Crystalline and liquid Si₃N₄characterization by first-principles molecular dynamics simulations

Abstract

Abstract. Silicon nitride (Si3N4) has a wide range of engineering applications where its mechanical and electronic properties can be effectively exploited. In particular, in the microelectronics field, the amorphous silicon nitride films are widely used as charge storage layer in metal-alumina-nitrideoxide nonvolatile memory devices. Atomic structure of amorphous silicon nitride is characterized by an high concentration of traps that control the electric behavior of the final device by the trappingde-trapping mechanism of the electrical charge occurring in its traps. In order to have a deep understanding of the material properties and, in particular, the nature of the electrical active traps a detailed numerical characterization of the crystalline and liquid phases is mandatory. For these reasons first-principles molecular dynamics simulations are extensively employed to simulate the crystalline Si3N4 in its crystalline and liquid phases. Good agreement with experimental results is obtained in terms of density and formation entalpy. Detailed characterization of c-Si3N4 electronic properties is performed in terms of band structure and band gap. Then constant temperature and constant volume first-principles molecular dynamics is used to disorder a stoichiometric sample of Si3N4. Extensive molecular dynamics simulations are performed to obtain a reliable liquid sample whose atomic structure does not depend on the starting atomic configuration. Detailed characterization of the atomic structure is achieved in terms of radial distribution functions and total structure factor.

Localization properties of vibrational modes in $a$-Si3N4

“…This is rather close to the value (∼1.2 Å ) reported e.g. for the N-N bond length in a first-principle investigation of the liquid phase [47] and is 0.2 Å shorter than the value found in [25] for a nonstoichiometric system Si 3 N 4.5 . We note however that the concentration of such type of bonds in a-Si 3 N 4 is rather controversial [25,48,49] and investigating this issue goes beyond the scope of the present work.…”

“…The projection shows a sharp peak at about ∼1150 cm −1 . We remark that shorter N-N bonds[48,47] should give rise to higher frequencies peaks as it is the case for instance of two-fold coordinated nitrogen atoms[14], and viceversa longer N-N bonds will show lower stretching frequencies. We thus infer that stretching of N-N homopolar bonds should only affect the high frequency tail (above ∼1100 cm −1 ) of the infrared dielectric function shown in Fig.2(c).…”