Crystallographic Origin of the Alternate Bright/Dark Contrast in 6H-SiC and other Hexagonal Crystal HREM Images

Abstract

Alternating bright/dark anomalous subunitcell contrast in HREM images along or near the close-packed direction of 6H-SiC, Ti5Si3, α-Ti, and 4H-SiC, all of which are hexagonal, was examined using computer-generated crystal models, HREM image simulations, and digital diffractograms from the corresponding experimental images. The primary variables were crystal tilt and thickness. Crystal model projections showed that the scattering potential was smeared anisotropically within the unit cells by small crystal tilts, which reproduced the experimentally observed anomalous subunit-cell contrast modulations in the corresponding simulations. The effect increased with thickness, but it did not occur in exact zone axis simulations for any crystal thickness. Structural considerations indicated that the contrast resulted from tilt-induced violations of Gjonnes-Moodie dynamical extinctions and excitation of kinematically forbidden reflections in the imaging zone. Digital diffractograms from experimental HREM images confirmed their presence in the imaging zone diffraction patterns. These effects were absent in HREM images from cubic crystals in this material system because the structurally induced requisite kinematically forbidden reflections do not occur in the imaging zone.

Platinum-enhanced graphitisation in sandwich structures of silicon carbide and borosilicate glass

“…1). The atomic 6H polytype structure of the silicon carbide can clearly be recognised by alternating bright/dark contrasts (which are induced by a small crystal tilt away from the 1 12 0 zone axis [28,29]) corresponding to the six-fold stacking of the atomic planes. Each dot represents a SiC dipole.…”

High resolution interface nanochemistry and structure. Progress report, December 1, 1994--November 30, 1995

“…6-1 1 of ref. 7. Digital diffractograms, computed from experimental HREM images of the hexagonal phases that exhibited the alternating contrast effect, did indeed exhibit excited forbidden Bragg reflections, as shown in fig.…”

“…12 of ref. 7. Similar models A. Subunit cell alternating contrast in HREM images of hexagonal crystals During our investigation of TVGH-SiC interfaces we noticed that HREM images of…”

“…1,2 of ref. 7. To gain physical insight into the mechanism responsible for this contrast effect we constructed computer crystal models using CERIUS software on a Silicon Graphics platform for each of the hexagonal phases and viewed them along the exact HREM image zone axis and at slight tilts off the zone axis.…”

A transmission electron microscopy investigation of SiC films grown on SiC substrates by solid-source molecular beam epitaxy

“…1 There is also the intriguing possibility of forming novel heterostructure devices because SiC is a well-known polytypic material. [4][5][6][7][8][9][10] At temperatures higher than 1300°C the growth of defect-free cubic or hexagonal polytypes has been reported by a number of authors [11][12][13][14][15] using chemical vapor deposition (CVD) or the sublimation technique. [4][5][6][7][8][9][10] At temperatures higher than 1300°C the growth of defect-free cubic or hexagonal polytypes has been reported by a number of authors [11][12][13][14][15] using chemical vapor deposition (CVD) or the sublimation technique.…”