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We have studied the lattice location of implanted nickel in silicon, for different doping types (n, n, and p). By means of on-line emission channeling, 65Ni was identified on three different sites of the diamond lattice: ideal substitutional sites, displaced bond-center towards substitutional sites (near-BC), and displaced tetrahedral interstitial towards anti-bonding sites (near-T). We suggest that the large majority of the observed lattice sites are not related to the isolated form of Ni but rather to its trapping into vacancy-related defects produced during the implantation. While near-BC sites are prominent after annealing up to 300-500 °C, near-T sites are preferred after 500-600 °C anneals. Long-range diffusion starts at 600-700 °C. We show evidence of Ni diffusion towards the surface and its further trapping on near-T sites at the Rp/2 region, providing a clear picture of the microscopic mechanism of Ni gettering by vacancy-type defects. The high thermal stability of near-BC sites in n-type Si, and its importance for the understanding of P-diffusion gettering are also discussed. © 2014 AIP Publishing LLC. ispartof: Journal of Applied Physics vol:115 issue:2 pages:023504-1 status: published

We report on the lattice location and thermal stability of Fe in n+- and p+-type silicon. By means of emission channeling we have observed Fe on ideal substitutional sites, sites located in between bond-centered (BC) and substitutional sites, and sites displaced from tetrahedral towards anti-bonding sites. Here, we focus our analysis on the identification of Fe displaced 0.4-0.6 Å from BC sites and the influence of the doping on the stability of these sites. Fe on near-BC sites is found to be more thermally stable in n+-type Si than in low doped or p+-type Si, and seems to be related to multiple vacancy defects. We suggest that the complexes which trap Fe near BC sites, as well as the formation of substitutional Fe, may play a crucial role in P-diffusion gettering.

Oxygen measurements using tunable diode laser absorption spectroscopy were performed at a full scale waste incineration facility. A comparative analysis to a reference method allowed identifying benefits and challenges inherent to the laser method.

We have investigated the magnetic properties of Ni nanoaggregates produced by ion implantation in ZnO single crystals. Several deviations from classical models usually adopted to describe the magnetic properties of nanoparticle systems were found. The strain between host and Ni nanoaggregates induces a magnetic anisotropy with a preferred direction. We show that these anisotropy effects can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among the nanoaggregates similar to that of an oriented, interacting nanocrystal ensemble.

It was recently shown that thin films of high-quality rare-earth (RE) silicides can be formed by high-dose implantation of RE-metals into Si under channeling conditions, whereas it is impossible to form continuous RESi layers when using conventional non-channeled (7° tilted) implantation. This different behavior can be explained to a large extent by the differences in projected range, damage density and sputtering yield between channeled and non-channeled implantation, as is shown by comparison of experimental results to model calculations of the initial implantation process using the binary collision code MARLOWE.

The emission channeling and blocking technique is a direct method for lattice location of radioactive atoms in single crystals. In the case of α-emitting isotopes, position-sensitive detection of emitted particles has provided an efficient means of carrying out a large number of experiments, and Monte Carlo simulations of the channeling effect allow for rather accurate identification and quantitative determination of occupied lattice sites. This work reviews recent results on the lattice sites of Li in elemental and III–V semiconductors, including Si, GaAs, GaP, InP and InSb, obtained by emission channeling and blocking following ion implantation of 8Li at temperatures between 30 and 700 K. Relevant properties of Li in these semiconductors are also briefly reviewed, and emission channeling and blocking is discussed in relation to other experimental methods which give direct information on lattice sites and atomic configurations of defects in semiconductors. The experimental methods described in this work are also suitable to study other probe-atom-host-crystal systems (possible examples are Li and Na in II–VI semiconductors or diamond, Er and other rare-earth elements in semiconductors or metals, etc.) and an outlook on ongoing and future experiments is given.

As the intrinsic origin of the high-temperature ferromagnetism often observed in wide-gap dilute magnetic semiconductors becomes increasingly debated, there is a growing need for comprehensive studies on the single-phase region of the phase diagram of these materials. Here we report on the magnetic and structural properties of Fe-doped ZnO prepared by ion implantation of ZnO single crystals. A detailed structural characterization shows that the Fe impurities substitute for Zn in ZnO in a wurtzite Zn(1-x)Fe(x)O phase which is coherent with the ZnO host. In addition, the density of beam-induced defects is progressively decreased by thermal annealing up to 900 ° C, from highly disordered after implantation to highly crystalline upon subsequent annealing. Based on a detailed analysis of the magnetometry data, we demonstrate that isolated Fe impurities occupying Zn-substitutional sites behave as localized paramagnetic moments down to 2 K, irrespective of the Fe concentration and the density of beam-induced defects. With increasing local concentration of Zn-substitutional Fe, strong nearest-cation-neighbor antiferromagnetic interactions favor the antiparallel alignment of the Fe moments. ispartof: Journal of Physics - Condensed Matter vol:25 issue:41 ispartof: location:England status: published

