Positron Diffusion Length Research Articles

A slow positron beam study of the oxidation behaviour of 310S stainless steel and alloy 800HT exposed to supercritical water

This paper reports a variable-energy slow positron beam characterisation of the surface oxide films in 310 S stainless steel and 800HT nickel-based alloy exposed to 550 °C supercritical water for 1000–4392 h. The evolution of the oxide layer was depth-profiled using positron beam energies from 0.5 to 20 keV, which enabled a probe of up to 2 mm of the near-surface region. SEM/EDS was used as a complementary characterisation tool to support and comprehend the positron annihilation spectroscopy data. A strong dependency on the positron trapping rate and the effective positron diffusion length on the microstructure of the studied materials enabled us to identify up to three non-bulk layers, which were preliminarily attributed to different oxide layers and Fe/Cr depletion regions. By calculating the effective positron diffusion length in the given multi-layer systems, we obtained the thicknesses of individual layers, which are in good agreement with the EDS analysis.

Defect characteristics of cadmium oxide nanocrystallites synthesized via a chemical precipitation method

In this study, we synthesized cadmium oxide nanocrystallites measuring 16 nm–30 nm via a chemical precipitation method at synthesis temperatures ranging from 400 °C to 800 °C. X-ray diffraction analysis identified well-defined peaks with no indication of mixed or impurity phases. The samples were also characterized by transmission electron microscopy. The band gap energies measured by optical absorption varied between 2.32 eV and 2.38 eV, thereby indicating quantum confinement effects over the entire range of nanocrystallite sizes. Fourier transform-infrared spectroscopic measurements demonstrated the localized surface plasmonic resonance behavior of the samples. Positron annihilation studies were conducted to identify and quantify the vacancy type defects, sizes, and concentrations. Increased annihilation of positrons occurred on the surfaces of nanocrystallites with dimensions less than the thermal diffusion length of positrons (∼25.4 nm). In crystallites larger than this limit, positrons were trapped in the non-stoichiometric defects within the crystallites, as demonstrated by both the positron lifetime and coincidence Doppler broadening measurements. Defects played a major role in determining the favorable properties of these cadmium oxide nanocrystallites because defects and parameters such as the synthesis temperature were strongly correlated.

Characterization of defect microstructure in MgRE (RE=Ce, Nd) alloys after processing by high-pressure torsion using positron annihilation spectroscopy and a high resolution X-ray diffraction

Two MgRE (RE = Ce, Nd) alloys with ultrafine-grain (UFG) microstructures were prepared by high-pressure torsion (HPT) at room temperature. The in-depth distribution of defects was characterized by Doppler broadening –variable energy positron annihilation spectroscopy (DB-VEPAS). The characteristic S parameter increases in bulk after HPT processing relative to an as-received sample and shows a relative stability between ½ and 10 turns, which suggests a rise in the open volume defect density. However, a theoretical analysis of the S(E) depth profile reveals an increase in the positron diffusion length from ∼115 nm for the as-received state to ∼207 nm after 10 HPT turns. Almost all the open volume defect consisted of dislocations (positron lifetime of τ = 260 ps). The dislocation density deduced from high-resolution X-ray diffraction in the HPT disc radial direction was reasonably homogeneous (around 4–6 × 1014 m−2).

Defect depth profiling of sputter-deposited Cu/Nb bilayers using a positron accelerator

Ultrathin bilayers of Cu/Nb viz. Nb/Cu/Si and Cu/Nb/Si have been deposited on Si substrate using DC and RF magnetron sputtering. The structural characterization of bilayers has been carried out using X-ray reflectivity (XRR) to determine the thickness and density of individual layers. The thickness of Nb and Cu layer in the bilayers ranges from ∼ 44–71 nm and 6–11 nm, respectively. Defect depth profiling of bilayers has been carried out using a positron accelerator. The measured depth profiles, i.e., S-parameter profiles have been fitted using VEPFIT, a program that factors the positron implantation profile and its annihilation to estimate S-parameter and positron diffusion length in individual layers. We present a detailed and systematic analysis of the measured S-profiles using VEPFIT to evaluate S-parameter and positron diffusion length in individual layers of the bilayer structures. These parameters are representative of defect characteristics of the layers. In this work we also highlight the nuances of VEPFIT analysis useful to extract defect characteristics in bilayers. The results indicate that defect microstructure of the layer depends on the thickness, deposition method and irradiation. Defect density is seen to increase with the reduction of thickness of the layer. RF sputtered layer has more defects as compared to DC sputtered layers. Positron depth profiling does not reveal distinct interface between the two metallic layers as well as layer and the substrate. A systematic characterization of defect in ultrathin structures is crucial to establish a correlation between defect microstructure and properties of nanolayer composites for advanced applications.

