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The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks$^{1–3}$. This nuclear clock will be a unique tool for precise tests of fundamental physics$^{4–9}$. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older$^{10}$, the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay$^{11}$. The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured$^{12–16}$. In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 ($^{229m}$Th). By performing vacuum-ultraviolet spectroscopy of $^{229m}$Th incorporated into large-bandgap CaF$_{2}$ and MgF$_{2}$ crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements$^{14–16}$ and the uncertainty is decreased by a factor of seven. The half-life of $^{229m}$Th embedded in MgF$_{2}$ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus. The nucleus of the radioisotope thorium-229 (${}^{229}$Th) features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. For this reason, it is a leading candidate for use in next-generation optical clocks. This nuclear clock will be a unique tool, amongst others, for tests of fundamental physics. While first indirect experimental evidence for the existence of such an extraordinary nuclear state is significantly older, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay and its hyperfine structure in a laser spectroscopy study, revealing information on the isomer's excitation energy, nuclear spin and electromagnetic moments. Further studies reported the electron conversion lifetime and refined the isomer's energy. In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. In this Letter, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (${}^{229\mathrm{m}}$Th). By performing vacuum-ultraviolet spectroscopy of ${}^{229\mathrm{m}}$Th incorporated into large-bandgap CaF${}_2$ and MgF${}_2$ crystals at the ISOLDE facility at CERN, the photon vacuum wavelength of the isomer's decay is measured as 148.71(42) nm, corresponding to an excitation energy of 8.338(24) eV. This value is in agreement with recent measurements, and decreases the uncertainty by a factor of seven. The half-life of ${}^{229\mathrm{m}}$Th embedded in MgF${}_2$ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

Recent developments in position sensitive detectors pave a way towards the efficient realization of Rutherford Backscattering Spectrometry/Channeling experiments in blocking geometry. Here we report on the use of a Timepix position sensitive detector (PSD) for this kind of studies. The blocking geometry in combination with a PSD brings the advantage that it is straightforward to measure full two-dimensional blocking patterns, which reveal considerably more details than one-dimensional angular scans. The Timepix detector response was first calibrated using internal test-pulses, followed by determining the energy resolution for ~5.5 MeV α particles from radioactive sources, and for 1.8 MeV alphas from a 2 MeV 4He+ beam backscattered from a Au target. An effective count-rate of ≈2000 events/s and energy resolution of ≈47 keV at 1.8 MeV were achieved. Blocking effects from backscattered α particles were measured around the 〈1 0 0〉 axis of Si and the 〈0001〉 axis of 6H-SiC single crystals and compared to theoretical patterns calculated by means of FLUX Monte Carlo simulations, from which the angular resolution achieved with the setup could be determined to be 0.10°-0.12°.

We provide the first systematic characterization of the structural and photoluminescence properties of optically active defect centers fabricated upon implantation of 30-100 keV Mg+ ions in artificial diamond. The structural configurations of Mg-related defects were studied by the emission channeling technique for 27Mg implantations performed both at room-temperature and 800 {\deg}C, which allowed the identification of a major fraction of Mg atoms (~30-42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (~13-17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in a temperature range comprised between 5 K and 300 K, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength enabling to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications., Comment: 7 figures

The acquisition of two-dimensional (2D) channeling patterns is gaining increased popularity within the ion beam community. However, with the exception of emission channeling experiments for the lattice location of radioactive impurities, quantitative analysis of such patterns is rarely found. We present a general description of the statistical data analysis methodology for 2D channeling patterns, which consists of comparing experimental data by means of a fit procedure to theoretical yield distributions. The developed software allows for chi-square or maximum likelihood-based fits, optimizing the orientation of the theoretical vs the experimental pattern, as well as the best choice of random level, and providing fractions for the contributions from several theoretical patterns related to different lattice sites. Optionally also the angular resolution can be used as a fit parameter. Use of the software is illustrated by examples of electron emission channeling from 27Mg in GaN, as well as 4He RBS channeling from Ge.

We demonstrate formation of the ideal split-vacancy configuration of the Sn-vacancy center upon implantation into natural diamond. Using beta(-) emission channeling following low fluence Sn-121 implantation (2 x 10(12) atoms/cm(2), 60 keV) at the ISOLDE facility at CERN, we directly identified and quantified the atomic configurations of the Sn-related centers. Our data show that the split-vacancy configuration is formed immediately upon implantation with a surprisingly high efficiency of approximate to 40%. Upon thermal annealing at 920 degrees C approximate to 30% of Sn is found in the ideal bond-center position. Photoluminescence revealed the characteristic SnV- line at 621 nm, with an extraordinarily narrow ensemble linewidth (2.3 nm) of near-perfect Lorentzian shape. These findings further establish the SnV- center as a promising candidate for single photon emission applications, since, in addition to exceptional optical properties, it also shows a remarkably simple structural formation mechanism. KU Leuven (SF/18/008) Portuguese Foundation for Science and Technology (CERN/FIS-PAR/0005/2017) FWO (G0E7417N; S004018N) EU Horizon 2020 Framework (654002; 824096; 820394) Wahl, U (corresponding author, Katholieke Univ Leuven, Quantum Solid State Phys, B-3001 Leuven, Belgium; Univ Lisbon, Ctr Ciencias & Tecnol Nucl, Inst Super Tecn, Dept Engn & Ciencias Nucl, P-2695066 Bobadela Irs, Portugal. uwahl@ctn.tecnico.ulisboa.pt

