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Mössbauer spectroscopy at ISOLDE
Abstract
Applications of radioactive ion beams produced at the ISOLDE facility for Mössbauer studies of probe atoms in solids are presented.
Examples are given for a site-selective incorporation on different substitutional sites in compound semiconductors by ion
implantation and thermal annealing of the radiation damage resulting from the implantation. The interactions of the probe
atoms with lattice defects created in the implantation process have been studied to elucidate likely causes for the site-selective
implantation mechanism. The technique has enabled to determine the electronic densities at electrically active substitutional
probe atoms, having shallow donor or acceptor states as well as states deeper in the band gap. The results are in good agreement
with theoretical results from local density calculations. Methodological aspects of the Mössbauer emission techniques employed
at ISOLDE are compared to alternative accelerator based techniques and the consequences of the application of different precursor
isotopes to the 57Fe Mössbauer isotope are treated in detail for 57Fe in silicon. Finally, results obtained for the magnetic hyperfine interactions of 5 sp impurities associated with vacancies
in ferromagnetic metals are discussed.
... For applications, the Fe 57 is followed in order of importance by the Sn 119 . The most used are highlighted with a yellow background, which is orange for the most "popular" Fe and Sn. Figure 8 shows a simplified scheme of a typical Mössbauer spectrometer for measurements in transmission mode on a sample (which acts as absorber) containing iron atoms [26]. The Co 57 radioactive source, with a mean life of 270 days, decays by electron capture to the excited state of 136.5 keV of Fe 57 , from which it jumps with a high probability to the ground state by successive emission of a photon of 123 keV and one of 14.4 keV (a low percentage of transitions occurs directly from the 136.5 keV level to ground). ...
... There are three principal hyperfine interactions, illustrated in Figure 9, which produce measurable effects on the related spectral components. The most used are highlighted with a yellow background, which is orange for the most "popular" Fe and Sn. Figure 8 shows a simplified scheme of a typical Mössbauer spectrometer for measurements in transmission mode on a sample (which acts as absorber) containing iron atoms [26]. The Co 57 radioactive source, with a mean life of 270 days, decays by electron capture to the excited state of 136.5 keV of Fe 57 , from which it jumps with a high probability to the ground state by successive emission of a photon of 123 keV and one of 14.4 keV (a low percentage of transitions occurs directly from the 136.5 keV level to ground). ...
... A typical example is the transformation, induced by cold working, of γ-Fe metastable precipitates in Cu matrix into the thermodynamically stable α-phase. This has been suggested by Gonser [26,34,35] as an instructive experience in a laboratory course to demonstrate the ease of quantitative phase analysis by means of MS. It consists of the following steps: The contribution of the central line will depend on the temperature and annealing time, so the transformation may be followed quantitatively with the phase analysis of the spectra. ...
This article is focused firstly on the basic physics and some historical aspects concerning the discovery of the Mössbauer effect. Then, elements of the spectroscopic methods utilizing this physical phenomenon are given, with some examples of applications to the field of metallurgy.
... eMS were conducted following the implantation of radioactive 57 Mn + (T 1/2 = 1.5 min) ion beams at the ISOLDE facility at CERN. The beam was produced by 1.4 GeV proton-induced fission in UC 2 targets and subsequent laser ionization 58 . Pure beams with intensities of ~5 × 10 8 ions/s were implanted at 50 keV (fluence <10 12 cm −2 ) into the GeTe sample held at temperatures from RT up to 210 °C in vacuum (10 −6 mbar), in an implantation chamber. ...
... Each eMS spectrum was recorded following an average 5 min implantation and measurement time. Each sample received a maximum implantation fluence of ~1.5 × 10 12 at./cm 2 , which is well below the threshold of overlapping damage cascades (typically 10 13 -10 14 cm −2 ) in semiconductors and insulators 58 . Heating was performed with a halogen lamp mounted behind the sample. ...
The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of ⁵⁷Fe emission Mössbauer spectroscopy, following dilute implantation of ⁵⁷Mn (T1/2 = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the ⁵⁷Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.
... Following multistage laser ionization and magnetic mass separation, the 57 Mn ions were accelerated to 45 keV energy, and implanted into undoped and Ba-doped BiFeO 3 (BFO) thin films. 57 Mn decays by β − emission with a half-life of T 1/2 = 90 s, and feeds the excited 14.4 keV Mössbauer state of 57 Fe, thus enabling online 57 Fe emission Mössbauer spectroscopy (eMS) measurements [15,16]. The 57 Mn → 57 Fe decay process imparts an average recoil energy of 40 eV to the 57 Fe ions, thereby enabling them to substitute Fe ions in the samples under study and become very sensitive probes of the hyperfine fields in their immediate vicinity. ...
Emission 57 Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive 57 Mn + ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO 3 (BFO) thin films. 57 Mn β decays (t 1/2 = 90 s) to the 14.4 keV Mössbauer state of 57 Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO 3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K-700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe 3+ ions located in octahedral sites (B hf); and (ii) to Fe 3+ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field B Distr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of B hf = 44.5(9) T. At higher sample temperatures, the B hf becomes much weaker, with the Fe 3+ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba-BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.
... In the case of implantation, there are off-line (longlived) and on-line (shortlived) experiments. In the first scenario, activity produced elsewhere is mass-separated and implanted at low energies, while the second approach is carried out at on-line radioactive beam facilities, such as ISOLDE/CERN (Weyer, 2000). ...
... 57 Fe eMS measurements [20,21] were performed at the ISOLDE-CERN facility [22,23], where the parent radioactive isotope was produced with 1.4 GeV proton-induced fission in a heated UC 2 target. Mass-separated 57 Mn ions were then implanted with an energy of 50 keV into the samples at RT. Emission Mössbauer spectra were recorded using a resonance detector equipped with a 57 Fe-enriched stainless-steel electrode mounted on a conventional drive system outside the implantation chamber at 60 • relative to the sample normal. ...
Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5–5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system.
... Of particular importance is emission Mössbauer spectroscopy employing short-lived parent isotopes ( 57 Mn → 57 Fe, T ½ = 1.45 min and 119 In → 119 Sn, T ½ = 2.4 min), as applied by the Mössbauer collaboration at ISOLDE/CERN. [2][3][4] The method allows for the study of implantation damage at low fluences down to ∼10 11 cm −2 (local concentration down to ∼10 −4 at. %) with annealing times dictated by the lifetime of the parent isotope. ...
