Defects in electron irradiated GaP studied by positron lifetime spectroscopy

Defects induced by electron irradiation were investigated in GaP. The irradiation was performed at 15 K with an incident electron energy of 2 MeV and a fluence of 10¹⁸cm^–2. Annealing experiments were carried out in the temperature range between 100 and 1000 K.Ga and P vacancies were detected after electron irradiation and the different annealing behaviour of the two types of vacancies was observed. The recovery stage between 100 and 400 K was attributed to the annealing of Ga vacancies and the recovery at temperatures above 900 K to the annealing of P vacancies.We also performed Hall measurements to determine the location of the Fermi level in the bandgap during the annealing. Two different ionization levels of the P vacancy were found which can be attributed to the transitions V _P ⁺ /V _P ^o and V _P ^o /V _P ^– .Temperature-dependent measurements were performed to study the effect of shallow positron traps.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Recovery of electrical resistivity,short-range order formation and migration of defects in electron-irradiated Fe–4Cr alloy doped with carbon

Resistivity recovery (RR) data of Cr4 alloy doped with carbon (Cr4C) after irradiation with 5 MeV electrons are presented and compared with RR data of non-doped Cr4 alloy. Analysis of the defect- and short-range order-induced parts of RR has confirmed the proposed earlier scheme of the evolution of Frenkel pair defects on post-irradiation anneal in Cr4 and Cr9. Vacancies start free migration around 205–210?K; however, the related peak of stage III is invisible in conventional RR plots. Interstitials atoms (IAs) trapped in stage I at configurations of several Cr atoms start their long-range migration at 220?K. Migrating vacancies are captured by carbon atoms in Cr4C while IAs are not. Dissociation of vacancies from carbon atoms at 350?K gives rise to a decoration of carbon atoms with Cr atoms. Indications are seen that a release of vacancies captured by atoms of residual nitrogen takes place around 250–260?K.     

Characterization of surface defects formation in strontium titanate(100)

The electronic properties of SrTiO₃(100) surfaces after various treatments have been studied by electron energy loss spectroscopy and Auger electron spectroscopy. A stoichiometric surface without contamination can be obtained by annealing at 910 K under oxygen atmosphere of 5 × 10⁻⁵ Pa. The surface heated under ultrahigh vacuum (UHV) at 910 K exhibits a new surface state in the band gap region, which comes from oxygen vacancies at the top Ti-O₂ layer. This state is also produced by electron irradiation or Ar-ion bombardment.     

Positron annihilation spectroscopy characterization of effect of intermetallic nanoparticles on accumulation and annealing of vacancy defects in electron-irradiated Fe–Ni–Al alloy

The effect of intermetallic nanoparticles like Ni₃Al and nanoparticles of an Fe-rich bcc phase on the evolution of vacancy defects in an fcc Fe–34.2 wt% Ni–5.4 wt% Al model alloy under electron irradiation at elevated temperatures (423 and 573 K) was investigated using positron annihilation spectroscopy. Nanosized (1–8 nm) particles, which are homogeneously distributed in the alloy matrix, cause a several-fold decrease in the accumulation of vacancies as compared to their accumulation in a quenched alloy. This effect depends on the size and the type of nanoparticles. The effect of the nanoparticles increases when the irradiation temperature increases. The irradiation-induced nucleation and the growth of intermetallic nanoparticles were also observed in an alloy pre-aged at 1023 K under irradiation at 573 K. Thus, a quantum-dot-like positron state within ultrafine intermetallic particles, which we revealed earlier, allows control of the evolution of coherent precipitates like Ni₃Al, along with vacancy defects, during irradiation and subsequent annealing. Possible mechanisms of the absorption of point defects by nanoparticles are discussed.     

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops. Received: 16 November 2001 / Accepted: 12 June 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: yinlw@sdu.edu.cn     

The annealing of damage in ion implanted gallium arsenide

Photoluminescence measurements at 77°K and Rutherford scattering of 450 keV protons were used to study radiation damage and annealing in ion implanted GaAs. The characteristic band edge luminescence (8225 Å) in GaAs is completely quenched by ion implantation. Photoluminescence measurements on samples which were isochronally annealed show a single annealing stage at 600°C. A luminescence peak at 9140 Å is introduced into the spectra of all implanted and annealed samples. This peak is attributed to an acceptor level created by As vacancies. The intensity of the peak is greatly reduced by protecting the surface of implanted layers with SiO₂ during annealing. Rutherford scattering measurements on isochronally annealed samples reveal two annealing stages. A 300°C annealing stage is observed on samples which have an initial aligned yield less than random while a 650°C stage is observed on samples which have an initial aligned yield equal to random.     