This chapter deals with the use of ion beam analysis (IBA) techniques, in particular in channeling geometry, to study defects in semiconductors. After a tutorial (Section 11.1) introducing the basic principles of IBA and channeling techniques, selected examples of their use to characterize defects (e.g., lattice location of dopants and implantation damage) are described in Section 11.2. Finally, Section 11.3 consists of a brief outlook into future developments and applications.

Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of $^{27}$Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra-low fluence $(≈2 × 10^{10} \textrm{cm}^{-2})$ implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre-doped with stable Mg during growth, and lowest (2–6%) in n-GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p- and n-GaN prolonged implantations cause interstitial $^{27}$Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 10$^{14}$ cm$^{-2}$ result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.

© 2015 Elsevier B.V. Although the formation of transition metal-boron pairs is currently well established in silicon processing, the geometry of these complexes is still not completely understood. We investigated the lattice location of the transition metals manganese, iron, cobalt and nickel in n- and p+-type silicon by means of electron emission channeling. For manganese, iron and cobalt, we observed an increase of sites near the ideal tetrahedral interstitial position by changing the doping from n- to p+-type Si. Such increase was not observed for Ni. We ascribe this increase to the formation of pairs with boron, driven by Coulomb interactions, since the majority of iron, manganese and cobalt is positively charged in p+-type silicon while Ni is neutral. We propose that breathing mode relaxation around the boron ion within the pair causes the observed displacement from the ideal tetrahedral interstitial site. We discuss the application of the emission channeling technique in this system and, in particular, how it provides insight on the geometry of such pairs. publisher: Elsevier articletitle: Drawing the geometry of 3d transition metal-boron pairs in silicon from electron emission channeling experiments journaltitle: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms articlelink: http://dx.doi.org/10.1016/j.nimb.2015.09.051 content_type: article copyright: Copyright © 2015 Elsevier B.V. All rights reserved. ispartof: Nuclear Instruments & Methods in Physics Research B vol:371 pages:59-62 ispartof: location:Opatija: CROATIA status: published

The lattice location of ion implanted radioactive 24Na (t1/2=14.96 h) in GaN and AlN was determined using the emission channeling technique at the ISOLDE/CERN facility. In the room temperature as-implanted state in both GaN and AlN, the majority of the sodium atoms are found on interstitial sites near the octahedral position, with a minority on cation Ga or Al substitutional sites. Following annealing at 800-900°C the interstitial fraction is reduced while the substitutional incorporation increases. Our results thus further establish the amphoteric character of Na in GaN and AlN, in analogy to the other light alkali Li, and alkaline earths Be and Mg. The site changes upon annealing are attributed to the onset of migration of interstitial Na, for which an activation energy of 2.2-3.4 eV is estimated in GaN and 2.0-3.1 eV in AlN, and its subsequent capture by cation vacancies resulting from the implan-tation. Comparison of the lattice site change behavior of Li, Be, Na and Mg shows that the onset of interstitial mi-gration correlates with the ionic radii of these elements.

A new approach to observe the radiative decay of the $^{229}$Th nuclear isomer, and to determine its energy and radiative lifetime, is presented. Situated at a uniquely low excitation energy, this nuclear state might be a key ingredient for the development of a nuclear clock, a nuclear laser and the search for time variations of the fundamental constants. The isomer's $\gamma$ decay towards the ground state will be studied with a high-resolution VUV spectrometer after its production by the $\beta$ decay of $^{229}$Ac. The novel production method presents a number of advantages asserting its competitive nature with respect to the commonly used $^{233}$U $\alpha$-decay recoil source. In this paper, a feasibility analysis of this new concept, and an experimental investigation of its key ingredients, using a pure $^{229}$Ac ion beam produced at the ISOLDE radioactive beam facility, is reported., Comment: 11 pages, 5 figures, 2 tables

In (Ga,Mn)As, a model dilute magnetic semiconductor, the electric and magnetic properties are strongly influenced by the lattice sites occupied by the Mn atoms. In particular, the highest Curie temperatures are achieved upon thermal annealing in a narrow temperature window around $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, by promoting the diffusion of interstitial Mn towards the surface. In this work, we determined the thermal stability of both interstitial and substitutional Mn in ferromagnetic (Ga,Mn)As thin films, using the emission channeling technique. At a higher Mn concentration, the temperatures at which substitutional and interstitial Mn become mobile not only decrease, but also become closer to each other. These findings advance our understanding of self-compensation in (Ga,Mn)As by showing that the strong dependence of the Curie temperature on annealing temperature around $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ is a consequence of balance between diffusion of interstitial Mn and segregation of substitutional Mn.

