Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Small-angle scattering of X-rays by electron-irradiated copper

Small-angle scattering of annealed electron-irradiated copper shows that isolated monovacancies persist through recovery stage III. The scattering appearing at small angles after annealing between 260 K and 300 K can be accounted for by the condensation of self-interstitials to form small dislocation loops.     

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

On the role of defect charge state in the stability of point defects in silicon

Defect annealing in 1-MeV electron-irradiated, phosphorus-doped silicon is studied. Charge state effects are explored directly using a p-n junction structure. A defect state which is associated with the E center (phosphorus-vacancy pair) is found to disappear at approximately 150°C with an activation energy of 0.95 ± 0.03 eV in the neutral charge state and 1.25 ± 0.05 eV in the negative charge state.     

Quasi-harmonic finite strain equations of state of solids

Thomsen's ‘fourth-order anharmonic’ theory, which explicitly evaluates thermal effects in finite strain equations of elasticity according to the fourth-order approximation in lattice dynamics, is reconsidered for the special case of isotropic stresses and strains. It is shown that the approximations made in the finite strain theory are independent from those made in the lattice dynamics theory, with the result that strain dependence may be described in terms of any frame-indifferent strain tensor, not just the ‘Lagrangian’ strain tensor, η, and that the finite strain expansions may be taken to any order, not just the fourth. This result is valid for general stresses and strains. Illustrative pressure-volume equations are derived in terms of three strain measures, including η and the frame-indifferent analogue, E, of the ‘Eulerian’ strain tensor, ε. The reference state is here left arbitrary, rather than identifying it with the ‘rest’ state. This results in greater convenience in applying the equations. Not being restricted to fourth order, the present equations do not depend for their application on knowing the second pressure derivatives of the bulk modulus. Expressions are obtained for isentropes and Hugoniots in terms of the same parameters as enter the original equations, which have the form of isotherm. Ultrasonic, thermal expansion and calorimetric data for MgO are used to evaluate the parameters of third-order equations of state of MgO. The equations of state are tested and refined with Hugoniot data. The third-order ‘E’ Hugoniot is much closer to the data than the third-order ‘η’ Hugoniot. Inclusion of fourth-order terms allows both ‘E’ and ‘ρ’ Hugoniots to fit the data within their scatter. The separation of Hugoniots corresponding to different initial densities is predicted within the accuracy of the data by the thermal part of this theory.     

X-ray K-absorption fine structure study of CuxZn1-xFe2O4 (x = 0.2, 0.4, 0.6 and 0.8)

For Cu_xZn_1-xFe₂O₄ spinel ferrites (x = 0.2, 0.4, 0.6 and 0.8), EXAFS of Cu and Fe K-absorption edges have been studied employing LSS theory. With change in copper content, in the spinel system, the basic dependence of cation distribution on degree of inversion has been reported by plotting variation of bond distance “d” with EXAFS parameter ‘α’. This is further substantiated by plotting chemical shifts (ΔE) with EXAFS parameter ‘α’. The necessity of the determination of final state wavefunction for the knowledge of distribution of charge is stressed.     

Copper,the dominant acceptor in refined,undoped zinc telluride

Comparative measurements have been made of optical absorption and photoluminescence of refined undoped and Cu in-diffused ZnTe single crystals. Strong increases in a bound exciton BE line near 2.375 eV previously identified with the electrically dominant point defect acceptor ‘a’, with binding energy E_A ? 149 meV, suggests that this acceptor is substitutional Cu_Zn. Similarly strong increases in a relatively broad band at slightly higher energy suggests the simultaneous incorporation of shallow donors, possibly interstital Cu_I. These findings indicate that intrinsic defects such as V_Zn neither control the Fermi level in refined ZnTe nor produce shallow acceptors with E_A ? 250 meV, contrary to much previous speculation.     

Transition metal impurities in silicon: New defect reactions

Recent investigations on transition-metal impurities in silicon emphasizing the effect of the combined diffusion of two transition metals are presented and briefly discussed. The electronic properties and basic thermal kinetics are analysed by DLTS. The conversion of a Pd-related multivalent defect atE _c-0.35 eV andE _c-0.57 eV to the Pd-related defect atE _c –0.22 eV is observed, and a Pd-Fe complex level atE _c –0.32 eV is identified. The annealing characteristics of the multivalent Rh levels atE _c-0.33 eV andE _c-0.57 eV are observed, and used to analyse the influence of prior Rh doping on the Au diffusion. A complex formed by the codiffusion of Au and Cu is observed atE _v+0.32eV andE _v+0.42 eV, and shown to exhibit bistable behavior as does a similarly produced Au-Ni complex observed atE _v + 0.35 eV andE _v+0.48 eV.     

