In-beam Mössbauer spectroscopy:57Fe in Si and Ge

Fe has been studied in the semiconductors Si and Ge with the Coulomb excitation recoil implantation technique in a wide temperature range. In the case of Fe inSilicon it was found that one third of the implanted atoms land on interstitial sites. The long range diffusion of these atoms could be observed microscopically at temperatures around 600 K. The isomer shift of interstitial Fe in Si was determined. The remaining atoms exhibit a strong quadrupole splitting on disturbed sites. This component seems to relax into a state with higher symmetry above 700 K. InGermanium a similar situation is found. Whereas iron on disturbed sites dominates the spectra, the direct implantation into interstitial sites is also observed below 200 K. At higher temperatures the substitutional position is preferred. The isomer shifts for interstitial and substitutional Fe in Si and in Ge are in good agreement with calculated electron densities.     

Isomer-shift of interstitial and substitutional iron in silicon and germanium

We describe results from in-beam Mössbauer 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.Dedicated to U. Gonser on the occasion of his 70th birthday     

Density-functional theory study of half-metallic heterostructures: interstitial Mn in Si

Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer delta doping of interstitial Mn (Mn(int)) are half-metallic. For Mn(int) concentrations of 1/2 or 1 layer, the states induced in the band gap of delta-doped heterostructures still display high spin polarization, about 85% and 60%, respectively. The proposed heterostructures are more stable than previously assumed delta layers of substitutional Mn. Contrary to widespread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials.     

In-beam Mössbauer spectroscopy on isomer shift and diffusion of interstitial Fe

In-beam Mössbauer spectroscopy (IBMS) is used to study single isolated⁵⁷Fe impurities after implantation in metals and semiconductors with very restricted or even vanishing solubility for Fe. From the Mössbauer parameters it can be inferred that the Fe implants take up substitutional as well as interstitial sites. The strongly increased electron density at the interstitial position in metals is qualitatively explained by the pressure resulting from the small interstitial volume. In Si, Sc and Pb exponential line broadening due to interstitial diffusion has been observed. Additional information on the dynamic behavior and local magnetic structure in some of the systems presented comes from perturbed angular distribution experiments (PAD) performed on an isomeric state of⁵⁴Fe.     

Isotope shifts for the P,Q, R lines in indium-doped silicon

We have measured the isotope shift of the P and R luminescence lines in Si doped with In and diffused with isotopically enriched Fe. We find no measurable shift in the R line. Similar experiments on the luminescence lines attributed to an isolated Fe complex show no shift in the previously identified “phonon replicas.” Theoretical calculations of defect phonon resonances near 9 meV for interstitial and substitutional Fe predict an isotope shift of 0.17 meV for ⁵⁶Fe and ⁵⁴Fe. This value is in disagreement with the observed lack of a shift.     

Mössbauer study of Fe in 3C-SiC following 57Mn implantation

Our investigations on substitutional and interstitial Fe in the group IV semiconductors, from ⁵⁷Fe Mössbauer measurements following ⁵⁷Mn implantation, have been continued with investigations in 3C-SiC. Mössbauer spectra were collected after implantation and measurement at temperatures from 300 to 905 K. Following comparison with Mössbauer parameters for Fe in Si, diamond and Ge, four Fe species are identified: two due to Fe in tetrahedral interstitial sites surrounded, respectively, by four C atoms (Fe_i.C) or four Si atoms (Fe_i,Si) and two to Fe in (or close to) defect free or implantation damaged substitutional sites. An annealing stage at 300–500 K is evident. Above 600 K the Fe_i,Si fraction decreases markedly, reaching close to zero intensity at 905 K. This is accompanied by a corresponding increase in the Fe_i,C fraction.     

The magnetic and hyperfine properties of iron in silicon carbide

The magnetic and hyperfine properties of iron impurities in 3C- and 6H- silicon-carbide are calculated using the abinitio method of full-potential linear-augmented-plane-waves. The iron atoms are introduced at substitutional carbon, Fe _C , and silicon, Fe _Si , sites as well as at the tetrahedral interstitial sites with four nearest neighbours carbon atoms, Fe _I (C), and four nearest neighbours silicon atoms, Fe _I (Si). The effect of introducing vacancies at the neighbours of these sites is also studied. Fe atoms with complete neighbors substituted at Si or C sites are found to be nonmagnetic, while Fe atoms at interstitial sites are magnetic. Introduction of a vacancy at a neighboring site reverse the picture.     

Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (E _F ) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an E _F of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.     

Dopant-assisted concentration enhancement of substitutional Mn in Si and Ge

The influence of p- and n-type electronic dopants on Mn incorporation in bulk Si and Ge is studied using first-principles calculations within density functional theory. In Si, it is found that the site preference of a single Mn atom is reversed from interstitial to substitutional in the presence of a neighboring n-type dopant. In Ge, a Mn atom is more readily incorporated into the lattice when an n-type dopant is present in its immediate neighborhood, forming a stable Mn-dopant pair with both impurities at substitutional sites. A detailed analysis of the magnetic exchange interactions between such pairs reveals a new type of magnetic anisotropy in both systems.     

In-beam Mössbauer spectroscopy on single iron atoms in alkali metals

In-beam Mössbauer spectroscopy is applied to study implanted Fe atoms in the alkali metals Li, Na and K. From the Mössbauer parameters we infer that the Fe implants take up substitutional as well as interstitial sites. The strongly increased electron density at the interstitial position is qualitatively explained by the pressure resulting from the small interstitial volume. It is concluded that recently reported local moment formation in these systems cannot be due to substitutional Fe only.     

Local field corrections to binding energies of substitutional and interstitial donors in Si and Ge

Intervalley Umklapp matrix elements, with inclusion of local field screening effects, are computed for substitutional and interstitial point-charge impurity potentials in Si and Ge. The shallow character of substitutional donors is not affected in Ge and is even reinforced in Si, where a 20% reduction of the binding energy is obtained as a consequence of the local field effect. In both semiconductors the local field corrections strongly reinforce the non-effective-mass, deep-level character of interstitial point-charge donors.     

硅中替位式和间隙式IB族杂质的电子态

我们对硅中IB族杂质Cu,Ag和Au的单粒子电子结构用自洽的Xα-SW法作了计算。用集团XSi₄Si′₁₂和XSi₁₀Si′₁₆分别模拟替位式和四面体间隙式杂质X所局域微扰的硅晶体。这两类集团已广泛地得到采用,我们已经成功地用它研究硅中4d过渡金属杂质的性质。计算得到;替位杂质在带隙中导致一个t₂能级,它几乎不具有d性质而是类悬挂键的。间隙杂质在带隙中导致一个a₁能级,对于Si:Au,此能级位置接近导带底,轨道是非常离域的。此外,还得到一个位于价带底之下类s的超深能级。对硅中1B族杂质的电子性质的趋势作了详细讨论。 关键词：     

Magnetic and electric properties of (Fe,Co)/(Si,Ge) multilayers

We review selected results concerning the interlayer exchange coupling in Fe/Si _x Fe_1−x , Fe/Ge and Co/Si layered structures. Among the ferromagnet/semiconductor systems, Fe/Si structures are the most popular owing to their strong antiferromagnetic interlayer coupling. We show that such interaction depends not only on semiconducting sublayer thickness, but also on deposition techniques and on the chemical composition of the sublayer as well. In similar heterostructures e.g. Fe/Ge, antiferromagnetic coupling was observed only in ion-beam deposited trilayers at low temperatures. In contrast, in Fe/Ge multilayers deposited by sputtering, no such coupling was found. However, when the Ge is partially substituted by Si, antiferromagnetic interlayer coupling appears. For Co/Si multilayers, we observed a very weak exchange coupling and its oscillatory behavior. The growth of Co on Si occurs in an island growth mode. The evolution of magnetic loop shapes can be successfully explained by the interplay between interlayer coupling and anisotropy terms.     

Early stages of vanadium deposition on Si(1 1 1)-7 × 7

We investigate the low-coverage regime of vanadium deposition on the Si(1 1 1)-7 × 7 surface using a combination of scanning tunnelling microscopy (STM) and density-functional theory (DFT) adsorption energy calculations. We theoretically identify the most stable structures in this system: (i) substitutional vanadium atoms at silicon adatom positions; (ii) interstitial vanadium atoms between silicon adatoms and rest atoms; and (iii) interstitial vanadium - silicon adatom vacancy complexes. STM images reveal two simple vanadium-related features near the Si adatom positions: bright spots at both polarities (BB) and dark spots for empty and bright spots for filled states (DB). We relate the BB spots to the interstitial structures and the DB spots to substitutional structures.     

