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© 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

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.

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°.

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.

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)

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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

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.

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.

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 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.

We report on emission channeling experiments using the radioactive isotope $^{67}$Cu implanted into single crystalline i-Ge at a dose of 2.4 $\times 10^{12}$ cm$^{-2}$. The lattice location of $^{67}$Cu was determined from the angular-dependent $\beta^{-}$ emission yield, which was measured by means of a position-sensitive detector around the , and directions. We find that already in the as-implanted state a considerable fraction of Cu (20-25%) occupies ideal substitutional lattice positions, a similar fraction is located on positions that are displaced around 0.5-0.6 from substitutional sites. Following annealing at 300°C for 10 min, the substitutional fraction of implanted Cu increased to 45% while the fraction of displaced Cu decreased to 23%. Upon further annealing at 400°C, channeling effects disappeared completely and around 10% of $^{67}$Cu diffused out of the Ge sample. From this we can estimate the activation energy for dissociation of substitutional Cu to be around 1.6-1.9 eV.

Single crystalline InGaN epilayers with different In content were implanted with Er$^{+}$ fluences in the range of 1 $\times 10^{13}$ to 5 $\times 10^{15}$ cm$^{-2}$ at room temperature. The structural changes and lattice site location were studied with emission channeling and RBS/channeling. Photoluminescence measurements were also performed to study the optical properties of the implanted samples. After implantation of 1 $\times 10^{13}$ Er$^{+}$ cm$^{-2}$, the emission channeling results show the incorporation of a significant fraction of Er in substitutional Ga/In sites. For fluences of 1 $\times 10^{15}$ Er$^{+}$ cm$^{-2}$ the aligned RBS spectrum along the [0001] direction reveals the displacement of the Er ions into random sites in the entire implanted region. Proximity cap annealing at 400°C and 500°C leads to some damage recovery on the samples implanted with lowest fluence accompanied by an increase of the substitutional fraction of Er. Despite the lattice disorder, a fraction of the Er ions are incorporated into optically active sites and luminescence emission was observed at 1.54 micrometer after annealing at 400°C.

ZnO (O face) single crystals were implanted with 150 keV Er + ions to fluences of 5 × 10 14 and 5 × 10 15 Er + /cm 2 at room temperature. For fluences of 5 × 10 15 Er + /cm 2 the implantation damage raises the minimum yield from 2% to 22%. Despite the large amount of damage a fraction of 90% of the implanted Er ions are incorporated in substitutional sites along the [0 0 0 1] axis. Photoluminescence (PL) studies reveal the presence of a weak emission near 1.54 μm at 77 K due to the 4 I 13/2 → 4 I 15/2 transition of the Er 3+ ions. Annealing in oxidizing atmosphere at 800 °C leads to a reduction of the implantation damage, which is fully recovered after annealing at 1050 °C for 30 min. The annealing at 1050 °C leads to the outdiffusion of Er, with a 50% loss from the implanted region after the annealing. Only a fraction of 25% of the remaining Er is still in regular sites after the annealing and the Er 3+ PL vanishes.

Many elements are rapidly released from oxide matrices. Some oxide powder targets show a fast sintering, thus losing their favorable release characteristics. Loosely packed oxide fiber targets are less critical since they may maintain their open structure even when starting to fuse together at some contact points. The experience with various oxide fiber targets (titania, zirconia, ceria and thoria) used in the last years at ISOLDE is reviewed. For short-lived isotopes of Cu, Ga and Xe the zirconia and ceria targets respectively provided significantly higher yields than any other target (metal foils, oxide powders, etc.) tested before. Titania fibers, which were not commercially available, were produced in a relic process by impregnation of a rayon felt in a titanium chloride solution and subsequent calcination by heating the dried felt in air. Thoria fibers were obtained either by the same process or by burning commercial gas lantern mantle cloth. In the future a beryllia fiber target could be used to produce very intense 6He beams (order of 1013 ions per second) via the 9Be(n,α) reaction using spallation neutrons.

Radioactive 8Li (t1/2=0.8 s) was ion implanted into single‐crystalline n‐Si of low resistivity (9×1019 P cm−3) at temperatures between 50 and 670 K. Channeling measurements of the alpha‐particles emitted during the decay of 8Li gave direct evidence that 35% of the implanted Li occupied substitutional sites between 350 and 670 K. Below room temperature the major fraction of Li was found to be on tetrahedral sites. The increase in the substitutional fraction around room temperature is attributed to diffusion of Li and capture by vacancy‐related defects.

We have investigated the lattice location of implanted in silicon. By means of emission channeling, three different lattice sites have been identified: ideal substitutional sites, displaced bond-centered sites and displaced tetrahedral interstitial sites. To assess the origin of the observed lattice sites we have compared our results to emission channeling studies on and and to Mossbauer spectroscopy experiments on , present in literature. The possible interpretation of several Mossbauer lines is discussed in the light of our new results on the lattice location. The conclusions are relevant for the microscopic understanding of some gettering techniques.