Studies of Defect Structure in Epitaxial AlN/GaN Films Grown on (111) 3C-SiC.

Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure, the consistency of the interfaces, and elemental diffusion are achieved by High Resolution Transmission Electron Microscopy (HR-TEM). In addition, the dislocation densities together with the defect correlation lengths are investigated via High-Resolution X-ray Diffraction (HR-XRD) and the characteristic positron diffusion length is achieved by Doppler Broadening Spectroscopy (DBS). Moreover, a comparative analysis with our previous work (i.e., GaN/AlN/Si and GaN/AlN/Al2O3) has been carried out. Within the epitaxial GaN layer defined by the relationship (111) 3C-SiC || (0002) AlN || (0002) GaN, the total dislocation density has been assessed as being 1.47 × 1010 cm−2. Compared with previously investigated heterostructures (on Si and Al2O3 substrates), the obtained dislocation correlation lengths (Le = 171 nm and Ls =288 nm) and the mean distance between two dislocations (rd = 82 nm) are higher. This reveals an improved crystal quality of the GaN with SiC as a growth template. In addition, the DBS measurements upheld the aforementioned results with a higher effective positron diffusion length = 75 ± 20 nm for the GaN layer.

Compensation of Shallow Donors by Gallium Vacancies in Monoclinic β - Ga2O3

Knowledge of the origin of deep levels and their impact on electrical properties is critical for device applications of \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. By annealing under an oxygen (${\mathrm{O}}_{2}$) atmosphere, the resistivity in shallow-donor (zirconium) doped \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$:$\mathrm{Zr}$ single crystals is found to increase by more than 10 orders of magnitude to (7 \ifmmode\pm\else\textpm\fi{} 4) \ifmmode\times\else\texttimes\fi{} 10 \ensuremath{\Omega} cm, which is comparable to the resistivity achieved by iron ($\mathrm{Fe}$) acceptor doping of (5 \ifmmode\pm\else\textpm\fi{} 3) \ifmmode\times\else\texttimes\fi{} 10 \ensuremath{\Omega} cm. We combine thermoelectric effect spectroscopy and positron annihilation spectroscopy (PAS), which are sensitive to deep levels and concentration of open-volume defects, with modeling of the electrical properties, to study these strongly compensated crystals. We find the compensating level in the ${\mathrm{O}}_{2}$-annealed \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$:$\mathrm{Zr}$ sample to be located at (0.727 \ifmmode\pm\else\textpm\fi{} 0.021) eV (E2*) below the conduction band, which correlates with a vacancy signal from PAS data. The defect is most likely the relaxed split $\mathrm{Ga}$ vacancy ${V}_{\mathrm{Ga}}^{i}$, rather than a simple gallium vacancy, considering theoretical predictions of a small energy barrier to relax. We observe that, due to the unique nature of these vacancies and anisotropy in the monoclinic lattice, the Doppler-broadening parameter is rather small compared with other wide-gap compounds, and in such a case the positron diffusion length is a suitable parameter to estimate the open-volume defect concentration.

Analysis of positron profiling data using e[formula omitted]DSc computer code

The Green’s function method was applied to solve the one-dimensional positron diffusion equation for a system consisting of up to four layers that contain defects with different trapping rates. These allow us to obtain the analytical relationships valid for the evaluation of data obtained from variable energy positron measurements. They have been implemented in user-friendly free computer code available to users. Fitting strategies are presented to extract the relevant physical parameters. The code was used to determine positron diffusion length in samples of polycrystalline pure, well-annealed iron, depleted uranium, and titanium. Program summaryProgram Title: e+DSC-1CPC Library link to program files: https://doi.org/10.17632/jxpj25kjvr.1Licensing provisions: MIT licenseProgramming language: Microsoft Visual Basic 2015External routines/libraries: Accord.NETNature of problem: The program enables the analysis of data obtained from a variable energy positron beam. The shape parameter of the annihilation line as a function of the incident positron energy is evaluated using a positron diffusion trapping model in which the positron trapping rate function is expressed as a four-step function.Solution method: The one-dimensional diffusion equation was solved by the Green’s function method. This allows the use of an exact form for the positron implantation profile, which is used in the program as the Makhov’s function. The parameters of this function are obtained from the MC simulation with GEANT4 and stored in the program.