An extension of the online implantation chamber used for emission Mössbauer Spectroscopy (eMS) at ISOLDE/CERN that allows for quick removal of samples for offline low temperature studies is briefly described. We demonstrate how online eMS data obtained during implantation at temperatures between 300 K and 650 K of short-lived parent isotopes combined with rapid cooling and offline eMS measurements during the decay of the parent isotope can give detailed information on the binding properties of the Mössbauer probe in the lattice. This approach has been applied to study the properties of Sn impurities in ZnO following implantation of ¹¹⁹In (T½ = 2.4 min). Sn in the 4+ and 2+ charge states is observed. Above T > 600 K, Sn²⁺ is observed and is ascribed to Sn on regular Zn sites, while Sn²⁺ detected at T < 600 K is due to Sn in local amorphous regions. A new annealing stage is reported at T ≈ 550 K, characterized by changes in the Sn⁴⁺ emission profile, and is attributed to the annihilation of close Frenkel pairs.
... Therefore, in the present study we applied the 57 Fe emission Mössbauer spectroscopy (eMS) at the radioactive ion beam facility ISOLDE at CERN to study the V 2 O 3 thin films of thickness ~100 nm particularly prepared for the MIT applications. This eMS facility allows ion implantation at very low fluences (~ 10 −4 at.%), thus avoiding overlapping of damage cascades caused by the ion beam and any prospect of Mn/Fe doping-induced phase changes and formation of Fe precipitates [27,28]. This is especially useful for in situ monitoring the local structure symmetry or defect effects around the ion-implanted probes. ...
Microscopic understanding the metal-to-insulator transition (MIT) in strongly correlated materials is critical to the design and control of modern “beyond silicon” Mott nanodevices. In this work, the local MIT behaviors in single crystalline V2O3 thin films were probed on an atomic scale by online 57Fe emission Mössbauer spectroscopy (eMS) following dilute (<10−3 at.%) implantation of 57Mn+ (T1/2 = 90 s). Both the epitaxial and the textured V2O3 thin films grown by direct current magnetron sputtering were studied. Three structural components were resolved and identified in the eMS spectra with parameters characteristic of Fe in the 2+ valence state, which are attributable to Fe in either lattice damage or structural defects and Fe in the intrinsic crystal structure of V2O3, respectively. The results prove that the oxygen vacancies are common in the V2O3 thin films. With co-existence of both the non-stoichiometry and epitaxial strain in the thin films, the epitaxial strain plays a dominant role in controlling the global MIT properties of the film. The atomic scale structural transition captured by the eMS affirms the early-stage dynamics of the MIT of V2O3 thin film reported previously. These results approve the feasibility to tune the electronic transport of the V2O3 thin films for the next-generation Mott nanodevices by the epitaxial strain via the sample growth parameters.
... When dealing with the on-line experiments, isotopes are produced and implanted at facilities such as ISOLDE/CERN [100] and may take place simultaneously or time delayed (implant and measure). It is possible to highlight the in-beam experiments, in this group measurements occur on the time scale of Mössbauer state [215]. The main advantage of on-line studies is the amount of required isotope, where 10 −4 at.% is sufficient (unlike in off-line experiments with a 57 Co or Sn source where one has to have at least 0.1 at.%). ...
Determination of factors, that govern the kinetics of photocatalytic processes and the knowledge of their roles in transition metal oxides is a critical component to consider when devising efficient materials. Previous studies have shown that point defects, such as substitutional atoms, interstitials and vacancies account remarkably for the electronic structure and chemical properties, that influence the performance of these material in various applications. The expectation that a fundamental understanding of such defects will benefit to elucidate the influence that they have on the system’s functionality is the driving force for pursuing experimental and theoretical research on doped and reduced materials. This dissertation aims to ascertain how electronic structure of rutile and anatase TiO2, which is considered the model system in transition metal oxides, changes when the material contains even subtle amount of dopants (H, Fe and Cd) and point defects. Consequently, time-differential γ-γ perturbed angular correlation technique, along with Mössbauer spectroscopy and tracer diffusion studies have been applied and mostly complemented with theoretical studies and standard techniques. Furthermore, the author presents a newly developed emission Mössbauer set-up (eMIL), which has been developed and constructed during the current study.
Obtained results demonstrate that dopant behaviour is not straight-forward. In monocrystalline rutile, Cd resides not only at the cationic site, but also at impurity-vacancy configurations near the probe. The probe in both environments seem to withstand high temperature annealing/measurements, although associated with alterations in fractions ratio. During emission Mössbauer studies, thin films of anatase show the temperature dependent behaviour throughout the whole temperature range. This results in two annealing stages caused by the vacancy movements and their interaction with Ti interstitials. Substitutional Fe3+, at Ti sites showing spin-lattice relaxation transforms upon hydrogenation, implying that hydrogen behaves as a shallow donor. eMS experiments performed in a temperature range of 300 - 700 K reveal that vacancies and their agglomerations may govern the hydrogen motion. Further perturbed angular correlation studies at the temperature when the hydrogen motion starts, show that depending on the hydrogenation degree the dopant (hydrogen) acts differently and could form a unique coupling with Cd. Hydrogenation for prolonged times demonstrates that doping/reduction is followed by the recovery processes.
The current work clearly indicates that experimental techniques based on hyperfine interactions can unravel a wealth of information about the nature and behaviour of defects in transition metal oxides, that may be readily evaluated and complemented with detailed density functional theory calculations.
... In the case of the on-line eMS experiments, isotopes are produced and implanted at facilities such as ISOLDE/CERN [3,4] and measurements may take place simultaneously alongside with implantation or * Corresponding author at: Chair Materials for Electrical Engineering and Electronics, Institute of Materials Science and Engineering, Institute of Micro and Nanotechnologies MacroNano ® , TU Ilmenau, Gustav-Kirchhoff-Strasse 5, 98693 Ilmenau, Germany. ...
The current work presents a contemporary design of an advanced emission Mössbauer Spectrometer: eMIL equipped with a parallel-plate avalanche detector, which has been devised and built for the Mössbauer collaboration at ISOLDE/CERN. The setup is based on emission geometry, combined with on-line/off-line isotope implantation and provides numerous advantages over conversion electron, common emission (where isotope is deposited chemically on a sample) or transmission Mössbauer spectroscopy. eMIL is designed to measure hyperfine interactions in solids under various exposures. The implemented design overcomes limitations and improves performance and handling. In the current revision, the chamber is supplied with an UV extension — allowing to perform studies of photo-catalytic materials under external light exposure. A specifically designed motorized lid-samples-holder is fully automatized, and makes it possible to study up to 4 samples loaded in a magazine within a temperature range from RT up to 1100 K and to perform angular dependent measurements in high vacuum. This work additionally briefly describes data acquisition with additional electronic blocks, vacuum and data-acquisition system construction.