Vacancy effects on one-dimensional migration of interstitial clusters in iron under electron irradiation at low temperatures

We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10^?4–10^?2, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10^?3. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.     

In situ synchrotron X‐ray powder diffraction for studying the role of induced structural defects on the thermoluminescence mechanism of nanocrystalline LiF

The correlation between the thermoluminescence (TL) response of nanocrystalline LiF and its microstructure was studied. To investigate the detailed TL mechanism, the glow curves of nanocrystalline LiF samples produced by high‐energy ball‐milling were analyzed. The microstructure of the prepared samples was analyzed by synchrotron X‐ray powder diffraction (XRPD) at room temperature. Then, the microstructure of a representative pulverized sample was investigated in detail by performing in situ XRPD in both isothermal and non‐isothermal modes. In the present study, the dislocations produced by ball‐milling alter the microstructure of the lattice where the relative concentration of the vacancies, responsible for the TL response, changes with milling time. An enhancement in the TL response was recorded for nanocrystalline LiF at high‐temperature traps (after dislocations recovery starts >425 K). It is also found that vacancies are playing a major role in the dislocations recovery mechanism. Moreover, the interactions among vacancies–dislocations and/or dislocations–dislocations weaken the TL response.     

Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (V_N) or multiple (m-V_N with m=2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to V_N vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at ∼400 nm assigned to m-V_N vacancies; further irradiation breaks more B-N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B-N dissociation and preserves the bulk sample stoichiometry.     

Electrical and XPS studies of 100 MeV Si ion irradiated Pd/n-GaAs devices

The effect of Swift Heavy Ion (100 MeV Si⁷⁺) irradiation on electronic-transport of Pd/n-GaAs devices has been studied by I–V and C–V techniques. The chemical compositions of the interface have been studied by XPS/EDAX techniques. It is observed that the irradiated devices show a reduction in current and capacitance by few orders of magnitude. The C–V characteristics show a change in conductivity type from n- to p-type after the irradiation. On hydrogenation, the irradiated devices show a capacitance peak in C–V characteristics, which has been ascribed to As vacancies. The XPS studies of these devices, for various etching durations, show that the ratio of As:Ga has reduced after the irradiation, which indicates the formation of irradiation-induced As vacancies. This reduction in As:Ga ratio is also confirmed by EDAX measurement. The observed conductivity type change from n- to p-type (on the irradiation) seems to be due to the change of substitutional sites of dopant silicon atoms from Ga to As sites due to the irradiation-induced As vacancies.     

Investigation of radiation defects in Fe-Cr alloy

Abstract

The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing.     

First stage f centre formation in pure and doped KCl crystals by room temperaure X-irradiation

The electrical conductivity and optical absorption of potassium chloride crystals, pure and doped with divalent cation impurities, have been measured before and after X-irradiation at room temperature. The concentration of free positive ion vacancies at room temperature has been calculated from conductivity for each crystal before irradiation and is found to be much less than the first stage F centre concentration. This shows that both free and associated positive ion vacancies are the latent source of F centres in the first stage colouration. Pb^{+ +} ions trap electrons producing Pb^{+ +} and Pb⁰ centres and making free the associated cation vacancies. Such centres are not produced in Ca-doped crystals where impurity-vacancy complexes trap F centres producing Z centres. The higher ratio of F centres to positive ion vacancies in Pb-doped crystals indicates that free cation vacancies are more effective in producing F centres. However, the concentration of divalent cations is found to decrease in both the crystals after irradiation, the decrease being more in Ca-doped crystals.The author is indebted to Prof. H. N. Bose for helpful discussions. Thanks are also due to Dr. M. L. Mukherjee for providing the crystals.     

Lattice location of169Er ions after implantation in al at 7K measured by the Mössbauer effect of169Tm

Conclusions It is shown by means of the Mössbauer effect of¹⁶⁹Tm that Er impurities in Al trap vacancies in the region 100K–300K. We conclude that vacancies become mobile in this region and are at least partly responsible for the annealing stage III found in classical radiation damage experiments on Al.     