The properties of GaMnN lms grown by metalorganic chemical vapour deposition using Raman spectroscopy

An investigation of room-temperature Raman scattering is carried out on ferromagnetic semiconductor GaMnN films grown by metalorganic chemical vapour deposition with different Mn content values. New bands around 300 and 669 cm^-1, that are not observed in undoped GaN, are found. They are assigned to disorder-activated mode and local vibration mode (LVM), respectively. After annealing, the intensity ratio between the LVM and E₂(high) mode, i.e., I_LVM=I_E2(high), increases. The LO phonon-plasmon coupled (LOPC) mode is found in GaMnN, and the frequency of the LOPC mode of GaMnN shifting toward higher side is observed with the increase in the Mn doping in GaN. The ferromagnetic character and the carrier density of our GaMnN sample are discussed.     

Relative internal conversion electron intensities of the 113 keV transition in177Hf

The intensities of the internal conversion lines of the 113 keVM1 +E2 transition in¹⁷⁷Hf have been measured. From comparison with theoretical conversion coefficients the transition has been found to be (95.2 ± 0.5)%E2 corresponding to ¦δ¦=4.5 ± 0.3. The theoreticalL _I andM _I conversion coefficients used in the comparison have been increased by 5% according to the result that for pureE2 transitions in the deformed region theL _I/L _II,L _I/L _III,M _I/M _II, andM _I/M _III theoretical ratios are too low (～5%). Moreover, the present result indicates that theL _II/L _III andM _II/M _III ratios obtained from the tabulations by Hager and Seltzer and from the computer program by Pauli are too high (1–2%).     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

利用单能慢正电子束流,对原生的和经过电子辐照的6H-SiC内的缺陷形成及其退火行为进行研究．发现在n型6H-SiC中,经过退火后缺陷浓度降低．这主要是因为在退火过程中缺陷和间隙子的相互作用所引起．n型6H-Si经过1400 oC、30 min真空退火后,在SiC表面形成一个大约20 nm的Si层,这是在高温退火过程中Si原子向表面逸出的有力证明．在高温退火中,在样品的近表面区域有一个明显的表面效应,既在这些区域的S参数整体较大,这种现象与高温退火中Si不断向表面逸出有关．经过10 MeV的电子辐照,在n型6H-SiC中,正电子有效扩散长度从86.2 nm减少至39.1 nm,说明在样品中由于电子辐照产生大量缺陷．但是对p型6H-SiC,经过10 MeV电子辐照后有效扩散长度变化不大,这与其中缺陷的正电性有关．同时还对n型6H-SiC进行了1.8 MeV电子辐照后的300 oC退火实验,发现退火后缺陷浓度不减反增,这主要是因为在退火过程中,一些双空位缺陷和Si间隙子互相作用从而产生了VC缺陷的缘故．     

Structural and optical properties of CdS/Cu(In,Ga)Se<Subscript>2</Subscript> heterostructures irradiated by high-energy electrons*

Thin films of Cu(In, Ga)Se₂ (CIGS) with a Ga/(Ga + In) ratio of ~0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E_g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P₁ at ~0.91 eV and P₂ at ~0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu_In) and indium vacancies V_In, respectively, as the simplest radiation-induced defects.     

Reduction of the annealing temperature of radiation-induced defects in ion-implanted MOS structures

Si-SiO₂ structures irradiated with 11-MeV electrons for 10 s and then implanted with B⁺ ions with an energy of 10 keV at a dose of 1.0×10¹² cm^-2 through the oxide were annealed at different temperatures. MOS capacitors including such oxide layers were studied by quasi-static C/V and thermally stimulated current (TSC) methods. A comparison of the radiation defect annealing of double-treated (electron-irradiated and ion-implanted) samples and of implanted-only samples was carried out. It is shown that a preceding low-dose high-energy electron irradiation of the samples leads to a lowering of the annealing temperature of radiation defects introduced by ion implantation. After annealing at 500 °C for 15 min, no TSC spectra for the double-treated samples were observed. The spectra of the other samples (which were not previously irradiated) showed that after the same thermal treatment only some of the radiation defects introduced by ion implantation are annealed. The difference between the annealed interface state density of previously electron-irradiated and current MOS structures is also demonstrated. A possible explanation of the results is proposed . PACS 61.82.Fk; 85.40.Ry; 61.80.Fe     