... Charge transfer effects were observed to play a role for the isomershift of 119 Sn in III-V compound semiconductors attributed to the fractional ionicity of the nearest neighbour (nn) atoms in these compounds [8,9]. Therefore, we consider in the first approximation, only the host atoms. ...
... In this work, we employ 57 Fe eMS by implantation of short-lived radioactive 57 Mn + ions in TiN at an extremely dilute level using the ISOLDE facility at CERN [25] [19]. This is especially powerful for atomic scale investigation of local structure defects and the possible diluted magnetic properties of TiN nanostructures. ...
The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission ⁵⁷Fe Mössbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (1 0 0). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2⁺ valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy VN and interstitial NI) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial NI mediated site-exchange between NI and VN inside the crystal grain are deduced via a N2 dimmer like diffusion of NI through the crystal grains in the temperature range of 540–620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy (VTi) is only detected at the grain boundaries. Annealing up to 813 K, both the VN and NI are annihilated in the crystalline grains and the VTi is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of ⁵⁷Mn/⁵⁷Fe and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.
... ISOLDE has been home to an extremely active Mössbauer programme since the late 1970s. The pioneer of this work was Weyer et al [10] who first exploited strong beams of 119 In Periodic table highlighting elements produced as primary beams at ISOLDE for hyperfine interactions methods. Elements that have isotopes, which are adequate for applied research at ISOLDE are shown in green, blue and red. ...
Solid state physics (SSP) research at ISOLDE has been running since the mid-1970s and accounts for about 10%–15% of the overall physics programme. ISOLDE is the world flagship for the on-line production of exotic radioactive isotopes, with high yields, high elemental selectivity and isotopic purity. Consequently, it hosts a panoply of state-of-the-art nuclear techniques which apply nuclear methods to research on life sciences, material science and bio-chemical physics. The ease of detecting radioactivity—<1 ppm concentrations—is one of the features which distinguishes the use of radioisotopes for materials science research. The manner in which nuclear momenta of excited nuclear states interact with their local electronic and magnetic environment, or how charged emitted particles interact with the crystalline lattices allow the determination of the location, its action and the role of the selected impurity element at the nanoscopic state. ISOLDE offers an unrivalled range of available radioactive elements and this is attracting an increasing user community in the field of nuclear SSP research and brings together a community of materials scientists and specialists in nuclear solid state techniques. This article describes the current status of this programme along with recent illustrative results, predicting a bright future for these unique research methods and collaborations.
... In the 57 Mn þ β À decay, a mean recoil energy of 40 eV (and a maximum possible value of 93 eV) [18] is imparted to the daughter 57 Fe nuclei. This is expected to result in a relocation of a significant fraction of Fe atoms from their implantation sites into interstitial sites in the lattice, as reported in previous eMS studies on Mn/Fe implanted Si [47], InP [43], GaAs and GaP [42]. The literature cites inconsistent displacement energies for the host ions in GaN and AlN. ...
The common charge states of Sn are 2+ and 4+. While charge neutrality considerations favour 2+ to be the natural charge state of Sn in ZnO, there are several reports suggesting the 4+ state instead. In order to investigate the charge states, lattice sites, and the effect of the ion implantation process of dilute Sn atoms in ZnO, we have performed 119Sn emission Mössbauer spectroscopy on ZnO single crystal samples following ion implantation of radioactive 119In ( T ½ = 2.4 min.) at temperatures between 96 K and 762 K. Complementary perturbed angular correlation measurements on 111mCd implanted ZnO were also conducted. Our results show that the 2+ state is the natural charge state for Sn in defect free ZnO and that the 4+ charge state is stabilized by acceptor defects created in the implantation process.
... Motivated by the impact of Fe and its complexes on Si based technology, 57 Fe MS has been used to study the location, diffusivity, and electronic activity of elemental Fe impurities. 3,20,31,32 A summary of experimental and calculated IS values for Fe i and Fe s defects in Si and other group IV semiconductors is presented in Table I. Consistent agreement between theoretical and experimental values of d has enabled the identification of Fe i and Fe s defects in Si, where dðFe s Þ ' À0:04 mm=s and dðFe i Þ ' 0:8 mm=s. ...
The recent discovery that Cu contamination of Si combined with light exposure has a significant detrimental impact on carrier life-time has drawn much concern within the solar-Si community. The effect, known as the copper-related light-induced degradation (Cu-LID) of Si solar cells, has been connected to the release of Cu interstitials within the bulk [Solar Energy Materials & Solar Cells, 147:115-126, 2016]. In this paper, we describe a comprehensive analysis of the formation/dissociation process of the CuB pair in Si by means of first-principles modelling, as well as the interaction of CuB defects with photo-excited minority carriers. We confirm that the long-range interaction between the Cu cation and the B anion has a Coulomb-like behaviour, in line with the trapping-limited diffusivity of Cu observed by transient ion drift measurements. On the other hand, the short-range interaction between the d-electrons of Cu and the excess of negative charge on B produces a repulsive effect, thereby decreasing the binding energy of the pair when compared to the ideal point-charge Coulomb model. We also find that metastable CuB pairs produce acceptor states just below the conduction band minimum, which arise from the Cu level emptied by the B acceptor. Based on these results, we argue that photo-generated minority carriers trapped by the metastable pairs can switch off the Coulomb interaction that holds the pairs together, enhancing the release of Cu interstitials, and acting as a catalyst for Cu-LID.
... Motivated by the impact of Fe and its complexes on Si based technology, 57 Fe MS has been used to study the location, diffusivity, and electronic activity of elemental Fe impurities. 3,20,31,32 A summary of experimental and calculated IS values for Fe i and Fe s defects in Si and other group IV semiconductors is presented in Table I. Consistent agreement between theoretical and experimental values of d has enabled the identification of Fe i and Fe s defects in Si, where dðFe s Þ ' À0:04 mm=s and dðFe i Þ ' 0:8 mm=s. ...
Abstract We employ a combination of pseudopotential and all-electron density functional calculations, to relate the structure of defects in supercells to the isomer shifts and quadrupole splittings observed in M\"ossbauer spectroscopy experiments. The methodology is comprehensively reviewed and applied to the technologically relevant case of iron-related defects in silicon, and to other group-IV hosts to a lesser degree. Investigated defects include interstitial and substitutional iron, iron-boron pairs, iron-vacancy and iron-divacancy. We find that in general, agreement between the calculations and M\"ossbauer data is within a 10% error bar. Nonetheless, we show that the methodology can be used to make accurate assignments, including to separate peaks of similar defects in slightly different environments.
