Hexagonal Silicon Nitride Research Articles

Vacancy-driven shear localization in silicon nitride

Vacancies play an essential role on mechanical properties of ceramics, while it is a challenge to reveal the atomistic process of vacancy-driven failure mechanism due to difficulty in characterizing the atomic defects. Here, we used hexagonal silicon nitride as a prototype ceramic and report the shear localization as a main failure mechanism, which is characterized by combining nanoindentation and scanning transmission electron microscopy. It is revealed that the intrinsic point defects in silicon nitride triggers the nanoscale shear bands on the prismatic slip plane when subjected to high stresses. The quantum mechanics simulations ascertained that the shear bands are mediated by nitrogen defects in tetragon sites during the pure shear deformation.

Silicon Nitride Metalenses for Close-to-One Numerical Aperture and Wide-Angle Visible Imaging

As one of nanoscale planar structures, metasurface has shown excellent superiorities on manipulating light intensity, phase and/or polarization with specially designed nanoposts pattern. It allows to miniature a bulky optical lens into the chip-size metalens with wavelength-order thickness, playing an unprecedented role in visible imaging systems (e.g. ultrawide-angle lens and telephoto). However, a CMOS-compatible metalens has yet to be achieved in the visible region due to the limitation on material properties such as transmission and compatibility. Here, we experimentally demonstrate a divergent metalens based on silicon nitride platform with large numerical aperture (NA~0.98) and high transmission (~0.8) for unpolarized visible light, fabricated by a 695-nm-thick hexagonal silicon nitride array with a minimum space of 42 nm between adjacent nanoposts. Nearly diffraction-limit virtual focus spots are achieved within the visible region. Such metalens enables to shrink objects into a micro-scale size field of view as small as a single-mode fiber core. Furthermore, a macroscopic metalens with 1-cm-diameter is also realized including over half billion nanoposts, showing a potential application of wide viewing-angle functionality. Thanks to the high-transmission and CMOS-compatibility of silicon nitride, our findings may open a new door for the miniaturization of optical lenses in the fields of optical fibers, microendoscopes, smart phones, aerial cameras, beam shaping, and other integrated on-chip devices.

Effects of diverse metal adsorptions on the electronic and optical properties of the β-Si3N4 (2 0 0) surface: A first-principles study

We use first-principles calculations in this contribution to study electronic structures and optical properties, especially absorption and reflectivity spectra, dielectric constant and loss function of Li, Na, K, Be, Mg, Ca and Al adsorption behaviors on (2 0 0) surface of hexagonal silicon nitride (β-Si3N4). The lower adsorption energy of −3.931 eV for Ca-adsorbed surface indicates that it has more excellent structural stability. The seven kinds of adsorptions are all featured by chemisorption with commonly negative value of adsorption energy, implying relatively abated capacity of defending the chemical corrosion formed by metals mentioned above. Band gaps of surfaces with different adsorptions are 0.113 eV, 0.099 eV, 0.113 eV, 0.147 eV, 0.154 eV, 0.151 eV and 0.111 eV for Li-, Na-, K-, Be-, Mg-, Ca- and Al-adsorbed surface structure, respectively, along with 0.143 eV for clean surface of β-Si3N4, which leads more efficient measures of potential for obtaining outstanding semiconductor properties for materials. Moreover, absorption spectra curve of surface drops to a lower peak value of 5.46 × 104 cm−1, 4.94 × 104 cm−1 and 3.53 × 104 cm−1 for Na-, Mg- and Ca-adsorption, decreasing by 10.0%, 18.6% and 41.8% in contrast to clean surface, 6.07 × 104 cm−1, respectively, in which the surface carries decreased reflectivity spectra and dielectric loss that are greatly valued and expected in solar cell industry, indicating its broader application potential in photoelectric and microelectronics devices fields.

Investigation of electronic structures and optical properties of β-Si3N4 doped with IV A elements: A first-principles simulation

Based on first-principles simulations with the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional, we studied the electronic structures and optical properties of hexagonal silicon nitride (β-Si3N4) doped with IV A elements, C, Ge, Sn and Pb. It was found that the Ge-doped system is characterized by a more stable structure with a lower formation energy of 2.584 eV compared with those of the C-, Sn- and Pb-doped systems of 3.877 eV, 5.249 eV and 7.672 eV, respectively. The band gap (EG) of the Pb-doped system was the lowest at 1.6 eV, displaying semiconducting characteristics. Additionally, there was a transition from a direct band gap to an indirect band gap in the C-doped system. Charge difference density analysis showed that the covalent property of the C-N bonds was enhanced by expansion of the electron-free region and the larger Mulliken population values of 0.71 and 0.86. Furthermore, lower absorption and reflectivity peaks at 11.30 eV were observed for the C-doped system, demonstrating its broader potential for application in photoelectric and microelectronic devices.

