reactive sputter magnetron. These films were characterized by means of X-ray diffraction and imaging techniques (SEM and TEM). To analyze the composition of the films, energy dispersive X-ray spectroscopy was applied. Positron annihilation spectroscopy, specifically Doppler broadening spectroscopy, was used to gather information on the depth profiling of open volume defects inside the films and the AlN films-Si substrate interfaces. The results are interpreted in terms of the structural changes induced in the films as a consequence of changes in the deposition time (i.e., thicknesses) and of the orientation of the substrates.

Positron Studies of Subsurface Zone in Titanium Created in Sliding Wear

Positron annihilation studies of the subsurface zone constituted during dry sliding in pure titanium are reported. A new type of defect depth profile in this zone, which is not an exponential decay usually reported for other metals and alloys, is observed. Two layers in the subsurface zone are present in this metal. In the layer adjacent to the worn surface, the characteristic plateau of the mean positron lifetime that extends to a depth of 60–70 μm was observed. Small vacancy clusters and dislocations were recognized in this layer. At deeper zone, the mean positron lifetime decreased exponentially in the manner typical for the other metals. In this zone, vacancy clusters increase in size and decreases in concentration with increasing depth. There is a slight increase in measured microhardness in the subsurface zone in relation to the interior.

Ion channeling study of defects in compound crystals using Monte Carlo simulations

Ion channeling is a well-established technique for determination of structural properties of crystalline materials. Defect depth profiles have been usually determined basing on the two-beam model developed by Bøgh (1968) [1]. As long as the main research interest was focused on single element crystals it was considered as sufficiently accurate. New challenge emerged with growing technological importance of compound single crystals and epitaxial heterostructures. Overlap of partial spectra due to different sublattices and formation of complicated defect structures makes the two beam method hardly applicable. The solution is provided by Monte Carlo computer simulations. Our paper reviews principal aspects of this approach and the recent developments in the McChasy simulation code. The latter made it possible to distinguish between randomly displaced atoms (RDA) and extended defects (dislocations, loops, etc.). Hence, complex defect structures can be characterized by the relative content of these two components. The next refinement of the code consists of detailed parameterization of dislocations and dislocation loops. Defect profiles for variety of compound crystals (GaN, ZnO, SrTiO3) have been measured and evaluated using the McChasy code. Damage accumulation curves for RDA and extended defects revealed non monotonous defect buildup with some characteristic steps. Transition to each stage is governed by the different driving force. As shown by the complementary high resolution XRD measurements lattice strain plays here the crucial role and can be correlated with the concentration of extended defects.

Defect clustering in irradiation of GaN by single and molecular ions

Atomistic simulations were used to study irradiation effects on photoluminescence (PL) decay time of GaN. Irradiations were done by single (F, P) and molecular ions (PF4). Equal energy per mass (0.6 keV/amu) was used for all projectiles. Irradiation by the molecular ion shows faster PL decay time in comparison with the single ion. The simulation results show that single ions produce isolated point defects, whereas molecular ions produce big clusters of points defects. The total amount of defects produced by a PF4 projectile and five individual cascades started by one P and four F single ions (P + 4 × F) were very close and their defect depth profile follows the same pattern. These findings suggest that defect clusters are one of the important reasons for fast PL decay.

Application of slow positron beam for study of commercial oxide-dispersion-strengthened steels

Microstructure of four different commercial oxide-dispersion-strengthened steels (MA 956, ODM 751, MA 957 and ODS Eurofer) with different chromium content was studied in term of a presence of vacancy defects and their accumulation due to irradiation. The radiation damage (∼45dpa) was simulated by an implantation of helium ions with kinetic energy of 500keV and the implantation depth around 1.2μm. Positron Doppler broadening spectroscopy with a slow positron beam was applied for detection of a defect depth profile (in layers up to 1.6μm) and observation of radiation resistance of the investigated steels. Samples of the steels were also measured by positron annihilation lifetime spectroscopy in order to find out some change in ODS structure due to the radiation damage, although a sensitivity of this technique is very low in the implanted zone. Results of the Doppler broadening spectroscopy visible demonstrated a defect accumulation during the implantation. It showed that ODS Eurofer is the most radiation resistant from the group of all investigated steels. This was also indicated by a mean lifetime of the positron annihilation lifetime spectroscopy, which further demonstrated alloys MA 956 and ODM 751 as the most radiation affected materials. The results from the positron techniques (mostly Doppler broadening spectroscopy) can indicate an existence of a slight relation of the radiation damage and the content of the most significant alloying element in the samples – chromium. The radiation damage expressed as an increase of S parameters and an increase of positron mean lifetimes demonstrated a small growth along with the increase of the chromium content.