The current socio-political debates on the sustainability of mobility and the related increasing awareness of air quality and pollutant emissions, as well as the pursuit of stricter emissions legislation, especially under real driving conditions, present the current challenges in powertrain development. The optimization of energy efficiency and the reduction of pollutant emissions are here the two key targets. A promising approach for this is the targeted formulation (or reformulation) of operating fluids. In this case, fuels, fuel mixtures or engine oils are formulated with additional synthetic components, which are produced with environmentally sustainable methods and at the same time lead to the reduction of exhaust emissions.

$\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$ and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$ nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature M\"ossbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters $a=3.76(2)\phantom{\rule{4pt}{0ex}}\AA{}$ and $c=3.50(2)\phantom{\rule{4pt}{0ex}}\AA{}$, and a magnetic moment of 2.45(5) ${\ensuremath{\mu}}_{B}$ per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$ is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Fe}$ taking tetragonal distortion into account.

We have studied the influence of electronic doping on the preferred lattice sites of implanted $${^{61}\text{Co}}$$ , and the related stabilities against thermal annealing, in silicon. Using the $$\beta ^-$$ emission channeling technique we have identified Co on ideal substitutional (ideal S) sites, sites displaced from bond-centered towards substitutional (near-BC) sites and sites displaced from tetrahedral interstitial towards anti-bonding (near-T) sites. We show clearly that the fractions of Co on these lattice sites change with doping. While near-BC sites prevail in $$n^+$$ -type Si, near-T sites are preferred in $$p^+$$ -type Si. Less than $$\sim$$ 35% of Co occupies ideal S sites in both types of heavily doped silicon, showing that the majority of implanted Co forms complex defect structures. Implantation-induced defects seem to getter more efficiently Co in lightly doped n-type than in heavily doped $$n^+$$ - or $$p^+$$ -type silicon. The formation of CoB pairs in $$p^+$$ -type silicon and its possible influence on the lattice sites is discussed.

Radioactive $^{27}\mathrm{M}\mathrm{g}$ (${t}_{1/2}=9.5\text{ }\text{ }\mathrm{min}$) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission channeling. Following implantations between room temperature and $800\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the majority of $^{27}\mathrm{M}\mathrm{g}$ occupies the substitutional Ga sites; however, below $350\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ significant fractions were also found on interstitial positions $\ensuremath{\sim}0.6\text{ }\text{ }\AA{}$ from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped $p$ type with $2\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ stable Mg during epilayer growth, and lowest in Si-doped $n$-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above $350\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ converts interstitial $^{27}\mathrm{M}\mathrm{g}$ to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.

The magnetic and electric properties of impurities in semiconductors are strongly dependent on the lattice sites which they occupy. While the majority site can often be predicted based on chemical similarities with the host elements and is usually simple to confirm experimentally, minority sites are far more complicated to predict, detect and identify. We have carried out extensive β − emission channeling studies on the lattice location of transition metal impurities in wide-gap dilute magnetic semiconductors, namely Co and Mn in GaN and ZnO, making use of radioactive 61 Co and 56 Mn implanted at the ISOLDE facility at CERN. In addition to the majority occupation of cation (Ga, Zn) sites, we located significant fractions (of the order of 20%) of the Co and Mn impurities in anion (N, O) sites, which are virtually unaffected by thermal annealing up to 900 °C. Here, we present the β − emission channeling experiments on 61 Co-implanted GaN. We discuss these results in the context of our recent reports of minority anion substitution in Mn-implanted GaN Pereira et al. (2012) [19] and Mn/Co-implanted ZnO Pereira et al. (2011) [20] , particularly in terms of the advantages of the emission channeling technique in such cases of multi-site occupancy.

Using radioactive isotopes produced at the CERN-ISOLDE facility, the lattice location of the implanted transition metal (TM) ions $^56$Mn, $^59$Fe and $^65$Ni in n-type single-crystalline hexagonal 6H-SiC was studied by means of the emission channeling technique. TM probes on carbon coordinated tetrahedral interstitial sites ($T_C$) and on substitutional silicon sites ($S_{Si,h+k}$) were identified. We tested for but found no indication that the TM distribution on $S_Si$ sites deviates from the statistical mixture of 1/3 hexagonal and 2/3 cubic sites present in the 6H crystal. The TM atoms partially disappear from $T_C$ positions during annealing at temperatures between 500 °C and 700 °C which is accompanied by an increase on $S_Si$ and random sites. From the temperature associated with these site changes, interstitial migration energies of 1.7–2.7 eV for Mn and Ni, and 2.3–3.2 eV for Fe were estimated. TM lattice locations are compared to previous results obtained in 3C-SiC using the same technique.