Magnetic nanoparticles were produced in ZnO single crystals using ion implantation of Ni along the [0001] channelling direction of ZnO. The particles were identified by X-ray diffraction and magnetization measurements as a distribution of superparamagnetic nickel nanoparticles having diameters in the range 2 − 3 nm. The depth distribution and size of the particles were determined using Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). The obtained results agree with magnetization and X-ray diffraction (XRD) data. From the determined depth distribution, the density change of implanted region of the material modified by the implantation procedure was estimated.

High dose 166Er or 160Gd implantations are used to form rare-earth (RE) silicides in Si. After implanting 0.8−2.0 × 1017 at./cm2 with 90 keV into Si(111) substrates kept at ∼ 450 to 530°C, we found that using conventional non-channeled implantation (tilted over 7°), it is impossible to form a continuous RESi1.7 layer. On the contrary, using channeled implantation, a continuous epitaxial ErSi1.7 layer with very good crystalline quality can be synthesized; a lowest χmin value of 1.5% for a surface ErSi1.7 layer is obtained. This different behaviour is explained using a model based on the difference in implantation depth, defect density and sputtering yield between random and channeled implantation, and the results are compared with Monte Carlo simulations. Such a high-quality RESi1.7/Si system offers a rare opportunity to study the structure, orientation and strain comprehensively using Rutherford backscattering and channeling spectrometry, X-ray diffraction and TEM. We found that the azimuthal orientation of the hexagonal RESi1.7 layer to the cubic Si substrate is RESi1.7[0001]/t|Si[111] and RESi1.7{11 2 0}/t|Si{110}. It is further observed that the ErSi1.7 epilayer is compre strained and quasi-pseudomorphic. In the case of GdSi1.7, the most difficult rare-earth silicide to form, and enhanced stabilization of the hexagonal over the orthorhombic phase is observed.

The alpha-emission channeling and blocking technique is a direct method for lattice site determination of radioactive atoms in single crystals. Position-sensitive detection of emitted alpha-particles provides an efficient means of carrying out such experiments at very low doses (10(10)-10(11) cm(-2) implanted probe atoms per spectrum), Comparison of the experimental data with Monte Carlo simulations of complete two-dimensional channeling patterns (e,g. +/-2 degrees around [100], [110] and [211] axes, which also includes all relevant planar directions) allows for straightforward identification and quantitative determination of occupied lattice sites, while at the same time the energy spectrum of emitted alpha-particles gives information on the probe atom depth distribution. We illustrate this for the case of ion implanted Li-8 (t(1/2) - 0.8 s) in Ge, where we identify mainly tetrahedral Li at room temperature, and bond-centered Li at slightly elevated temperature.

Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300°C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300°C.

We report on optical and structural analysis of Tm implanted ZnO [0001] single crystals. The samples were implanted at room temperature with 150 keV Tm+ ions with a nominal fluence of 5x1016 Tm+/cm2 and subsequently air annealed for 30 min at 800 oC, 900 oC and 950 oC. The implantation damage and annealing effects were investigated with Rutherford Backscattering/Channelling Spectroscopy. We observe that following implantation the majority of Tm ions are incorporated on Zn sites. The optical properties of as-implanted and annealed samples have been studied by low temperature photoluminescence measurements. Well defined Tm-related near infrared emission were observed upon above band gap excitation and the data are consistent with the presence of multi Tm-related optical centers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using the electron emission channeling and Rutherford backscattering/ channeling techniques, the influence of the mosaicity of a thin film on the axial channeling of charged particles was investigated. It is found that crystal mosaicity leads to a broadening and a degradation of the experimental channeling effects. This phenomenon, which is shown to be of major importance when assessing the lattice site of impurities in a single crystal, can be modeled quantitatively by using the mosaic tilt and twist components derived from X-ray rocking curve scans. As a case study, we illustrate that our approach allows to accurately determine the lattice site of Er in AlN, despite the significant influence of mosaicity on the channeling measurements. © 2013 AIP Publishing LLC. ispartof: Applied Physics Letters vol:103 issue:17 pages:172108- status: published

The lattice site occupation of oversized atoms, implanted in Ni and Fe at 60, 180 and 293 K has been studied extensively using the α-emission channeling technique. Low dose (

The lattice sites of ion implanted Li-8 in Si and Ge were studied using the alpha-emission channeling technique. In both materials tetrahedral interstitial sites were found to be the major occupied lattice sites following room temperature implantation (60-80% in Si and 40-60% in Gel. Depending on the doping character of the sample, up to 55% of Li was found on bond-center sites in Si after implantations between 425 K and 475 K and up to 40% in Ge for implantations between 325 K and 400 K at higher temperatures Li occupied increasingly random lattice sites. The lattice site changes from tetrahedral to bond-center sites are attributed to the onset of interstitial Li diffusion and its capture by additional defects, which are likely to be vacancy-type defects from the implantation process. The trapping of Li at these defects inhibits its long-range diffusion.

