Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of 27Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra‐low fluence (≈2 × 1010 cm−2) implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre‐doped with stable Mg during growth, and lowest (2–6%) in n‐GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p‐ and n‐GaN prolonged implantations cause interstitial 27Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 1014 cm−2 result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (T As ) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (T Ga ) is negligible. T As is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding T As versus T Ga occupancy of interstitial Mn in (Ga,Mn)As.[http://dx.doi.org/10.1063/1.4905556] a lino.pereira@fys.kuleuven.be (Ga,Mn)As has become the model system, in which to explore the physics of carrier-mediated ferromagnetism in semiconductors and the associated spintronic phenomena. 1,2In particular, as the most widely studied dilute magnetic semiconductor (DMS), (Ga,Mn)As is the perfect example of how the magnetic behavior of DMS materials is strongly influenced by local structure. In typical high Curie temperature (T C ) (Ga,Mn)As thin films (several % Mn regime), the majority of the Mn atoms substitute for Ga (Mn s ), while a minority fraction (several % of all Mn) occupies interstitial sites (Mn i ).3,4 Mn s provides both the localized magnetic moment and the itinerant hole that mediates the magnetic coupling, whereas Mn i has a twofold compensating effect: (i) magnetically, as Mn i -Mn s pairs couple antiferromagnetically and (ii) electrically, since double donor Mn i compensates Mn s acceptors.3 For a given Mn s concentration, Mn i therefore determines the hole concentration, the Fermi level and the effective Mn s concentration (of non-compensated Mn s moments), all of which define the magneto-electronic behavior of (Ga,Mn)As. The existence of such a crucial role of Mn i is clearly reflected in the effect of the Mn i concentration on the two relevant figures of merit: T C and magnetization. 3−5 Despite this central role in the understanding of (Ga,Mn)As, and, consequently, of Mn-doped III-V DMS materials, interstitial Mn is far from being a well understood defect. The presence of Mn i in ferromagnetic (Ga,Mn)As was first reported based on ion channeling measurements. 6Although consistent with Mn i occupying tetrahedral (T) interstitial sites, the measurements did not allow to discriminate between the two nonequivalent T sites: coordinated by four Ga atoms (T Ga ) or by four As atoms (T As ). Transmission electron microscopy measurements using the (002) diffracted beam indicated that Mn i predominantly occupies the T As site.7 X-ray absorption fine structure (XAFS) techniques were later applied, suggesting T Ga occupancy (e.g., Refs. 8 and 9). However, XAFS is not well suited to distinguish neighboring elements with similar atomic numbers, as is the case for Ga and As, especially in such cases of multi-site occupancy (substitutional and interstitial), where the site to be identified is in fact the minority one (interstitial). In pi...

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 27 Mg in GaN, as well as 4 He RBS channeling from Ge.

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 γ decay towards the ground state will be studied with a high-resolution VUV spectrometer after its production by the β 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 α-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.

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