Mn-doped Si has attracted significant interest in the context of dilute magnetic semiconductors. We investigated the lattice location of implanted Mn in silicon of different doping types (n, n+ and p+) in the highly dilute regime. Three different lattice sites were identified by means of emission channeling experiments: ideal substitutional sites; sites displaced from bond-centered towards substitutional sites and sites displaced from anti-bonding towards tetrahedral interstitial sites. For all doping types investigated, the substitutional fraction remained below ∼ 30%. We discuss the origin of the observed lattice sites as well as the implications of such structures on the understanding of Mn-doped Si systems. We have studied the influence of electronic doping on the preferred lattice sites of implanted 61Co, and the related stabilities against thermal annealing, in silicon. Using the beta- emission channeling technique we have identified Co on ideal substitutional (ideal S) sites, sites displaced from bond-centered towards substitutional (near-BC) sites and sites displaced from tetrahedral interstitial towards anti-bonding (near-T) sites. We show clearly that the fractions of Co on these lattice sites change with doping. While near-BC sites prevail in n+-type Si, near-T sites are preferred in p+-type Si. Less than ∼35% of Co occupies ideal S sites in both types of heavily doped silicon, showing that the majority of implanted Co forms complex defect structures. Implantation-induced defects seem to getter more efficiently Co in lightly doped n-type than in heavily doped n+- or p+-type silicon. The formation of CoB pairs in p+-type silicon and its possible influence on the lattice sites is discussed.

Silicon carbide doped with magnetic ions such as Fe, Mn, Ni or Co could make this wide band gap semiconductor part of the diluted magnetic semiconductor family. In this study, we report the implantation of 6H-SiC single crystals with magnetic $^{56}$Fe$^{+}$ ions with an energy of 150 keV. The samples were implanted with 5E14 Fe$^+$/cm$^{2}$ and 1E16 Fe$^+$/cm$^{2}$ at different temperatures to study the damage formation and lattice site location. The samples were subsequently annealed up to 1500°C in vacuum in order to remove the implantation damage. The effect of the annealing was followed by Rutherford Backscattering/Channeling (RBS/C) measurements. The results show that samples implanted above the critical amorphization temperature reveal a high fraction of Fe incorporated into regular sites along the [0001] axis. After the annealing at 1000°C, a maximum fraction of 75%, corresponding to a total of 3.8E14 Fe$^{+}$/cm$^{2}$, was measured in regular sites along the [0001] axis. A comparison is made between the observed annealing behavior and the one measured in low fluence (2E13 cm$^{−2}$) $\beta$- emission channeling experiments using the radioactive isotope $^{59}$Fe (t$_{1/2}$ = 45 d), where the sample was implanted at room temperature and the $\beta$- emission channeling yield was measured by means of a position-sensitive detector.

A GaN thin film was implanted with 5 × 1014 cm−2 of 60 keV stable 166Er, followed by the implantation of 2 × 1013 cm−2 radioactive 167Tm (t1/2 = 9.3 d) and an annealing sequence up to 900 °C. The emission channeling (EC) technique was applied to assess the lattice location of Er following the Tm decay from the conversion electrons emitted by 167mEr, which showed that more than 50% of 167mEr occupies substitutional Ga sites. The results are briefly compared to a 167mEr lattice location experiment in a GaN sample not pre-implanted with 166Er. In addition, high-resolution X-ray diffraction (HRXRD) was used to characterize the perpendicular strain in the high-fluence implanted film. The HRXRD experiments showed that the Er implantation resulted in an increase of the c-axis lattice constant of the GaN film around 0.5–0.7%. The presence of significant disorder within the implanted region was corroborated by the fact that the EC patterns for off-normal directions exhibit a pronounced angular broadening of the order of magnitude of 0.5°.

B cell-derived chronic lymphocytic leukemia (CLL) is an incurable disease that requires innovative therapeutic regimens. Experimental approaches of immunotherapy aiming at induction of systemic T-cell responses have been developed. Trioma cells provide a potent vaccine derived from malignant B cells that allows multiple antigens (Ags) from the parental tumor to be ingested by Ag-presenting cells. Like other strategies using modified whole tumor cells, this approach induces polyvalent responses. Using trioma cell-pulsed dendritic cells (DCs) for T-cell activation in vitro, we asked whether specific Ags overexpressed by CLL can be identified as target structures of such responses and what is the nature of these Ags. Expression levels of several genes in CLL samples were quantitated by reverse transcriptase-polymerase chain reaction. T lymphocytes were polyvalently stimulated by trioma-pulsed DCs and specificities were tested by determining cytokine secretion in the presence of target cells transfected with RNA coding for those Ags that were found to be overexpressed. We demonstrate that DCs pulsed with the modified tumor cells efficiently activate T lymphocytes against CLL and that overexpressed Ags related to leukemogenesis, such as BCL-2, MDM2, and ETV5, serve as targets for those T cells. Immune escape by Ag loss or mutation is less likely to occur if immunity is directed against altered self-proteins that are involved in malignant transformation. Therefore, vaccines based on modified tumor cells such as triomas hold promise for immunotherapy of CLL and other malignancies. Polyvalent vaccines originally designed as individualized therapeutics may be more broadly applicable, at least in patients showing similar Ag patterns.

