Defect trapping by100Pd in fcc metals

Lattice defects in Al, Cu, Ag and Au were studied by the perturbed angular correlation technique (PAC) using the probe atom¹⁰⁰Pd/¹⁰⁰Rh. The comparison of data obtained on interstitial trapping in Cu and Au at different probe atoms (¹⁰⁰pd,¹¹¹In) allows defect characterisation less affected by the respective probe. The trapping efficiency of¹⁰⁰Pd for vacancy like defects is quite different to that of¹¹¹In atoms.     

Magnetic hyperfine fields in ultrathin Ni films on Cu(100)

The magnetic hyperfine field was measured at ¹¹¹In(¹¹¹Cd) probe atoms in ultrathin Ni films epitaxially grown on Cu(100) utilizing the perturbed -angular correlation (PAC) method. The behaviour of the hyperfine field as a function of temperature was studied for different film thicknesses ranging from 2 up to 10 monolayers. It was found that the strength of the hyperfine fields as well as the critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness. The orientation of the hyperfine field is discussed.     

PAC investigations of Au(110) 1×2-surfaces

Au(110) surfaces with (1×2)-reconstruction have been investigated using perturbed -angular correlation (PAC) spectroscopy. From the two observed electric-field-gradient tensors at¹¹¹In probe atoms, deposited at room temperature in a concentration of about 10^–4 ML, the occupation of substitutional sites in the densely-packed rows along [110]-directions and ninefold coordinated sites within the (111)-oriented microfacets, respectively, can be concluded. Annealing to about 600 K leads to bulk migration of the In atoms, detected by an increase of probe atoms with cubic surroundings. Due to this behaviour the order-disorder transitions of these surfaces occurring atT _c=649 K cannot be detected in our PAC experiments.     

Internal oxidation of 5sp impurities in noble metals

The internal oxidation of ion-implanted radioactive¹¹⁹In,¹¹⁹Cd, and¹¹⁹Sb in high-purity (5N) copper, silver, and gold single crystals has been investigated. The formation of the impurity-oxygen defect structures was monitored by Mössbauer emission spectroscopy on the 24 keV-radiation from the daughter¹¹⁹Sn after the-decay of the implanted radioactive isotopes. Molecule-like complexes are formed consisting of the implanted impurity and several oxygen atoms. The various Mössbauer parameters (isomer shift, quadrupole splitting, and Debye temperature) enable a distinction between different Sn-IV and Sn-II complexes, as well as between substitutional and vacancy-associated Sn atoms, which result from the ion implantations. The evolution of oxygen complexes with temperature during isochronal annealing was studied in the temperature range 300–1000 K. The formation (around 500 K) and the disintegration (around 900 K) of the complexes, in' silver in particular, are explained on the basis of diffusion properties of oxygen. A comparison of results from oxidations due to either thermal diffusion of oxygen or due to recoil implantations from a surface-oxide layer indicates that Sn-IV complexes are invariably formed in the first case, whereas in the latter case, a mixture of Sn-IV and Sn-II complexes is formed. Relations of the present results to similar results from PAC measurements on¹¹¹Cd in silver and from Mössbauer experiments on AgSn alloys are discussed.     

Precision PAC measurements in Er2O3 and Ho2O3 single crystals and structure refinement

The structure of C-form Ho₂O₃ and Er₂O₃ single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of¹¹¹In(EC)¹¹¹Cd probe ions using the perturbed - angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to X-ray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.     

The nature of the C-defects in nickel and their rôle in the interpretation of radiation damage in metals