The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique

The annealing behavior of the subsurface zone (SZ) in pure bismuth induced by dry sliding was studied using the positron lifetime measurement. This measurement allows us to detect the SZ and its recovery, and recrystallization processes. The comparative measurements of the sample exposed to compression revealed the thermal stability of the SZ. The compressed sample rebuilt its structure due to the recovery and recrystallization processes at the temperature of 60 °C, whereas the sample exposed to dry sliding does it at higher temperature of 260 °C, which is close to the melting point. The isothermal annealing at the temperature of 100 °C confirmed these results. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that the concentration of defects at the worn surface gradually decreases, but at the depth between 50 and 170 μm, the generation of new defects takes place at the temperature of 75, 100 and even at 175 °C. At the temperature of 175 °C, the defects still are extended up to the depth of about 60 μm from the worn surface. The results were qualitatively confirmed by the measurements of the Vickers microhardness depth profile. Similar annealing behavior of the SZ was observed in pure magnesium.

Positron annihilation studies of recrystallization in the subsurface zone induced by friction in magnesium—effect of the inhomogeneity on measured positron annihilation characteristics

The discussion of the positron annihilation studies of crystal structure defects, like vacancies, dislocations, grain boundaries and the defect depth profile, is presented. The role of the positron implantation depth and positron diffusion in such studies has been considered in detail. For description of the measured annihilation characteristics the proposed theoretical models take into account both effects. The annealing studies of defects created in pure magnesium by compression or dry sliding-wear were used for demonstration of the discussed thesis. The positron lifetime measurements were applied for monitoring open volume defects behavior. It was demonstrated that annealing at the temperature of about 300 °C removes the defects created by compression. Application of the proposed model to description of the data obtained allows to determine the activation energy of the grain boundary mobility in pure magnesium equal to Q=0.56±0.18 eV. However, defects created by the dry sliding are not completely annealed up to the temperature of 500 °C. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that at the worn surface concentration of defects gradually decreases but at the depth between 60 and 100 μm the generation of new defects takes place at temperature of 150 and 225 °C. Above 300 °C the defects still are extended up to the depth of about 80 μm.

Subnanometer-resolution depth profiling of boron atoms and lattice defects in silicon ultrashallow junctions by ion beam techniques

The continued research and development effort on silicon ultrashallow junctions has posed a great challenge to materials characterization, due to the need for depth profiling of dopants and silicon lattice defects with a subnanometer resolution. In this work, we report on a method combining ion beam analysis (Rutherford backscattering, ion channeling, and nuclear reaction analysis) with room-temperature UV-assisted oxidation and chemical wet etching for obtaining high-resolution (∼0.5 nm) depth distributions of total boron atoms, electrically activated boron atoms and silicon lattice defects in silicon ultrashallow junctions. The application of this method was demonstrated by profiling silicon junctions as shallow as 8 nm, created by 200-eV boron ion implantation followed annealing by various techniques. The capability to profile boron at such high resolution has resulted in observation of boron segregation. Additionally, the ability for depth profiling Si lattice defects offered by this method has aided in the understanding of thermal and laser annealing effects on defect formation in Si junctions. Our experimental results are compared with those obtained using other techniques such as secondary ion mass spectroscopy and four-point probe, and differences are discussed in detail.