... In the 57 Mn þ β À decay, a mean recoil energy of 40 eV (and a maximum possible value of 93 eV) [18] is imparted to the daughter 57 Fe nuclei. This is expected to result in a relocation of a significant fraction of Fe atoms from their implantation sites into interstitial sites in the lattice, as reported in previous eMS studies on Mn/Fe implanted Si [47], InP [43], GaAs and GaP [42]. The literature cites inconsistent displacement energies for the host ions in GaN and AlN. ...
The lattice sites, valence states, resulting magnetic behaviour and spin-lattice relaxation of Fe ions in GaN and AlN were investigated by emission Mössbauer spectroscopy following the implantation of radioactive 57Mn+ ions at ISOLDE/CERN. Angle dependent measurements performed at room temperature on the 14.4 keV γ-rays from the 57Fe Mössbauer state (populated from the 57Mn β- decay) reveal that the majority of the Fe ions are in the 2+ valence state nearly substituting the Ga and Al cations, and/or associated with vacancy type defects. Emission Mössbauer spectroscopy experiments conducted over a temperature range of 100-800 K show the presence of magnetically split sextets in the "wings" of the spectra for both materials. The temperature dependence of the sextets relates these spectral features to paramagnetic Fe3+ with rather slow spin-lattice relaxation rates which follow a T2 temperature dependence characteristic of a two-phonon Raman process.
The magnetic properties of Mn x Ga alloys critically depend on composition x, and the atomic‐scale origin of those dependences is still not fully disclosed. Molecular beam epitaxy has been used to produce a set of Mn x Ga samples (x = 0.7 ÷ 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy at ISOLDE/CERN, the Mn and Ga site‐specific chemical, structural, and magnetic properties of Mn x Ga are investigated as a function of x, and correlated with the magnetic properties as measured by superconducting quantum interference device magnetometry. Hyperfine magnetic fields of Mn/Fe (either at Mn or Ga sites) are found to be greatly influenced by the local strain induced by the implantation. However, In/Sn probes show clear angular dependence, demonstrating a huge transferred dipolar hyperfine field to the Ga sites. A clear increase of the occupancy of Ga lattice sites by Mn for x >1 is observed, and identified as the origin for the increased antiferromagnetic coupling between Mn and Mn at Ga sites that lowers the samples’ magnetization. Our results shed further light on the atomic‐scale mechanisms driving the compositional dependence of magnetism in Mn x Ga This article is protected by copyright. All rights reserved.
In-beam Mössbauer spectroscopy (IBMS) involves online measurement of Mössbauer γ-radiation emitted from excited atoms produced by nuclear reactions, Coulomb excitation, and radioisotope (RI) implantation. This chapter introduces past and current experimental techniques of IBMS and reviews some recent topics using 57Mn (T1/2=85 s) nuclei at RIKEN RI Beam Factory (RIBF) and thermal neutron capture reaction. It describes some experimental and application investigations of IBMS using a 57Mn beam at the RIKEN RIBF. The chapter introduces β-γ anticoincidence detection system. Finally it presents results of experimental setup for online Mössbauer spectroscopy using the thermal neutron capture reaction, 56Fe (n, γ) 57Fe.
A nucleus, like an atom, has discreet (quantized) ground and excited levels, the transition from the upper to the lower level being accompanied by gamma-ray emission. Nuclear gamma emission line spectrum is similar in this respect to atomic optical emission spectrum occurring as a result of the transition from the upper to the lower electronic level.
In this tutorial we describe the basic principles of the ion
implantation technique and we demonstrate that emission Mössbauer
spectroscopy is an extremely powerful technique to investigate the
atomic and electronic configuration around implanted atoms. The physics
of dilute atoms in materials, the final lattice sites and their chemical
state as well as diffusion phenomena can be studied. We focus on the
latest developments of implantation Mössbauer spectroscopy, where
three accelerator facilities, i.e., Hahn-Meitner Institute Berlin,
ISOLDE-CERN and RIKEN, have intensively been used for materials research
in in-beam and on-line Mössbauer experiments immediately after
implantation of the nuclear probes.
The monoclinic Sn2P2S6 and rombohedral layered SnP2S6 crystals have been investigated by 119Sn Mössbauer and X-ray photoelectron spectroscopies. By comparing the isomer shift, quadrupole splitting and binding energies for Sn 3d and 4d core levels in these crystals with such parameters for the metal tin and for different tin compounds it was found that in the Sn2P2S6 crystals Sn existed in the Sn2+ oxidation state and in the SnP2S6 crystals – in the Sn4+ state. Temperature dependences of the isomer shift and Debye–Waller factor satisfied the Debye model in the temperature interval of 6 – 300 K for the SnP2S6 crystals but deviated from such behavior at T > 250 K for Sn2P2S6 crystals. This peculiarity together with specific character of the temperature dependence of the quadrupole splitting were related to the role of the stereoactive Sn 5s2 lone pair electrons in the determined by three-well potential mechanism of the second order ferroelectric phase transition at 337 K in Sn2P2S6 crystals. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
At room temperature the 57Fe Mössbauer spectrum of chromite particles separated from the Martian meteorite Allan Hills 84001 is different from the Mössbauer
spectrum of natural chromite. One hypothesis for this discrepancy is that the chromite in Allan Hills 84001 is found in a
disordered or even amorphous state, possibly caused by a shock event. As a test of this hypothesis, we have attempted to amorphize
natural chromite by ion implantation. 57Fe emission Mössbauer spectra of chromite implanted with radioactive precursor 57Mn ions have been measured. The spectra can be analyzed in terms of two contributions, a crystalline chromite fraction and
a disordered fraction, attributed to disordered iron-chromium oxide or amorphous zones created by the implantation process.
The Mössbauer parameters of the amorphous fraction are indistinguishable from those deduced from the spectrum of the chromite
separate from Allan Hills 84001. A shock event is a likely explanation for these findings and with appropriate energy calibration
this may possibly be used to estimate the intensity of the shock event that once modified minerals in the meteorite Allan
Hills 84001.