We report on ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission channeling experiments on the lattice location and thermal stability of Mn impurities ($l$0.05$%$) in both semi-insulating and heavily $n$-type doped GaAs. In addition to the majority of the Mn impurities substituting for Ga, up to 30$%$ occupy tetrahedral interstitial sites with As nearest neighbors. Whereas the interstitial fraction is stable up to 400${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C, with an activation energy for diffusion of 1.7\char21{}2.3 eV, substitutional Mn diffuses only at $\ensuremath{\sim}$700${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C with an activation energy of $\ensuremath{\sim}$3 eV. By comparing these results to those of recent emission channeling experiments on heavily $p$-type doped GaAs [L. M. C. Pereira et al., Appl. Phys. Lett. 98, 201905 (2011)], we conclude that the observed high thermal stability of the interstitial fraction cannot be ascribed to trapping by charged defects, but is an intrinsic characteristic of isolated interstitial Mn in the low doping regime ($l$0.05$%$ Mn). Compared to ferromagnetic Ga${}_{1\ensuremath{-}x}$Mn${}_{x}$As (few percent Mn), for which a significantly lower activation energy has been reported, these findings motivate a comprehensive assessment of how the thermal stability and the diffusion of interstitial Mn are affected by the Mn concentration.

We report on the lattice location of Mn in wurtzite GaN using ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission channeling. In addition to the majority substituting for Ga, we locate up to 20$%$ of the Mn atoms in N sites. We propose that the incorporation of Mn in N sites is enabled under sufficiently high concentrations of N vacancies, and stabilized by a highly charged state of the Mn cations. Since N substitution by Mn impurities in wurtzite GaN has never been observed experimentally or even considered theoretically before, it challenges the current paradigm of transition metal incorporation in wide-gap dilute magnetic semiconductors.

Doping of semiconductors by ion implantation usually requires implantation doses below 1013 cm−2 to obtain typical impurity concentrations of

We have studied the interaction of Li and implantation defects using the emission channeling technique after implantation of 60 keV 8Li ions into n- and p-InSb. Emission channeling patterns of α-particles emitted in the nuclear decay of 8Li ( t 1 2 = 838 ms ) were measured for temperatures between 50 and 600 K. A quantitative determination of Li site fractions was achieved by comparing the experimentally observed channeling and blocking patterns to calculated emission channeling effects from Monte Carlo simulations. Below 130 K about 60% of the Li atoms occupy tetrahedral interstitial sites. Between 130 and 200 K a site change to substitutional sites is observed for p-InSb as well as n-InSb, resulting in substitutional fractions of Li of about 60–70%. Substitutional Li is stable up to about 425 K in p-InSb and 475 K in n-InSb. From these temperatures we have estimated dissociation energies of 1.1 and 1.2 eV, respectively. For higher temperatures out-diffusion of Li to the surface is observed. The behavior of Li in InSb was found to be similar to the other III–V semiconductors GaAs, GaP and InP. Whether diffusion of Li or the mobilization of vacancy defects is the dominant process responsible for the observed lattice site changes is discussed.

The physical properties of an impurity atom in a semiconductor are primarily determined by the lattice site it occupies. In general, this occupancy can be correctly predicted based on chemical intuition, but not always. We report on one such exception in the dilute magnetic semiconductors Co- and Mn-doped ZnO, experimentally determining the lattice location of Co and Mn using ${\ensuremath{\beta}}^{\ensuremath{-}}$-emission channeling from the decay of radioactive ${}^{61}$Co and ${}^{56}$Mn implanted at the ISOLDE facility at CERN. Surprisingly, in addition to the majority substituting for Zn, we find up to 18% (27%) of the Co (Mn) atoms in O sites, which is virtually unaffected by thermal annealing up to 900${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C. We discuss how this anion site configuration, which had never been considered before for any transition metal in any metal oxide material, may in fact have a low formation energy. This suggests a change in paradigm regarding transition-metal incorporation in ZnO and possibly other oxides and wide-gap semiconductors.

