Substitutional Lattice Sites Research Articles

Mössbauer spectroscopy on Fe impurities in diamond

Radioactive 57 Mn + ions have been implanted into a natural diamond held at 300–1100 K for Mossbauer spectroscopy studies on the 14 keV γ -radiation emitted in the decay of the 57 Fe daughter atoms. The Mn parent isotopes are found to be implanted into defect sites of non-cubic symmetry, which anneal partially with increasing temperature during the 57 Mn lifetime. This is concluded to lead to their incorporation on substitutional lattice sites. This site is inherited by the majority of the 57 Fe daughter atoms, however, a sizeable fraction is expelled into interstitial sites owing to the recoil in the nuclear -decay. Thus the Fe isomer shifts in both lattice sites have been determined.

Impact of high B concentrations and high dislocation densities on Au diffusion in Si

We report diffusion experiments of Au in highly dislocated, undoped and homogeneously doped Si with B concentrations of 3.0×1019 and 1.7×1020 cm−3. After Au diffusion at temperatures between 800 and 1100 °C, Au profiles were measured by means of neutron-activation analysis in conjunction with serial sectioning. The diffusion of Au in dislocated Si is accurately described on the basis of the kick-out mechanism and an additional reaction which takes into account the capture of interstitial Au at dislocations. The profiles yield data for the boundary concentration CAu0 and the Au diffusion coefficient DAu whose product CAu0DAu equals the transport capacity CAuieqDAui of interstitial Aui. With increasing B doping Au diffusion increases showing that interstitial Au is singly positively charged. An enhanced boundary concentration CAu0 and simultaneously retarded Au diffusivity was observed at temperatures below 1000 °C which provides direct evidence of Au segregation at dislocations. A segregation enthalpy of 2.7 eV for the energy difference of Au atoms dissolved on substitutional lattice sites and Au atoms trapped at dislocations was determined.

Copper related diffusion phenomena in germanium and silicon

This paper concerns diffusion related properties of Cu in Ge and Si. In Ge, Cu prefers to occupy a substitutional lattice site whereas Cu in Si is mainly dissolved on an interstitial position. This difference in the lattice site occupancy is also reflected in the diffusion behaviour. Whereas Cu diffusion in Ge is accurately described on the basis of the dissociative mechanism, which involves Ge vacancies for the conversion of Cu from interstitial to substitutional sites, the diffusion of Cu in Si is mainly mediated by the direct interstitial mechanism. The specific diffusion behaviour of Cu in Ge and Si as well as its electronic properties strongly affect other diffusion related phenomena such as the precipitation and gettering of Cu. Detailed understanding of the latter processes are of fundamental technological significance to keep the contamination of electronic devices during processing to a harmless level. The various aspects of Cu in Ge and Si, which are the result of former and recent experiments, are reviewed in this paper.

The different behavior of nitrogen and phosphorus as n-type dopants in SiC

In this work, the experimentally observed differences in the behavior of nitrogen and phosphorus as n-type dopants in silicon carbide have been investigated within the framework of density functional theory. A key to the understanding of complex formation is the investigation of the dynamics of these dopant atoms. We have found essential differences in the recombination behavior of nitrogen and phosphorus atoms with vacancies. In contrast to nitrogen, which prefers to be built in on the carbon site exclusively (NC), our dynamical calculations show phosphorus to be built in on both substitutional lattice sites (PSi and PC). By this, PC-containing complexes should be considered as possible candidates for P-related electron paramagnetic resonance spectra in 6H–SiC which up to now have not been explained consistently. Furthermore, we explain why the formation of inactive complexes is likely in case of nitrogen but not in case of phosphorus.