In this review paper, a series of Mössbauer experiments on Fe in Si, spread over almost thirty years, is discussed. In early Mössbauer experiments, the role of precipitate formation during diffusion was insufficiently realized. Later, an apparent inconsistency was observed between ion implantation experiments by recoil implantation of Coulomb excited atoms and by conventional ion implantation. This inconsistency is removed by recent high-resolution Coulomb excitation recoil implantation studies and by ion implantation experiments at the temperature of liquid helium. These studies lead to an unambiguous identification of interstitial Fe and Co in Si. Finally, the present status of the theoretical predictions on the isomer shift of Fe in Si is reviewed.
Sn impurity atoms have been selectively inserted on the two different substitutional lattice sites in the III-V semiconductors GaP, GaAs, GaSb, InP, InAs, and InSb. Radioactive 119In+ and 119Sb+ ions which decay to the Mössbauer state of 119Sn have been implanted. From the isomer shifts determined in Mössbauer-emission experiments it is concluded that the implanted In and Sb ions selectively populate III and V sites, respectively.
Radioactive 57Mn+ ions have been implanted at the ISOLDE facility into variously doped silicon single crystals to study the lattice location and electronic configuration of 57Fe daughter atoms by 57Fe Mössbauer emission spectroscopy. Two single lines in the spectra are assigned to substitutional Fes and interstitial Fei, respectively. The isomer shift, i.e. electron density, for Fei has been found to depend on the doping of the crystals, this is attributed to the 0/+ band gap state of Fei. A model for the creation of both Fes and Fei in different charge states by the β-- decay of ion implanted 57Mn is proposed.
We report on a 119Sn Moessbauer emission spectroscopy study of 119Cs ion implanted single crystalline wurtzitic GaN. From the isomer shifts Sn is found substitutional on the III and V sites and partly associated with defects. The sample has been treated by Rapid Thermal Annealing and the effect of such treatment on the different lines is presented.
Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark
Laboratory for Solid State Physics, ETH-Zürich, CH-8093 Zürich, Switzerland
§
Instituto di Fisica de Liquidos y Sistema Biologica, Grupo de Fisica del Solido, CC 565, La Plata, Argentina
We describe results from in-beam Mssbauer implantation measurements for57Fe inn-type Si, inp-type Si, and in Ge. Ab initio calculations for the contact densities from density functional theory in the local-density approximation are presented and their results are employed to locate the type of position of Fe in the lattice of the host. Thus the different spectral features corresponding to interstitial and substitutional Fe in Si and Ge are unambiguously identified.
The lattice location of ion-implanted radioactive isotopes in metals and their interactions with defects in their own radiation
damage cascade are of importance for studies of the hyperfine interactions of such probe atoms by nuclear methods. Recent
results from Mössbauer and PAC experiments in particular are reviewed. Emphasis is put on lattice site identifications, which
can be inferred from measured lattice-dynamical and hyperfine interaction parameters of probe atoms. Some general conclusions
on the hyperfine interactions in interstitial- and vacancy-type complexes are drawn.
Channeling and blocking effects of electrons and positrons emitted from ion-implanted radioactive In dopants in silicon have been utilized for lattice location of In in parts-per-million concentrations. A majority of In atoms occupies substitutional sites after implantation at room temperature. Defect recovery, which is observed after annealing at 700 K, increases the channeling effects. These results corroborate conclusions from previous Mössbauer studies on the same system under identical implantation conditions. The nature of the damage cascades of the individual implanted impurity probe atoms is discussed.
The lattice vibrations of the two constituent atoms in the III–V semiconductors GaP, GaAs, GaSb, InP, InAs, and InSb have been studied experimentally by neutron diffraction and theoretically by calculations within the framework of various phonon models proposed in the literature for these compounds. The mean-square amplitudes (measured at 295 K) show a general increase with increasing lattice constant and seem furthermore to reflect the partial ionicity of the compounds. The different phonon models for the lattice dynamics are compared with each other and tested critically against the experimental data. Several models are found to be insufficient. The most satisfactory ones are some shell models.
119Sn Mössbauer impurity atoms have been implanted site-selectively on the two different substitutional lattice sites and their Debye temperatures have been determined. A rigorous result relating Debye temperatures of host and impurity atoms permits a simplified interpretation of the experimental results in terms of “Einstein-Debye force constants”. Both lower and higher force constants are deduced for the impurities as compared with the host atoms. Larger force constants are found on V sites than on the III sites for Sn in the Ga compounds, whereas the opposite holds in the In compounds. Further details can be obtained in an extended version of this paper available from the authors.
The magnetic hyperfine fields at isomeric 54Fe nuclei have been measured in Fe and Ni at 85 and 295 K by observation of the spin precession time differentially with high time resolution following recoil implantation. In iron at 85 K a unique defect structure has been detected by its well-resolved magnetic hyperfine field which differs considerably from the value for the substitional site. This defect is tentatively assigned to be a nonovacancy in the next-nearest neighborhood of the probe atom. The change of the hyperfine field by such a vacancy is similar to the change caused by adjacent impurity atoms in dilute Fe alloys.
Channeling and blocking effects of electrons and positrons emitted from
ion-implanted radioactive In dopants in silicon have been utilized for
lattice location of In in parts-per-million concentrations. A majority
of In atoms occupies substitutional sites after implantation at room
temperature. Defect recovery, which is observed after annealing at 700
K, increases the channeling effects. These results corroborate
conclusions from previous Mössbauer studies on the same system
under identical implantation conditions. The nature of the damage
cascades of the individual implanted impurity probe atoms is discussed.
We determine the nuclear quadrupole moment Q of the most important Mössbauer nucleus 57Fe by comparing experimental quadrupole splittings with calculated electric field gradients (EFG) for a large number of different Fe compounds. These ab initio calculations are based on the linearized-augmented plane-wave band structure method. From the slope of the linear correlation between theoretical EFGs and experimental quadrupole splittings a new value of Q57Fe = 0.16 b is deduced, twice as large as previously suggested. Our results should also stimulate nuclear physicists to revise nuclear structure shell model calculations of Q.
Radioactive 119Sb was implanted in an undoped GaN single crystal and Mössbauer spectra were taken of the 23.8 keV gamma rays of 119Sn emitted in its decay. A comparison with Mössbauer spectra of 119Sb implantated in other III-V compounds leads to the conclusion that Sb in GaN lands in the Ga site in contrast to all common III-V compounds, where Sb occupies the anion site. Calculations of electronegativity differences for Sb incorporation in either Ga or N site support our interpretation of the experimental data.
A complex Sn impurity defect created by room-temperature implantation of radioactive 119In+ ions in GaAs has been studied by Mossbauer emission spectroscopy on the 24 keV gamma transition of the daughter 119Sn. From the Mossbauer parameters for the Sn impurity atoms, the defect structure is proposed to consist of (nearly) substitutional Sn atoms on Ga sites associated with vacancies.
