Influence of oxygen on the ion-assisted regrowth of deposited Si layers

Summary The ion-assisted regrowth of chemical-vapour deposited Si films onto (100) Si substrates is reported. The regrowth was induced by a 600 keV Kr⁺⁺ beam at doses in the range (2·10¹⁵÷6·10¹⁵)/cm² and at a dose rate of 1·10¹²/cm²s. The target temperature was set at 450°C. During irradiations the crystal-amorphous interface velocity was measuredin situ by monitoring the reflectivity of a He-Ne laser light focused onto the sample surface. After irradiation some samples were also analysed by Rutherford backscattering in combination with the channelling effect and by transmission electron microscopy. The growth rate of deposited layers depends on the cleaning procedure performed prior to deposition,i.e on the total amount of oxygen present at the deposited layer/substrate interface. Moreover, twinned material is observed in the recrystallized layers and its concentration is strongly dependent on substrate cleaning. These phenomena are explained in terms of a decrease in the ion-assisted growth rate in the presence of high oxygen concentrations. The data are discussed and compared with those obtained during pure thermal annealing. To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.     

Discrete symmetry groups of vertex models in statistical mechanics

We analyze discrete symmetry groups of vertex models in lattice statistical mechanics represented as groups of birational transformations. They can be seen as generated by involutions corresponding respectively to two kinds of transformations onq×q matrices: the inversion of theq×q matrix and an (involutive) permutation of the entries of the matrix. We show that the analysis of the factorizations of the iterations of these transformations is a precious tool in the study of lattice models in statistical mechanics. This approach enables one to analyze two-dimensionalq ⁴-state vertex models as simply as three-dimensional vertex models, or higher-dimensional vertex models. Various examples of birational symmetries of vertex models are analyzed. A particular emphasis is devoted to a three-dimensional vertex model, the 64-state cubic vertex model, which exhibits a polynomial growth of the complexity of the calculations. A subcase of this general model is seen to yield integrable recursion relations. We also concentrate on a specific two-dimensional vertex model to see how the generic exponential growth of the calculations reduces to a polynomial growth when the model becomes Yang-Baxter integrable. It is also underlined that a polynomial growth of the complexity of these iterations can occur even for transformations yielding algebraic surfaces, or higher-dimensional algebraic varieties.     

Growth,structure and optical properties of an efficient NLO crystal-aqua maleatocopper(II)

A promising non-linear optical (NLO) crystal, aqua maleatocopper(II) (CuC₄H₂O₄·H₂O), was grown at room temperature by the controlled ionic diffusion technique. Fourier transform infrared spectrum could identify the various functional groups in the crystal. Structural analysis using single crystal XRD revealed that the compound crystallizes in the monoclinic system with space group P2₁ and unit cell parameters a = 7.7277(5) Å, b = 5.2967(3) Å, c = 7.7179(4) Å, α = γ = 109.170(5)°, β = 111.995(2)°. The thermal stability and decomposition pattern of the material were explored using thermogravimetry (TG) and differential thermal analysis (DTA). The optical band gap energy of the material was estimated as 2.2 eV from the diffuse reflectance spectroscopy. The Kurtz and Perry powder technique established the crystal to be an efficient non-linear optical (NLO) material.     

Synthesis,growth and characterization of non-linear optical material: l-Tryptophan p-nitrophenol (LTPNP) single crystal

A new nonlinear optical organic crystal l-tryptophan p-nitrophenol (LTPNP) of dimension 19 mm × 2 mm × 1.5 mm has been grown from an aqueous solution for the first time by slow evaporation technique at ambient temperature. The crystal structure of LTPNP was confirmed by single crystal X-ray diffraction. LTPNP crystallizes in non-centrosymmetric monoclinic system with space group P₂₁. The recorded FTIR spectrum confirms the presence of various functional groups in the grown crystal and confirms the formation of LTPNP. Thermal stability and melting temperature of the LTPNP crystal were identified from TG/DTA analysis. The optical absorption study confirms the suitability of the crystal for device applications. LTPNP exhibits SHG efficiency over 1.7 orders of magnitude higher than that of urea and 4 orders of magnitude higher than that of KDP.     

