Formation of misfit edge dislocations in Ge x Si1 ? x films ( x ～ 0.4–0.5) grown on tilted Si(001) → (111) substrates

The dislocation structure of Ge _x Si_{1 − x} films (x ∼ 0.4–0.5) grown by molecular epitaxy on Si(001) substrates tilted by 6° about the 〈011〉 axis was studied. It is shown that, in the tilt direction, edge misfit dislocations (MDs) arise only in the form of short segments lying on the intersections of 60° MDs. As a result, the total length of edge MDs along the substrate tilt direction is smaller than that along the tilt axis. The deviation of the substrate surface from the singular plane made it possible to detect a dislocation configuration that consists of a short segment of an edge MD and two diverging 60° MDs propagating from it in the tilt direction. The formation of the segment is assumed to begin with simultaneous nucleation of complementary dislocation half-loops that form a short edge MD on the interface and then propagate on one side as two diverging 60° MD lines. Original Russian Text ? Yu.B. Bolkhovityanov, A.K. Gutakovskii, A.S. Deryabin, L.V. Sokolov, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 10, pp. 1783–1787.     

Identification ofμ +-sites in the 1-2-3 compoundin the 1-2-3 compound

Possibleμ ⁺ sites have been identified by a comparison of measured spontaneous internal fields in YBa₂Cu₃O_6+δ and REBa₂Cu₃O_7−δ and of measured second moments and width of transverse field powder spectra with the corresponding calculated quantities. In the YBa₂Cu₃O₇ system only one possibleμ ⁺ site emerges at a distance of ∼1.05 A from a chain oxygen O(4) at the position (0.15(1), 0.44(1), 0.071(1)). In the system YBa₂Cu₃O₆ the only possibleμ ⁺ sites are near to an oxygen O(1) with z/c=0.133 and the same distance as above. The analysis leads also to improved valuesμ _Ho = 2.2(1)μ _B andμ _Cu = 0.67(4)μ _B and to a determination of the latters direction:μ _Cu ‖〈110〉.     

Shallow-Donor Hydrogen-Like Impurities in ZnO Studied by MuSR

Two species of Mu centers with extremely small hyperfine parameters have been observed in single crystalline ZnO below 40 K. Both Mu centers have an axial symmetric hyperfine structure along with the [0001] axis, indicating that they are located at AB_O,∥ and BC_∥ sites. It is inferred from their small ionization energy (≃6 meV and 50 meV) and hyperfine parameters (∼10⁻⁴ times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO. This revised version was published online in September 2006 with corrections to the Cover Date.     

<Superscript>29</Superscript>Si MAS-NMR study of oxide conductors derived from the apatite-type La<Subscript>9.33</Subscript>Si<Subscript>6</Subscript>O<Subscript>26</Subscript> compound

The preparation of (La_9.33−2x/3Sr _x □_0.67−x/3)Si₆O₂₄O₂ (0 ≤ x ≤ 2) samples with different amounts of cation vacancies is reported. Structure and unit-cell parameters were deduced by Rietveld analysis of XRD patterns. Structural features that enhance oxygen conductivity in Sr-doped apatites are discussed. Up to three components were detected in ²⁹Si MAS-NMR spectra which change with the amount and distribution of cation vacancies. In general, oxygen conductivity increases with the amount of vacancies at La1 (6h) sites, passing through a maximum for x = 0.4. In the case of activation energy, a minimum is detected near x = 1.2, indicating that entropic and enthalpic change in different ways. The presence of cation vacancies should enhance oxygen hopping along c-axis; however, the analysis of the frequency dependence of conductivity suggests that oxygen motions are produced along three axes.     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)     

Excitation pathways and efficiency of Eu ions in GaN by site-selective spectroscopy