Recovery of point defects and local phase transitions were studied in the neighbourhood of implanted 111mCd (low fluence of 1.5 × 1012 at/cm2) into undoped and 2%Nb-doped SrTiO3 single crystals by means of the perturbed angular correlation technique. Measurements in the as implanted state and after subsequent 720 °C and 1000 °C 20 min air annealings have shown the recovery of defects in the Cd neighbourhood. After annealing, low temperature experiments have studied the cubic-to-tetragonal and the orthorhombic-to-rhombohedral phase transitions at the atomic scale. In the undoped samples it was found at 77 K, the coexistence of cubic and tetragonal single oriented phases. By further cooling the samples to 10 K a single low – symmetry phase is formed. On Nb-doped samples the phase transitions are hindered, being the lattice symmetries also affected in the Cd atomic neighbourhood. These effects appear to be particularly dependent on the dopant concentration.

Radioactive 73As ions were implanted into a ZnO single crystal at room temperature with 60 keV up to a fluence of 2×1013 cm−2. Subsequently, the angular emission channeling patterns of emitted conversion electrons were recorded by means of a position-sensitive detector in the as-implanted state and following annealing up to 900 ∘C, and were compared to simulated emission yields for a variety of different lattice sites. We find that As does not occupy substitutional O sites, but mainly occupies the substitutional Zn sites. The fraction of As on O sites was at most a few per cent. Arsenic in ZnO is thus an interesting example of an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. Possible consequences with respect to the role of arsenic as a p-type dopant in ZnO are being discussed.

Technical progress on silicon pad electron detectors, currently used in emission channelling experiments to study lattice location of radioactive dopants and impurities in single crystals, is reported. Noise and trigger efficiency improvements are achieved by using 500 μm and 1 mm thick detectors coupled to a cooled readout system. The static properties, noise, gamma ray and electron trigger efficiency and energy resolution for different temperatures under air and vacuum were measured. The advantages of the future implementation of 1 mm silicon pad detectors with cooled self-triggering readout chips are discussed.

We have used the emission channeling technique in order to study the lattice sites of radioactive 59 Fe and 67 Cu following 60 keV ion implantation into Si single crystals at fluences around 10 12 –10 14 cm −2 . We find that in the room temperature as-implanted state in high-resistivity Si both Fe and Cu occupy mainly lattice sites displaced around 0.05 nm (0.5 A) from substitutional positions. Both are released from these positions during annealing at temperatures between 300 °C and 600 °C. Fe is then found mainly on near-tetrahedral interstitial sites and further annealing causes it to be increasingly incorporated on ideal substitutional sites, on which it is stable to around 800 °C. We have strong indications that during annealing around 600 °C, along with the dominance of interstitial Fe, a redistribution towards the surface takes place, suggesting that the subsequent formation of ideal substitutional Fe may be related to the trapping of Fe at R p /2, half of its implanted depth. Possible R p /2 trapping might also have taken place in our Cu experiments but appears to be less efficient since Cu tended to escape to the bulk of the samples.

GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared.;According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties.;Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity.;We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.

ZnO substrates and films were intentionally implanted with rare earth and transition metal ions. The influence of the implantation and subsequent air thermal annealing treatments on the structural and optical properties of ZnO samples were studied by using Rutherford backscattering spectrometry and low temperature photoluminescence techniques. Intraionic Tm-related emission was observed for bulk and ZnO films. Similarly, Eu and Tb-doped ZnO films follow the same trend observed in bulk samples. No intraionic related emission was observed for Eu-doped samples even being the ion in Zn sites and for the Tb-doped samples ion segregation was observed for thermal annealing temperatures above 800 °C. For the Mn-doped ZnO bulk samples the lattice recovery follows the same trend to that one observed for the Fe-doped samples, starting near 800 °C being fully recovered at 1050 °C. Although Fe3+ was observed no intraionic Mn-related emission was detected under the used conditions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission electron microscopy (TEM) structural investigations with atomic resolution have been carried out on different rare earth (RE) doped GaN samples. RE doping was done by two techniques: ion implantation on MOCVD substrates and in-situ by molecular beam epitaxy (MBE). In both cases typical stacking faults (SFs) are formed, there are shown to correlate with a possible RE segregation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The local environment of implanted 111Ag (t 1/2 = 7.45 d) in single-crystalline [0001] ZnO was evaluated by means of the perturbed angular correlation (PAC) technique. Following the 60 keV low dose (1 × 1013 cm−2) 111Ag implantation, the PAC measurements were performed for the as-implanted state and following 30 min air annealing steps, at temperatures ranging from 200 to 1050 °C. The results revealed that 42% of the probes are located at defect-free SZn sites (υ Q ∼ 32 MHz, η = 0) in the as-implanted state and that this fraction did not significantly change with annealing. Moreover, a progressive lattice recovery in the near vicinity of the probes was observed. Different EFGs assigned to point defects were furthermore measured and a general modification of their parameters occurred after 600°C. The 900°C annealing induced the loss of 30% of the 111Ag atoms, 7% of which were located in regions of high defects concentration.