Mössbauer spectroscopy on Fe impurities in germanium

Mössbauer experiments have been performed on the 14keV γ-radiation of 57Fe emitted in the decay of ion-implanted (60keV, <1012/cm2), radioactive 57Mn+ ions in germanium single crystals held at temperatures of 77–800K. At temperatures ⩽300K, a broad asymmetric quadrupole-split doublet, assigned to Fe in implantation-damage related sites, dominates the spectra. At ∼350K these defects anneal during the 57Mn lifetime (2.2min) and the Mn atoms are incorporated mainly on substitutional lattice sites. An average recoil energy of 40eV imparted on the 57Fe daughter atoms in the nuclear decay expels the majority of the probe atoms into tetrahedral interstitial sites. A sizable fraction traps the mobile vacancy created in the recoil event, forming Fei-V complexes within the lifetime of the 57mFe Mössbauer state (140ns), which are stable up to ≈700K.

Aggregate nitrogen in synthetic diamond

Nitrogen is a commonly observed impurity in natural and synthetic diamond, yet there are still many questions in regard to its incorporation in the material. In all three common forms of diamond: natural, high-pressure high-temperature synthetic, and chemical vapor deposition (CVD) diamond; nitrogen has been observed in substitutional lattice sites. This configuration results in an unpaired electron which gives rise to an easily observed paramagnetic resonance. In addition, aggregate forms of nitrogen, such as A-center (paired nitrogen), B-center, and platelet-related nitrogen are the predominant form of nitrogen observed in natural Type Ia diamonds. These aggregate forms have yet to be observed unambiguously in any form of synthetic diamond. Here, we show evidence, based on quantitative secondary ion mass spectrometry and electron paramagnetic resonance spectroscopy measurements, of nonparamagnetic nitrogen in some CVD diamond films. Further examination of the symmetry-disallowed one-phonon absorption in the infrared spectra of these films indicates the presence of aggregate nitrogen in these materials.

Ferromagnetic Ga1−xMnxAs produced by ion implantation and pulsed-laser melting

We demonstrate the formation of ferromagnetic Ga1−xMnxAs films by Mn ion implantation into GaAs followed by pulsed-laser melting. Irradiation with a single excimer laser pulse results in the epitaxial regrowth of the implanted layer with Mn substitutional fraction up to 80% and effective Curie temperature up to 29 K for samples with a maximum Mn concentration of x≈0.03. A remanent magnetization persisting above 85 K has been observed for samples with x≈0.10, in which 40% of the Mn resides on substitutional lattice sites. We find that the ferromagnetism in Ga1−xMnxAs is rather robust to the presence of structural defects.

Characterization of metal-doped TiO 2 films by RBS/channeling

Epitaxial anatase and rutile TiO 2 films doped with Cr, Nb, Ta and W are successfully prepared by pulsed laser deposition. Anatase TiO 2(0 0 1) films are grown on SrTiO 3(0 0 1) and LaAlO 3(0 0 1) substrates. Rutile TiO 2(1 0 0) films are also obtained on α-Al 2O 3(0 0 0 1) substrates. The typical temperature of substrates during the deposition is 500 °C and the pressure of O 2 gas is 4.7 Pa. The crystal quality of films, crystallographic relationships, and the concentration of doped metals were assessed by X-ray diffraction and Rutherford backscattering spectroscopy with channeling. The high quality metal-doped epitaxial TiO 2 films are obtainable through the post-deposition annealing at 800 °C in air for 5 h. The concentration of doped metals is in the range from 0.2 to 1.5 at.%. RBS/channeling analysis reveals that doped Cr, Nb, Ta and W atoms are incorporated into the substitutional lattice sites of the anatase and rutile TiO 2.