Growth,optical, mechanical and electrical studies of nonlinear optical crystal: l-Valinium oxalate

l-Valinium oxalate (LVO) single crystal has been grown by solution growth technique at room temperature. The crystal structure and lattice parameters were determined for the grown crystal by single crystal X-ray diffraction studies. Optical constants like band gap, refractive index, reflectance, extinction coefficient and electric susceptibility were determined from UV–vis–NIR spectrum. The microhardness studies reveal the mechanical strength of the grown crystal. The dielectric constant, dielectric loss and AC conductivity of the compound were calculated at different temperatures and frequencies to analyze the electrical properties. Nonlinear optical property was discussed to confirm the SHG efficiency of the grown crystal.     

Luminescence properties of Ba2NaNb5O15 crystals activated with Sm, Eu, Tb or Dy ions

Single crystals of Ba₂NaNb₅O₁₅ (BNN) singly doped with Sm³⁺, Eu³⁺, Tb³⁺ or Dy³⁺ have been grown by means of the flux growth method. Their visible emission and excitation spectra and the decay profiles of the luminescence have been measured at room temperature. All spectral features are significantly inhomogeneously broadened in consequence of the structural disorder of the host and of the doping mechanisms. The analysis of the observed spectra allows formulating an hypothesis about the site occupancy of the active ions in the BNN lattice.     

Traveling wave solutions of a reaction-diffusion model for bacterial growth

In this paper, we consider a reaction-diffusion model for the bacterial growth. Mathematical analysis on the traveling wave solutions of the model is performed. This includes traveling wave analysis and numerical simulations of wave front propagation for a special case. Specifically, we show that such solutions exist only for wave speeds greater than some minimum speed giving wave with a sharp front. The minimum speed is estimated and the wave profile is calculated and compared with different numerical methods.     

Dendritic and eutectic growth in Sb<Subscript>60</Subscript>Ag<Subscript>20</Subscript>Cu<Subscript>20</Subscript> ternary alloy

The rapid solidification of Sb60Ag20Cu20 ternary alloy was realized by high undercooling method, and the maximum undercooling is up to 142 K (0.18TL). Within the wide undercooling range of 40-142 K, the solidified microstructures are composed of (Sb), θand ε phases. High undercooling enlarges the solute solubility of (Sb) phase, which causes its crystal lattice to expand and its crystal lattice constants to increase. Primary (Sb) phase grows in two modes at small undercoolings non-faceted dendrite growth is the main growth form; whereas at large undercoolings faceted dendrite growth takes the dominant place. The remarkable difference of crystal structures between (Sb) and θphases leads to (θ Sb) pseudobinary eutectic hard to form, whereas strips of θform when the alloy melt reaches the (θ Sb) pseudobinary eutectic line. The cooperative growth of θand ε phases contributes to the formation of (ε θ) pseudobinary eutectic easily. In addition, the crystallization route has been determined via microstructural characteristic analysis and DSC experiment.     

Radiation induced polymerization of Vinylbenzoate–II. Study of the molecular weights of polymers

Abstract

In this study, the molecular weights of the polymer from vinyl benzoate has been reported for different dose rates and different temperatures. The mol.wt. remains around 4000. With increasing dose, the molecular weight increases to about 14,000. The degrees of polymerization and the kinetic chain lengths calculated from the experimentally determined G _R values have been compared which show that the kinetic chain length is larger than the D ^? Pn.

From a plot of the (D ^? Pn)^?1 vs. R _p , an intercept is obtained which is equal to 3.25 × 10^?2 which is higher than obtained in conventional polymerization. By examining the effect of temperature on mol.wt., the dependence of R _p on dose rate and the absence of induction period it has been concluded that chain transfer occurs to impurities that are generated during radiolysis.