GaN epilayers grown by metal organic chemical vapor deposition (MOCVD) were implanted with Tm and Eu ions with different energies and fluences and at different temperatures in order to optimize the implantation conditions. The recovery of the implantation damage was studied using both rapid thermal annealing and furnace annealing with nitrogen overpressure of 4 x 10(5) Pa. Rutherford backscattering spectrometry in the channeling mode (RBS/C) was used to monitor the evolution of damage introduction and recovery in the Ga-sublattice and transmission electron microscopy (TEM) was carried out for further structural analysis. The RBS/C spectra as well as TEM images show two different damage regions, one at the surface arising from an amorphous surface layer and another one deeper in the crystal coinciding with the end of range of the implanted ions. For implantation with 150 keV at room temperature, even for fluences as low as 3 x 10(14) at/cm(2), a thin amorphous surface layer, which becomes thicker with increasing implantation fluence, was observed by TEM. High temperature annealing of these highly damaged layers often results in loss of the amorphous layer and accumulation of the implanted species at the surface rather than a regrowth of the crystal. It was possible to prevent the formation of an amorphous layer by implanting at 500 degreesC. In those samples a large part of the lattice damage was removed during annealing at 1000 degreesC and the recovery of the lattice is similar for both applied annealing methods. (C) 2004 Elsevier Ltd. All rights reserved.

Single crystalline BaTiO 3 films have been grown by pulsed laser deposition on SrTiO 3 . Radioactive 111 In ions were implanted with energies of 60 and 400 keV, respectively. The lattice site of 111 In has been determined by emission channeling of conversion electrons emitted after the nuclear decay to 111 Cd. Besides a large random fraction, 11(3)% of the events show that In substitutes Ti-sites directly after the implantation. The recovery of the lattice after the implantation has been monitored in the direct vicinity of the probes by perturbed angular correlation (PAC) spectroscopy in a wide temperature range. After annealing in air at a temperature of 1420 K for 10 min, PAC spectroscopy yields a unique electric field gradient at the site of the 111 In( 111 Cd)-probe with a quadrupole coupling constant of 28.7(5) MHz and a frequency distribution with a width of 4.2(2) MHz. The film starts to degrade at a temperature of 1520 K. Scanning electron microscopy and energy dispersive X-ray spectroscopy investigations have been used to monitor the degradation.

ZnO [0 0 0 1] single-crystals were implanted at room temperature with 150 keV Tm+ ions at a fluence of 5×1015 cm−2. Each sample was then subjected to one single 30 min air annealing at 800°C, 950°C and 1050°C. The Tm lattice site location and defect recovery were investigated with Rutherford Backscattering/Channeling Spectroscopy. Detailed angular scans along the [0 0 0 1] direction show that 94% of the Tm ions occupy substitutional Zn sites (SZn) in the as-implanted sample. All the annealing temperatures lead to a reduction of this fraction to 30%. Also, progressive damage recovery and Tm segregation to the surface were observed, being enhanced at 1050°C. Photoluminescence (PL) studies with above band gap excitation performed on these samples revealed no luminescence on the as-implanted state. The 800°C air annealing promotes the Tm3+ optical activation and a well-defined near-infrared intraionic emission is observed. For higher annealing temperatures, in spite of no change of the Tm fraction at SZn sites, a decrease of the Tm intraionic emission was observed. These results suggest that optical activation of Tm ions is related with the defect density in their environment.

This paper reviews the current knowledge on rare earths (REs) implanted into GaN with a special focus on their lattice location and on the optical activation by means of thermal annealing. While emission channeling experiments have given information on the lattice location of rare earths following low-dose (≈10 13 cm −2 ) implantation, both in the as-implanted state and after annealing up to 900 °C, the lattice location of higher-dose implants (10 14 –10 15 cm −2 ) and their defect annealing behaviour were studied using the Rutherford backscattering/channeling (RBS/C) method. The available channeling and luminescence results suggest that the optical activation of implanted REs in GaN is related to their incorporation in substitutional Ga sites combined with the effective removal of the implantation damage.