We report on the lattice location of Mn in heavily p-type doped GaAs by means of $\beta^{-}$-emission channeling from the decay of $^{56}$Mn. The majority of the Mn atoms substitute for Ga and up to 31% occupy the tetrahedral interstitial site with As nearest neighbors. Contrary to the general belief, we find that interstitial Mn is immobile up to 400$^{\circ}$C, with an activation energy for diffusion of 1.7–2.3 eV. Such high thermal stability of interstitial Mn has significant implications on the strategies and prospects for achieving room temperature ferromagnetism in Ga$_{1−x}$Mn$_{x}$As.

In this paper we review materials characterization techniques using radioactive isotopes at the ISOLDE/CERN facility. At ISOLDE intense beams of chemically clean radioactive isotopes are provided by selective ion-sources and high-resolution isotope separators, which are coupled on-line with particle accelerators. There, new experiments are performed by an increasing number of materials researchers, which use nuclear spectroscopic techniques such as Mössbauer, perturbed angular correlations (PAC), β-NMR and emission channeling with short-lived isotopes not available elsewhere. Additionally, diffusion studies and traditionally non-radioactive techniques as deep level transient spectroscopy, Hall effect and photoluminescence measurements are performed on radioactive doped samples, providing in this way the element signature upon correlation of the time dependence of the signal with the isotope transmutation half-life. Current developments, applications and perspectives of using radioactive ion beams and techniques in solid state and biophysics research are presented with a few examples.

The ISOLDE isotope separator facility at CERN provides a variety of radioactive ion beams, currently more than 800 different isotopes from ∼70 chemical elements. The radioisotopes are produced on-line by nuclear reactions from a 1.4 GeV proton beam with various types of targets, outdiffusion of the reaction products and, if possible, chemically selective ionisation, followed by 60 kV acceleration and mass separation. While ISOLDE is mainly used for nuclear and atomic physics studies, applications in materials science and biophysics account for a significant part (currently ∼15%) of the delivered beam time, requested by 18 different experiments. The ISOLDE materials science and biophysics community currently consists of ∼80 scientists from more than 40 participating institutes and 21 countries. In the field of materials science, investigations focus on the study of semiconductors and oxides, with the recent additions of nanoparticles and metals, while the biophysics studies address the toxicity of metal ions in biological systems. The characterisation methods used are typical radioactive probe techniques such as Mossbauer spectroscopy, perturbed angular correlation, emission channeling, and tracer diffusion studies. In addition to these “classic” methods of nuclear solid state physics, also standard semiconductor analysis techniques such as photoluminescence or deep level transient spectroscopy profit from the application of radioactive isotopes, which helps them to overcome their chemical “blindness” since the nuclear half life of radioisotopes provides a signal that changes in time with characteristic exponential decay or saturation curves. In this presentation an overview will be given on the recent research activities in materials science and biophysics at ISOLDE, presenting some of the highlights during the last five years, together with a short outlook on the new developments under way.

The lattice location of ion implanted In into type IIa diamond and the annealing of implantation damage was studied with the emission channeling technique. Radioactive 111In was implanted into IIa diamonds with 〈110〉 surface orientation at 300 K, at energies of 120 and 350 keV and at doses between 5 × 1012 and 5 × 1013 cm−2. Axial channeling effects of the emitted conversion electrons were measured after annealing to different temperatures up to 1473 K. For implantation doses below 1013 cm−2 two stages for annealing of implantation defects are observed, one between 300 and 600 K and the other above 1000 K. The appearance of strong channeling effects in all major axial directions after annealing at 1473 K indicates a significant substitutional fraction of In atoms. From a comparison with calculated emission channeling profiles a substitutional fraction of up to 45% was determined. The experimental data are also consistent with a close-to-substitutional fraction of about 50–60% assuming small mean displacements of the In atoms of about 0.1 A. A significant fraction of In on tetrahedral interstitial sites can be ruled out.

The lattice location of iron in thin-film, single-crystalline hexagonal GaN was studied by means of the emission channeling technique. Following 60-keV room-temperature implantation of the precursor isotope $^{59}$Mn at a dose of 1.0 $\times 10^{13}$ cm$^{-2}$ and annealing up to 900°C, the angular distribution of $\beta^{-}$-particles emitted by the radioactive isotope $^{59}$Fe was measured by a position-sensitive electron detector. The $\beta^{-}$-emission patterns around the [0001], [1102], [1101] and [2113] directions give direct evidence that the majority of Fe (80%) occupies substitutional Ga sites.

