Surface physics with radioactive ion beams

Nuclear methods using radiation detection are very well suited for surface and interface investigations, since they generally require only a small number of radioactive probe nuclei. Virtually isolated probe atoms can be investigated. The use of an isotope separator to solve the central problem in the application of radioactive atoms for the study of surfaces, the clean deposition of the probe nuclei, is described. First physics experiments include studies of desorption isochrones and characterization of adatom sites on flat and vicinal surfaces by PAC. A particularly complete picture could be obtained for Cd and In on the Pd(111) surface, where five different sites successively populated in the surface diffusion process were observed. Other nuclear methods suitable for a future extension of the surface investigations, such as Mössbauer spectroscopy, decay recoil angular distribution and emission channeling, are briefly discussed.     

Radioactive ions for solid-state investigations at magnetic surfaces and interfaces

Hyperfine interactions observed at isomeric states of radioactive probe nuclei are used as a tool for solid-state investigations. This method is sensitive to atomic-scale properties. In recent years surface and interface investigations using radioactive probes delivered many results which can hardly be achieved by any other method. Several groups, e.g., from Konstanz, Leuven, Groningen, Aarhus, Uppsala, Tel Aviv, Pennsylvania, contributed to this field. Our group studies magnetic properties at surfaces and interfaces performing perturbed angular correlation (PAC) measurements in the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN). We take advantage of the enhanced variety of PAC probes delivered by the on-line mass separator ISOLDE. First, we report on measurements of magnetic hyperfine fields ( B _hf) at Se adatoms on a ferromagnetic substrate using ⁷⁷Se as a PAC probe. The investigation of induced magnetic interactions in nonmagnetic materials is a further subject of our studies. Here the nonmagnetic 4d element Pd is investigated, when it is in contact with ferromagnetic nickel. An outlook will be given on studies to be done in the future. The experiments were performed at the HMI, Berlin, and at CERN, Geneva. Received: 1 May 2001 / Accepted: 4 December 2001     

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.     

Electric field gradient at111In on Cu (100) surfaces and its utilization for indium surface diffusion

The electric field gradient at¹¹¹In probe atoms on Cu (100) surfaces was studied. At clean surfaces all probes are exposed to a well-defined surface field gradient. This is used to investigate indium surface diffusion, where the applied PAC method allows to observe diffusion steps on an atomistic scale. The jump rate for indium on Cu (100) was found to be in the order of 10^–3 Hz at 200 K.     

Hyperfine interactions of impurities in defect sites in ion-implanted metals

The lattice location of ion-implanted radioactive isotopes in metals and their interactions with defects in their own radiation damage cascade are of importance for studies of the hyperfine interactions of such probe atoms by nuclear methods. Recent results from Mössbauer and PAC experiments in particular are reviewed. Emphasis is put on lattice site identifications, which can be inferred from measured lattice-dynamical and hyperfine interaction parameters of probe atoms. Some general conclusions on the hyperfine interactions in interstitial- and vacancy-type complexes are drawn.     

PAC Investigation of Amorphous Ferromagnets

The amorphous ferromagnetic alloy Fe79/B16/Si5 was investigated by PAC. For that the probe atoms ¹¹¹In(¹¹¹Cd) and ^111mCd were implanted at the ISOLDE on-line separator. The distributions of magnetic hyperfine fields were determined. Below the crystallization temperature α-Fe clusters could be observed. For comparison Mössbauer transmission experiments were performed.     

Defects in semiconductors—results from Mössbauer spectroscopy

Progress in the development of Mössbauer techniques with ion-implanted, radioactive precursors to a Mössbauer isotope is discussed. Results obtained for elemental group IV semiconductors and their alloys as well as for III–V and II–VI compound semiconductors are presented. Emphasis is put on Mössbauer emission spectroscopy with radioactive probe atoms where the recoil energy in the nuclear decay is sufficient to expel the daughter atoms from a (substitutional) lattice site. The interactions of such (interstitial) atoms have been studied for ¹¹⁹Sb →¹¹⁹Sn in III–V compounds and for ⁵⁷Fe in silicon in particular. Finally, preliminary results, contributing to the question of the origin and nature of the magnetism in the Fe-doped, dilute magnetic semiconductor ZnO, are given.     

Adsorption sites of111In and111mCd on Pd(111) surfaces characterized by the electric field gradient

PAC measurements of the electric field gradient at the nucleus of isolated probe atoms on Pd(111) surfaces lead to the identification of five adsorption sites successively occupied by the parents¹¹¹In and^111mCd during annealing between 80K and 600K. The data are consistent with an estimated activation energy for Pd surface self diffusion of E_a=0.76(8)eV.     

Cavities in silicon investigated with the PAC-probe 111In

The perturbed angular correlation technique (PAC) was used to study the creation and development of He-induced cavities. In order to investigate the interaction of Indium atoms with cavities in Silicon the Bonn isotope separator was used to implant overlapping profiles of He (10 keV) and radioactive ¹¹¹In (160 keV) into undoped FZ-silicon. To get insight into the cavity formation mechanism samples were prepared with various He-doses (0.6, 2 and 6× 10¹⁶ ions/cm²). The samples were measured directly after implantation and after different annealing steps (t_hold= 10 min, T = 500–1100oC). Further, different implantation and annealing sequences were used. At higher He doses (2 and 6× 10¹⁶ ions/cm²) we find a large fraction of ¹¹¹In probe atoms subjected to an electric field gradient (EFG) corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 411(6) MHz with η= 0.25(4). The corresponding defect configuration is formed most effectively after He implantation into annealed, ¹¹¹In doped Si. This and the affinity of In to vacancies leads us to the assumption that, similarly to the situation in metals, the Indium atoms act as nucleation centres for vacancy clusters (cavities) and are situated on the inner walls of the cavities. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experiments with intense secondary beams of radioactive ions

