Emission Channeling Experiments Research Articles

A generalized fitting tool for analysis of two-dimensional channeling patterns

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.

The two gap transitions in Ge1−xSnx: Effect of non-substitutional complex defects

The existence of non-substitutional β-Sn defects in Ge1−xSnx alloys was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B 81, 155204 (2010)], which established that, although most Sn enters substitutionally (α-Sn) in the Ge lattice, a second significant fraction corresponds to the Sn-vacancy defect complex in the split-vacancy configuration (β-Sn), in agreement with our previous theoretical study [Ventura et al., Phys. Rev. B 79, 155202 (2009)]. Here, we present the electronic structure calculations for Ge1−xSnx, including the substitutional α-Sn as well as the non-substitutional β-Sn defects. To include the presence of the non-substitutional complex defects in the electronic structure calculation for this multi-orbital alloy problem, we extended the approach for the purely substitutional alloy by Jenkins and Dow [Phys. Rev. B 36, 7994 (1987)]. We employed an effective substitutional two-site cluster equivalent to the real non-substitutional β-Sn defect, which was determined by a Green's functions calculation. We then calculated the electronic structure of the effective alloy purely in terms of substitutional defects, embedding the effective substitutional clusters in the lattice. Our results describe the two transitions of the fundamental gap of Ge1−xSnx as a function of the total Sn-concentration: namely, from an indirect to a direct gap, first, and the metallization transition at a higher x. They also highlight the role of β-Sn in the reduction of the concentration range, which corresponds to the direct-gap phase of this alloy of interest for the optoelectronics applications.

Drawing the geometry of 3d transition metal-boron pairs in silicon from electron emission channeling experiments

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.

Direct observation of the lattice sites of implanted manganese in silicon

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 toward substitutional sites; and sites displaced from anti-bonding toward tetrahedral interstitial sites. For all doping types investigated, the substitutional fraction remained below $$\sim$$ 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.

Emission channeling studies on transition-metal doped GaN and ZnO: Cation versus anion substitution

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 61Co and 56Mn 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 61Co-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.

A versatile apparatus for on-line emission channeling experiments

The concept and functionality of an apparatus dedicated to emission channeling experiments using short-lived isotopes on-line at ISOLDE∕CERN is described. The setup is assembled in two functional blocks - (a) base stand including beam collimation, implantation and measurement chamber, cryogenic extension, and vacuum control system and (b) Panmure goniometer extension including maneuvering cradle and sample heating furnace. This setup allows for in situ implantation and sample analysis in the as-implanted state and upon cooling down to 50 K and during annealing up to 1200 K. The functionality of the setup will be illustrated with the example of establishing the lattice location of (56)Mn probes implanted into GaAs.

Stability and diffusion of interstitial and substitutional Mn in GaAs of different doping types

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.

Lattice position and thermal stability of diluted As in Ge

We present a lattice location study of the n-type dopant arsenic after ion implantation into germanium. By means of electron emission channeling experiments, we have observed that the implanted As atoms substitute the Ge host atoms and that, in contrast to several implanted metal impurities in Ge, no significant fraction of As is found on interstitial sites. The substitutional As impurities are found to be thermally stable up to 600 °C. After 700 °C annealing a strong reduction of emission channeling effects was observed, in full accordance with the expected diffusion-induced broadening of the As profile.

Damage formation and recovery in Fe implanted 6H–SiC

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 56Fe+ ions with an energy of 150keV. The samples were implanted with 5×1014Fe+/cm2 and 1×1016Fe+/cm2 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 3.8×1014Fe+/cm2, 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 (2×1013cm−2) β emission channeling experiments using the radioactive isotope 59Fe (t1/2=45d), where the sample was implanted at room temperature and the β− emission channeling yield was measured by means of a position-sensitive detector.

Slow-neutron-induced charged-particle emission-channeling-measurements with Medipix detectors

The lattice location of helium, lithium, beryllium, boron or sodium host or impurity atoms in single crystals can be studied by slow-neutron-induced charged particle emission channeling measurements. Modern silicon pixel detectors can cover an entire emission channeling pattern in a single measurement and allow reviving this technique for practical applications. We report on the use of the TimePix detector for such emission channeling experiments with samples containing 6Li, 7Be or 10B.

Lattice location study of ion implanted Sn and Sn-related defects in Ge

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.

Lattice location study of implanted In in Ge

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted I111n 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 I111n 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.

Transition Metal Impurities on the Bond-Centered Site in Germanium

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.

