Electron Emission Channeling Research Articles

Electron Emission Channeling for lattice location of radioactive isotopes in single crystals: Improvements from a Timepix3 quad detector and new PyFDD data analysis software

Electron Emission Channeling (EC) is a powerful technique for the investigation of the lattice location of radioactive isotopes implanted into single crystals. After implantation the isotopes occupy certain lattice locations in the crystal, which can in some cases be altered by annealing. Upon decay, the emission of a charged particle, typically a beta, may result in a channeling trajectory when its starting lattice location is aligned with major symmetry axes or planes of the crystal. By measuring the emission anisotropy in the direction of these axes for distinct annealing temperatures, the lattice location of the isotope can be determined with great precision and insightful information can be obtained on how annealing affects the occupied sites. This work reports on the installation of a Timepix3 quad detector and Katherine Gen2 readout in an experimental setup located at ISOLDE at CERN. The large increase in the number of pixels of the Timepix3, in comparison to previously used pad detectors, required more sophisticated tools for data treatment and fitting of channeling patterns. From this need, the PyFDD software was born . Its latest update features an intuitive graphical interface, with tools for noise masking, pattern visualization, simulations browsing, chi-square or maximum likelihood based fits and gamma background correction.

Magnesium-Vacancy Optical Centers in Diamond.

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

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.

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

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

Influence of crystal mosaicity on axial channeling effects and lattice site determination of impurities

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.

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.

Diluted manganese on the bond-centered site in germanium

The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.

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 sites of ion-implanted Cu atoms in diamond

Radioactive 67Cu atoms were accelerated to 60keV at the online isotope separator ISOLDE at CERN, and implanted into a type IIa natural diamond sample to a dose of 2×1012cm−2. The channeling of β−-particles and conversion electrons emitted in the decay of 67Cu and 67Zn∗, respectively, were monitored about the three major axial directions with a two-dimensional position-sensitive detector. The electron emission channeling data were collected from the room-temperature-implanted sample and after annealing at 1200K.The observed channeling patterns were fitted with simulations based on the many beam formalism of electron motion through a crystal lattice. In the as-implanted sample, 25% of the Cu atoms were located a mean, isotropic displacement of 0.25(5)Å from substitutional sites, and the remainder, fR=75%, at sites that gave an isotropic emission yield. Annealing at 1200K results in enhanced axial and planar channeling effects. The fits to the data yield either a fraction f1=45(5)% of Cu atoms located 0.24(4)Å from substitutional sites and fR=57%, or a fraction f1=10(2)% at substitutional sites, a fraction f2=50(5)% at mean isotropic displacement of 0.5Å from substitutional sites, and a ‘random’ fraction fR=40%.

Lanthanide Doped Cubic Boron Nitride

ABSTRACTFirst basic investigations on lanthanide implanted cubic boron nitride were performed with focus on the site of the implanted ions as well as their luminescence behaviour. In order to investigate the lattice sites of lanthanides in c-BN, we implanted single crystal c-BN with radioactive 139Ce ions using the on-line isotope separator ISOLDE at CERN. After the samples were annealed at 1193 K, direct lattice location studies were performed using the conversion electron emission channeling technique and showed a near-substitutional fraction of the implanted ions of at least 10 %. Additionally, poly crystalline c-BN samples were implanted with stable Eu and Tm ions in order to investigate the luminescence properties of triply ionised lanthanides in the c-BN host. It is found that the annealing of the implanted samples at high temperature and high pressure results in strong luminescence of the Eu3+ ions in the orange and moderate luminescence of the Tm3+ ions in the blue. In case of the Eu implanted samples, the luminescence is assigned to the 5D0-7FJ intra-4f electron transitions of Eu3+, in case of the Tm implanted samples, detectable transitions are 1D2-3H6, 1D2-3F4, 1G4-3H6. of Tm3+.