Recently we have shown that the reduction in the photoluminescence linewidth of many deep luminescence centers in highly enriched $^{28}\text{S}\text{i}$ results in well-resolved isotopic fingerprints. This allows for a better characterization of a defect center, as not only the involvement of a specific element but also the number of atoms of that element within the complex can be determined. Surprisingly, we have found that many well-known luminescence centers have a different composition than originally supposed. In addition, we have found a large number of four- and five-atom luminescence centers involving the elements Cu, Au, and Li. Here we introduce series of four- and five-atom deep luminescence centers involving a single Pt atom together with Cu and Li, similar to what has been seen previously for Au-containing luminescence centers.

In this work, we present a lattice location study of Sn in Ge. From emission channeling experiments, we determined the exact lattice location of ion implanted $^{121}\text{S}\text{n}$ atoms and compared the results to predictions from density-functional calculations. The majority of the Sn atoms are positioned on the substitutional site, as can be expected for an isovalent impurity, while a second significant fraction occupies the sixfold coordinated bond-centered site, which is stable up to at least $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. Corroborated by ab initio calculations, we attribute this fraction of bond-centered Sn atoms to the Sn-vacancy defect complex in the split-vacancy configuration. Furthermore, we are able to assign specific defect complex geometries to resonances from earlier M\"ossbauer spectroscopy studies of Sn in Ge.

Modifying the properties of ZnO by means of incorporating antimony, arsenic or phosphorus impurities is of interest since these group V elements have been reported in the literature among the few successful p-type dopants in this technologically promising II-VI compound. The lattice location of ion-implanted Sb, As, and P in ZnO single crystals was investigated by means of the electron emission channeling technique using the radioactive isotopes $^{124}$Sb, $^{73}$As and $^{33}$P and it is found that they preferentially occupy substitutional Zn sites while the possible fractions on substitutional O sites are a few percent at maximum. The lattice site preference is understandable from the relatively large ionic size of the heavy mass group V elements. Unfortunately the presented results cannot finally settle the interesting issue whether substitutional Sb, As or P on oxygen sites or Sb$_{Zn}$−2V$_{Zn}$, As$_{Zn}$−2V$_{Zn}$ or P$_{Zn}$−2V$_{Zn}$ complexes (as suggested in the literature) are responsible for the acceptor action. However, the fact that the implanted group V ions prefer the substitutional Zn sites is clearly a strong argument in favour of the complex acceptor model, while it discourages the notion that Sb, As and P act as simple “chemical” acceptors in ZnO.

This chapter focuses on the lattice site location of rare earth (RE) ions introduced into group III-nitrides either by ion implantation or doped in situ during growth by molecular beam epitaxy (MBE). The lattice site occupied will, to a large extent, govern the luminescence properties of the optically doped semiconductors. An overview of the various possible sites that exist in the wurtzite nitride lattice and of the techniques that can be used to accurately determine the occupied sites will be given. Particular attention will be devoted to channelling techniques, both ion beam channelling and electron emission channelling, which offer direct evidence of the lattice site without the need of further modelling.

The lattice locations of the potential p-type dopants arsenic and antimony in single-crystalline ZnO were studied by means of the electron emission channeling method following the implantation of radioactive 73As and 124Sb isotopes. The majority of the implanted As and Sb probe atoms was found to occupy substitutional Zn sites, with the possible fraction on substitutional O sites being at maximum a few percent. The obtained results illustrate the difficulty in introducing oversized group V impurities on O sites and thus put further into question whether these elements may act as simple chemical dopants in ZnO.

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted $^{111}$In atoms in Ge. The majority of the In atoms was found on the substitutional site, which is a thermally stable site at least up to 500 °C. We also found strong indication that directly after implantation, a fraction of the implanted $^{111}$In atoms occupies the bond-centered BC site. This fraction disappears after annealing at 300 °C. From comparison with ab initio calculations, electrical studies, and perturbed angular correlation experiments, the In atoms on the BC site can be related to In-vacancy and In-self-interstitial defect complexes. The activation energy for dissociation of this BC related defect was found to be below 1.6 eV.

The channeling and blocking effect of electrons and positrons emitted in nuclear decay allows the lattice site location of radioactive impurities implanted into single crystals at small concentrations (ppm) and low implantation fluences (1012/cm2). We applied this emission channeling technique to the localization of 112mIn and 111mCd after implantation into si-GaP single crystals at different temperatures. After implantation at low temperature and subsequent annealing an increase of the fraction of substitutional probe atoms and a recovery of the local lattice structure between 300 and 500 K were observed. GaP tends to anneal at higher temperatures than GaAs (200–350 K), but compared to GaAs the channeling effects observed in GaP are more pronounced, indicating a more complete recovery of implantation defects.