In previous Perturbed-Angular-Correlation (PAC) studies of the - emission of ¹¹¹In probe nuclei in cold-worked or particle-irradiated nickel, it has been found that thermal annealing in the temperature regime of recovery stage III leads to the formation of so-called C-defects (Cubic defects). This is indicated by the occurrence of a new frequency of about 80 Mrad/s, in addition to the frequency (200 Mrad/s) that is due to ¹¹¹In on substitutional sites. Obviously, the C-defects are complexes consisting of ¹¹¹In and the intrinsic point-defect species that migrates freely in recovery stage III. Therefore, they have played an important rôle in the long-standing controversy on whether the recovery-stage-III defects are vacancies (one-interstitial model) or self-interstitials (two-interstitial model). The present paper reports on a novel experimental effort to reveal the nature of the C-defects by combining PAC studies on nickel samples differently pretreated in a systematic way, investigations of the Extended X-ray Absorption Fine Structure (EXAFS) on In-doped nickel, and measurements of the decay rate of ¹¹¹In nuclei in the Electron-Capture-Induced Decay (ECID). On the basis of the results of these experiments it is concluded that the defects trapped by substitutional ¹¹¹In atoms (In_s) in recovery stage III are self-interstitials (I), as expected according to the two-interstitial model. Moreover, there is evidence that the C-defects are In interstitials on tetrahedral sites (In_i) that form exclusively in the vicinity of the specimen surface from In_s – I pairs via the reaction In_s+I In_i.     

Nucleation of He clusters at111In in metals

Helium has been implanted in copper. Its interaction with¹¹¹In atoms during isochronal annealing is observed by the perturbed angular correlation (PAC) technique. The results reveal that the nucleation of He-clusters at the¹¹¹In site is accompanied by reactions of intrinsic defects. Therefore, in the case of Cu, where the knowledge of intrinsic defects is nearly complete, the successive building-up of He-clusters can be pursued.This work was supported by the Bundesminister für Forschung und Technologie.     

Structural lattice defects in silicon observed at111in by perturbed angular correlation

Probing of structural defects in silicon by the perturbed γγ angular correlation (PAC) technique is demonstrated between 77 K and 1300 K. The behaviour of radioactive¹¹¹ In probe atoms implanted at 295 K, is monitored during isochronal annealing in n-type, p-type and intrinsic Si. Trapping of defects, produced by the¹¹¹In implantation itself or by postirradiation is studied in P-doped crystals (10¹⁶/cm³-10¹⁷/cm³).     

Dynamics of defects in semiconductors

Hyperfine interaction techniques involving radioactive probe atoms like the perturbed angular correlation technique (PAC) and the Mössbauer effect have, due to their inherent sensitivity, successfully been applied to the study of defects in semiconductors. By probing the characteristic charge distribution around the probe atom interacting with a defect, they contributed to the microscopic understanding of the nature, structure and stability of complexes formed between radioactive dopant atoms and defects present in elemental and compound semiconductors. Moreover, dynamic effects can be studied by hyperfine interaction probe techniques. In this case, dynamics always means the fluctuation of a charge distribution resulting in a time dependent hyperfine interaction within the time scale defined by the lifetime of the isomeric nuclear state used for the measurement. Such fluctuations can either be caused by structural changes like local rearrangements of a defect complex or by electronic transitions in the semiconductor resulting in a change of the charge state of a defect complex. Examples using PAC to monitor such processes will be discussed for the semiconductor silicon.     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

119Sn Mössbauer study of the implantation behaviour of119In,119Sn,119mSn,119Sb and119mTe ions in SiC

The implantation behaviour of stable¹¹⁹Sn⁺ ions and radioactive¹¹⁹In⁺,^119mSn⁺,¹¹⁹Sb⁺ and^119mTe⁺ ions in SiC has been investigated by, respectively, conversion-electron Mössbauer spectroscopy on the 24 keV transition of¹¹⁹Sn, and by Mössbauer emission spectroscopy on the 24 keV radiation emitted by the¹¹⁹Sn daughter after the decays of the radioactive isotopes. The Mössbauer spectra could be decomposed in most cases into two groups of lines, one originating from¹¹⁹Sn atoms on substitutional Si sites, the other from various Sn-vacancy complexes distinguished by their Mössbauer parameters. Annealing experiments reveal a strong dependence of the structure of the defects and the formation and annealing kinetics on the chemical nature of the impurities. Defects formed in 297 K implantations with^119mSn and¹¹⁹Sb anneal above 500 C, resulting in a preferential location of the impurities on substitutional Si sites, whereas^119mTe atoms are efficient defect-trapping centres and no stable, substitutional fraction is observed on either lattice site. Possible structures for the Sn-vacancy complexes are discussed and comparison is made to similar defect complexes in group IV and in III–V semiconductors.     

