Electronic structure and optical properties of alpha and beta phases of silicon nitride, silicon oxynitride, and with comparison to silicon dioxide

Growth of Si3N4 Thin Films on Si(111) Surface by RF-N2 Plasma Nitriding

Ultra-thin Si3N4 films were grown on Si(111) surface by radio frequency (RF)-N2 plasma exposure at 900 °C with 1–1.2 sccm of a flux of atomic nitrogen. We discuss the effect of various conditions such as N2 flow rate, nitriding time and RF power on the optical, chemical, and structural properties of a nitrided Si3N4 layer. The optical properties, surface morphology and chemical composition are investigated by using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Cross-sectional TEM images show that an RF power of 350 W induced some damage to the Si(111) surface. The thickness of nitrided Si3N4 was measured to be about 5–7 nm. XPS results shown that the binding energy of Si 2p3/2 located at 101.9 ± 0.1 eV is attributed to the Si–N bonds in the Si3N4 compound. Smooth Si3N4 ultra-thin films were obtained at a nitridation time close to 1 h with an RF power of 300 W, with a measured refractive index (n) nearly to 1.88 at 632 nm. The increase in refractive index with decreased RF-plasma power and nitrogen flow rate is probably attributed to the change in the stoichiometry of the film and less surface damage.

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide-based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent-bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide-only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion-engineering principles in heteroanionic materials are reviewed.

Transparent polycrystalline cubic silicon nitride

Glasses and single crystals have traditionally been used as optical windows. Recently, there has been a high demand for harder and tougher optical windows that are able to endure severe conditions. Transparent polycrystalline ceramics can fulfill this demand because of their superior mechanical properties. It is known that polycrystalline ceramics with a spinel structure in compositions of MgAl₂O₄ and aluminum oxynitride (γ-AlON) show high optical transparency. Here we report the synthesis of the hardest transparent spinel ceramic, i.e. polycrystalline cubic silicon nitride (c-Si₃N₄). This material shows an intrinsic optical transparency over a wide range of wavelengths below its band-gap energy (258 nm) and is categorized as one of the third hardest materials next to diamond and cubic boron nitride (cBN). Since the high temperature metastability of c-Si₃N₄ in air is superior to those of diamond and cBN, the transparent c-Si₃N₄ ceramic can potentially be used as a window under extremely severe conditions.

Investigation of photoelectrical properties of α-Si3N4 nanobelts with surface modifications using first-principles calculations

The photoelectrical properties of α-Si₃N₄ nanobelts with surface H, F and Cl modifications are investigated using first-principles methods.

Linking photoluminescence of α-Si3N4 to intrinsic point defects via band structure modelling

Photoluminescence properties have been connected to intrinsic point defects for Si abundant (red bar) and N plentiful (blue bars) α-Si₃N₄via band structure modelling using DFT calculations.

Theoretical calculations of stability, mechanical and thermodynamic properties of IVA group Willemite-II nitrides

The structural stability and mechanical and thermodynamic properties of WII- A 3 N 4 ( A=C , Si , Ge and Sn ) are calculated by first-principles calculations based on the density functional theory. The calculated lattice parameters and elastic constants of WII- A 3 N 4 ( A=C , Si , Ge and Sn ) are in good agreement with the experimental data and previously calculated values. WII- A 3 N 4 ( A=C , Si , Ge and Sn ) compounds are also found to be thermodynamically and mechanically stable. The results suggest that hardness of WII- C 3 N 4 is the hardest of these C 3 N 4 polymorphs. The hardness of WII- Sn 3 N 4 is the smallest among WII- A 3 N 4 ( A=C , Si , Ge and Sn ).

Furthermore, the mechanical anisotropy, Debye temperature, the minimum thermal conductivity and thermodynamic properties of WII- A 3 N 4 ( A=C , Si , Ge and Sn ) compounds can be investigated.

Electronic structure and optical properties of Si–O–N compounds with different crystal structures

The optical properties of Si–O–N compounds are determined not only by their component, but also by their microstructure.

