Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride

A systematic study of kinetics and thermodynamics of Si (111) surface nitridation under ammonia exposure is presented. The appeared silicon nitride (8 × 8) structure is found to be a metastable phase. Experimental evidences of graphene-like nature of the silicon nitride (8 × 8) structure are presented. Interlayer spacings in the (SiN)2(AlN)4 structure on the Si (111) surface are found equal to 3.3 Å in SiN and 2.86 Å in AlN. These interlayer spacings correspond to weak van der Waals interaction between layers. In contrast to the widely accepted model of a surface structure (8 × 8) as monolayer of β-Si3N4 on Si (111) surface, we propose a new graphene-like Si3N4 (g-Si3N3 and/or g-Si3N4) model for the (8 × 8) structure. It is revealed that the deposition of Al atoms on top of a highly ordered (8 × 8) structure results in graphene-like AlN (g-AlN) layers formation. The g-AlN lattice constant of 3.08 Å is found in a good agreement with the ab initio calculations. A transformation of the g-AlN to the bulk-like wurtzite AlN is analyzed

Diffusion in GaN/AlN superlattices: DFT and EXAFS study

We report theoretical and experimental study of diffusion processes at GaN/AlN interfaces. Using climbing image nudged elastic band method with density functional theory (DFT) we have calculated migration barriers for vacancy-mediated self-diffusion in group-III element sublattice in AlN and GaN, for Ga diffusion in AlN and for Al diffusion in GaN. Attempt frequencies for this diffusion processes have been estimated based on harmonic transition state theory, and Al-Ga interdiffusion coefficient has been calculated. The calculations are in agreement with experimental results for GaN/AlN superlattices obtained by extended X-ray absorption fine structure (EXAFS) spectroscopy and transmission electron microscopy