Positron Annihilation Measurements Performed on Oxide-Dispersion Strengthened Steels

This paper is focused on investigation of oxide-dispersion strengthened steels in a basic state (before an experimental treatment) by two positron annihilation techniques – positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy. Three high chromium commercial steels – MA 956, MA 957 and PM 2000 were measured in order to identify and quantify defects in samples as well as to demonstrate defect depth profile in subsurface layers up to 1.6µm. Results indicate presence of di-vacancies in PM 2000, three-vacancies in MA 956 and six-vacancies together with dislocations in MA 957. Positron techniques were supplemented by scanning electron microscopy, which demonstrated also presence of larger defects as pores and different density of precipitates.

Defect Detection in Fe-Cr Alloys with Positron Annihilation Doppler Broadening Spectroscopy

Positron annihilation Doppler broadening spectroscopy (DBS) has been used for the detection of structural defects in Fe-9wt%Cr (Fe-9Cr) alloy in the as-prepared and implanted states. Defects were created by He and H ion implantation with a kinetic energy of 250 keV. DBS is a non-destructive method and is a unique tool for the observation of open volume defects like vacancies and vacancy clusters in solids. A positron beam with variable positron energy was used for the measurement of defect depth profiles up to 1.5 µm. The obtained results provide qualitative and semi-quantitative information about radiation induced defects and their chemical environment. Although the collision damage from helium implantation was one order of magnitude higher than for the case of hydrogen, the changes in S and W parameters are much less significant, probably due to considerably lower mobility of helium in the implanted materials, which results in helium capture by the created open volume defects.

Defect profiling in organic semiconductor multilayers

Defect depth profile study has been carried out in organic semiconductor (OSC) multilayers to characterize the buried interfaces and layers using beam based positron annihilation spectroscopy. The bilayer and trilayer heterostructures (p–n, p–p and n–p–n) comprise of organic–organic and organic–inorganic (substrate) interfaces. Our study reveals the presence of defects at the interfaces whose concentration is seen to vary with the layer thickness. The S–W correlation has been used to examine the effect of organic materials as well as thickness of the layers on the defect microstructure in multilayers. The nature and type of defects in p–p bilayer are seen to be different as compared to p–n and n–p–n multilayers. Positron mobility in OSC layers has been calculated from the fitted diffusion length which is seen to be of the same order as the effective mobility of charge carrier obtained from the measured current density–voltage (J–V) characteristics. The role of structural defects and the intrinsic electric field at the interfaces on positron systematics is also examined. Positron diffusion modeling together with experimental data suggests that the defect at the interfaces has a stronger influence on the positron systematics than the intrinsic electric field across organic–organic interfaces.

Temperature Dependence of Defects in Hydrogen-Implanted Silicon Characterized by Positron and Ion-Beam Analyses

Abstract Nanometer-sized voids formed in Si after H implantation to a dose of 3¥10 16 cm -2 and annealing at 600 or 800 iC were characterized by slow-positron and ion-beam analyses. Depth profiles of defects were calculated from the S parameter curves of Doppler broadening and from Rutherford backscattering/channeling spectra. Concentrations of gettering sites were also calculated from the Rutherford backscattering spectra of the samples after Au gettering. Defect profiles by Doppler broadening measurements were found to be shallower than the projected range of implanted H, while profiles of defects and gettering sites detected by the backscattering measurements were close to the projected range. The peak concentrations of defects and gettering-sites were dependent on annealing temperature and analytical techniques. The observed annealing behaviors can be explained by the evolution of vacancy clusters and the interaction of H with defects.

Cluster dynamics modeling of accumulation and diffusion of helium in neutron irradiated tungsten

A cluster dynamics model based on rate theory has been developed to study the accumulation and diffusion processes of helium in tungsten under synergistic effects of helium implantation and neutron irradiation. By including self-interstitial atoms, vacancies and helium atoms as well as their clusters and further using more reliable parameters, the evolution of different types of defects with time and depth is investigated. The calculated results are comparable with experiments. The cases of helium plasma corresponding to the first wall and to the divertor are taken into account. The accumulation and diffusion behaviors of helium in tungsten are illustrated by the time and depth dependence of helium concentration in tungsten with or without the neutron irradiation, the contribution of different types of helium clusters/complexes to helium concentration and the depth profiles of different mobile defects and helium–vacancy complexes. It is concluded that the competition of trapping and diffusion effects dominates the behavior of helium atoms in tungsten for these two typical cases. Understanding these mechanisms is important for estimating damages to the plasma-facing materials.

