Stacking Fault Layer Research Articles

Reconstructed Structure of SiO2/Si(111) Interface

ABSTRACTIn order to investigate the initial oxidation process Qf the Si (111) surface, we have studied the molecular beam deposited Si0 2/Si(111)-7×7 interface structure using grazing incidence X-ray diffraction geometry. We suggest a three-fold symmetry structural model composed of stacking fault layer, dimer layer and additional ordered atoms. The three-fold symmetry structure comes from the preference for oxidation in the faulted half of the 7×7 structure.

Kinematical analysis of RHEED intensities from the Si(111)7 × 7 structure

RHEED patterns of the Si(111)7 × 7 surface in the [112̄] direction taken at several angles of incidence are analyzed by kinematical calculations. The calculations are carried out for the DAS (dimer-adatom-stacking-fault) structure with changes of 200 atomic positions. We also test for new models proposed recently. Comparing the experimental RHEED patterns and rocking curves with the calculated ones, good agreement is obtained for the DAS model with the adatom positions about 1.3 Å above the stacking fault layer, which is deduced from recent dynamical calculations of RHEED and LEED. The efficiency of the kinematical calculation is also discussed.

Reflection high-energy electron diffraction pattern calculations for Si(111)-7×7 surface

Reflection high-energy electron diffraction (RHEED) pattern intensity calculations were performed within the kinematical approximation for the Si(111)-7×7 reconstructed surface using a structure model containing dimers, adatoms, and stacking faults (DAS) in the surface layers as proposed by Takayanagi et al. By fitting experimental RHEED pattern intensities taken with incident electron energies of 30 and 20 keV along the [1̄1̄2] azimuth, we found that the lateral and vertical positions of atoms in the first two surface layers (i.e., adatom layer and stacking fault layer) are in good agreement with that of an energy minimization calculation (EMC). The best fit to the low-order diffraction spots was obtained when all adatoms were moved out toward vacuum around 0.45 Å from bulk positions, and the average spacing between adatom layer and stacking fault layer was set at (1.23±0.04)Å. If we include the dimer layer in the calculation, using the dimer spacing given by EMC, we found that the relative calculated RHEED pattern intensities slightly deviated from the experimental RHEED patterns. This slight disagreement may be due to incorrect lateral positioning of the dimer atoms by EMC or the kinematical approximation. Also, the calculated relative RHEED pattern intensity is not very sensitive to variations in the spacing between the stacking fault and dimer layers. Finally, our results imply that the kinematical approximation can be used for analyzing the intensities of superlattice RHEED spots in some suitable RHEED patterns.

Structure analysis of Si(111)-7 × 7 reconstructed surface by transmission electron diffraction

The atomic structure of the 7 × 7 reconstructed Si(111) surface has been analysed by ultra-high vacuum (UHV) transmission electron diffraction (TED). A possible projected structure of the surface is deduced from the intensity distribution in TED patterns of normal electron incidence and from Patterson and Fourier syntheses of the intensities. A new three-dimensional structure model, the DAS model, is proposed: The model consists of 12 adatoms arranged locally in the 2 × 2 structure, a stacking fault layer and a layer with a vacancy at the corner and 9 dimers on the sides of each of the two triangular subcells of the 7 × 7 unit cell. The silicon layers in one subcell are stacked with the normal sequence, CcAaB + adatoms, while those in the other subcell are stacked with a faulted sequence, CcAa/C + adatoms. The model has only 19 dangling bonds, the smallest number among models so far proposed. Previously proposed models are tested quantitatively by the TED intensity. Advantages and limits of the TED analysis are discussed.

Structural analysis of Si(111)-7×7 by UHV-transmission electron diffraction and microscopy

Structural analysis of the surface reconstructions investigated by ultrahigh vacuum (UHV) transmission electron microscopy (TEM) and diffraction (TED) is shown. By TED intensity analysis a new structural model of Si(111)-7×7 is derived. The model basically consists of 12 adatoms arranged locally in the 2×2 structure, nine dimers on the sides of the triangular subunits of the 7×7 unit cell and a stacking fault layer. UHV–HREM of Si (111)-7×7 surface is commented.