The resonance line of the 129-kev transition in Ir/sup 191/ was determined by a centrifuge method'' at a temperature of 88 deg K. A value of 6.5 x 10sup -6/ ev was obtained for the width of the 129-kev level and a lifetime of 1.0 x 10/sup -10/ sec. (J.S.R.)
The valence-electron configuration of substitutional Sn-donor and acceptor impurities in III–V compound semiconductors has been studied by Mössbauer spectroscopy on the 24-keV γ radiation of 119Sn. The Sn dopants have been implanted site-selectivity on the two inequivalent lattice sites. The isomer shift of the Mössbauer γ radiation has been determined. The Sn electronic configuration is found to depend on the dehybridization of the covalent bonds with increasing bond length and on the fractional ionicity fi of the host bonds. For materials with fi ≲ 0,3 no influence of ionicity is indicated and the Sn-donor and acceptor states have the same basic valence-electron configuration (Sn°) as in a homopolar semiconductor of equivalent bond length. For materials with higher ionicities, positive and negative charges are accumulated on the Sn atoms for donor and acceptor states, respectively.
We have utilized the electron-capture decay of119 Te to119Sb to produce isolated single Frenkel pairs in InSb. This effect is caused by the neutrino emission in the decay process which imparts a monoenergetic recoil of 12 eV to the119Sb atoms, thereby displacing about 20% of them to interstitial sites. Two distinct interstitial components can be observed. The process is traced by Mössbauer emission spectroscopy following the decay of119Sb to119Sn. The displacement thresholdEd is confined to 6 eVEd 119mTe isotopes.
A lattice defect recovery model is presented for ion-implanted n-type InP. It is based on Mössbauer studies utilizing implantations of radioactive precursor isotopes to the Mössbauer isotope 119Sn and takes into account recent results from emission channeling and positron annihilation studies.
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.
The electronic structure of impurity atoms (atomic number Z{=}1--56) in ferromagnetic iron is calculated self-consistently by the Green function method in the local density approximation of the spin-density functional approach. The calculation corresponds to an improvement and the extension of the previous work by Yoshida, Terakura and Kanamori. The mechanism proposed by them for the observed systematic variation in the hyperfine field and the nuclear spin-lattice relaxation time of impurity nuclei for substitutional impurities with Z>=10 is confirmed on the basis of the present self-consistent calculation which is carried out systematically for the first time.
Measurements of conversion electrons from the outermost valence shells of ion implanted, dilute impurity probe atoms in solids are reported. The measured spectra are directly related to the local density and angular momentum character of the occupied electron states at the probe atom. Thus the valence electron configuration of implanted probe atoms in solids has been investigated. First results for the probes Sn and Ge in Si, Ge, GaAs, Pd, and beta-Sn are compared to theoretical calculations.
The value, sign and temperature dependence of the magnetic hyperfine field Bhf have been determined by time differential perturbed angular correlation measurements for 111Cd impurities in a cubic vacancy defect in ferromagnetic fcc cobalt. The defect is formed by ion implantation of radioactive 111In ions. Comparison to analogous experimental results on Bhf for Cd and Sn impurities in fcc cobalt and nickel reveals distinct systematic trends in both value and sign of Bhf. These are discussed in the light of results from functional-density theories.
The radiation damage density in the microscopic vicinity of recoil implanted ions was measured at different implantation energies. The results demonstrate its independence from the recoil energy in the region of nuclear stopping above 10 keV.
Ion implantations of radioactive 57Mn+ into differently doped silicon single crystals held at 300–600K have been utilized for 57Fe Mössbauer studies of interstitial and substitutional Fe. Site and charge state assignments have been made on the basis of the determined hyperfine interaction parameters and Debye temperatures. Substantial fractions of substitutional 57Mn probe atoms are proposed to occur due to annealing reactions. This site is maintained in the subsequent decay to 57Fe by ⩽50% of the 57Fe atoms, the remainder is displaced by recoil effects into interstitial sites.
The amorphous phases of silicon, germanium, and a-tin have been studied by Mössbauer emission spectroscopy on ion-implanted, radioactive119mSn. Amorphous samples have been produced by ion implantations of various elements and by vacuumevaporation techniques. The same well-defined type of complex spectrum is observed for all investigated amorphous samples. These spectra are characterized by an increase in average isomer shift of (0.15±0.03) mm/s, a line broadening of 20±2%, and the same Debye temperature as compared with spectra of substitutional Sn in the respective crystalline host lattices. The spectra are proposed to originate from Sn atoms incorporated substitutionally in the amorphous host with distorted local surroundings. The recrystallization of the amorphous phase upon thermal and laser annealing has been monitored. After appropriate annealing, spectra characteristic of crystalline materials are observed for most samples. An exception are high-dose, inert-gas implanted samples where different complex defects seem to be formed in the annealing process.
Isomer shifts of the 37 ke V Mössbauer transition in 121Sb are determined for 121Sb constituents and impurities in III-V and II-VI compound semiconductors. Radioactive 121Xe isotopes decaying to 121Te, the 121Sb-parent isotope, were implanted at the ISOLDE facility at CERN. The nature of the electronic configuration of Sb-isoelectronic and Sb-donor and -acceptor impurities in semiconductors is discussed in terms of bond hybridization and ionicity. The influence of structural relaxation is pointed out.
Isomerieverschiebungen des 37 ke V-Mössbauer Übergangs in 121Sb werden für 121Sb-Selbst- und Fremdatome in III-V- und II-VI-Verbindungshalbleitern bestimmt. Radioaktive 121Xe-Isotope, die zu 121Te, dem Mutterisotop von 121Sb, zerfallen, wurden an der ISOLDE-Anlage des CERN implantiert. Die Art der elektronischen Konfiguration für Sb als isoelektronisches bzw. Donatorund Akzeptor-Fremdatom wird im Hinblick auf Hybridisierung und Ionizität der Bindungen diskutiert. Ein Einfluß struktureller Relaxationen wird berücksichtigt.
