Migration and agglomeration of heliumatoms in copper

The magration and agglomeration of Helium atoms in He irradiated Cu was studied between 10 K and 900 K by perturbed γγ angular correlation (PAC) measurements using the radioactive probe atom¹¹¹In. Trapping of interstitial and substitutional He atoms is observed already after irradiation at 10 K. Substitutional He (He-yacancy pair) is bound in the nearest neighbourhood to the¹¹¹In probe atom and seems to be stable up to 725 K (E _He1V1 ^b ≥2.1 eV). The onset of vacancy assisted He agglomeration in Cu is observed at 250 K.     

Laser surface-melting of metals studied by PAC

High purity Pt, Ni, Cu, Fe and Al metal foils were electroplated with carrier-free¹¹¹In probe atoms and melted at 293 K using 32 ns ruby laser pulses with energy densities in the range 1–10 J cm⁻². Three distinct lattice locations of the probe atoms were detected in the melted surface layers using perturbed γ-γ angular correlation spectroscopy: (i) defect-free substitutional sites, (ii) non-unique sites with broad distributions of quadrupole interactions, and (iii) a probemonovacancy complex (for Pt only). The defect-free fraction was found to approach 100% when Pt is irradiated at 9 J cm⁻². The fraction of probes on defect-free sites was found to increase with the energy density of the pulse and with the solubility of In in the metals. The data are consistent with the idea that laser surface-melting produces high concentrations of vacancies and lattice sinks, although no thermal trapping of point defects was detected. Supported in part by National Science Foundation grant DMR 86-19688 (Metallurgy Program).     

Influence of the temperature and the ionisation potential on the hyperfine interaction of 111Cd ions in gaseous radioactive indium‐halides

The hyperfine interaction (HFI) of ¹¹¹Cd in different indium‐halides was studied by perturbed angular correlation (PAC). The ¹¹¹Cd ion arises from a ¹¹¹In by electron capture decay (EC) and joined Auger cascade. Coulomb fragmentation of the indium‐halide molecule produces the free excited ¹¹¹Cd ion. The nucleus decays via a γ–γ‐cascade which is perturbed by the HFI. Collisions of the ¹¹¹Cd ion with indium‐halide molecules can strongly change the perturbation. The perturbation was measured by the attenuation coefficient G₂₂(∞) and G₂₂(t). The molecular densities varied between 10¹⁴ and 10²⁰ cm⁻³. The present article shows the temperature dependence of the indium(III)‐halides and a fit for an empirical basic approach. The investigation of the InBr shows the independence of the attenuation coefficient from the temperature by indium(I)‐halides in contrast to the dependence of the indium(III)‐halides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stacking fault defects in hep cobalt studied by PAC

A study was made of lattice defects in hep Co metal using the technique of perturbed γ-γ angular correlations. Lattice sites of¹¹¹Cd impurities were characterized by their nuclear magnetic dipole and electric quadrupole interaction frequencies, respectively ω_L and ω_O. The signal for defect-free substitutional Cd impurities was found to have collinear interactions at 77K, with ω_L=438 Mrad/s and ω_O=7 Mrad/s. Analysis of measurements after heavy deformation at 100 K and after annealings up to 700 K gave little evidence of thermal trapping of point defects. However, in all spectral measured after deformation, a highly stable defect site with ω_L=418 and ω_O Mrad/s, and showing collinear interactions, was also observed. It is attributed to a¹¹¹Cd probe in a stacking fault (SF). Assuming a random distribution of proble atoms and SF's, a SF density of 20% was estimated.     

The size of thermal spikes in xenon-irradiated Ag/Fe bilayers

Ag/Fe bilayers in which a submonolayer of ¹¹¹In had been deposited in the Fe layer at a distance of x=0–17 nm from the interface, were irradiated at 80 K with 450 keV Xe ions to a fluence of 3 and 6⋅ 10¹⁵ ions/cm². Perturbed Angular Correlation spectroscopy was used to monitor the microsurrounding of the ¹¹¹In tracers and to identify the radiation-induced defects. The fraction of tracer atoms being transported into the Ag layer cannot be explained by ballistic mixing, but is reproduced when assuming local thermal spikes to develop. The diffusion of ¹¹¹In into the Ag layer can be modeled if the local spikes have an average diameter of some 5 nm. This revised version was published online in August 2006 with corrections to the Cover Date.     