Implantation sites of In, Cd, and Hf ions in diamond

The implantation sites of In, Cd, and Hf ions in diamond have been investigated with complementary electron emission channeling (EC) and perturbed $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlations (PAC) spectrometry on radioactive ${}^{111}\mathrm{In},$ ${}^{111m}\mathrm{Cd},$ and ${}^{181}\mathrm{Hf}$ probes implanted into natural type-IIa diamonds. The fraction of probes occupying specific sites were determined from comparisons of the measured axial channeling yields with channeling patterns calculated using the many-beam formalism of electron motion through the crystal. For the In-implanted sample the EC measurements, after room-temperature implantation and annealing at 1473 K or after implantation at 1373 K, show a substitutional or near-substitutional fraction of 32(4)%, a tetrahedral interstitial fraction of 10(3)%, and the remainder in highly disturbed environments. The $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ PAC measurements confirm the near-substitutional population, but show that about $10%$ of the In probes are at sites with nearest-neighbor point defects and $20%$ at sites with more distant defects, but none in a defect-free environment. The ${}^{111m}\mathrm{Cd}$ measurements confirm these results and show that the loss of anisotropy in the PAC signal is not due to ``aftereffects'' of the electron capture decay of In to Cd, but due to extended lattice damage produced by the implantation process. The In PAC measurements confirmed the previously observed In-defect interaction in diamond, with a quadrupole coupling frequency of ${\ensuremath{\nu}}_{Q}=117 \mathrm{MHz},$ and in addition, showed evidence of a new defect interaction with ${\ensuremath{\nu}}_{Q}=315 \mathrm{MHz}$ at annealing temperatures above 1473 K. A diamond sample implanted with overlapping profiles of ${}^{111}\mathrm{In}$ and hydrogen and annealed up to 1673 K showed no evidence of the higher-frequency component or of any signal attributable to the formation of In-H pairs. This suggests that in diamond no significant fraction of the implanted In atoms act as electrically active acceptors. In the ${}^{181}\mathrm{Hf}$-implanted diamond, the present EC and PAC measurements yield consistent results of a near-substitutional fraction of only $10%--15%,$ in contrast to earlier observations.

Lattice location studies of indium in Cr2O3

The lattice sites of implanted In atoms in Cr2O3 were investigated by means of electron emission channeling (EC) measurements using 111In probe atoms. EC spectra were recorded for several axes and compared to simulations. Indium atoms are most likely located at near-Cr sites. Small differences in the EC patterns for prompt and delayed electrons may be an indication for displacements of 111Cd emitter atoms following the electron capture decay of In.

Advances in electron emission channeling measurements in semiconductors

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.

Electron emission channeling with position-sensitive detectors

Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

Lattice Sites and Stability of Implanted Er in FZ and CZ Si

AbstractWe report on the lattice location of 167Er in Si measured by conversion electron emission channeling. In both FZ and CZ Si, a high fraction of Er (>65%) occupies near-tetrahedral interstitial (T) sites directly following 60 keV room temperature implantation at doses of 6× 1012 cm−2 For higher doses, the as-implanted near-T fractions of Er visible by emission channeling are smaller, due to the beginning of amorphization. Following the recovery of implantation damage at 600°C, more than 70% of Er is found on near-T sites in both FZ and CZ Si. In FZ Si, Er exhibits a remarkable thermal stability and only prolonged annealing for several hours reduces the near-T fraction. On the other hand, annealing of CZ Si at 900°C for more than 10 minutes results in the majority of Er probes in sites of very low symmetry or disordered surroundings.

Impurity lattice location and recovery of structural defects in semiconductors studied by emission channeling

Doping of semiconductors by ion implantation usually requires implantation doses below 1013 cm−2 to obtain typical impurity concentrations of <1018 cm−3. The lattice location of impurities as well as the defect recovery after such low dose implantations can be studied using the emission channeling technique. In this technique, single crystals are doped with radioactive probe atoms and the channeling effects of electrons, positrons or α-particles emitted from these atoms are measured. We present a quantitative analysis of electron emission channeling measurements after heavy-ion implantation into Si and III–V compound semiconductors by comparison with calculated channeling profiles based on the dynamical theory of electron diffraction. For In atoms implanted into Si, complete substitutionality was found after rapid thermal annealing to 1200 K. For lower annealing temperatures, the observed channeling effects indicate small mean displacements (of about 0.2 A) of the In atoms from substitutional sites, caused by residual implantation defects. For GaAs, GaP and InP implanted at low temperatures with In or Cd isotopes, pronounced recovery stages around 300, 400 and 350 K, respectively, were observed and substitutional fractions close to 100% were derived after annealing above the stage.

Lattice location of implants in diamond by conversion electron emission channeling

There is an increasing interest in diamond, inter alia, because of the efficient new metastable techniques of growing thin diamond films. Consequently, there is an increased potential for the industrial exploitation of the unique properties of diamond as, for example, a specialized coating or as semiconductor devices. This requires extensive studies of the physical processes of solid state chemistry and defect engineering in diamond. Conversion electron emission channeling spectroscopy can be used to study lattice site occupancy and other more complex processes. Initial results show a significant population of the substitutional site and also the importance of substrate temperature during implantation for local annealing. In order to shorten the data acquisition time, and minimise radiation damage by implanting less activity, a two-dimensional position and energy sensitive detector, capable of accumulating the whole angular scan simultaneously has been deployed.