The kinetics of atomic diffusion to stacking faults and the related special problems in cold-worked alpha brasses

The diffusion kinetics of solute atoms to the stacking fault layers were investigated by the Laplace transformation method in binary homogeneous alloys. The room temperature annealing kinetics of stacking faults in cold-worked alpha brasses, which were studied by an X-ray diffraction technique, were analyzed in terms of the present theoretical findings. The chemical diffusivities in cold-worked (filings) Cu−10Zn and Cu−22.7Zn alpha brasses were determined as equal to 1.7×10−21 and 1.5×10−19 cm2/s at room temperature (300 K), respectively. Finally, the concentration of athermal monovacancies was estimated in plastically deformed Cu−10Zn (the particle size 20 μm) and Cu−22.7Zn (the particle size 75 μm) alloys and was found to be about 2.0×1019 and 3.0×1019 cm−3, respectively.

Chemical Composition in a Tetrahedron in Quenched α-Brass

The deviation in the chemical composition around a tetrahedron in quenched α-brass (27 at.%Zn) has been examined with a Kevex X-ray energy spectrometer attached to an electron microscope. It has been confirmed that the copper concentration in a tetrahedron is about 3 at.% higher than that in matrix, and this local deviation has been interpreted by the ledge mechanism using the diffusivity difference between copper and zinc atoms in α-brass. It has been also made clear that while the zinc concentration in a tetrahedron is apt to increase to accommodate the volume dilatation, this tendency is not observed because the stacking fault layer prevents the flow of atoms.

Stacking-faults in tellurium-doped gallium arsenide

Samples of bulk-grown gallium arsenide single crystals, taken from both static freeze, and Czochralski ingots doped to a high level with tellurium, have been examined using transmission electron microscopy. Observation of single and multiple stacking-fault layers which have fault vectors of the kindR=a/3〈111〉 is reported. It is shown by diffraction contrast experiments that the stacking-fault defects are extrinsic. They are thought to be rafts of tellurium substituting for arsenic in {111} planes. The prevalence of the observed layer defects correlates well with the increase in carrier concentration in certain regions of the crystals.

Mécanismes de croissance et qualité cristalline des composes du type diamant et des cristaux des groupes III–V et II–VI

The authors show that Ge and Si can form stacking fault layers in sufficient number for undercooling less than 30°. The mechanism of two-dimensional nucleation is the only possible for these faults. They study in which conditions such layers can produce whether a macroscopic growth twin, or a macroscopic lamella bordered by a dislocation loop. The influence of the wetting agents of Ge (Sb, Ga) is then explained by an adsorption of these atoms along the [11̄0] edges on (111), lowering the two-dimensional nucleation energy; the frequency of twinning is then increased, according to the experiences of Boiling. As to the III–V and II–VI compounds, owing to their polarity it is shown that these defects appear less easily on As, Sb, P, S, Cl ( 111 ) faces, according to the experiences of Gatos.

金属微粒子の格子欠陥

Mechanical and thermal stability of crystal lattice imperfections in metallic fine particles were theoretically examined and compared with those in bulk materials. As generally expected, dislocations, grain boundaries and phase boundaries were found to be unstable in fine particles except for a few special cases. The finer the particles, the less stable these imperfections. Because of their very small self-energy, stacking fault layers might be able to survive as the least unstable imperfection after annealing to a considerable extent. In contrast with these imperfections, it is possible to show that vacancies in fine crystal particles could have a thermal equilibrium concentration much higher than that in the bulk crystal at fairly low temperatures. For instance, copper fine particles of 1 micron in diameter could have a thermal equilibrium concentration of vacancies a few hundreds times higher than that in the bulk material at around 300°C, when they have clean spherical surfaces. Application of these calculations to some practical problems, such as the phase transformation, oxidation, and sintering processes of metallic fine particles, were also discussed. The very high thermal equilibrium concentration of vacancies seems to play the main rôle in most of these cases.