Structure of α- and β-Si3N4observed by 1 MV electron microscopy

Abstract Crystal structures of Si2N4 prepared by chemical vapour deposition (CVD) have been observed on the atomic scale by high-voltage, high-resolution electron microscopy. Many-beam imagee with [001] axial illumination exhibit the projected potential with trigonal and hexagonal symmetry for α- and β-Si2N4 respectively at thicknesses of less than about 10 nm. The image contrasts are discussed on the basis of multislice calculations as a function of crystal thickness and objective lens defocus. Excellent agreement is obtained between the observed and calculated images for both crystals. An atomic model of a structure defect in β-Si2N4 is proposed directly from the images.