We report on the lattice location of ion implanted Fe, Cu, and Ag impurities in germanium from a combined approach of emission channeling experiments and ab initio total energy calculations. Following common expectation, a fraction of these transition metals (TMs) was found on the substitutional Ge position. Less expected is the observation of a second fraction on the sixfold coordinated bond-centered site. Ab initio calculated heats of formation suggest this is the result of the trapping of a vacancy by a substitutional TM impurity, spontaneously forming an impurity-vacancy complex in the split-vacancy configuration. We also present an approach to displace the TM impurities from the electrically active substitutional site to the bond-centered site. ispartof: Physical Review Letters vol:102 issue:6 ispartof: location:United States status: published

The channeling and blocking effects of alpha and beta particles emitted in nuclear decay of radioactive probes allow a localization of ion-implanted impurities in single crystals at very small concentrations and low implantation fluences. Using the isotope 8Li as an interstitial probe, indications for a different mobility of vacancies in GaAs and GaP below 500 K were obtained by this technique. The thermal recovery of structural defects in heavy-ion-implanted GaAs was investigated using radioactive 111mCd and 112In probes. A prominent recovery stage was observed in the temperature regime between 200 and 400 K, in which also the mobilization of Ga vacancies was detected in GaAs with 8Li probes.

We have determined the lattice location of implanted arsenic in GaN by means of conversion electron emission channeling from radioactive $^{73}$As. We give direct evidence that As is an amphoteric impurity, thus settling the long-standing question as to whether it prefers cation or anion sites in GaN. The amphoteric character of As and the fact that As$\scriptstyle_{Ga}\,$ " anti-sites ” are not minority defects provide additional aspects to be taken into account for an explanantion of the so-called “ miscibility gap ” in ternary GaAs$\scriptstyle_{1-x}$N$\scriptstyle_{x}$ compounds, which cannot be grown with a single phase for values of $x$ in the range 0.1

Modifying the properties of ZnO and GaN by means of incorporating arsenic impurities is of interest in both of these semiconductors, although for different reasons. In the case of ZnO, the group V element As has been reported in the literature as one of the few p-type dopants in this technologically promising II-VI compound. However, there is an ongoing debate whether the p-type character is due to As simply replacing O atoms or to the formation of more complicated defect complexes, possibly involving As on Zn sites [1]. In the case of GaN, the incorporation of high concentrations of As has been studied with respect to the formation of GaAs(x)N(1-x) alloys and the related modification of the GaN band gap and its luminescence behaviour. It has been suggested that As in GaN is amphoteric, with its lattice site preference depending on the doping character of the material, i.e. mostly substitutional Ga in p-type but also substitutional N in n-type [2].We have determined the lattice location of implanted As in ZnO and GaN by means of conversion electron emission channeling from radioactive 73As. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites in ZnO but in its large majority substitutional Zn sites [3]. Arsenic in ZnO is thus an interesting example for 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. The results are different in the case of As implanted into GaN, where we found roughly half of the implanted As atoms occupying Ga and the other half N sites. The amphoteric character of As therefore certainly plays a role in explaining the extreme difficulties in growing high quality GaAs(x)N(1-x) alloys with values of x above a few percent.A preliminary report will also be given on ongoing emission channeling lattice location experiments using radioactive 124Sb in ZnO and GaN.

We report a detailed crystal field analysis of ${\mathrm{Pm}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ as well as lattice location studies of $^{147}\mathrm{Pm}$ and $^{147}\mathrm{Nd}$ in 2H-aluminum nitride $(w\text{\ensuremath{-}}\mathrm{AlN})$. The isotopes of mass 147 were produced by nuclear fission and implanted at an energy of $60\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The decay chain of interest in this work is $^{147}\mathrm{Nd}\ensuremath{\rightarrow}^{147}\mathrm{Pm}\ensuremath{\rightarrow}^{147}\mathrm{Sm}$ (stable). Lattice location studies applying the emission channeling technique were carried out using the ${\ensuremath{\beta}}^{\ensuremath{-}}$ particles and conversion electrons emitted in the radioactive decay of $^{147}\mathrm{Nd}\ensuremath{\rightarrow}^{147}\mathrm{Pm}$. The samples were investigated as implanted, and also they were investigated after annealing to temperatures of $873\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as well as $1373\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The main fraction of about 60% of both $^{147}\mathrm{Pm}$ as well as $^{147}\mathrm{Nd}$ atoms was located on substitutional Al sites in the $\mathrm{AlN}$ lattice; the remainder of the ions were located randomly within the $\mathrm{AlN}$ lattice. Following radioactive decay of $^{147}\mathrm{Nd}$, the cathodoluminescence spectra of ${\mathrm{Pm}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ were obtained between $500\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $1050\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at sample temperatures between $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. High-resolution emission spectra, representing intra-4$f$ electron transitions, were analyzed to establish the crystal-field splitting of the energy levels of ${\mathrm{Sm}}^{3+}\phantom{\rule{0.3em}{0ex}}(4{f}^{5})$ and ${\mathrm{Pm}}^{3+}\phantom{\rule{0.3em}{0ex}}(4{f}^{4})$ in cationic sites having ${C}_{3v}$ symmetry in the $\mathrm{AlN}$ lattice. Using crystal-field splitting models, we obtained a rms deviation of $6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ between 31 calculated-to-experimental energy (Stark) levels for ${\mathrm{Sm}}^{3+}$ in $\mathrm{AlN}$. The results are similar to those reported for ${\mathrm{Sm}}^{3+}$ implanted into $\mathrm{GaN}$. Using a set of crystal-field splitting parameters ${B}_{nm},$ for ${\mathrm{Pm}}^{3+}$ derived from the present ${\mathrm{Sm}}^{3+}$ analysis, we calculated the splitting for the $^{5}F_{1}$, $^{5}I_{4}$, and $^{5}I_{5}$ multiplet manifolds in ${\mathrm{Pm}}^{3+}$ and obtained good agreement between the calculated and the experimental Stark levels. Temperature-dependent lifetime measurements are also reported for the emitting levels $^{4}F_{5∕2}\phantom{\rule{0.3em}{0ex}}({\mathrm{Sm}}^{3+})$ and $^{5}F_{1}\phantom{\rule{0.3em}{0ex}}({\mathrm{Pm}}^{3+})$.