Spectra and energy levels ofTm3+(4f12)in AlN

We report a detailed analysis of the cathodoluminescence spectra of ${\mathrm{Tm}}^{3+}$-implanted 2H-aluminum nitride (AlN) covering the wavelength range between 290 and $820\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at temperatures between 12 and $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. More than 200 transitions are observed, of which more than 100 of these transitions can be identified from emitting multiplet manifolds $^{1}I_{6}$, $^{1}D_{2}$, and $^{1}G_{4}$. Although the emitting levels are not observed directly, emission is also attributed to the $^{3}P_{2}$ and $^{3}P_{1}$ multiplet manifolds based on analyses of transitions to terminal levels $^{3}F_{4}$, $^{3}H_{5}$, and $^{3}F_{3}$. The observed crystal-field splitting of the ground-state multiplet manifold, $^{3}H_{6}$, and manifolds $^{3}F_{4}$, $^{3}H_{5}$, $^{3}H_{4}$, $^{3}F_{3}$, $^{3}F_{2}$, and $^{1}G_{4}$ is established from an analysis based on matching repeated energy differences between transitions. This method is similar to one used in analyzing arc and spark spectra. Temperature-dependent spectra also establish the crystal-field splitting of the $^{3}P_{1}$ and part of the manifold splitting of emitting levels such as $^{1}I_{6}$. To establish an initial set of crystal-field splitting parameters, ${B}_{\mathrm{nm}}$, that can be related to a physical model, we carried out a lattice-sum calculation by computing the crystal-field components, which are the coefficients in a multipolar expansion of the crystal field about the ${\mathrm{Al}}^{3+}$ sites that have ${C}_{3v}$ symmetry in the lattice. Emission channeling experiments indicate that the ${\mathrm{Al}}^{3+}$ sites serve as the substitutional sites for ${\mathrm{Tm}}^{3+}$ in AlN. With only minor adjustments to the calculated centroids to account for $J$-mixing, the calculated crystal-field splitting of most multiplet manifolds, $^{2S+1}L_{J}$, of ${\mathrm{Tm}}^{3+}(4{f}^{12})$ based on the ${B}_{\mathrm{nm}}$ obtained from the lattice-sum calculations, is in good agreement with the reported experimental splitting.

Recent Emission Channeling Studies in Wide Band Gap Semiconductors

We present results of recent emission channeling experiments on the lattice location of implanted Fe and rare earths in wurtzite GaN and ZnO. In both cases the majority of implanted atoms are found on substitutional cation sites. The root mean square displacements from the ideal substitutional Ga and Zn sites are given and the stability of the Fe and rare earth lattice location against thermal annealing is discussed.

Position-sensitive Si pad detectors for electron emission channeling experiments

Position-sensitive detector systems, initially developed for the detection of X-rays, have been adapted for their use in electron emission channeling experiments. Each detection system consists of a 30.8×30.8 mm 2 22×22-pad Si detector, either of 0.3, 0.5 or 1 mm thickness, four 128-channel preamplifier chips, a backplane trigger circuit, a sampling analog to digital converter, a digital signal processor, and a personal computer for data display and storage. The operational principle of these detection systems is described, and characteristic features such as energy and position resolution and maximum count rate, which have been determined from tests with conversion electrons and β − particles in the energy range 40–600 keV, are presented.

Lattice location and stability of implanted Cu in Ge

We report on emission channeling experiments using the radioactive isotope 67Cu implanted into single crystalline i-Ge at a fluence of 2.4×1012cm−2. The lattice location of 67Cu was determined from the angular-dependent β− emission yield, which was measured by means of a position-sensitive detector around the 〈111〉, 〈100〉 and 〈110〉 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 0.5–0.6Å from substitutional sites. Following annealing at 300°C for 10min, 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 67Cu diffused out of the Ge sample. From this we can estimate the activation energy for dissociation of substitutional Cu to be around 1.6–1.9eV.

Lattice location and optical activation of rare earth implanted GaN

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.

Emission channeling experiments from the decay of [formula omitted] to [formula omitted] in GaN

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 2.0×10 13 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 Å. 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.

Er–O clustering and its influence on the lattice sites of Er in Si

We present emission channeling experiments on the lattice location of Er in CZ Si single crystals with a well-defined O concentration of 6.5–6.6×1017cm−3 and 60keV-implanted Tm+Er doses ranging from 4.3×1012 to 3.6×1013cm−2. The experimental results are compared with the predictions of a simulator which models the formation of ErnOm 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 ErnOm 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.