Non-reactive metal/semiconductor interfaces: a combined AES,AFM andPAC study

Thin In films on Ge(100), Si(100) andSi(111) are investigated using Auger-electron spectroscopy (AES), atomic force microscopy (AFM) andperturbed -angular correlation (PAC) spectroscopy, respectively. The growth mode of the metal films is characterized by in situ AES measurements, indicating distinct differences between the different substrate surfaces. Additional AFM investigations are used to monitor the film topography at higher metal coverage. Finally, the local crystalline structure of the films is studied by the PAC technique.     

Hyperfine interaction of111Cd in In-Pd intermetallic compounds

Perturbed - angular correlation (PAC) spectroscopy was applied to study the hyperfine parameters for¹¹¹In probes in In-Pd intermetallic compounds. The PAC spectra measured in In₃Pd, In₃Pd₂ and InPd₂ compounds reflect the number, population and symmetry of nonequivalent indium probe sites predicted by the crystallographic data. The temperature dependence of the observed electric field gradients was measured in the temperature range 80–873 K.     

Excited levels of111In and113In

Levels and transitions in¹¹¹In and¹¹³In have been studied in the¹¹¹Cd(p, n)¹¹¹In and¹¹³Cd(p, n)¹¹³In reactions. By means of- coincidences,-angular distributions, relative excitation functions and conversion electron measurements more than 20 levels below 2 MeV have been established in each nucleus. Several negative-parity states were found above 1 MeV. Two low-spin positive-parity states with features similar to the possible rotational band in the heavier In nuclei are observed below 1,400 keV in both nuclei.     

Structural properties of the donor indium in nanocrystalline ZnO

The structural properties of the nanocrystalline semiconductor ZnO (nano-ZnO) doped with the donor Indium were investigated by perturbed γγ angular correlation spectroscopy (PAC) and extended X-ray absorption fine structure measurements (EXAFS). Up to an average concentration of one In atom per nanocrystallite, PAC measurements show that about 12% of the ¹¹¹In atoms are incorporated on substitutional Zn sites. At higher In concentrations, new In defect complexes are visible in the PAC spectra, which dominate the spectra if the average In concentration exceeds one In atoms per nanocrystallite. In addition, the local environment of Zn and In atoms in In doped nano-ZnO was investigated by EXAFS. The measurements at the K edge of Zn show that the crystal structure of nano-ZnO corresponds to bulk ZnO. In heavily In doped nano-ZnO the EXAFS experiments at the K edge of In exhibit an expansion of the first O shell about the In site. Since about four O atoms are detected in this first shell a substitutional incorporation of the In atoms in the ZnO lattice is suggested. The second shell to be occupied by Zn atoms as well as higher shells are almost invisible, which might have the same microscopic origin as the occurrence of defect complexes observed by PAC.     

Stimulated emission of a laser photon by the excited states of a three-level atom

We study the stimulated emission spectra arising from the emission of a laser photon by two excited states of a three-level atom interacting with a laser field at low intensities. The lifetimes of the stimulated photons emitted by the two excited states are much longer than those emitted spontaneously, while the intensities of the induced peaks take negative values indicating that amplification occurs at low frequencies. The ratio of the intensities of the light emitted by the excited states |3 > and |2 > of the atom is proportional to (₃/₂)^1/2, where ₃ and ₂ are the radiative decay rates of the spontaneously emitted photons by the excited states |3 > and |2 > into the ground state |1 > of the atom, respectively. An absorption spectrum is induced into the ground state of the atom by the laser field. The competition between induced absorption and stimulated emission at low frequencies without population inversion is considered in the low-intensity limit of the laser field. It is shown that for values of ₃/₂ > 1 the relative intensity (height) of the induced peak takes positive values implying that the process of the induced absorption dominates. As the ratio ₃/₂ increases, the height of the induced peak decreases and vanishes for values of ₃/₂ < 400. For values of ₃/₂ > 400, the height of the induced peak becomes negative indicating that the process of the stimulated emission (amplification) is likely to occur at low frequencies. The computed spectra are graphically presented and discussed.Issued as NRCC No. 39088     