Lattice site location and local environment characterization of implanted $^{111}\!$Ag in SrTiO$_{3}$ by means of the emission channeling (EC) and $\gamma$-$\gamma$-perturbed angular correlation (PAC) techniques are reported. The angular distribution of $\beta\!^{-}$-particles emitted from the $^{111}\!$Ag decay was monitored with a position-sensitive detector as a function of annealing temperature up to 900 ºC. In the as-implanted state Ag occupies several lattice sites in SrTiO$_{3}$. Upon annealing, near-Sr and near-Ti occupancies increased to 58% and 28%, while an octahedral interstitial fraction vanished. Ag on near-Sr and near-Ti sites are still displaced by $\sim$0.2--0.5 Å from ideal cubic positions. Subsequent PAC measurements confirmed that ~20% of $^{111}\!$Ag atoms are in specific sites of non-cubic environment, characterized by a unique electrical-field-gradient (EFG), while ~80% were subject to a wide EFG distribution.

ZnO, with its high energy bandgap of ∼3.3 eV, has been in the last years one of the most studied materials. The main driving force of the research performed in this oxide semiconductor is directly related to the ability and potentialities of ZnO for optoelectronic and spintronic applications. In the domain of optoelectronics, short wavelength light emitting devices are amongst the most important devices of compound semiconductors. Rare earth (RE) doping appears as an alternative route to photonic applications. In this work we discuss the role of defects in optical activation and structural properties of ion implanted RE doped ZnO single crystals.

Arsenic has been reported in the literature as one of the few $p$-type dopants in the technologically promising II-VI semiconductor ZnO. However, there is an ongoing debate whether the $p$-type character is due to As simply replacing O atoms or to the formation of more complicated defect complexes, possibly involving As on Zn sites. We have determined the lattice location of implanted As in ZnO by means of conversion-electron emission channeling from radioactive $^{73}\mathrm{As}$. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites but in its large majority substitutional Zn sites. Arsenic in ZnO (and probably also in GaN) is thus an interesting example for 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.

The group $\textrm{I}$b impurities Cu and Ag on substitutional Zn sites are among possible candidates for p-type doping of ZnO. In order to explore possible lattice sites of Cu and Ag in ZnO the radioactive impurities $^{67}\!$Cu and $^{111}\!$Ag were implanted at doses of $4\!\times\!10^{12}$cm$^{-2}\to1\!\times\!10^{14}$cm$^{-2}$ at 60 keV into ZnO single crystals. The emission channeling effects of $\beta\!^{-}$ -particles from the decay were studied by means of position-sensitive electron detectors, giving direct evidence that in the as-implanted state large fractions of Cu and Ag atoms (60--70% for Cu and 30% for Ag) occupy almost ideal substitutional Zn sites with root mean square (rms) displacements of 0.014--0.017 nm. However, following vacuum annealing at 600 °C and above both Cu and Ag were found to be located increasingly on sites that are characterized by large rms displacements (0.03--0.05 nm) from Zn sites. We conclude that in high-temperature treated ZnO Cu and Ag are most likely not simply replacing Zn atoms but are incorporated in complexes with other crystal defects or as clusters.