Electronic structure and magnetic properties of Fe andc(2×2)FeCr overlayers deposited on Cr(100)

Ab initio spin-polarized calculations of the electronic structure of Fe overlayers on the Cr(100) substrate were performed using full-potential LAPW method. There was found that the magnetic moment of Fe monolayer on Cr(100) aligned antiferromagnetically with respect to the interface Cr layer and is equal 2.48 μ_B. The dependence on the Fe film thickness of spin-density distribution near the surface and Fe-Cr interface, is analysed. The stability of CrFec(2 × 2) substitutional surface alloy on Cr(100) substrate is studied on the basis of electronic structure calculations. The total energy calculations show that Fe monolayer is unstable against the CrFec(2 × 2) surface alloy formation with ferromagnetic ordering in the layer. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999.     

Ab initio computer simulation of adsorption of a Fe monolayer on Si(111)

Computer simulation of adsorption of an Fe monolayer on Si(111) is carried out in the generalized gradient approximation in the density functional theory. It is shown that mixing of Fe and Si atoms followed by the replacement of Si atoms and the formation of a silicide-like film containing two types of domains (with the A8 and B8 structure) takes place in the course of deposition. Analysis of the atomic structure of this compound shows that the formation of this interface makes it possible to obtain an FeSi film (with the CsCl-type structure) as well as FeSi₂ film (with a CaF₂-type structure).     

Chemisorption of Fe on Au-passivated Si（001） surface

The chemisorption of one monolayer of Fe atoms on a Au-passivated Si(001) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The Fe adatom chemisorption on an ideal Si(001) surface is also considered for comparison. The chemisorption energy and layer projected density of states for a monolayer of Fe atoms on Au-passivated Si(001) surface are calculated and compared with that of the Fe atoms on an ideal Si(001) surface. The charge transfer is investigated. It is found that the most stable position is at the fourfold hollow site for the adsorbed Fe atoms, which might sit below the Au surface. Therefore there will be a Au－Fe mixed layer at the Fe/Au－Si(100) interface. It is found that the adsorbed Fe atoms cannot sit below the Si surface, indicating that a buffer layer of Au atoms may hinder the intermixing of Fe atoms and Si atoms at the Fe/Au－Si(001) interface effectively, which is in agreement with the experimental results.     

Electrical properties of DX centers in GaAs and AlGaAs

Abstract

The DX center, the lowest energy state of the donor in AIGaAs with x < 0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AIGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 × 10¹⁹ cm^?3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed.     

The effect of substitutional impurity atoms on the diffusivity of an interstitial species

A model has been developed to treat the problem of the effect of substitutional solute elements on the diffusion of an interstitial species. The calculation applies only to cases where the substitutional solute creates “antitrapping” sites, whose energies are higher than the “normal” interstitial sites in the defect-free solvent lattice.The model predicts that, for sufficiently “high” antitrapping sites, there will be substantially no effect of composition or even solute atom type on the mobility of the interstitial species even up to about 20 atm% of substitutional solute.It is also possible for a given substitutional solute to act both as an accelerator or retarder with respect to the diffusivity of the interstitial species, depending upon temperature.     

Vibrational and Light-Emitting Properties of Si/Si1−xSnx Heterostructures

Multilayer heterostructures Si/Si_1−xSn_x grown by molecular beam epitaxy on Si (001) substrates have been studied by Raman and photoluminescence spectroscopy. Raman spectra exhibit peaks corresponding to the vibrations of Si-Sn and Sn-Sn bonds; the vibrations of Sn-Sn bonds imply the presence of tin nanocrystals in heterostructures. Two photoluminescence bands at 0.75 eV (1650 nm) and 0.65 eV (1900 nm) have been observed in heterostructures at low temperatures. The former band can be attributed to optical transitions in quantum wells in the type II heterostructure Si/Si_1−xSn_x. The latter band can be associated with excitons localized in tin nanocrystals.