Point and extended defects in heteroepitaxial β−Ga2O3 films

$\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ is emerging as an excellent potential semiconductor for high power and optoelectronic devices. However, the successful development of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ in a wide range of applications requires a full understanding of the role and nature of its point and extended defects. In this work, high quality epitaxial $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films were grown on sapphire substrates by metal-organic chemical vapor deposition and fully characterized in terms of structural, optical, and electrical properties. Then defects in the films were investigated by a combination of depth-resolved Doppler broadening and lifetime of positron annihilation spectroscopies and thermally stimulated emission (TSE). Positron annihilation techniques can provide information about the nature and concentration of defects in the films, while TSE reveals the energy level of defects in the bandgap. Despite very good structural properties, the films exhibit short positron diffusion length, which is an indication of high defect density and long positron lifetime, a sign for the formation of Ga vacancy related defects and large vacancy clusters. These defects act as deep and shallow traps for charge carriers as revealed from TSE, which explains the reason behind the difficulty of developing conductive $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films on non-native substrates. Positron lifetime measurements also show nonuniform distribution of vacancy clusters throughout the film depth. Further, the work investigates the modification of defect nature and properties through thermal treatment in various environments. It demonstrates the sensitivity of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ microstructures to the growth and thermal treatment environments and the significant effect of modifying defect structure on the bandgap and optical and electrical properties of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$.

Near-surface investigation of positron diffusion length in helium-implanted Fe9Cr and its ODS variant

In the present paper, we employ the depth sensitive slow positron Doppler broadening technique to characterise the positron diffusion in various near-surface layers of He+ implanted f/m steels. One of the frequently used positron annihilation parameters, with a high sensitivity to radiation-induced defects in a wide range of displacement damage, is the positron diffusion length. This parameter expresses the probability of the back diffusion of positrons to the surface and reflects the microstructural damage. Via the application of VEPFIT program on Doppler broadening spectroscopy data obtained on helium-implanted Fe9Cr and ODS Fe9Cr steels, we determined the changes in diffusion length and thickness of corresponding layers due to implantation.

Synchrotron GIXRD and slow positron beam characterisation of Ar ion irradiated pure V and V-4Cr-4Ti alloy: Candidate structural material for Fusion reactor application

Pure V and V-4Cr-4Ti has been irradiated with Ar9+ion at 0.4, 2.3, 8.4 and 23 dpa. GIXRD was carried at two different incident angles i.e. at 0.2° and 0.45° using Synchrotron radiation. The XRD data were analyzed to determine the variation of microstructural parameters like domain size and microstrain as a function of dose by modified Rietveld technique. The dislocation density calculated from these parameters show contrasting behaviour in pure V and V-4Cr-4Ti as a function of dose. In pure V the dislocation density increases at the highest dose, whereas in V-4Cr-4Ti it saturates after 8.4 dpa. However, in V-4Cr-4Ti at the highest dpa clear evidence of segregation of the alloying elements is observed. Slow positron annihilation spectroscopy has been used to characterize the irradiation induced depth dependent changes in the microstructure with dose. The positron diffusion lengths in pure V (202 nm) and V-4Cr-4Ti (370 nm) indicate a defect free bulk state. An increase in S-parameter (∼1.07Sbulk) confirms the creation of vacancy type open volume defects in the damaged region (0−300 nm from surface) of the ion-irradiated samples. It was observed that that there is no appreciable change in open volume defect density and type of defects up to 8.4 dpa for pure V except at the highest dose. However, in contrary to pure V, the increase in S-parameter at highest dpa clearly indicates that the type of defects at highest dose is significantly different from the other samples.

Defect Structure Determination of GaN Films in GaN/AlN/Si Heterostructures by HR-TEM, XRD, and Slow Positrons Experiments

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge and screw dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associated with the better effective positron diffusion length (Leff) of = 43 ± 6 nm.

Study of defects in potassium-doped tungsten alloy by positron annihilation technique