Studies of the hyperfine interaction of111Cd in gaseous InCl and InCl3 systems

Using the perturbed γ-γ angular correlation at excited¹¹¹Cd-ions from the decay of¹¹¹In, the time-integral attenuation coefficientG ₂₂(∞) was measured in gaseous InCl and InCl₃ surroundings. The perturbation depends as well on the character and the density of the surrounding gas, as on the angular momentum of the electronic shell of the Cd-ion.G ₂₂(∞) varied between 0.012 and 0.828, while the molecule densities varied between 2×10¹⁴ and 2×10²⁰ molecules/cm³. For high densities it is to be noted, that the perturbation vanishes much faster for the InCl systems. Using a stochastic model, it is possible to calculate the mean hyperfine frequency Ω₀, and to estimate cross-sections for molecule-ion collisions at thermal energies.     

PAC-study of defects in n-type germanium

Using perturbed angular correlations of γ rays the behaviour of Frenkel defects in n-type germanium is studied.¹¹¹In/¹¹¹Cd probes either serve as primary knock-on atoms in the production of Frenkel defects by neutrino recoil or as trapping agents for defects produced by electron irradiation. The neutrino recoil process leads to a defect which is characterized by vQ=e²qQ/h=42(2) MHz (η=0.6), while a defect with vQ=423(1) MHz (η=0) is observed after electron irradiation and thermal annealing.     

Shape coexistence near the double-midshell nucleus 111Rh

The decay of ¹¹¹Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of ¹¹¹Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at ¹⁰⁹Rh₆₄, with two neutrons less than ¹¹¹Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. Received: 25 June 1997 / Revised version: 10 September 1997     

Mössbauer Studies on Nanocrystalline Diol Capped γ-Fe2O3

Nanocrystalline indium (nano-In) was prepared with different particle sizes by electrochemical deposition. The temperature dependence of the local electric field gradient (EFG) of nano-In was investigated in a temperature range of 20–300 K using the probe ¹¹¹In for perturbed γγ angular correlation (PAC) experiments. The temperature dependence of the EFG of nano-In can be described by a (1−BT ^3/2) dependence as in bulk-In. It is shown that the low temperature value of the EFG and the proportionality constant B vary systematically with particle size.     

Hyperfine Interactions in CeT2Ge2 (T = Mn,Co) Heavy Fermions Compounds Measured by TDPAC

Time Differential Perturbed γ–γ Angular Correlation (TDPAC) technique was used to measure the magnetic hyperfine field at both Ge and Ce sites in CeMn₂Ge₂ and CeCo₂Ge₂ intermetallic compounds. The ¹¹¹In (¹¹¹Cd) probe nuclei was used to investigate the hyperfine interaction at Ge sites, while the ¹⁴⁰La (¹⁴⁰Ce) nuclei was used to measure the magnetic hyperfine field at Ce site. The present measurements cover the temperature ranges from 10–460 K for CeMn₂Ge₂ and 9–295 K for CeCo₂Ge₂, respectively. The result for ¹¹¹Cd probe showed two distinct electric quadrupole frequencies above magnetic transition temperatures, in both compounds and a combined interaction in the magnetic region. The temperature dependence of the magnetic hyperfine field at ¹¹¹Cd at Mn site for the CeMn₂Ge₂ compound showed a transition from ferromagnetic to antiferromagnetic phase around 320 K and from antiferromagnetic to paramagnetic phase at 420 K. While a small magnetic field was measured on ¹¹¹Cd at Co site, no magnetic field on ¹⁴⁰Ce site was observed in CeCo₂Ge₂. This revised version was published online in September 2006 with corrections to the Cover Date.     

Point defects in NiAl near the equiatomic composition

Perturbed γ-γ angular correlation spectroscopy was applied to study the structure and properties of point defects near¹¹¹In probes in quenched and annealed NiAl samples with five compositions in the range c_Ni=48–52%. Quadrupole interaction signals are analysed in terms of near-neighbor Ni-vacancy and nextnear-neighbor Ni antisite atom defects. Large mono- and divacancy concentrations were observed in all samples after quenching. For c_Ni<50%, the vacancy defects could not be annealed out (structural defects). For C_Ni>50%, large quenched-in vacancy concentrations annealed out near 500°C. For c_Ni=50%, most vacancies annealed out near 500°C, but about 35% of the¹¹¹In probes retained a trapped monovacancy. This is attributed to a very low value of the formation enthalpy of a vacancy next to the In probe, estimated to be 0.16(2) eV. A lesser annelaing stage detected at 300°C is tentatively attributed to diffusion of Al-vacancies.     

Disorders produced during high-current and high-dose phosphorus ion implantation in silicon

Transmission electron microscopy, optical reflection and channeling effect measurements are employed to investigate disorders in 30 keV, high dose (3×10¹⁶ions/cm²) and high current (≦5 mA) phosphorus as-implanted silicon with (111), (100), and (110) orientation as a function of temperature rise (100–850°C) by the beam heating effect during implantation. Temperature rise below 400°C results in continuous amorrphous layer formation. This contrasts with results of the recovery into single crystals for temperature rise samples above 500°C, regardless of wafer orientation. Secondary defects (black-dotted defects, dislocation loops and rodlike defects) are formed in singlecrystal recovery samples, having a deeper distribution in (110) wafers and a shallower distribution in (111) and (100) wafers. Rodlike defects observed in 850°C samples are of “vacancy” type and have the largest density in (110) wafers.     

