American Club Sandwich Model

Crystal J26

An idealised sandwich configuration in which long period/disordered layers are clearly seen sandwiched between a shorter period polytype. Enlargements from an edge topograph of a predominantly 6H polytype SiC crystal J26 with dimensions 4.21 x 4.05 mm and total thickness 1370 m m.

1. A {4150} l = 0 reflection which provides finite contrast for all polytypes and reveals in this instance two regions of higher diffraction contrast termed defect density bands. These are shown magnified in the blow up of the circular region as would be observed under an optical microscope (magnification x actual size).
2. A {3142} l ¹ 0 reflection reveals two clear absence bands which are believed to correspond to fine one-dimensionally disordered (1DD) or long-period polytype (LPP) layers. Again a magnified view is shown to the left of the topograph with clear indication from the morphology showing their location along the crystal edge in relation to the main 6H polytype.
3. An overexposed region close to a main {314 4} 6H reflection showing fine (12μm) closely spaced reflections from measurements corresponding to a 152H/456R long period polytype (LPP) previously unreported. These are visible in the blow up and correspond to the lower of the regions indicated in (b).
4. A model to scale of the polytypic content indicating a fine ~ 5μm 1DD layer.