Role of carbon in GaN

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal–organic chemical-vapor deposition. These samples have been characterized by a variety of electrical and optical techniques, and the resulting experimental data are compared to density-functional-theory calculations of the formation energies and electronic states of substitutional and interstitial carbon in hexagonal GaN. We find that in samples where the silicon concentration exceeds that of carbon, carbon sits in the N substitutional site, acting as an acceptor and partially compensating the material. However, when carbon densities exceed those for Si, GaN becomes semi-insulating due to carbon occupation of both N and Ga substitutional lattice sites, and a new luminescence peak appears at ∼3 eV. Calculated formation energies of carbon in both sites are strong functions of both the Fermi level and growth stoichiometry. The former dependence gives rise to self-compensation when [C]&amp;gt;[Si] because the formation energy of the Ga substitutional configuration (the donor state) becomes equal to that of the N substitutional site, effectively pinning the Fermi level as it approaches midgap. Our results suggest that effective p-type doping of GaN can only be achieved under Ga-rich growth conditions.

Spectra and energy levels of Tb3+(4f 8) in GaN

The cathodoluminescence spectra of Tb3+ ions implanted and annealed in GaN epilayers have been analyzed and modeled according to its incorporation into a substitutional Ga lattice site having D2 point group symmetry. High-resolution emission spectra obtained at 11 K between 380 and 750 nm are identified as transitions from the G65,D35 combined manifold and the D45 manifold to individual energy (Stark) levels of the FJ7 multiplet manifolds. All intense peaks and nearly all weaker peaks and bands are associated with emission from Tb3+ ions in a similar site. The observed crystal-field splitting of the F67,F57, and F47 manifolds is much too large for any lattice-sum predictions based on Tb3+ ions in C3v sites. Calculated splittings and relative intensities based on a site symmetry of D2 give the best overall agreement with the observed data.

Solute-defect interactions in a metastable Pb–Ni alloy formed by high-fluence ion implantation

Implantation of 240 keV Pb+ ions into a Ni (110) single crystal to a fluence of 1016 cm−2 at room temperature and 470 K, respectively, resulted in the formation of a metastable supersaturated Pb–Ni solid solution with a maximum lead concentration of 2.4 at. %. Rutherford backscattering/channeling analysis and transmission electron microscopy have shown that in the as-implanted state most of the Pb atoms were distributed on substitutional lattice sites in the host matrix while a small fraction of Pb was confined within nanoscale precipitates. Most of the precipitates, with sizes ranging from 2 to 15 nm, were single crystalline although bi-, tri-, and tetracrystals were occasionally observed. Upon heating, decomposition of the metastable alloy was observed, with strong outdiffusion of a large fraction of Pb to the surface. By means of angular scan channeling analysis, the lattice location of the implanted Pb atoms was followed directly during in situ isochronal annealing at different temperatures up to 860 K. Thermally activated formation of Pb atom–vacancy complexes was found to cause originally substitutional Pb atoms to change to different types of lattice site occupation. The regular and distorted substitutional, octahedral interstitial, and random locations of the Pb atoms appeared to be strongly correlated with the temperature of the annealing.

Carbon incorporation pathways and lattice sites in Si1−yCy alloys grown on Si(001) by molecular-beam epitaxy

We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si1−yCy alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature Ts (380 °C–680 °C) and C fraction y (0–0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si1−yCy(001) layers grown at Ts⩽480 °C. Increasing Ts⩾580 °C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and cross sectional transmission electron microscopy reveal surface roughening at Ts⩾580 °C with the formation of bulk planar structures, interconnected by {113} segments, that are periodic along [001] with a periodicity which decreases with increasing Ts. We interpret the planar structures as layers of C-rich Si1−yCy which form in the presence of excess surface C resulting from segregation. Our ab initio density functional calculations show that substitutional C arranged in an ordered Si4C phase is 0.34 eV per C atom more stable than isolated substitutional C atoms.

Erbium implantation in strontium titanate

We report on the lattice location of Er in SrTiO 3 single crystals using the emission channeling technique. The angular distribution of conversion electrons emitted from 167 m Er ( T 1/2=2.27 s) was monitored with a position-sensitive detector following the room-temperature 60 keV implantation of the precursor isotope 167 Tm ( T 1/2=9.25 d) to a dose of 2×10 12 at./cm 2. The results for the sample annealed in vacuum at 610 °C for 15 min provide direct evidence that Er occupies both Sr and Ti substitutional lattice sites. In addition, thermal recovery of lattice damage was also studied with Rutherford backscattering/channeling for SrTiO 3 implanted to doses of 5×10 14 and 5×10 15 Er/cm 2. We further comment on preliminary photoluminescence results from these samples.

