Lattice imaging studies on 201R polytype of silicon carbide

Abstract

Lattice imaging in the electron microscope has been found to be a very suitable and complimentary technique to X-ray diffraction for structural characterization in some systems at the unit cell level. A number of mixed layer compounds, minerals and oxides have been studied by this technique [1-4]. The lattice imagining technique has been extended to two-dimensional structure images by AUpress [5], Iijima et al. [6], Hashimoto et al. [7], Van Tendeloo and Amelinckx [8] and others. Smith et al. [9] and Jepps et al. [10] have emphasized the role of two-dimensional tilted beam images in SiC and have shown that the stacking sequence of Si-C double layers can be derived from chevron shaped fringes. However, the image contrast becomes extremely sensitive to experimental parameters in such images and comparatively low resolution one-dimensional images are able to provide valuable information about the structure in large period polytypes of SiC [11-17]. However, the direct correlation of the image contrast with the crystal structure is possible only under suitable experimental conditions and instances of complete structure determination with the help of lattice images has been very limited. Silicon carbide is one of the prominent polytypic materials and it is usually found that large unit cells consist of regular stackings of unit cells of more common structure types such as 6H, 15R, 4H and 21R, which are periodically interrupted by stacking faults. Therefore, it has become a common practice to describe any large period structure in terms of stacking of smaller unit cells followed by sequences of some faulted layers. During the present investigation the crystal has been examined first by X-ray diffraction and then by the lattice imaging technique so that a final structure derived from the lattice image can be confirmed by establishing a match between observed and calculated X-ray intensities. A thin light green crystal platelet from vapour grown (Lely's method) SiC of size 3 mm x 2 mm× 0.5 mm was first examined by X-ray diffraction. The X-ray examination revealed the presence of a very thin lamella of a high period polytype (c ~ 16.9 nm) in parallel intergrowth with a 6H platelet. Fig. 1 shows 10.1 row of spots of the parent crystal as recorded on a 15 °, c-axis oscillation photograph. The diffracted beams from the high period structure on one side of the zero layer line pass through the 6H platelet and get absorbed. High period polytype structures often intergrow as extremely thin lamellae, as in the present case, and it becomes impossible to characterize the structure fully by X-ray diffraction. Lattice imaging plays an excellent role in investigating such structures. This is because on crushing such crystals thin lamellae of intergrown structures cleave out and give rise to suitable flakes for electron microscopic examination. Therefore the crystal was crushed into fine powder fragments and were dispersed on a holey carbon grid for examination in a Philips EM300 electron microscope at 100kV equipped with a high resolution double tilt goniometer stage. A one-dimensional lattice image taken in exact zone axis orientation appeared to be exactly of 6H without giving any indication of a higher periodicity on the microscope screen. However by slightly deviating from the zone axis orientation indications of higher periodicity appear. Figs. 2a and b show a bright field lattice image obtained by symmetrical 0 0 . l reflections and the corresponding diffraction pattern. The fringe modulations in the thicker part of the crystal clearly indicate a periodicity of 16.9 nm which corresponds to that observed from X-ray diffraction. Fig. 2c shows a magnified image of a few unit cells of periodicity 16.9 nm where the sequence of fringe spacings is more clear. For deriving structural information the thinnest region near the edge is best suited. Examination of such a region (Figs. 2a and c) clearly reveals