Synthesis of Ultrafine γ-Al2O3 Powders by Pulsed Laser Ablation

Ultrafine -alumina (-Al₂O₃) powders have been successfully produced by laser ablation (aluminum target, Nd:YAG laser, 1.06 m, 7 ns, filter-target distance 6 cm, oxygen flow rate 1 l/min, pressure 200 Torr, fluence 16 J/cm²). The structural properties of the Al₂O₃ powders have been studied by X-ray diffraction, transmission electron microscopy and Brunauer, Emmet, Teller analysis. The behavior of luminous plume in background oxygen has been investigated by streak and fast photography, and emission spectroscopy. Stoichiometric -Al₂O₃ powder has been obtained at an oxygen pressure of 200 Torr, with a diameter of 11.8 nm and a specific surface area of 160 m²/g. At lower oxygen pressures the average diameter decreased to 6.1 nm, and the surface particle area increased to over 300 m²/g, but the powder composition was altered by the presence of some unreacted aluminum. The measurements on plume expansion revealed initial plume velocities of 0.04–0.18 cm/s, slowing down with pressure and distance. Emission spectroscopy indicates the presence of both neutral and ionized species, the most prominent lines being Al(I) 394.4 and 396.1 nm, Al(I) 358.7 nm, Al(III) 360.1 and 361.2 nm and Al(II) 466.7 nm; in time, only Al(I) 394.4 and 396.1 nm lines persisted for more than 2 s. Much weaker lines could be observed for O(II) ions. -Al₂O₃ (0.9 g) powders have been obtained for 1 kWh (laser output energy), with a powder collection efficiency of 75%.     

Feasibilities of nuclear techniques for the study of molecular defects in metals and semiconductors

Nuclear techniques, like Mössbauer effect and perturbed γγ angular correlation spectroscopy (PAC), have proven themselves to be sensitive tools for labelling and identifying probe atom-defect complexes. In these experiments, the “molecular defect” is investigated via the nuclear hyperfine interaction, which is measured at the site of the radioactive probe atom. Here, we shall put the emphasis on the PAC spectroscopy, which often uses¹¹¹In/¹¹¹Cd as radioactive probe atom. In metals, based on the identification of simple probe atom-defect pairs, the agglomeration of defects after cold-working and the interaction of vacancies with He atoms will be discussed. In semiconductors, it will be focussed on the interaction between dopant atoms, which strongly determines the electrical properties of these materials.     

Application of PAC to study equilibrium point defects in intermetallic compounds

A new method has been developed to measure properties of equilibrium, or thermal, defects in intermetallics using the technique of perturbed angular correlations of gamma rays (PAC). After quenching, thermal defects are detected microscopically by distinctive nuclear hyperfine interaction signals produced when they localize next to probe atoms present in high dilution. Using a Schottky defect model and applying the law of mass action to defect equilibria, a linear Arrhenius temperature dependence is predicted for a normalized monovacancy site fraction. We have observed such linear dependences in PAC experiments on NiAl, CoAl and TiAl using the¹¹¹In probe. Features of the method are summarized, of which the most important are the abilities to discriminate between different defects and to determine absolute vacancy concentrations, formation enthalpies and entropies. Extremely large formation entropies have been obtained for NiAl, CoAl and TiAl, suggesting vacancy concentrations of 15% at the melting temperatures.     

Hyperfine-field measurements inCr Rh andCr Re alloys using perturbed angular correlation technique

The hyperfine fields on¹¹¹Cd probe nuclei in Cr (1 at% Rh), Cr (0.3 at% Rh) and Cr (0.5 at% Re) matrices are measured using time-differential perturbed (–) angular correlation of 173–247 keV cascade in the decay of¹¹¹In to the levels in¹¹¹Cd. The sources prepared by ion implantation of¹¹¹In (2.81 d) activity followed by appropriate annealing procedure were found to be of good quality. The addition of impurities, located to the right of chromium in the periodic table such as Rh and Re, to the chromium matrix increases the electron to atom ratio, resulting in an increase in the magnetic moment compared to pure chromium. This in turn is expected to result in an increase in the hyperfine fields at probe nuclei in these alloys. The measured hyperfine fields are in qualitative agreement with the expected changes in these alloys.On leave from Kanpur Institute of Technology.