Detection of Substitutional and Interstitial Fe in Silicon by Mössbauer Spectrocopy

A novel approach to create radioactive, substitutional and interstitial 57Fe atoms in silicon at elevated temperatures is reported, which allows for Mössbauer studies of the emitted 14 keV γ-radiation. Radioactive 57Mn + (T1/2 = 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences <1012/cm2 into silicon crystals held at 400–800 K. As a result of the annealing of the radiation damage during the 57Mn lifetime, the Mn atoms are found on substitutional lattice sites. In the subsequent decay of 57Mn to the Mössbauer state of 57Fe an average recoil energy of 40 eV is imparted on the daughter atom. This leads to a relocation into interstitial sites for the majority of the Fe atoms, the remainder remains on substitutional sites. Thus the Fe electronic and vibrational properties on both lattice sites can be studied. A few diffusional jumps of interstitial Fe during the lifetime of the Mössbauer state (T1/2 = 100 ns) have been observed by a broadening of their Mössbauer line at >450 K. At the highest temperatures a new line occurs in the spectra, the possible nature of this line will be discussed.

Detection of diffusional jumps of interstitial Fe in silicon by Mössbauer spectroscopy

A technique to create radioactive, interstitial 57Fe atoms in silicon at diffusion temperatures is reported, which allows for the detection of their diffusional jumps by the resulting line broadening in the Mössbauer spectra of the emitted 14 keV γ-radiation. Radioactive 57Mn + ( T 1/2=1.5 min) ions have been implanted into silicon crystals held at 400–700 K. As a result of the annealing of the radiation damage during the 57Mn lifetime, the Mn atoms are found on substitutional lattice sites. In the subsequent decay of 57Mn to the Mössbauer state of 57Fe, an average recoil energy of 40 eV is imparted on the daughter atom. This leads to a replacement into interstitial sites for the majority of the Fe atoms, the remainder remains on substitutional sites. From the temperature dependent line broadening the diffusion coefficients for neutral and ionised interstitial Fe have been deduced.

Synthesis of silicon-based infrared semiconductors in the Ge-Sn system using molecular chemistry methods.

Growth reactions based on a newly developed deuterium-stabilized Sn hydride [(Ph)SnD(3)] with Ge(2)H(6) produce a new family of Ge-Sn semiconductors with tunable band gaps and potential applications in high-speed, high-efficiency infrared optoelectronics. Metastable diamond-cubic films of Ge(1-x)Sn(x) alloys are created by chemical vapor deposition at 350 degrees C on Si(100). These exhibit unprecedented thermal stability and superior crystallinity despite the 17% lattice mismatch between the constituent materials. The composition, crystal structure, electronic structure, and optical properties of these materials are characterized by Rutherford backscattering, high-resolution electron microscopy, and X-ray diffraction, as well as Raman, IR, and spectroscopic ellipsometry. Electron diffraction reveals monocrystalline and perfectly epitaxial layers with lattice constants intermediate between those of Ge and alpha-Sn. X-ray diffraction in the theta-2theta mode shows well-defined peaks corresponding to random alloys, and in-plane rocking scans of the (004) reflection confirm a tightly aligned spread of the crystal mosaics. RBS ion-channeling including angular scans confirm that Sn occupies substitutional lattice sites and also provide evidence of local ordering of the elements with increasing Sn concentration. The Raman spectra show bands corresponding to Ge-Ge and Sn-Ge vibrations with frequencies consistent with random tetrahedral alloys. Resonance Raman and ellipsometry spectra indicate a band-gap reduction relative to Ge. The IR transmission spectra suggest that the band gap decreases monotonically with increasing Sn fraction. The synthesis, characterization, and gas-phase electron diffraction structure of (Ph)SnD(3) are also reported.

