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Micro-Raman spectroscopy was employed for the determination of the germanium content, x and strain, , in ultrathin SiGe virtual substrates grown directly on Si by molecular beam epitaxy. The growth of highly relaxed SiGe layers was achieved by the introduction of point defects at a very low temperature during the initial stage of growth. SiGe virtual substrates with thicknesses in the range 40-200 nm with a high Ge content ͑up to 50%͒ and degree of relaxation, r, in the range 20%-100% were investigated using micro-Raman spectroscopy and x-ray diffraction ͑XRD͒ techniques. The Ge content, x, and strain, , were estimated from equations describing Si-Si, Si-Ge, and Ge-Ge Raman vibrational modes, modified in this study for application to thin SiGe layers. The alteration of the experimentally derived equations from previous studies was performed using independent data for x and r obtained from XRD reciprocal space maps. A number of samples consisting of a strained-silicon ͑s-Si͒ layer deposited on a SiGe virtual substrate were also analyzed. The stress value for the s-Si varied from 0.54 to 2.75 GPa, depending on the Ge-content in the virtual substrates. These results are in good agreement with theoretically predicted values.

A series of 3C-SiC films have been grown by a novel method of solid–gas phase epitaxy and studied by Raman scattering and scanning electron microscopy (SEM). It is shown that during the epitaxial growth in an atmosphere of CO, 3C-SiC films of high crystalline quality, with a thickness of 20 nm up to few hundreds nanometers can be formed on a (111) Si wafer, with a simultaneous growth of voids in the silicon substrate under the SiC film. The presence of these voids has been confirmed by SEM and micro-Raman line-mapping experiments. A significant enhancement of the Raman signal was observed in SiC films grown above the voids, and the mechanisms responsible for this enhancement are discussed.

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PACS. 61.30Eb -Experimental determinations of smectic, nematic, cholesteric, and other structures. PACS. 61.30Gd -Orientational order of liquid crystals; electric and magnetic field effects on order. PACS. 64.70Md -Transitions in liquid crystals.Abstract. -We report the anchoring transition in a discotic liquid crystal, hexapentyloxytriphenylene (HPT), from edge-on to side-on alignment in a discotic phase. The topology, easy direction of orientation and the coating of the substrate with a polymer are the determining factors for the alignment and the anchoring transition in a discotic liquid crystal.Discotic liquid crystals are perfect examples of 2-dimensional self-organizing systems with a long-range order in the 3rd dimension. Finding reasons as to why and how such large molecules organize themselves interests physicists involved in exploring condensed matter [1]. Another reason for the continued interest in discotics lies in the potential applications of such systems. The type of orientation of the molecules inside a LC cell governs the characteristics of devices [1][2][3][4]. It has recently been shown [1,2] that the columnar phase structure of discotic liquid crystals is suitable for the fast transport of photogenerated charge carriers. These results demonstrate that molecular organization in discotics plays an essential role in improving the response time of organic photoconductors; the charge mobility of hexahexylthiotriphenylene (HHTT) [1] approximates to that of an organic crystal anthracene. Consequently, it is therefore of paramount importance that causes of the surface and the surface-induced bulk alignment in discotics be explored to advance their potential for use in devices. From an extensive work carried out on nematics and smectics [5][6][7], certain factors (the type of the orienting layer, the mechanical rubbing or its grooving) are known to control the surface alignment. The mechanisms for the surface-induced bulk alignment are based on a) the short-range surface-molecule and then molecule-molecule interactions [8] and b) the minimization of the long-range interactions between the surface and the bulk [9].The alignment techniques for discotic liquid crystals have so far included rotating magnetic field [10][11][12] and different surface treatments [13]. A strong rotating magnetic field (∼ 2 T) c EDP Sciences