Tungsten alloy is known as a promising plasma-facing material (PFM) in IETR because of high strength, high-temperature stability, low sputtering erosion, low tritium retention, etc. However, tungsten has some disadvantages, such as high ductile-brittle transition temperature, low temperature brittleness, and radiation embrittlement. For the severe environment of PFM, various techniques have been adopted to improve W-based materials, among which the potassium doping is an effective bubble strengthening method, it can bring in nano-sized K bubbles, and enhance the toughness and strength, thermal shock performance, irradiation resistance of the materials. The K bubbles, which can pin grain boundaries (GBs) and dislocations, are the most characteristic defects in W-K alloy and have been widely reported. However, little attention is paid to other defects such as vacancies, GBs and dislocations. In fact, high-density dislocations exist in W-K alloy and vacancies play a considerable role in forming the K bubbles. Thus, positron annihilation technique (including the positron annihilation lifetime spectrum and slow positron beam Doppler broadening spectrum), which is a useful technique for detecting defects in solids, can be used to study these defects in W-K alloy samples. The positron lifetime of potassium bulk is about 376 ps and the positron lifetime of tungsten bulk is about 110 ps. But by simulating positron lifetime of defects in tungsten, it is found that potassium atoms in tungsten lattice do not exhibit the characteristic positron lifetime. Therefore, potassium is not considered in analyzing positron annihilation lifetime spectra of W-K alloy samples with different potassium content (46, 82, 122, 144 ppm). Three-state capture model is established in this paper, the dislocation density and vacancy cluster concentration of these samples are obtained. From the results, the dislocation densities in all samples are very high, but vacancy cluster concentrations are relatively low, and the vacancy cluster concentration in the sample with 82 ppm potassium content is the lowest in all samples. The behavior of potassium atoms in the sintering process is also discussed. Then the slow positron beam Doppler broadening spectra of W-K alloy samples and pure tungsten samples are measured and the obtained data are fitted by VEPFIT. It is noted that the defects in W-K alloy samples are much more than those in pure tungsten sample, and are distributed homogeneously with depth. The positron diffusion length information simultaneously obtained is compared with these values computed by dislocation density and vacancy cluster concentration, confirming the positrons trapped by potassium bubbles and grain boundaries are existent.

Study of Edge and Screw Dislocation Density in GaN/Al2O3 Heterostructure.

This study assesses the characteristics (edge and screw dislocation density) of a commercially available GaN/AlN/Al2O3 wafer. The heterostructure was evaluated by means of high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), and Doppler-Broadening Spectroscopy (DBS). The results were mathematically modeled to extract defect densities and defect correlation lengths in the GaN film. The structure of the GaN film, AlN buffer, Al2O3 substrate and their growth relationships were determined through HR-TEM. DBS studies were used to determine the effective positron diffusion length of the GaN film. Within the epitaxial layers, defined by a [GaN (0 0 0 2) || AlN (0 0 0 2) || (0 0 0 2) Al2O3] relationship, regarding the GaN film, a strong correlation between defect densities, defect correlation lengths, and positron diffusion length was assessed. The defect densities 6.13 × 1010 cm−2, 1.36 × 1010 cm−2, along with the defect correlation lengths Le = 155 nm and Ls = 229 nm found in the 289 nm layer of GaN, account for the effective positron diffusion length Leff~60 nm.

Microscopy studies on magnetron sputtered thin films with preferred orientation for positron moderation

Doppler-broadening Spectroscopy (DBS) and positron annihilation lifetime spectroscopy (PALS) are two positron annihilation spectroscopy techniques used for identifying near surface vacancies and lattice dislocations. Positrons manifest a broad energy distribution of about several MeV and in order to generate a low-energy positron beam, the positrons need to pass through a material called “moderator” so that a narrow positron kinetic energy bandwidth can be obtained, without losing too much intensity. The moderator must possess certain properties, among which a large negative positron work function and a large branching ratio to free positrons. The first can be assessed to an intrinsic characteristic of the material, that is usually correlated to the material bandgap, and in order to attain the second, a defect free, epitaxial thin film should be obtained. Due to its 3.4 eV bandgap, GaN represents a very promising candidate for positron moderation and field assisted moderation, that might manage longer average positron diffusion lengths. The aim of the present study is to obtain semi-insulating GaN thin films by magnetron sputtering GaN on different substrates (ZnO, TiO2, Al2O3) with fewer impurities incorporated, which should hold promises of increasing the positron diffusion length in the material. Preliminary depositions made in this study are to be discussed, highlighting both their advantages and limitations derived from their structure characterization.

Calculation and tabulation of efficiencies for tungsten foil positron moderators

Monte Carlo radiation transport simulations were used to calculate the positron stopping profiles in tungsten positron moderator foils. Stopping profiles were numerically integrated with efficiency kernels derived from Green's function solutions of the 3D diffusion equation to determine the moderation efficiency in both the backscattering and transmission geometries. Stopping profiles and efficiencies were calculated for positron energies from 10 keV to 10 MeV and incident angles from 0° to 75°. The resulting efficiencies agreed with other calculated and measured values in the literature, especially when similar values of the positron diffusion length and surface emission branching ratio were used. Large discrepancies with some of the values reported in the literature are mainly attributed to differences in foil manufacture and surface condition—factors which are known to greatly influence the diffusion length—as well as work function and branching ratios. This work provides tabulated efficiencies for tungsten foil moderators that can be interpolated and integrated with a positron flux having arbitrary energy and angular distributions.

