Electronic structure and bonding of intergranular glassy films in polycrystallineSi3N4:Ab initiostudies and classical molecular dynamics simulations

Abstract

The electronic structure and bonding of a realistic model of an intergranular glassy film ͑IGF͒ was studied with multiple computational methods. The model has a Si-O-N glassy region sandwiched between crystalline basal planes of ␤-Si 3 N 4 and contains a total of 798 atoms. It was constructed with periodic boundary conditions via classical molecular dynamics ͑MD͒ techniques using an accurate multibody atomic potential. The model was then further relaxed by the VASP ͑Vienna ab initio simulation package͒ program. It is shown that the VASP-relaxed structure reduces the total energy from the MD-relaxed structure by only 47.38 eV, validating the accuracy of the multiatom potential used. The calculated electronic structure shows the IGF model to be an insulator with a sizable gap of almost 3 eV. Quasidefectlike states can be identified near the band edges arising from the more strained Si-N and Si-O bonds at the interface. Calculation of the Mulliken effective charge and bond order values indicates that the bonds in the glassy region and at the interface can be enhanced and weakened by distortions in the bond length and bond angle. The states at the top of the valence band are derived mostly from the crystalline part of the Si-N bonding while the states at the bottom of the conduction band are dominated by the Si-O bonding in the glassy region. Calculation of the electrostatic potential across the interface shows an average band offset of about 1.5 eV between the crystalline ␤-Si 3 N 4 and the glassy Si-O-N region which could be related to the space charge model for IGF.

Projected Potential Profiles across Intergranular Glassy Films

“…A possible explanation is that the segregation of rare earth ions to the interface attracts compensating charge. This knowledge of space charge accumulation at IGFs and glassy pockets of Si 3 N 4 will help to validate MD and first principle simulation methods 31 for this material. The slanted region of electrostatic potential profile indicated by A-Aöin Fig.…”

Theoretical study of the elasticity, mechanical behavior, electronic structure, interatomic bonding, and dielectric function of an intergranular glassy film model in prismaticβ-Si3N4

“…The basal model contains defective structures at the IGF/crystalline interface as explained in Ref. 35. Also, the convergence criteria used in the VASP relaxation for the basal model were less stringent.…”

“…54 In contrast, the basal model in the earlier studies had a rather blurred interface due to the existence of defective structures within the first three crystalline layers next to the IGF. 35 On the other hand, the so-called diffuse interface model argues that the crystal/IGF boundary should not be sharp. 55 Obviously, a clear distinction between a diffuse interface model and that with a more abrupt sharp interface cannot be made based on the study of one or two such models.…”

“…35 In the basal model we used classical MD with multiatom potentials to obtain the initial structure and then later relaxed it using VASP. The basal model contains defective structures at the IGF/crystalline interface as explained in Ref.…”

“…We have previously demonstrated that ab initio simulations can be successfully applied to IGF models in ␤-Si 3 N 4 to yield a wealth of previously unknown information. [35][36][37][38] The model we previously constructed has 798 atoms sandwiched between the basal planes of ␤-Si 3 N 4 ͑referred to as the basal IGF model͒. The basal IGF model, with its defective structure at the glass/crystal interface, can be considered as a kind of diffuse interface model where the boundary between the crystalline part and the glass part is not perfectly sharp.…”