Local atomic structure of α-Pu

The local atomic structure of \ensuremath{\alpha}-Pu was investigated using x-ray absorption fine structure (XAFS) spectroscopy. XAFS spectra were obtained for a zone-refined \ensuremath{\alpha}-Pu and the results were compared to 32-year-old and Ce-doped (0.34 at. %) samples. X-ray diffraction (XRD) patterns were also measured for the zone-refined and 32-year-old materials. The extent of the Bragg peaks showed that amorphization of the 32-year-old sample had not occurred despite the prolonged exposure to self-radiation. Analogous to metastable \ensuremath{\delta}-Pu alloys, the local atomic structure around Pu for the zone-refined material shows the possible presence of noncrystallographic Pu-Pu distances. Conversely, the Ce and the 32-year-old sample show no evidence for such noncrystallographic distances. Disorder in the Pu local environment was found to be impurity dependent. The Ce-doped sample presented a larger Pu-Pu nearest neighbor disorder than the aged sample, although the total amount of Am, U, and He impurities was actually higher in the aged sample. The local environment around U and Ce impurities is consistent with these elements being in substitutional lattice sites. In addition, U and Ce do not introduce significant lattice distortion to their nearest neighbors. This is consistent with disorder being more related to the perturbation of the coupling between the electronic and crystal structure, or the Peierls--Jahn-Teller distortion that generates the monoclinic \ensuremath{\alpha}-Pu structure, and less to strain fields produced in the vicinity of the impurities.

Electron and hole impact excitation of Er in MBE grown Si:O and Si 1− yC y diodes

We have investigated the doping and electroluminescence (EL) properties at 1.54 μm of erbium ions in Si:O and Si 1− y C y :Er layers completely grown by MBE. Erbium in Si 1− y C y :Er films shows a lower doping density ( p=+1.6×10 17 cm −3) than erbium in Si:O ( n=−3×10 18 cm −3). The energy levels responsible for doping and optical erbium activation are different because different activation energies are observed. To realise both maximum luminescence output and low doping concentration, it is important to incorporate the carbon on substitutional lattice sites. The efficiency for electron and hole excitation by impact processes is compared. Electrons are about 5000× more efficient for impact excitation of Er 3+ than holes.

Interstitial carbon-substitutional tin center in silicon (?)

Electron-irradiated n-type silicon doped with carbon and tin has been studied by infrared absorption spectroscopy. In addition to the well known signals of interstitial carbon, C i, and the dicarbon center, C s–C i, three new lines at 873.5, 1025, and 6875 cm −1 are observed in the spectra. The 873.5 and 1025 cm −1 lines are located in a region characteristic of the local vibrational modes of interstitial carbon in silicon. Similarly, the frequency of the 6875 cm −1 line is very close to the electronic transition of C i at 6903 cm −1 . Based on the annealing kinetics of these lines as well as those belonging to C i and C s–C i, we tentatively identify the center responsible for the 873.5, 1025, and 6875 cm −1 lines as interstitial carbon perturbed by tin, which is located at one of the neighboring substitutional lattice sites.

Lattice location of implanted Cu in highly doped Si

We report on the lattice location of ion-implanted Cu67 in p+- and n+-Si using the emission channeling technique. Following room-temperature implantation, the majority of Cu was found on near-substitutional sites in both p+- and n+-Si. Annealing in the temperature range 200–600 °C resulted in changes of near-substitutional Cu to random sites in p+-Si, while in n+- Si all of the near-substitutional Cu was converted to ideal substitutional lattice sites. The activation energy for dissociation is estimated to be 1.7–2.0 eV for near-substitutional Cu in p+-Si and 2.9(2) eV for ideal substitutional Cu in n+-Si.