Magnetic Properties of Grain Boundaries of Nanocrystalline Ni and of Ni Precipitates in Nanocrystalline NiCu Alloys

Perturbed γγ-angular correlation spectroscopy (PAC) was used to investigate nanocrystalline Ni and NiCu alloys, which are prepared by pulsed electrodeposition (PED). Using diffusion for doping nanocrystalline Ni with ¹¹¹In four different ordered grain boundary structures are observed, which are characterized by unique electric field gradients. The incorporation of ¹¹¹In on substitutional bulk sites of Ni is caused by moving grain boundaries below 1000 K and by volume diffusion above 1000 K. The nanocrystalline NiCu alloys prepared by PED are microscopically inhomogeneous as observed by PAC. In contrast, this inhomogeneity cannot be detected by X-ray diffraction. The influence of the temperature of the electrolyte, the current density during deposition, and the optional addition of saccharin to the electrolyte on the homogeneity of nanocrystalline NiCu alloys was investigated. This revised version was published online in September 2006 with corrections to the Cover Date.     

Magnetically stabilized electron-hole liquid in indium antimonide

Luminescence spectra of sufficiently pure n-type indium antimonide crystals (N _D−N _A=(1–22)·10¹⁴ cm⁻³) in a magnetic field of up to 56 kOe, at temperatures of 1.8–2 K, and high optical pumping densities (more than 100 W/cm²) have been studied. More evidence of the existence of electron-hole liquid stabilized by magnetic field has been obtained, and its basic thermodynamic parameters as functions of magnetic field have been measured. When the magnetic field increases from 23 to 55.2 kOe, the liquid density increases from 3.2·10¹⁵ to 6.7·10¹⁵ cm⁻³, the binding energy per electron-hole pair rises from 3.0 to 5.2 meV, and the binding energy with respect to the ground exciton level (work function of an exciton in the liquid) rises from 0.43 to 1.2 meV. Zh. éksp. Teor. Fiz. 111, 737–758 (February 1997)     

Lithium in In2S3

The electrochemical insertion of lithium into β-In₂S₃ at different concentrations has been investigated by means of γγ-Perturbed Angular Correlations measurements using implanted radioactive ¹¹¹In nuclei. The study of the hyperfine interactions of the ¹¹¹Cd nuclei during the decay ¹¹¹In(EC)¹¹¹Cd allowed to determine the evolution of the three observed electric field gradients at different temperatures and lithium contents. This study reveals a great difference of behavior between the three indium sites of the structure, and shows the mobility of indium ions and their partial reduction after lithium insertion. In₂S₃ is good model material to study the properties of more complex materials based on this structure, which may be used as electrodes for lithium-ion batteries. This revised version was published online in September 2006 with corrections to the Cover Date.     

Indium solubility in iron studied with perturbed angular correlations

Segregation of In in Fe-based alloys containing 0.11–2.12 at.% In has been studied by measuring the perturbed angular correlations of γ-rays emitted in the nuclear decay of ¹¹¹In. The probe atoms were introduced into the alloys by either adding the carrier-free ¹¹¹In during melting or via ion implantation. Ageing of the samples at elevated temperatures followed by a slow cooling to room temperature allowed us to determine the upper limit of the solubility at room temperature, S(300 K)≤ 30 at. ppm. This value is far below the one reported in the published phase diagram. Quenching of the samples in water from 973–1373 K resulted in the determination of the indium solubility in iron at these elevated temperatures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Equivalent circuit of silicon diodes subjected to high-fluence electron irradiation

Silicon diodes with a p ⁺-n junction made in a 48-μm-thick phosphorus-doped silicon epilayer (resistivity ρ = 30 Ω cm) grown on antimony-doped Si(111) wafers (ρ = 0.01 Ω cm) are studied. The diodes are irradiated by high-energy (3.5 MeV) electrons with fluences from 5 × 10¹⁵ to 2 × 10¹⁶ cm⁻². It is shown that the conventional equivalent circuit of the diode that consists of a parallel RC network and a series-connected resistor inadequately describes the dependence of the dielectric loss tanδ on variable current frequency f in the range 1 × 10²–3 × 10⁷ Hz. Another equivalent circuit is suggested that includes not only the capacitance and resistance of the n-base (the latter increases because radiation-induced defects are compensated for by shallow donors) but also the f dependence of the capacitance of the space-charge region, which is due to retarded charge exchange between deep-level radiation-induced defects.     

