Simple Sandwich Model

Crystal Sample J6, Plate no:# 192

A classic example of the 6H + 1DD + 6H model of polytype coalescence in silicon carbide consisting of a 40 m m one-dimensionally disordered (1DD) layer sandwiched between a predominantly thick (870 m m) 6H layer and a thinner layer of the same type, which is the most common polytype in SiC. The model describing this observed polytype content has been referred to as a simple sandwich configuration.

The separation of the thinner (30 m m) 6H layer from the main crystal, whose total measured thickness is 940 m m, is clearly visible in the 3142, 3144 and 3146 and in their corresponding negative index reflections. The sum of these individual discrete reflections is only 900 m m leaving the clear absence with a measured width of 40 m m and unaccounted for.

The presence of darker diffraction contrast visible in the 5270 reflection, indicates a region of higher defect density and suggests the possibility of a layer without a regular crystallographic repeat. There is also feint streaking along the diffraction row corroborating the suspected existence of disorder, and with this technique of imaging the edge profile, its position can be clearly identified along the edge of the crystal.

The region shown is an enlargement of the topographic plate around the zero layer line and as such the 5270 and 4150 central reflections provide diffraction contrast from the whole crystal.

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