An experimental and theoretical study has been undertaken on the lattice dynamics of 119mSn atoms on substitutional sites in the diamond lattices of the group-IV elements silicon, germanium, and α-tin. Experimentally, the temperature dependences of the Debye-Waller factors for the 24-keV γ transition of 119Sn have been determined by Mössbauer-emission spectroscopy in the temperature interval between 77 and 297 K. The 119mSn was implanted on substitutional sites in the host lattices with an isotope separator. Polycrystalline α-tin material was also grown from metallic β-tin containing small amounts of radioactive 119mSn. Relative Debye-Waller factors for different materials or for different temperatures have been determined with an accuracy of ≲2% by a resonance counting technique for the emitted 24-keV γ radiation. The measured temperature dependences of the Debye-Waller factors may be compared to theoretical results obtained from various models. A critical review on the applicability and the relevance of various dynamical models for substitutional impurity atoms in a diamond lattice is given. New calculations are presented in the framework of an extension of Mannheim's model to the diamond and zinc-blende lattices. The calculations are based on the phonon densities of states from an adiabatic bond-charge model for the group-IV elements. Unexpected large changes of the impurity-host force constants as compared to the host-host force constants are found for substitutional Sn in silicon and germanium, whereas the result from the model for the perfect lattice agree reasonably well with the experimental values for α-tin. Possible reasons for the force-constant changes are discussed.
It is suggested that the recently proposed two-parameter model impurity potentials of the shallow-level group-V donors in silicon may be scaled to give the model impurity potentials of the deep-level group-VI donors of the same row in the Periodic Table. It is shown that in the case of sulfur this suggestion follows naturally from examining the behavior of the true impurity potential in the central-cell region. By extending the multivalley effective-mass approximation (EMA) to singly ionized sulfur donors in silicon, using recently available optical data, it is found that indeed the potential parameters of sulfur are essentially identical to those of phosphorus, as expected from this model. A heliumlike model in the multivalley EMA is also developed and applied to the two-electron neutral group-VI donors in silicon. The calculated energy of neutral sulfur agrees well with observed thermal activation energy. Using the scaled model impurity potentials of As, Sb, and Bi, the energies of substitutional Se, Te, and Po in Si are predicted. It is also suggested that the same procedure may be applied to acceptor states. Holes bound to deep-level acceptors may be described simply by scaling the impurity potentials of the shallow-level group-III acceptors. The effects of the Δ5 valence band and the Δ2′ conduction band on the deep-level donors are discussed. These effects are found to be small for singly ionized sulfur in silicon.
An experimental and theoretical study has been undertaken on the lattice dynamics of 119mSn atoms on substitutional sites in the diamond lattices of the group-IV elements silicon, germanium, and α-tin. Experimentally, the temperature dependences of the Debye-Waller factors for the 24-keV γ transition of 119Sn have been determined by Mössbauer-emission spectroscopy in the temperature interval between 77 and 297 K. The 119mSn was implanted on substitutional sites in the host lattices with an isotope separator. Polycrystalline α-tin material was also grown from metallic β-tin containing small amounts of radioactive 119mSn. Relative Debye-Waller factors for different materials or for different temperatures have been determined with an accuracy of ≲2% by a resonance counting technique for the emitted 24-keV γ radiation. The measured temperature dependences of the Debye-Waller factors may be compared to theoretical results obtained from various models. A critical review on the applicability and the relevance of various dynamical models for substitutional impurity atoms in a diamond lattice is given. New calculations are presented in the framework of an extension of Mannheim's model to the diamond and zinc-blende lattices. The calculations are based on the phonon densities of states from an adiabatic bond-charge model for the group-IV elements. Unexpected large changes of the impurity-host force constants as compared to the host-host force constants are found for substitutional Sn in silicon and germanium, whereas the result from the model for the perfect lattice agree reasonably well with the experimental values for α-tin. Possible reasons for the force-constant changes are discussed.
The behavior of interstitial iron in high-resistivity dislocation-free silicon has been studied by annealing and by electron irradiation and subsequent annealing. Annealing of iron-doped samples at temperatures above 120°C yielded one, new electron paramagnetic resonance (EPR) spectrum labeled Si-NL22. For the corresponding center we suggest a cluster of four iron atoms in a trigonal arrangement. Irradiation at about 20°C yielded many new EPR spectra, part of which are related with iron. One center with only one iron atom in trigonal symmetry was identified. Its spectrum is labeled Si-NL19. As a model we propose a-distorted substitutional iron atom. Four centers involving two equivalent iron atoms are formed. The spectra and tentative models are Si-NL20(2Fei+V)-, Si-NL21(2Fei+2V)+/-, Si-NL24(2Fei)+, and Si-NL25(2Fei+V)+. Many spectra without resolved hyperfine interactions with iron were observed. Only one of these spectra was analyzed. This spectrum, labeled Si-NL23, has only triclinic symmetry. The formation of iron-iron pairs and the disappearance of isolated interstitial iron during irradiation at only 20°C shows that iron is subject to radiation-induced diffusion. From our study we conclude that in the absence of dislocations or acceptors as precipitation centers, isolated interstitial iron does not become substitutional during annealing. Instead it forms pairs and eventually larger clusters.
Isomer shifts and Debye temperatures for substitutional 119Sn impurity atoms in the FCC metals Al, Ag, Au, Cu, Pb, Pd, Pt and Rb were determined by Mossbauer emission spectroscopy on the 24 keV gamma radiation of 119Sn. The radiation sources were prepared by ion implantations of radioactive 119In and 119Sb isotopes into single crystals and high-purity foils of the respective metals. Correlations are discussed between measured isomer shifts and electronic properties of the host materials (the Fermi-level free-electron density nF1/3 and the cell-boundary electron-density parameter nWS1/3 from the Miedema theory of alloy formation). The Debye temperatures for 119Sn are interpreted in terms of the Einstein-Debye and the Mannheim models for impurity lattice vibrations.
Isomer shifts have been determined for the 37 keV gamma -radiation of substitutional 121Sb impurities acting as donors in elemental group IV and as acceptors in group VI sites in II-VI compound semiconductors. Site-selective ion implantation techniques of radioactive precursors to the 121Sb isotope combined with suitable annealing techniques have been applied to achieve the impurity lattice locations. The isomer shifts, and thus the contact electron densities, for the Sb donors and acceptors are remarkably similar to those for (iso-electronic) Sb on V sites in III-V semiconductors. This leads to the general qualitative conclusion that the nominal electronic charges of +or-e giving rise to either the donor electron or the acceptor hole potential are not highly localised on the donor or acceptor impurity. The small but systematic differences observed for donor and acceptor ground-state contact densities indicate, however, a stronger bonding of the additional electronic charge accumulated on the acceptor impurity than for the donor electron on the donor impurity. A predominant s character is further indicated for the Sb donor ground state from very small differences in electron density for neutral and ionised donors, whereas a prevailing p character is evident for the acceptor state. Results for impurity-defect complexes are discussed briefly.