Hybrids generated from tumor cells and dendritic cells (DC) have been proposed as tools for treating malignant disease. Here, we study the underlying principles and the feasibility for the adjuvant therapy of human B cell chronic-lymphocytic leukemia (B-CLL). CLL cells and allogeneic DC were only mixed or additionally fused. Using a combination of FACS and fluorescence microscopic analyses, we show that DC-CLL hybrids can be successfully generated. However, fusion frequencies have to be critically evaluated because the number of fused cells is overestimated when based on FACS analyses alone. The capability of activating patients' PBMC was examined by measuring cytokine secretion in co-culture assays. We made a systematic comparison of the immunostimulatory capacities of different stimulator cell populations, including DC-CLL fusion samples, unfused mixtures of DC and CLL cells as well as DC or tumor cells alone. Surprisingly, even unfused mixtures had a pronounced tumor-directed immunostimulatory effect. This could be explained by the capture of antigens from surrounding leukemia cells by DC during co-cultivation. Although fusion frequencies were low, PBMC stimulation was significantly more effective when the mixtures were subjected to cell fusion. The most potent stimulus was provided by DC-CLL fusion samples derived from mature DC, probably due to their enhanced costimulatory capacity. In summary, DC-tumor cell hybrids might be feasible in the treatment of B-CLL. It should be considered that FACS analysis is not sufficient to assess fusion frequencies and that interactions between unfused DC and CLL cells also result in PBMC activation.

The angular distribution of $\beta^{-}$-particles emitted by the radioactive isotope $^{59}$Fe was monitored following implantation into Si single crystals at fluences from $1.4 \times 10^{12}$ cm$^{-2}$ to $1 \times 10^{14}$ cm$^{-2}$. We identified Fe on three distinct sites: ideal substitutional, displaced substitutional and displaced tetrahedral interstitial. Whereas displaced substitutional Fe was dominating for annealing temperatures below 500 °C, annealing between 500-700 °C caused the majority of Fe to occupy displaced tetrahedral interstitial sites. Ideal substitutional positions were increasingly populated following annealing at 800 °C and above. A comparison of the emission channeling results to Mössbauer and electron paramagnetic resonance experiments is given.

The emission channeling technique was applied to evaluate the lattice location of implanted $^{59}$Fe in single-crystalline ZnO. The angular distribution of $\beta^{-}$- particles emitted by $^{59}$Fe was monitored with a position-sensitive electron detector, following 60-keV low dose ($2.0 \times 10^{13}$ cm$^{-2}$) room-temperature implantation of the precursor isotope $^{59}$Mn. The emission patterns around the [0001], [1102], [1101] and [2113] directions revealed that following annealing at 800°C 95(8)% of the Fe atoms occupy ideal substitutional Zn sites with root mean square displacements of 0.06-0.09 Å.

The lattice location of low-dose implanted Er in GaN, GaN:O, and GaN:C was investigated using the emission channeling technique. The conversion electrons emitted by the probe isotope $^{167m}$Er give direct evidence that the majority (~90%) of Er atoms are located on substitutional Ga sites for all samples. Annealing up to 900 °C does not change these fractions, although it reduces the Er root-mean-square (rms) displacements. The only visible effect of oxygen or carbon doping is a small increase in the rms displacements with respect to the undoped sample.

The lattice location of copper in single-crystalline zinc oxide was studied by means of the emission channeling technique. Following 60-keV room-temperature implantation at a fluence of $2.3\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}2},$ the angular distribution of ${\ensuremath{\beta}}^{\ensuremath{-}}$ particles emitted by the radioactive isotope ${}^{67}\mathrm{Cu}$ was measured by a position-sensitive detector. The ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission patterns give direct evidence that in the as-implanted state a large fraction of Cu atoms (60%--70%) occupy almost ideal substitutional Zn sites with root-mean-square (rms) displacements of 0.16--0.17 \AA{}. However, following annealing at $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and above Cu was found to be located on sites that are characterized by large rms displacements (0.3--0.5 \AA{}) from Zn sites.

Radioactive Li ions were implanted into natural IIa diamonds at temperatures between 100 K and 900 K. Emission channelling patterns of $\alpha$-particles emitted in the nuclear decay of $^{8}$Li(t$_{1/2}$ = 838 ms) were measured and, from a comparison with calculated emission channelling and blocking effects from Monte Carlo simulations, the lattice sites taken up by the Li ions were quantitatively determined. A fraction of 40(5)% of the implanted Li ions were found to be located on tetrahedral interstitial lattice sites, and 17(5)% on substitutional sites. The fractions of implanted Li on the two lattice sites showed no change with temperature, indicating that Li diffusion does not take place within the time window of our measurements.