Positron diffusion enhances the accumulation effect in small particles embedded in a low-density matrix

This report considers the mathematical model of positron’s behavior in a system consisting of small particles embedded in a matrix. It takes into account their initial distribution after the slowing down process, random walk, and finally annihilation. The theoretical results are compared with the results obtained from an experiment in which positron lifetime measurements were carried out for samples consisting of nickel particles of various diameters embedded in an epoxy resin. The model well describes the measurements extracted from the probability of positron annihilation in particles, as a function of their volume fraction. Based on these results, it was possible to determine the parameter characterizing the accumulation effect in this system and to estimate the positron diffusion length in the matrix. The model can be useful in studying, using positron annihilation techniques, polymer composites with particles and nanoparticles, as well as shedding light on the behavior of swift positrons in composites.

Application of positron beam for the long range effect studies in proton implanted iron

The paper reports studies of pure, polycrystalline iron samples exposed to proton implantation with energies of 73 keV and 173 keV and doses of 3 × 1015, 1016 and 3 × 1016 cm−2. Additionally, two samples were exposed to multiple implantation with different energies and fluencies. Variable energy positron beam (VEP) was applied to the detection of irradiation-induced defects and their distribution. In turn, elastic recoil detection (ERD) method was used to estimate hydrogen concentration and its range. Analysis of VEP results was performed with VEPFIT code. The strong reduction of positron diffusion lengths in the implanted layers point out the presence of defects with high concentration. The thicknesses of these layers are in good agreement with hydrogen range evaluated by ERD. However, the presence of defects with lower concentration behind implanted zone was also detected when the free fitting was performed. The long range effect cannot be excluded.

Defects characterization studies of europium-substituted bismuth ferrite nanocrystals by positron annihilation and other methods

In this paper, we present a detailed study of the origin of structural defects and their role on the effects of Eu3+-substitution (x = 0.0, 0.05…..., 0.3) in place of Bi3+ in Bi1−xEuxFeO3 nanocrystalline multiferroic perovskite samples. The samples were synthesized through a sol-gel route and characterized by x-ray diffraction and transmission electron microscopy. The energy dispersive analysis of x-rays clearly demonstrated the effective replacement of Bi3+ ions by Eu3+ and a possibility of generation of oxygen vacancies. UV–vis diffuse reflectance spectra were recorded to determine the band gap, which showed a continuous decrease on increased substitution. The phenomenon indicated the creation of defects-induced energy levels in the band gap closer to the conduction band. Positron annihilation spectroscopy revealed the presence of structural defects like trivacancies within the Bi1−xEuxFeO3 nanocrystallites. The surfaces of nanocrystallites smaller than the thermal diffusion lengths of positrons also acted as trapping centres. The transformation from the distorted rhombohedra structure to an orthorhombic configuration with an accompanying lattice contraction at x > 0.2 influenced the positron lifetimes and the intensities. Substitution also resulted in the recombination of the pre-existed vacancies with a fraction of the doped ions. Coincidence Doppler broadening measurements confirmed cationic vacancies as the predominant defects. Magneto-electric voltages measured under a.c. magnetic fields of different strengths showed an increase until x = 0.2 and then decreased concomitantly with the transformation of the crystallites to the orthorhombic structure.

The origins and properties of intrinsic nonradiative recombination centers in wide bandgap GaN and AlGaN

The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10−14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a VGa and two (or three) VNs, namely, VGa(VN)n (n = 2 or 3). The ion-implanted Mg-doped GaN films are found to contain larger size vacancy complexes such as (VGa)3(VN)3. In analogy with GaN, major NRCs in Al0.6Ga0.4N alloys are assigned to vacancy complexes containing an Al vacancy or a VGa.

Positron annihilation and nano-indentation analysis of irradiation effects on the microstructure and hardening of A508-3 steels used in Chinese HTGR

ABSTRACT The irradiation and annealing behavior of Chinese A508-3 reactor pressure vessel (RPV) steel (0.04 wt% Cu) after 3 MeV Fe-ion irradiation ranging from 0.1 to 20 dpa at room temperature (called RTRPV) and high temperature (250 °C, called HTRPV) was studied by positron annihilation Doppler broadening (PADB) spectroscopy and nano-indentation hardness. PADB showed that the density of vacancy-type defects was higher for low-temperature irradiations. The higher hardness was found after high-temperature irradiation because of the formation of solute clusters during irradiation. Positron annihilation measurements revealed the interaction and clustering of vacancies with solute clusters which were introduced by Fe-ion irradiation. For both RTRPVs and HTRPVs, the positron defect parameter and positron diffusion length showed the recovery of the irradiation-induced defects. Total recovery was observed after annealing at 450 °C.