Fe-Cr合金辐照空洞微结构演化的相场法模拟

Fe-Cr合金作为包壳材料在高温高辐照强度等极端环境下服役,产生空位和间隙原子等辐照缺陷,辐照缺陷簇聚诱发空洞、位错环等缺陷团簇,引起辐照肿胀、晶格畸变,导致辐照硬化或软化致使材料失效.理解辐照缺陷簇聚和长大过程的组织演化,能更有效调控组织获得稳定服役性能.本文采用相场法研究Fe-Cr合金中空洞的演化,模型考虑了温度效应对点缺陷的影响以及空位和间隙的产生和复合.选择400—800 K温度区间、0—16 dpa辐照剂量范围的Fe-Cr体系为对象,研究在不同服役温度和辐照剂量下的空位扩散、复合和簇聚形成空洞的过程.在400—800 K温度区间,随着温度的升高,Fe-Cr合金空洞团簇形核率呈现出先升高后下降的趋势.考虑空位与间隙的重新组合受温度的影响可以很好地解释空洞率随温度变化时出现先升高后降低的现象.由于温度的变化将影响Fe-Cr合金中原子离位阀能,从而影响产生空位和间隙原子.同一温度下,空洞半径和空洞的体积分数随辐照剂量的增大而增大.辐照剂量的增大,级联碰撞反应加强,空位与间隙原子大量产生,高温下空位迅速的扩散聚集在Fe-Cr合金中将形成更多数量以及更大尺寸的空洞.     

Thermoluminescence of pretreated NaCl: Sr crystals

Thermal glow curves of thermally pretreated NaCl:Sr crystals have been recorded after irradiation with ultraviolet light at room temperature. The observed glow curves exhibit a prominent glow peak at 380°K only when the specimen is annealed and quenched from 750°C. This peak is attributed to the dipole clusters which disintegrate after heating the specimen beyond 380°K     

Muon trapping and diffusion in Al and In after electron irradiation at 9 K

The transverse spin relaxation of positive muons has been measured on an Al single crystal and on polycrystalline In after irradiation with 2 MeV electrons at 9 K or 11 K, sample transfer at 4.2 K, and various subsequent annealing treatments. The Al data are analysed in terms of diffusion-limited trapping by vacancies. This yields a muon diffusivityD _μ which within experimental accuracy is proportional toT between 4 K and 50 K, indicating that in this temperature intervalD _μ is dominated by one-phonon-assisted incoherent tunnelling. In In only very small effects due to the irradiation could be observed. The muons appear to be localized in octahedral interstices. From the motional averaging taking place above about 20 K the diffusivity ofD _μ in In is deduced.     

The low-temperature diffusion of Hydrogen through annealed,quenched and aged gold

An electrolytic method has been used to measure the diffusivity of hydrogen through gold in the well-annealed state, as-quenched from 1248 K, and in foils formed by aging the quenched material at temperatures in the range 348–673 K. The resulting diffusivities are consistent with a simple statistical model in which a fraction of the H atoms saturate traps formed by quenched-in vacancies, but the mobility is primarily determined by the excess of nontrapped H atoms diffusing through “normal” sites in the fcc lattice.     

High temperature defects in electron irradiated semiconductors HgCdTe,PbSnTe

The peculiarities of defect formation in n- and p-type conductivity HgCdTe and PbSnTe crystals after electron irradiation (2 MeV, 300 K) up to 2 × 10¹⁸ cm^-2 are examined. It has been found that irradiation results in formation of n-type conductivity crystals with final parameters that are determined by the composition of initial samples. The annealing of radiation defects occurs in the 360–470 K temperature range. It has been believed that the change of HgCdTe, PbSnTe properties after electron irradiation at 300 K are connected with formation of radiation defects, including Te vacancies.     

Characterization of 3 MeV H+ irradiation induced defects in nuclear grade graphite

Atomistic structure change in a nuclear grade graphite irradiated at 353 K to 3.4×10¹⁷ ion/cm² with 3 MeV H⁺ was characterized by measuring positron lifetime and Raman spectrum at room temperature. It is evident from the positron lifetime results that the pre-existing structural defect is disoriented crystalline boundaries, and vacancy clusters ranging from di- to quadruple-vacancies were newly formed after ion irradiation. The relative intensity ratio of the Raman D and G peaks increased from 0.25 to 0.67 after ion irradiation. The concentration of radiation-induced vacancies was reasonably estimated by the Raman intensity ratio.     

Effect of Heat Treatment on the Stress-Strain Characteristics of Cd-Sn Eutectic Alloy

Stress-strain curves of slowly cooled and quenched Sn-33wt. alloy were studied in the temperature range from 353 to 433 K. The coefficient of logarithmic work/hardening, the fracture time, the yield stress and the fracture stress of annealed and quenched samples decreased with increasinig deformation temperature and exhibited a minimum at 393 K. The activation energy of the fracture mechanism is found to be 5.4 and 12.8 KJ/mol characterizing a dislocation mechanism for annealed and quenched samples. The X-ray analysis of the slowly cooled and quenched samples showed that the residual internal strains increase in the low temperature relaxation stage and decrease in the second stage.