Computer simulation of polytypes

Polytypes have been simulated, treating them as analogues of a one-dimensional spin-half Ising chain with competing short-range and infinite-range interactions. Short-range interactions are treated as random variables to approximate conditions of growth from melt as well as from vapour. Besides ordered polytypes up to 12R, short stretches of long-period polytypes (up to 33R) have been observed. Such long-period sequences could be of significance in the context of Frank's theory of polytypism. The form of short-range interactions employed in the study has been justified by carrying out model potential calculations.

Monte Carlo simulation of polytypes

Based on the analogy between polytypes and spin-half Ising chains with competing short- and infinite-range interactions, a Monte Carlo simulation of polytypes has been attempted. A general double-layer mechanism connects different states of the polytype chain with about the same probability as the spin-flip mechanism in magnetic Ising chains. It has been possible to simulate various polytypes with periodicities extending up to 12 layers. The Monte Carlo method should be useful in testing different interaction models that may be proposed in the future to describe polytypism.

Structure and electron transport anomalies in InSe-In6Se 7 composite

Rapid solidification of an equiatomic In-Se alloy resulted in the formation of an equilibrium InSe-In6Se7 phase mixture. The InSe phase was found to be polytypic and exhibited the structural variants 2H, 3H, and 4H. The 4H polytype was found to be in considerably higher proportion compared to 2H and 3H types. The In6Se7 phase was found to be hexagonal with a=0.8919 nm and c=1.4273 nm. Both In6Se 7 and the polytypes of InSe could be identified with the space group P61. The conductivity σ variation with temperature was found to be similar to that observed in disordered semiconducting materials. For temperatures >200 K, ln σ decreased linearly with T-1, phonon-assisted carrier excitation. For temperatures <200 K, ln σ decrease followed T-1/3 behavior, representative of variable-range hopping conduction of electrons.

Tensile and compressive mechanical behavior of twinned silicon carbide nanowires

Molecular dynamics simulations with the Tersoff potential were used to study the response of twinned SiC nanowires under tensile and compressive strain. The critical strain of the twinned nanowires can be enhanced by twin stacking faults, and their critical strains are larger than those of perfect nanowires with the same diameters. Under axial tensile strain, the bonds of the nanowires are stretched just before failure. The failure behavior is found to depend on the twin segment thickness and the diameter of the nanowires. An atomic chain is observed for thin nanowires with small twin segment thickness under tension strain. Under axial compressive strain, the collapse of twinned SiC nanowires exhibits two different failure modes, depending on the length and diameter of the nanowires, i.e., shell buckling for short nanowires and columnar buckling for longer nanowires.

Raman scattering detection of stacking faults in free-standing cubic-SiC epilayer

We report on stacking fault (SF) detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si(100) substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability (BP) model in the framework of linear-chain concept. Good agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 angstrom in the BP model.

Structural and optical properties of 6H-SiC helium-implanted at 600 K

Single crystals of 6H-SiC were implanted at 600 K with 100 key He ions to three successively fluences and subsequently annealed at different temperatures ranging from 873 to 1473 K in vacuum. The recovery of lattice damage was investigated by different techniques including Rutherford backscattering spectrometry in channeling geometry, Raman spectroscopy and Fourier transform infrared spectroscopy. All three techniques showed that the damage induced by helium ion implantation in the lattice is closely related to the fluence. Rutherford backscattering spectrometry/channeling data on high temperature implantations suggest that for a fluence of 3 x 10(16) He+/cm(2), extended defects are created by thermal annealing to 1473 K. Apart from a well-known intensity decrease of scattering peaks in Raman spectroscopy it was found that the absorbance peak in Fourier transform infrared spectroscopy due to the stretching vibration of Si-C bond shifted to smaller wave numbers with increasing fluence, shifting back to larger wave numbers with increasing annealing temperature. These phenomena are attributed to different lattice damage behavior induced by the hot implantation process, in which simultaneous recovery was prevailing. (C) 2010 Elsevier B.V. All rights reserved.

Strain dependent microstructural modifications of BiCrO3 epitaxial thin films

Strain-dependent microstructural modifications were observed in epitaxial BiCrO3 (BCO) thin films fabricated on single crystalline substrates, utilizing pulsed laser deposition. The following conditions were employed to modify the epitaxial-strain: (i) in-plane tensile strain, BCOSTO [BCO grown on buffered SrTiO3 (001)] and in-plane compressive strain, BCONGO [BCO grown on buffered NdGaO3 (110)] and (ii) varying BCO film thickness. A combination of techniques like X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (TEM) was used to analyse the epitaxial growth quality and the microstructure of BCO. Our studies revealed that in the case of BCOSTO, a coherent interface with homogeneous orthorhombic phase is obtained only for BCO film with thicknesses, d < 50 nm. All the BCOSTO films with d = 50 nm were found to be strain-relaxed with an orthorhombic phase showing 1/2 <100> and 1/4 <101> satellite reflections, the latter oriented at 45° from orthorhombic diffraction spots. High angle annular dark field scanning TEM of these films strongly suggested that the satellite reflections, 1/2 <100> and 1/4 <101>, originate from the atomic stacking sequence changes (or “modulated structure”) as reported for polytypes, without altering the chemical composition. The unaltered stoichiometry was confirmed by estimating both valency of Bi and Cr cations by surface and in-depth XPS analysis as well as the stoichiometric ratio (1 Bi:1 Cr) using scanning TEM–energy dispersive X-ray analysis. In contrast, compressively strained BCONGO films exhibited monoclinic symmetry without any structural modulations or interfacial defects, up to d ~ 200 nm. Our results indicate that both the substrate-induced in-plane epitaxial strain and the BCO film thickness are the crucial parameters to stabilise a homogeneous BCO phase in an epitaxially grown film.