Spectral mixing formulations for van der Waals-London dispersion interactions between multicomponent carbon nanotubes

Recognition of spatially varying optical properties is a necessity when studying the van der Waals-London dispersion (vdW-Ld) interactions of carbon nanotubes (CNTs) that have surfactant coatings, tubes within tubes, andor substantial core sizes. The ideal way to address these radially dependent optical properties would be to have an analytical add-a-layer solution in cylindrical coordinates similar to the one readily available for the plane-plane geometry. However, such a formulation does not exist nor does it appear trivial to be obtained exactly. The best and most pragmatic alternative for end-users is to take the optical spectra of the many components and to use a spectral mixing formulation so as to create effective solid-cylinder spectra for use in the far-limit regime. The near-limit regime at "contact" is dominated by the optical properties of the outermost layer, and thus no spectral mixing is required. Specifically we use a combination of a parallel capacitor in the axial direction and the Bruggeman effective medium in the radial direction. We then analyze the impact of using this mixing formulation upon the effective vdW-Ld spectra and the resulting Hamaker coefficients for small and large diameter single walled CNTs (SWCNTs) in both the near- and far-limit regions. We also test the spectra of a [16,0,s+7,0,s] multiwalled CNT (MWCNT) with an effective MWCNT spectrum created by mixing its [16,0,s] and [7,0,s] SWCNT components to demonstrate nonlinear coupling effects that exist between neighboring layers. Although this paper is primarily on nanotubes, the strategies, implementation, and analysis presented are applicable and likely necessary to any system where one needs to resolve spatially varying optical properties in a particular Lifshitz formulation.

Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface

Hydrogen-gas etching of a 6H-SiC(0001) surface and subsequent annealing in nitrogen atmosphere leads to the formation of a silicon oxynitride (SiON) epitaxial layer. A quantitative low-energy electron diffraction analysis revealed that the SiON layer has a hetero-double-layer structure: a silicate monolayer on a silicon nitride monolayer via Si-O-Si bridge bonds. There are no dangling bonds in the unit cell, which explains the fact that the structure is robust against air exposure. Scanning tunneling spectroscopy measured on the SiON layer shows a bulk SiO2-like band gap of approximately 9 eV. Great potential of this new epitaxial layer for device applications is described.

Crystal Structure and Electron Density of α-Silicon Nitride: Experimental and Theoretical Evidence for the Covalent Bonding and Charge Transfer

Crystal structure and electron-density distribution of alpha-silicon nitride (alpha-Si3N4, space group: P31c) have been investigated by a combined technique of the Rietveld method, the maximum-entropy method (MEM), and MEM-based pattern-fitting of high-resolution synchrotron powder diffraction data. In combination with density functional theory calculations, the present experimental electron-density distribution of the alpha-Si3N4 indicates covalent bonds between Si and N atoms and charge transfer from the Si to N atom. The triangular distribution around the N atoms, which is attributable to the nitrogen sp2 hybridization for the nearest silicon and nitrogen pairs, was found in both experimental and theoretical electron density distributions. The minimum electron density in an intralayer Si-N bond was a little lower than that in an interlayer bond, which would be responsible for the inequality between elastic constants, C33 > C11. The present work suggests that the high bulk modulus of the alpha-Si3N4 is attributable to the high minimum electron density of the Si-N bond.

Effect of Microstructure on Dielectric Properties of Si3N4 at Microwave Frequency

Silicon nitride (Si3N4) has been researched intensively because of superior mechanical properties up to high temperature. The mechanical properties of Si3N4 are strongly related to microstructure. The microstructure control of silicon nitride is well known to be a key issue for tailoring the mechanical properties of structural ceramics. This work was performed to reveal the effect of microstructure on dielectric properties at microwave frequency. Three starting powders were used fine, course a-Si3N4 and b-Si3N4. Sintering additives, 5 wt.% Y2O3, 2 wt.% Al2O3 and 1 wt.% MgO were mixed with each starting powder. Si3N4 ceramic with different b/a phase specimen were obtained by hot pressing. The post-resonator method was used for the measurement of dielectric properties, dielectric constant (e′) and dielectric loss (tand), at microwave frequency range. Silicon nitride ceramics show dielectric constant of 8.1 – 8.6 and dielectric loss 1.1 x 10-3 – 5.6 x 10-3. The effect of grain size and the role of phase on microwave dielectric properties are discussed.

Effects of nitridation on the characteristics of silicon dioxide: dielectric and structural properties from ab initio calculations

By combining ab initio calculations and classical molecular dynamics, we determine how the inclusion of nitrogen in a silica matrix changes its dielectric constant, and elucidate the underlying mechanisms. We find that there is an entire range of nitrogen concentrations (up to approximately 25%) for which the structural pattern of the oxide is preserved in bulk SiON, and the dielectric constant increases mainly because of the variation of the ionic polarizability. This behavior is not sensitive to hydrogen passivation of nitrogen. The few defects, which are associated with electron states near the gap, are mainly centered on undercoordinated nitrogen and undercoordinated silicon, and tend to be removed by hydrogen.