Radioactive 119In and 119Sb, which both populate the 24 keV Mossbauer level of 119Sn in their nuclear decay, have been incorporated on the two inequivalent substitutional lattice sites in InP. The electronic and vibrational properties of Sn impurity atoms on these sites have been investigated by Mossbauer spectroscopy. The formation and the annealing properties of complex impurity defects have been studied. For substitutional Sn atoms the electronic configuration of the impurities reflects the donor and acceptor character on the two different lattice sites. Debye temperatures determined for the two sites indicate analogous decreasing Sn bond force constants as compared with host-host force constants. Two complex defects are attributed to the association of vacancies to Sn atoms on the two substitutional sites.
Radioactive119mCd+ and119In+ ions have been implanted into CdTe single crystals at temperatures between 50–300K. Radiogenic defects formed with the daughter119Sn have been investigated by Mössbauer spectroscopy of the emitted 24 keV γ radiation. All Mössbauer spectra could be analysed
consistently with three lines. These are proposed to be due to substitutional Sn on Cd sites in two different charge states
and to Sn-vacancy complexes. The corresponding In-parent vacancy complexes anneal at 120K and above 300K.
A review is presented of recent applications of Mssbauer spectroscopy that focus on determining the fate of doped impurities in semiconductors, primarily GaAs, Ga1–xAlxAs and Si. Other solar energy materials and processes which are discussed include amorphous SiH-based alloys, chalcopyrites, transparent conducting oxides, photochemical processing via semiconductor powders in electrolytes, mirror making, and plant photosynthesis.
Mssbauer spectroscopy on ion-implanted sources of119Cd in single-crystals was applied to study the electric field gradients (EFG) at119Sn in three non-cubic metals. The signs and magnitudes determined are in agreement with presently known systematics. The measured isomer shifts and the recoilless fractions are discussed.
The lattice location and electrical activity of ion implanted Sn in InP after rapid thermal annealing has been determined by, respectively, Mössbauer spectroscopy using the 119mSn isomer and differential Hall resistivity measurements. Sn is preferentially located on the In sublattice for concentrations below 2×1019 cm-3 resulting in a high electrical activation and mobility. For Sn concentrations above 2×1019 cm-3, in addition different electrically inactive Sn complexes are observed. No indication of a Sn location on P‐sublattice sites has been found.
A question of fundamental interest in ion implantation metallurgy concerns the lattice site which the implanted ions will occupy at the end of their trajectories. This review describes the results of a systematic study on the basic mechanisms which determine the lattice site occupation of impurities implanted in metals. Current models on the prediction of the substitutionality are reviewed and the mechanisms of impurity-point-defect interactions on the lattice site occupation are outlined. Recent experimental results are reviewed which demonstrate that implanted ions will preferentially occupy substitutional lattice sites within the relaxation phase of the collision cascade. Their displacements from the substitutional sites are due to the interaction with point defects which leads to the formation of defect-impurity complexes. These processes occur during the cooling phase of the cascade and at temperatures at which point defects are mobile. The probability of the complex formation increases as a function of the heat of solution and the size-mismatch energy.
Mössbauer experiments applying short-lived radioactive isotopes from the ISOLDE facility at CERN have been reported recently. To fully utilize the high activity (10–100 mCi), which is obtainable for some Mössbauer parent isotopes, a special counting technique is required.The count rate limit of the ordinary Mössbauer transmission technique can be overcome by resonance-scattering techniques. Resonance detectors of the parallel-plate avalanche counter type, detecting conversion electrons, are demonstrated to be a favourable solution for 119Sn Mössbauer spectroscopy. The merits of such detectors compared with other scattering and with transmission techniques are discussed. An experimental set-up for on-line measurements at liquid-nitrogen temperature is described.
We present detailed calculations of the electronic structure and the hyperfine fields of 3d and 4d impurities in nickel. The calculations are based on the local-density approximation of density-functional theory and the Korringa-Kohn-Rostoker Green's-function method for impurity calculations. We self-consistently calculate the local moments and hyperfine fields of the impurities and their nearest neighbors. We derive new formulas for the proper relativistic generalizations of the contact, orbital, and dipolar contributions to the hyperfine field and explicitly calculate relativistic corrections to the contact interaction which are important for 4d impurities. The hyperfine fields can be split up into local and transferred contributions which are directly related to the local moments and to the moments of the neighboring atoms. The calculated hyperfine fields are in reasonable agreement with the experimental data.
Diffusion of interstitial iron in silicon could be observed on an atomic scale for the first time: Coulomb-excited 57Fe nuclei were implanted into high-purity n-type silicon Mössbauer spectra were recorded at temperatures between 300 and 850 K. The diffusional broadening of one spectral component identified as interstitial Fe could be observed. The isomer shift of interstitial Fe in Si was determined and the assumption that one single mechanism is governing the diffusion of Fe in Si between 300 and 1500 K is confirmed.
The behavior of iron in silicon during low-temperature annealing was studied. The influence of electric fields on the underlying defect reactions is detailed. The depth profiles reveal Fei outdiffusion and no precipitation in the bulk up to 470 K. In the presence of electric fields, the kinetics differ considerably and can be understood in terms of carrier-emission-limited iron drift. Consequences of this mechanism are discussed. The outdiffusion and drift data indicate a charge-state-dependent diffusion mechanism, in contrast to the generally accepted lack of any charge-state effect.
The microscopic nature of defects in ion-implanted amorphous Si has been probed using Mössbauer spectroscopy. Analysis of the 119Sn Mössbauer spectra obtained from 119Sb in amorphous Si requires two independent sites for Sb in amorphous Si. Direct comparison of the isomer shifts and Debye temperatures of these lines with data from crystal Si demonstrates that the sites are a substitutional network site and an Sb-vacancy complex. The data suggest that point defects analogous to the crystal vacancy exist in the amorphous Si structure.
A Comment of the Letter by G. N. van den Hoven et al., Phys. Rev. Lett. 68, 3714 (1992).
The decay of Te to Sb via electron capture is utilized to produce isolated, single Frenkel pairs in InSb. This is caused by neutrino emission, which imparts a recoil of 12 eV to the Sb atoms, thereby displacing about 20% of them into interstitial sites. The effect is traced by M\"ossbauer emission spectroscopy following the decay of Sb to Sn. The displacement threshold is confined to eV from auxiliary experiments employing isotopes.
Moustakas and the ISOLDE Collaboration
- M Fanciulli
- M Lindroos
- G Weyer