No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Lattice Location of Nuclear Probes by Electron and Positron Channeling
Abstract
The angular dependence of the emission yields of electrons and positrons emitted from radioactive probes implanted into single crystals was first investigated by Uggerhøj.1 The channeling effect of electrons, emitted from substantial lattice sites, is responsible for enhanced emission yields in the directions of crystal axes or planes. Positrons, on the other hand, show pronounced channeling peaks, if emitted from interstitial sites and blocking dips, if emitted from substantial lattice sites as shown for a ‹110› direction in a 112m
In implanted Mo crystal (Fig. 1). In an early paper2 Uggerhøj and Andersen suggested that this effect may be used as a method for the localization of foreign atoms in single crystals. From the emission yields observed for different low index crystal directions, the lattice site occupancy of the emitter atoms can be determined.
Hyperfine interaction techniques like Mössbauer effect or perturbedγγ angular correlation are commonly applied to study the structure and properties of impurity-defect complexes in solids. It is often difficult to resolve a certain defect structure unambiguously with these techniques, because an absolute determination of the lattice site of the probe atoms is not straight-forward. The emission channeling technique allows the direct determination of lattice sites of radioactive impurity atoms, incorporated into single crystalline solids. The channeling effects of electrons, positrons or alpha particles, emitted from radioactive impurities are measured along different crystal axes and planes. From the measured anisotropic emission distributions the lattice sites of the emitting atoms can be determined. Emission channeling can be applied to a large variety of different probe atoms. Also, rather low impurity concentrations, comparable to those typically required for hyperfine interaction techniques, are sufficient. In this contribution, the principles of the emission channeling technique, the experimental requirements and the quantitative analysis of emission channeling spectra are reviewed. The capabilities and possibilities, which the emission channeling technique offers, are highlighted by three recent experimental studies. First, studies of the diffusion of Ag in CdTe using transmutation doping with the electron emitting isotopes107mAg and109mAg are described. Second, lattice location studies of As in diamond, which is a potential n-type dopant in this material, will be discussed. Third, an experiment is described to study the lattice location of oversized impurities after low dose implantation into Fe. In this experiment, the unique decay properties of221Fr and221 Ra are utilized to determine the lattice sites of five different impurity atoms in a singleα emission channeling measurement.
Doping of semiconductors by ion implantation usually requires implantation doses below 1013 cm–2 to obtain typical impurity concentrations of 18 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 -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 ) 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.
Though the defect characteristic hyperfine interaction can be measured by several techniques, the main body of results is obtained by the nuclear techniques like the Moessbauer effect, the perturbed angular correlation and perturbed angular distribution technique. This article presents a review of the great amount of experiments done by these techniques and also summarizes the state of interpretation. A short introduction to the current knowledge of defect physics is presented, and the necessary elements of hyperfine physics which are needed for the understanding of the experimental facts are discussed. The discussion of defect investigations with radioactive probe atoms - is separated into experiments with radioactive sources and nuclear reaction experiments.
The influence of atomic strings on the motion through a single crystal of positrons and electrons has been investigated, showing a pronounced dip and peak in yield, respectively, in the axis direction.
The ISOLDE facility at CERN has been utilized for ion implantations of radioactive In isotopes into Cu single crystals for impurity lattice-location studies by means of the channeling effects of emitted charged particles. Conversion electrons with energies around 0.2MeV (112In, 114In), electrons from beta--decays with energies up to 2 MeV (114In, 119In) and positrons (112In) were employed for the channeling measurements. These experiments demonstrate the possibility of combining channeling and Mössbauer effect measurements using 119In under identical experimental conditions.
Radioactive119In+ ions (T
1/2=2.1 min) obtained from the ISOLDE facility at CERN have been implanted into silicon single crystals at room temperature. Mössbauer emission spectra from the 24 keV γ-radiation of the daughter119Sn have been measured by fast resonance-counting technique. Five independent lines, characterized by their hyperfine parameters and Debye temperatures, have been found in the spectra. From the bonding configurations, deduced for the Sn impurity atoms, these are concluded to be located in four different defects in the silicon lattice. Simple models are proposed for the defects.
Impurity defect structures in amorphous and crystalline silicon have been created by the ion implantation of radioactive 119In at room temperature. The defects have been studied by Mössbauer spectroscopy on the 24 keV gamma radiation of the daughter 119Sn. Structurally similar complex defects are concluded to be formed in both phases of the silicon host. The nature of the complex defects depends on the species of implanted impurities. The fraction of complex defects in crystalline silicon increases with increasing implanted dose in the dose range of 1011 − 1015 atoms/cm2. No pronounced discontinuity is observed for the critical dose (⪆ 1013 atoms/cm2), where the implanted host volume is amorphised. The implications of these results on the question of a characterisation of the amorphous phase of semiconductors by Mössbauer spectroscopy on implanted impurities will be discussed.