Electromagnetic mass separation applied on-line to accelerators and nuclear reactors is now a standard technique for producing preselected isotopic beams (A, Z selection) of nuclear reaction products. The development, performance and anatomy of a large on-line isotope separator facility, the CERN-ISOLDE, is discussed. As a result of recent technical developments it is now possible to study individual nuclei of about 40 elements, in many cases out to where the limits of nucleon binding are approached and half-lives become as short as 10 ms. It is shown that the nuclear reaction processes induced with the high-intensity several hundred MeV proton beam can provide secondary radioactive beams in almost all regions of the nuclidic chart with intensities which are not matched by any other method. The intense beams of 10⁸–10¹¹ atoms/s have opened up a number of new experimental possibilities like laser spectroscopy on radioactive atoms, radioactive targets for nuclear reaction spectroscopy, and precision X-ray shift measurements.     

Deep level transient spectroscopy on radioactive impurities: Demonstration for Si:111In*

A defect specific analysis technique is introduced that combines the high concentration sensitivity of deep level transient spectroscopy (DLTS) with the radioactive transmutation of probe atoms; this technique results in the chemical identification of particular atoms which participate in the observed defect centres. The method is demonstrated for n- and p-type silicon samples, which are implanted with radioactive ¹¹¹In* ions. The activity of the indium ions decreases with half-life T _1/2 of 2.83 d by a decay into stable cadmium ions. A series of Cd-related levels whose concentrations increase exponentially with the decay time of ¹¹¹In* are identified in the DLTS spectra. Cd-diffused silicon samples are investigated for comparison; these samples reveal similar DLTS spectra. The analysis method described can be extended to all semiconductors and to a variety of radioactive probe atoms.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

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.     

Intrinsic point defects and light interstitials in metals

Clusters consisting of a radioactive probe atom and various point defects in metallic hosts were studied by means of NMR-ON and PAC. NMR resonance signals were observed for¹³¹IV₂ clusters in Fe and^114mInV₄ clusters in Ni. Decoration of¹¹¹InV₂ clusters by H atoms in W and Mo and by He atoms in W was monitored by PAC. The measurements yield the first reliable data on vacancy-hydrogen binding energies in these metals. The He-decoration results are in excellent agreement with the interpretation of He-desorption data and clearly show the existence of trap mutation.     

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.     

Investigation of metastable superconducting alloys produced by low temperature ion implantation

The suitability of the perturbed γγ angular correlation technique (PAC) for studies of the diffusion behaviour of He in metals is discussed. Using the example of He implanted into stainless steel, some features of the PAC using ¹¹¹In as radioactive probe atom are demonstrated. The results are compared with those in Ni, Cu and Au also obtained by PAC.     

Spin-polarized nuclei as probes of electromagnetic field distributions on solid surfaces

Recent experiments using thermal beams of nuclear-spin-polarized alkali atoms adsorbed on hot metal surfaces show that polarized nuclei are sensitive probes of surface electromagnetic field distributions. The high polarization of the probe beams, when coupled with the efficiency of atomic physics techniques used for monitoring the polarization of desorbed particles, makes possible a variety of interesting spinrelaxation experiments on single-crystal surfaces, including nuclear magnetic resonance. Extension of the current experimental method to semiconductor and insulator surfaces at arbitrary temperatures appears to be straightforward. The information from spin-polarized nuclear surface spectroscopy (SPNSS) will allow detailed tests of charge-density profiles now available in self-consistent surface structure calculations. Moreover, the variety of presently available polarized nuclear species suggests that the chemistry of many interesting adsorbate-surface systems could also be profitably investigated by this technique. The use of spin-polarized hydrogen nuclei in particular offers enticing prospects for fundamental studies in catalysis, surface structure and basic two-dimensional physics.     

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.     

A sectioning technique for sodium chloride single crystals and its application for ion implantation studies

A sectioning technique for sodium chloride single crystals has been developed. This technique, which is based on the formation of a silver chloride film, is capable of removing layers of constant thickness up to 2μg/cm². Being insensitive to radiation damage of the. NaCl-surface this film technique can be applied in studies of penetration and diffusion in NaCl. An electro-magnetic isotope separator has been used to implant radioactive phosphorous, krypton and xenon atoms in single crystals of sodium chloride, and examples are shown where the film technique has been applied for the study of range distributions and diffusion processes.     

Atomic beam,optogalvanic and stored ion spectroscopy for radioactive isotopes

Systematic studies of the optical isotope shift and the hyperfine splitting of long chains of stable and radioactive isotopes were performed. The method applied was laser induced resonance absorption; the experimental techniques were the atomic beam method with fluorescence detection and the optogalvanic method. The theory of hyperfine splitting and isotope shift of atoms is reviewed in short with emphasis on the aspects interesting for nuclear physics and on the evaluation problem. The experimental methods are described and their key parameters sensitivity and resolution are discussed. Preliminary results on stored thorium ions are given. Sample results for the nuclear quantity δ<r ²> derived from atomic beam experiments with tin and strontium are presented.     

Surface studies with hyperfine probes

The current status of surface hyperfine fields measured with NMR, Mössbauer spectroscopy and PAC is reviewed. Emphasis is put on experiments with well-characterized, free surfaces and on results on surface electric field gradients. Applications to detection of probe sites and diffusion phenomena are discussed.This article is dedicated to the memory of our late colleague Dr. Wolfgang Keppner