Preparation,proton transport properties and use in gas sensors of thin films of zirconium phosphate with γ-layered structure

Pellicular γ-zirconium phosphate (γ-ZP_(p)), i.e. sheets made up entirely of oriented lamellae of Zr(PO₄)(H₂PO₄)·2H₂O (γ-ZP), have been obtained by filtering colloidal dispersions of exfoliated γ-ZP in water/acetone. The ac-conductivity of γ-ZP_(p) and γ-ZP was measured in the temperature range 20/–20°C on samples previously conditioned at relative humidities between 90 and 5%. In both cases, the conductivity dependence on material hydration indicates the presence of a non-negligible bulk transport at low relative humidities. For each relative humidity the conductivity data have been parameterised on the basis of the Arrhenius equation. Activation energy and pre-exponential factor values suggest the presence of the same conduction mechanism in both materials. The conductivity of γ-ZP_(p) measured by applying the electric field parallel to the sheets ranges from 3·10⁻⁴ to 1·10⁻⁵ S cm⁻¹ for relative humidity decreasing from 90 to 11%, being an order of magnitude higher than that of γ-ZP. Since the pellicular and microcrystalline material have very similar surface areas (11–12 m²/g), the higher conductivity of γ-ZP_(p) is mainly due to the preferred particle orientation parallel to the electric field. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Calculated Electric Field Gradients and Electronic Properties of Acceptors in CdTe

Linearised augmented plane wave (LAPW) calculations are performed in order to determine the electric field gradients (EFG) induced by group V and group Ib acceptors in CdTe at the nearest neighbour (NN) atomic site. Experimentally, the EFG are measured by the perturbed γγ-angular correlation spectroscopy using the radioactive probe isotopes ¹¹¹In and ⁷⁷Br. Besides the identification of the experimentally observed defects, the LAPW calculations provide information about fundamental properties of the EFG. For the group Ib acceptors, the dependence of the EFG on the NN distance is opposite to the expectation from the simple point charge model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Growth of thin, crystalline oxide, nitride, and oxynitride films on NiAl and CoGa surfaces

At 300 K, an amorphous Al-oxide film is formed on NiAl(001) upon oxygen adsorption. Annealing of the oxygen-saturated NiAl(001) surface to 1200 K leads to the formation of thin well-ordered θ-Al₂O₃ films. At 300 K, and low-exposure oxygen atoms are chemisorbed on CoGa(001) on defects and on step edges of the terraces. For higher exposure up to saturation, the adsorption of oxygen leads to the formation of an amorphous Ga-oxide film. The EEL spectrum of the amorphous film exhibits two losses at ≈400 and 690 cm^-1. After annealing the amorphous Ga-oxide films to 550 K thin, well-ordered β-Ga₂O₃ films are formed on top of the CoGa(001) surface. The EEL spectrum of the β-Ga₂O₃ films show strong Fuchs-Kliewer (FK) modes at 305, 455, 645, and 785 cm^-1. The β-Ga₂O₃ films are well ordered and show (2×1) LEED pattern with two domains, oriented perpendicular to each other. The STM study confirms the two domains structure and allows the determination of the two-dimensional lattice parameters of β-Ga₂O₃. The vibrational properties and the structure of β-Ga₂O₃ on CoGa(001) and θ-Al₂O₃ on NiAl(001) are very similar. Ammonia adsorption at 80 K on NiAl(111) and NiAl(001) and subsequent thermal decomposition at elevated temperatures leads to the formation of AlN. Well-ordered and homogeneous AlN thin films can be prepared by several cycles of ammonia adsorption and annealing to 1250 K. The films render a distinct LEED pattern with hexagonal [AlN/NiAl(111)] or pseudo-twelve-fold [AlN/NiAl(001)] symmetry. The lattice constant of the grown AlN film is determined to be a_AlN= 3.11 Å. EEL spectra of AlN films show a FK phonon at 865 cm^-1. The electronic gap is determined to be E_g= 6.1±0.2 eV. GaN films are prepared by using the same procedure on the (001) and (111) surfaces of CoGa. The films are characterized by a FK phonon at 695 cm^-1 and an electronic band gap E_g= 3.5±0.2 eV. NO adsorption at 75 K on NiAl(001) and subsequent annealing to 1200 K leads to the formation of aluminium oxynitride (AlON). An oxygen to nitrogen atomic ratio of ≈2:1 was estimated from the analysis of AES spectra. The AlON films shows a distinct (2×1) LEED pattern and the EEL spectrum exhibits characteristic Fuchs-Kliewer modes. The energy gap is determined to be E_g= 6.6±0.2 eV. The structure of the AlON film is derived from that of θ-Al₂O₃ formed on NiAl(001). Received: 21 March 1997/Accepted: 12 August 1997