Experiments at the reconstructed on-line isotope separator facility ISOLDE-2 at the CERN 600 MeV synchro-cyclotron are now in progress. Over the past year the extracted proton beam has been raised to 1.3 >A, and the design value of 5 >A is now within reach.The target and ion-source technology developed before the reconstruction has been further improved so as to permit the utilization of the high proton-beam intensities and to increase the range of elements available for study to 30. With a proton-beam intensity of 0.7 >A the radioactive Cs+ beams after mass separation have increased by a factor of 300 and, in the most favoured cases, are exceeding 6 nA. This permits the visualization of the separated radioactive ion beams by means of the usual beam scanners. A new target concept, a finely divided metal powder at ultra-high temperatures, features a short delay component which enhances the yield of very short-lived products. In the case of a niobium powder target at 2200°C for the production of Rb, the new could be identified with a collection rate of 450 atoms per second.The new beam-optical system supplies four measuring stations simultaneously and allows up to ten groups to take data during an ISOLDE run (typically 100 h).The over-all performance and the stability of this system are perhaps best illustrated with the on-line spin measurements: about 20 new spins in isotopes of Rb, Cs, and Au have been measured by means of an atomic beam magnetic resonance (ABMR) apparatus. This Gothenburg-Uppsala experiment requires the ion beam to be directed onto its oven and kept steady within an area of 0.5 mm2. The joint set-up (ISOLDE+ABMR) acts as an instrument that gives a beam of nuclei selected in Z, A, and spin value.
Impurity-vacancy complexes in copper have been formed with radioactive 119In and 119Sb by ion implantation and trapping. The defects are studied by Mössbauer spectroscopy on the 119Sn daughter atoms. Comparison with 111In PAC studies reveals that various different defects are formed in stage III, some of which are ``invisible'' in PAC experiments.
Lattice damage after implantation of111In in Ni has been studied applying the DPAC technique to the 171 245 keV gamma-ray cascade in the daughter nucleus111Cd. Implantations were carried out at 10 K and at 300 K. The low temperature implantation yields a higher regular substitutional fraction (80%) than the room temperature implantation (40%). The annealing behaviour of both implants above RT is the same. In addition, two distinct defect-associated sites were observed in isochronal annealing sequences. A microscopic model for these defects is presented, which takes into account magnetic and electric hyperfine interaction strengths, binding energies and site populations as a functions of temperature.
With the use of conversion electrons emitted during the decay of 111In atoms diffused into a copper single crystal, it is shown that the electrons are steered by lattice rows and that the impurities are localized at substitutional sites. The damage produced by implantation of the radioactive atoms was investigated in a simultaneously performed channeling and perturbed-γγ angular-correlation experiment using 111In as probe atom. The formation of defect clusters at the probes' site can be observed by both techniques.
Heavy ions from all groups of the periodic system have been implanted at keV energies into Si at room temperature. The samples have been analyzed in their as-implanted state by high resolution ion backscattering of 0.5 MeV 4He ions. The results show that only elements from groups IIIa, IVa and Va have dominant substitutional occupation, whereas in all other cases interstitial sites are largely preferred. It is concluded that chemical effects govern the selection of lattice sites.
The channeling of conversion electrons with energies of 145 and 219 keV emitted from 111In embedded in a Cu single crystal was investigated and used to localize the In impurities on substitutional sites after implantation. Electron channeling was performed simultaneously with perturbed γγ-angular correlation (PAC) experiments using the γγ-cascade in the decay of 111In to study the radiation damage produced by the implantation of the probe atoms. The channeling effect is influenced by the formation of defects at the impurities as indicated by the PAC.
The formalism for a Monte Carlo computer program which simulates slowing down and scattering of energetic ions in amorphous targets is presented. It was developed for determining ion range and damage distributions as well as angular and energy distributions of backscattered and transmitted ions. The computer program provides particularly high computer efficiency, while still maintaining a high degree of accuracy. This is achieved mainly by applying a new analytic formula for determining nuclear scattering angles based on the Molière potential, and by suitably expanding the distance between collisions at high energies. With these improvements it becomes feasible for the first time to assess accurately both low and high energy problems with high precision using a single simulation program.
Hyperfine Interaction of Radioactive Nuclei
- E Recknagel
- G Schatz
- T Wichert
Atomic Collision Phenomena in Solids
- J Lindhard