Positrons and electron-irradiation induced defects in the layered semiconductor InSe

The positron annihilation characteristics of the layered semiconductor InSe have been investigated. No evidence for low temperature positron trapping is found in as-grown and heavily deformed InSe. The temperature dependence of the S-parameter in these sample exhibits an increase rate in good agreement with the linear expansion coefficient along the c-axis. The positron lifetime spectra of electron-irradiated 0.01% Sn-doped InSe show a long-lifetime component of 336 ps which is tentatively attributed to positrons trapped at isolated In vacancies. Isochronal annealing experiments performed on these samples show that the recovery of the positron lifetime measured at 77K is accomplished in two stages. The first, starting after annealing at 150K, could be induced by the formation of complexes (V_In-Sn_In). The second stage, observed at temperatures T375K, is attributed to the dissociation of these complexes and subsequent annealing of the In vacancies.     

The recovery of thermal-neutron-irradiated platinum

The dose-dependence of the recovery substages I_A to I_E in stage I and of the substages at 35 K and 120 K in stage II and of the stage between 300 K and 400 K, is presented. The 120 K stage is investigated especially carefully, in order to determine whether this stage is due to impurities.     

Internal conversion studies of the 114 keV transition in175Lu

Measurements of the conversion line intensities, the directional correlation and the conversion probability for the 114 keVM1+E2 transition in¹⁷⁵Lu are reported. TheE2 admixture, determined from relativeK andL intensities, is (19.2±0.2)%. The theoreticalL _I internal conversion coefficient is found to be about 8% too low relative theK,L _II andL _III conversion coefficients.     

Annealing of vacancies in electron-irradiated α-iron

Positron lifetime studies are reported on the behaviour of vacancies in high-purity electron irradiated α-iron. Results show that already at temperatures around 220 K vacancies migrate and form agglomerates, which anneal out only well above room temperature. The decrease of monovacancy concentration at stage I_E around 130 K is also visible.     

On the spin assignment to the 268 keV state in Au197

It is shown that the recent internal conversion data favour theI=1/2 value if the penetration effects inM1 conversion are fully taken into account. The 191 keV transition is found to be anM1 +E0 mixture with 41 (2)%E0, although the measuredK conversion coefficient is smaller than the theoreticalβ _k(M1) value.     

Far-infrared photoconductivity spectra of quenched-in acceptors in germanium: Chemical effects on the SA1 acceptors

Far-I.R. photoconductivity reveals the existence of at least four different kinds of SA₁ acceptors in germanium quenched from T > 800 each acceptor producing two or more hydrogenic excitation line series. One acceptor is correlated with the presence of Ni, another one with strong Cu contamination. The two other acceptors, which we call ‘normal’ and ‘shifted’, occur in samples quenched without intentional doping. The experiments indicate that the SA₁ acceptors in germanium are impurity complexes involving fast diffusing elements. We discuss a model according to which the SA₁ acceptors would be interstitial-substitutional pairs of transition metals. In particular, we propose to assign the ‘normal’ SA₁ acceptors to Fe_I-Cu_S pairs.     

Thermal stability of F ion-implant isolated AlGaN/GaN heterostructures

The thermal stability of F ion-implanted isolated AlGaN/GaN heterostructures was investigated,with B ion-implanted isolation shown for comparison.The sheet resistance of as-implanted samples with F ions was lower than that with B ions due to enhanced hopping conduction.The leakage current for both implanted samples initially decreased and then increased with increases in post-annealing temperature,indicating a damage-induced isolation mechanism.Leakage reached a minimum value after 400~oC annealing,which was over 10～8times lower than the saturated current(I_(sat))of the as-grown structure,suggesting a successful isolation.After relatively high-temperature annealing,the leakage for F implantation showed a small change,whereas that for B implantation showed significant increase,suggesting that F-implanted isolation exhibited excellent thermal stability.Leakage and F or B ion-implanted samples indicated that the leakage current originated from the region above the high-resistance GaN layer.Variation in E_arevealed that the optimal contributing states for conduction changed with operating temperature,annealing temperature,and ion species.