In this work we report on the lattice location of implanted Ag in ZnO single-crystals, evaluated by means of the emission channeling technique. Following 60 keV low dose (2 $\times 10^{13}$ cm$^{-2}$) ion implantation, the $\beta^{-}$-emission patterns from $^{111}$Ag were monitored with a position-sensitive detector as a function of annealing temperature up to 800$^\circ$C in vacuum. Our experiments revealed that in the as-implanted state around 30% of the Ag atoms are substitutional at the Zn site with root mean square displacements around 0.17-0.28 . Though this fraction did not change with increasing annealing temperature, upon annealing at 600°C the root mean square displacement of Ag from the Zn site increased considerably, followed by partial outdiffusion during 800°C annealing.

The lattice site location of excited states of 149 Eu was studied by means of the emission channeling technique. The 60 keV implantation of 149 Tb into a GaN thin film was performed at room temperature up to a dose of 2.0×1013 cm−2. This radioactive isotope eventually decays into short-lived excited states of 149 Eu . The conversion electrons emitted in the subsequent decay to the 149 Eu ground state were detected with a position sensitive detector. We measured their angular distributions around the [0 0 0 1], [ 1 1 0 2 ], [ 1 1 0 1 ] and [ 2 1 1 3 ] axes in the as-implanted state and after 600 and 900 °C vacuum annealing. Already in the as-implanted state around 65% of 149∗ Eu atoms were found on substitutional Ga sites. The root mean square (rms) displacements from the substitutional sites in the as-implanted state were found to be around 0.13–0.16 A. Annealing up to 600 °C increased the substitutional fraction by a few percent and slightly reduced the rms displacements, most likely due to the removal of crystal defects in the vicinity of the Eu atoms, resulting in a better incorporation into substitutional sites.

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As.

In search for dilute magnetic semiconductors, the magnetic properties at the atomic-scale of Fe atoms incorporated in ZnO, in a concentration range of more than five orders of magnitude from 1 x 10(-5) to 2.2 at% have been probed using emission 57 Fe Mossbauer spectroscopy on implanted Mn-57 and Co-57 produced at ISOLDE/CERN. In the ultra-dilute regime (10(-5) at%), the system shows isolated paramagnetic Fe3+ ions with a spin-lattice type of relaxation. At higher concentrations (between 0.02 and 0.2 at%) a transition to spin-spin type of relaxation between neighboring Fe3+ is observed, without any signature of magnetic ordering up to 2.2 at%. Despite the many reports of dilute magnetism in 3d-doped ZnO, this atomic level study shows no evidence of any long-range magnetic ordering between isolated Fe atoms incorporated in the ZnO lattice.

The lattice location of implanted silver in Si was studied by means of the emission channeling technique. Following 60 keV room temperature implantation of radioactive $^{111}$Ag at a dose of 2-3 $\times 10^{12}$ cm$^{-2}$, we identify around 30% of Ag on near-substitutional sites (around 0.45 from ideal S-sites). Upon annealing at 200-300°C, the fraction on near-S sites reaches a maximum around 60-80%. For higher annealing temperatures it decreases again and at 600°C Ag starts to diffuse out of the Si samples. We estimate the activation energy for the dissociation of near-substitutional Ag to be 1.8-2.2 eV. The experimental results are compared to those of Cu in Si, and common features and characteristic differences in the behavior of the two group 1B metals are discussed.

We report on the lattice location of Er in SrTiO$_{3}$ single crystals using the emission channeling technique. The angular distribution of conversion electrons emitted from $^{167m}$Er(T$_{1/2}$=2.27 s) was monitored with a position-sensitive detector following the room-temperature 60 keV implantation of the precursor isotope $^{167}$Tm(T$_{1/2}$=9.25 d) to a dose of 2$\times$ 10$^{12}$ at./cm$^{2}$. The results for the sample annealed in vacuum at 610°C for 15 min provide direct evidence that Er occupies both Sr and Ti substitutional lattice sites. In addition, thermal recovery of lattice damage was also studied with RBS/C for SrTiO$_{3}$ implanted to doses of 5$\times$ 10$^{14}$ and 5 $\times$ 10$^{15}$ Er/cm$^{2}$. We further comment on preliminary photoluminescence results from these samples.

We have investigated the lattice location of implanted transition metal (TM) 56Mn, 59Fe and 65Ni ions in undoped single-crystalline cubic 3C–SiC by means of the emission channeling technique using radioactive isotopes produced at the CERN-ISOLDE facility. We find that in the room temperature as-implanted state, most Mn, Fe and Ni atoms occupy carbon-coordinated tetrahedral interstitial sites (TC). Smaller TM fractions were also found on Si substitutional (SSi) sites. The TM atoms partially disappear from ideal-TC positions during annealing at temperatures between 500 °C and 700 °C, which is accompanied by an increase in the TM fraction occupying both SSi sites and random sites. An explanation is given according to what is known about the annealing mechanisms of silicon vacancies in silicon carbide. The origin of the observed lattice sites and their changes with thermal annealing are discussed and compared to the case of Si, highlighting the feature that the interstitial migration of TMs in SiC is much slower than in Si.