Interaction between hydrogen and the Fe-B pair in boron-doped p-type silicon

The effect of hydrogen incorporation into iron-contaminated boron-doped Cz-Si has been investigated using deep level transient spectroscopy. In-diffusion of hydrogen by wet chemical etching followed by reverse bias annealing of Al, Schottky diodes result in the appearance of the defect level characteristic to interstitial iron (Fei), and the concentration of iron-boron pairs (Fe-B) decreases correspondingly. Quantitative observations from various defect concentration versus depth profiles imply strongly that H promotes dissociation of Fe-B releasing Fei whereas no detectable passivation of Fe-B or Fei by H occurs.

Dual energy time reversed elastic wave propagation and nonlinear signal processing for localisation and depth-profiling of near-surface defects: A simulation study

Nonlinear Elastic Wave Spectroscopy (NEWS) relies on the activation of defects by wave energy that propagates through the medium. In general, the response of activated defects will not scale linearly with the excitation amplitude, and the resulting nonlinear signatures can be identified and used for quality inspection. The efficiency of NEWS based inspection methods is therefore intrinsically linked to the locally deposited activation energy at the defect zone and the ability to generate nonlinear signatures that exceed the noise level of acquisition. Time Reversal techniques allow focusing of high levels of energy in small areas, and are consequently very useful for the local activation of defected zones. In this report, numerical simulations are reported showing the potential of a combination consisting of dual energy reciprocal Time Reversal and nonlinearity filtering using the Scaling Subtraction Method. The method is applied to the detection of planar near-surface defects parallel to the surface in a 2D domain. The results are evaluated for sweep excitation at different frequency ranges; for point-like receiver as well as extended transducers, and for in-plane as well as out-of-plane focusing. The observable nonlinear response at the surface is linked to an effective nonlinearity within the medium based on the defect geometry and the distribution of the local stresses.

Depth profiling of defects in ion-implanted Ni and Fe by positron annihilation measurements

Pure Ni and Fe samples were irradiated with 150 keV Ar ions at room temperature, 300 and 500 °C to introduce defects near the surface. The irradiated samples were characterized by energy-variable slow-positron beams with Doppler broadening and positron lifetime measurements to investigate defect profiles. The irradiation-induced defects were detected at regions much deeper than ion projected ranges for all the temperatures. The origin of such defects beyond the projected ranges was discussed in terms of vacancy migration, ion channeling, surface contamination and depth-scale error. Among these effects, the ion channeling may account for the obtained results.

Defect profiles in ion-irradiated metal samples by slow positron beams in comparison with simulation profiles

Metal (Fe and Ni) samples ion-irradiated at 100 – 773 K were characterized by Doppler broadening measurements with slow positron beams to investigate depth profiles of irradiation-induced defects. Obtained results were compared with defect profiles calculated by Monte-Carlo simulation. Defects profiles obtained by positron measurements were always deeper than those by the simulation. The difference between measured and simulated profiles was observed even in the case of the Fe sample irradiated at 100 K where vacancies are immobile. The origin of such differences was discussed with respect to vacancy diffusion, depth calculation, surface contamination and ion channeling.

Study of helium evolution in nanocrystalline titanium films by slow positron beam analysis

In this paper, the evolution and depth profiles of vacancy-type defects were investigated by slow positron beam (SPB) analysis. Films of helium-containing titanium were deposited on Si substrates by direct current magnetron sputtering under the helium/argon (He/Ar) gases ambiance. Different helium concentrations were introduced into the sample films by altering the He/Ar inflow ratios (QHe/QAr). It was found that the defect-characteristic parameter S from positron-electron annihilation peaks rise with the increase of the ratio of QHe/QAr. Homogenous He-related defect profiles and their evolution deep into around ~400nm were found by SPB measurements, together with the analysis of Rutherford backscattering (RBS) and elastic recoil detection (ERD).