Using combined excitation emission spectroscopy, we performed a comparative study of europium ions in GaN in samples that have been in situ doped during interrupted growth epitaxy (IGE) or conventional molecular beam epitaxy (MBE) as well as samples that were grown using organometallic vapor phase epitaxy (OMVPE) and subsequently ion implanted with Eu ions. Through site-selective resonant excitation, we are able to unambiguously assign all major observed transitions to a combination of different incorporation sites and electron–phonon coupled transitions. We identified at least nine different incorporation sites of Eu ions in GaN and studied how these sites behave under different excitation conditions and how their relative number is modified by different growth and doping conditions. The coupling to phonons has also been studied for a series of Al _x Ga_1−x N samples with x=0…1. We find that a main site most resembling an unperturbed Eu ion on Ga site is always dominant, while the minority sites are changing substantially in relative numbers and can occur in some samples fairly close in emission intensity to the main site. In terms of the excitation pathway after the creation of electron-hole pairs, we found three types of centers: (1) sites that are dominantly excited through shallow defect traps; (2) sites that are excited through a deep defect trap; (3) sites that cannot be excited at all including the majority of the main sites. We interpret this finding to indicate that the ion in this environment is not very efficient in trapping excitation and that the indirect excitation involving other traps depends on the ion/trap distance. Many of the main sites are far away from these traps and cannot be excited through this channel at all. The efficiency of excitation is highest for the deep traps, indicating that it would be desirable to enrich the respective site, as has been done with some success in the IGE grown samples.     

Phase transitions in diglyceride monolayers studied by computer simulations,pressure-area isotherms and X-ray diffraction

Summary 1,2-sn-diglyceride monolayers exhibit unique and complex phase transitions as a function of surface pressure. The dynamical response of the layer on expanding the film has been investigated by computer simulations, (π-A) isotherms and grazing-incidence X-ray diffraction. Good agreement is found between the simulations and experiments. The Langmuir film undergoes two phase transitions occurring at 38.3 and 39.8 ?²/molecule. The transition at low surface density is associated with a tilt of ≈14° in the direction close to nearest neighbour. The first transition is unique for the diglyceride molecules and has not been observed for other amphiphilic molecules. It is driven by a competition of hydrophobic/hydrophilic and intra/intermolecular forces and can be pictured as a ?seesaw? mechanism. Due to the close packing of the chains at high surface pressure, thesn-1 ester group aligns with the alkyl chain, and only thesn-2 ester group is favoured to lay at the aqueous interface. Hydrophilic forces attract thesn-1 ester group, but its motion towards the aqueous subphase is hindered by intra- and intermolecular chain interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains is strong enough to cause a ?swelling? of the molecules. The diffraction pattern determined in the different mesophases reveals that the layer maintains its hexagonal structure up to the second phase. Concomitant with the tilt, the structure relaxed from a hexagonal to a distorted hexagonal lattice. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Hyperfine interactions of muonium and hydrogen in semiconductors: Cluster calculations

The electronic structure of muonium (Mu) located at different interstitial sites of the silicon crystal is calculated by the complete neglect of differential overlap (CNDO) and intermediate neglect of differential overlap (INDO) methods. Calculations of the electronicg- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “normal” (Mu′) and “anomalous” (Mu^*) muonium centers. It is shown that the most likely dynamic model for Mu′ is that in which neutral Mu diffuses rapidly in the silicon lattice, whereas for Mu^* it is the model wherein interstitial Mu is located at the bond-center site. A correlation is made between the characteristics of the hydrogen-bearing Si-AA9 center, recently observed by EPR in a silicon crystal, and those of Mu^*. The Si-AA9 center is shown to be a hydrogen-bearing paramagnetic analogue of the Mu^* center.     

Nematic liquid crystal in the wedge and edge geometry in the case of homeotropic alignment

Nematic liquid crystal confined to a wedge or edge is studied on the assumption that the confining surfaces provide strong and weak homeotropic anchorings, respectively. Both infinite and finite systems are considered. The model based on the Frank-Oseen and Rapini-Papoular formalisms predicts two textures of opposite rotations of the director as in the case of strong anchoring on both surfaces. However, the presence of weak anchoring results in a length scale λ which characterizes the crossover between the regions close to the apex and far from it. The ratio λ/b , where b is the extrapolation length, is a function of the opening angle α. Both stable and metastable textures are considered and the mechanism by which a texture loses its stability is found. It is related to the formation of a defect-like structure at the surface of weak anchoring whose distance from the apex is λ(α) and the loss of stability is signalled by the divergence of λ. Only in the limit α → 2τ, the defect-like structure transforms into a defect of strength -1/2 located at a finite distance from the apex.