We give an overview on the historical development and current program for lattice location studies at CERN’s ISOLDE facility, where the EC-SLI (Emission Channeling with Short-Lived Isotopes) collaboration maintains several setups for this type of experiments. We illustrate that the three most decisive factors for the success of the technique are access to facilities producing radioactive isotopes, position-sensitive detectors for the emitted decay particles, and reliable simulation codes which allow for quantitative analysis.

This paper reports on recent advances in the application of electron emission channeling to studies of the lattice location of radioactive probe atoms in semiconductors. The introduction of position-sensitive electron detection has resulted in an improvement in the experimental efficiency of this method by two orders of magnitude. Electron emission channeling now is able to carry out detailed lattice location studies, and is especially interesting for cases where conventional ion beam lattice location techniques cannot be applied due to a lack of sensitivity. Characteristic features of electron emission channeling with position-sensitive detection are discussed and illustrated by results on the lattice location of Er in Si and GaAs, and on the lattice sites and stability of implanted transition metals in Si.

Recent results on the study of high-TC superconductors using the e ‐ perturbed angular correlation technique are presented. The basic features of the experimental equipment and its installation at the ISOLDE facility are briefly described. Results obtained from 197m Hg implanted into high quality Y1Ba2Cu3O6+ epitaxy thin films are presented and discussed.

We have implanted the radioactive probe atom 67 Cu (t1/2=61.9 h) into single-crystalline Si. Monitoring the β− emission yield from the decay of 67 Cu to 67 Zn as a function of angle from different crystallographic directions allows to determine the lattice location of the Cu atoms by means of the emission channeling effect. We give direct evidence that the majority of implanted Cu occupies near-substitutional sites. As most-likely lattice location we suggest a displacement of 0.51(7) A along 〈1 1 1〉 directions from substitutional sites to bond center positions. The annealing behavior shows that near-substitutional Cu is remarkably stable, and we estimate a dissociation energy of 2.2(3) eV.

We present emission channeling experiments on the lattice location of Er in CZ Si single crystals with a well-defined O concentration of 6.5-6.6 $\times 10^{17}$ cm$^{-3}$ and 60 keV-implanted Tm+Er doses ranging from 4.3 $\times 10^{12}$ cm$^{-2}$ to 3.6 $\times 10^{13}$ cm$^{-2}$. The experimental results are compared to the predictions of a simulator which models the formation of Er$_{n}$O$_{m}$ clusters on the basis of simple diffusion and capture kinetics. We find that our experimental data compare favorably with a scenario where the formation of Er$_{n}$O$_{m}$ defects with one or more O atoms is responsible for removing the Er atoms from their tetrahedral interstitial (T) sites. This suggests that Er does no longer occupy the T site even in simple (ErO) pairs.

The implantation sites and diffusion of Li in as-grown and Se-rich ZnSe has been studied with emission channeling measurements on the α-particles emitted in the decay of radioactive 8 Li probes implanted in single-crystal samples. At implantation temperatures below 180 K the Li atoms are immobile at tetrahedral interstitial sites, but a site change sets in at 200 K, and above 250 K the Li atoms occupy mainly substitutional sites. In Se-rich ZnSe the interstitial Li fraction ( f T ) shows partial recovery between 260 and 360 K, a phenomenon which may be attributed to Li trapping at interstitial Se. Implantation dose, however, has a strong influence on the observed channeling and blocking patterns. Maximum f T is achieved at T ⩾400 K where the fraction at interstitial sites is an order of magnitude lower. At higher temperatures (≈ 550 K) the Li atoms dissociate from their substitutional sites and diffuse to the surface in the Se-rich sample and to extended defects in the implantation region in the as-grown sample.

Epitaxial, hexagonal rare-earth silicides, such as ErSi1.7, can be formed using channeled ion beam synthesis. In the case of Gd-silicide, an orthorhombic GdSi2 phase exists at high temperature; the transition temperature is related to the thickness and crystalline quality of the silicide. In the case of the lightest rare-earth metals, such as Nd, silicides only exist in a tetragonal or orthorhombic phase, which cannot grow epitaxially on Si(1 1 1). However, introduction of a fraction of yttrium (YSi1.7 also possesses the aforementioned hexagonal lattice) drives the Nd–Si system into a hexagonal lattice structure. A combined backscattering and channeling spectrometry (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) study shows that an epitaxial, continuous ternary silicide is formed (and not a mixture of binaries) with a hexagonal structure, which is stable up to 950°C. Further annealing, however, results in a gradual transformation into polycrystalline phases. The experimental results are compared to total energy calculations of these (meta-)stable rare-earth silicides, using the density functional theory (DFT).