Structural and light emitting properties of silicon-rich silicon nitride films grown by plasma enhanced-chemical vapor deposition

Si-rich Silicon nitride films fabricated by plasma enhanced chemical vapor deposition (PECVD) on p-type silicon substrates were investigated by means of photoluminescence (PL) and X-ray diffraction (XRD) methods. The film stoichiometry was controlled via varying the NH3/SiH4 ratio (R) in the range R=0.56–1.0. Thermal annealing at 1100°C for 30min in the nitrogen flow was applied to form the Si nanocrystals (NCs) embedded in the films. XRD experiment has revealed also the formation of hexagonal silicon nitride NCs with the sizes of 26–30nm in the films grown with R=1. When R decreasing, the sizes of silicon nitride NCs decreases down to 2.8–3.0nm (R=0.63). In the films grown with R=0.56 the amorphous silicon nitride, amorphous Si and crystallized Si phases have been detected. PL study showed the non-monotonic behavior of PL intensity with the R decreasing. The highest PL intensity was detected for the films grown with R=0.63. PL spectra of all films were found to be complex containing a set of PL bands peaked at about 2.8–3.0, 2.5–2.7, 2.1–2.2 and 1.8–2.0eV. Temperature behavior of PL spectra was investigated in the range 20–300K that allowed analyzing the nature of each PL component. It was shown that the peak position of all PL bands varies with the SiNx stoichiometry and Si NC sizes, demonstrating a “red” shift with R decreasing. It was revealed that the former three PL bands did not change their peak positions versus temperature of measurement that permits to assign them to the carrier recombination via radiative defects in the silicon nitride matrix. The PL band at 1.8–2.0eV has demonstrated a “blue” spectral shift with cooling similar to the Si band gap shrinkage. Based on this behavior, the 1.8–2.0eV PL band has been attributed to exciton emission inside of Si NCs. The role of silicon nitride NCs in photoluminescence and its excitation is discussed.

Characterization of oxygen impurity in silicon nitride storage layer: A first-principles investigation

In this paper, based on first-principles calculations, we have carried out a comprehensive study of substitutional oxygen defects in hexagonal silicon nitride (β-Si3N4). First of all, it is found that substitutional oxygens tend to form clusters at three different sites due to the intensive attractive interaction. By analyzing modified Bader charge and trap energy, we next discuss retention characteristic of the three clusters. The results manifest that all of the clusters are amphoteric defects with the ability to trap both electrons and holes. Moreover, every cluster is more powerful to trap holes, which indicates that oxygen clusters have a higher stability to hold holes than electrons. With regard to endurance characteristic, our studies reveal that the three clusters present differences after program/erase cycles, and then we explore the mechanism of endurance degeneration by nudged elastic band method (NEB). Taking full account of retention and endurance, we deem holes rather than electrons to be optimal to act as operational charge carriers for oxygen defects in Si3N4-based charge trapping memories.

A first-principle investigation of the oxygen defects in Si3N4-based charge trapping memories

Based on first principle calculations, a comprehensive study of substitutional oxygen defects in hexagonal silicon nitride (β-Si3N4) has been carried out. Firstly, it is found that substitutional oxygen is most likely to form clusters at three sites in Si3N4 due to the intense attractive interaction between oxygen defects. Then, by using three analytical tools (trap energy, modified Bader analysis and charge density difference), we discuss the trap abilities of the three clusters. The result shows that each kind of cluster at the three specific sites presents very different abilities to trap charge carriers (electrons or holes): two of the three clusters can trap both kinds of charge carriers, confirming their amphoteric property; While the last remaining one is only able to trap hole carriers. Moreover, our studies reveal that the three clusters differ from each other in terms of endurance during the program/erase progress. Taking full account of capturing properties for the three oxygen clusters, including trap ability and endurance, we deem holes rather than electrons to be optimal to act as operational charge carriers for the oxygen defects in Si3N4-based charge trapping memories.

First-principles study of oxygen and aluminum defects in β-Si3N4: Compensation and charge trapping

Formation energies for oxygen and aluminum defects in hexagonal silicon nitride (β-Si3N4) were calculated from first-principles. Aluminum induces a deep donor level at the thermodynamically preferred interstitial site. Oxygen donors substituted into lattice nitrogen sites are found to ionize at 3.5eV above the valence band and might readily compensate acceptor levels induced by native silicon micro-cluster defects. This is rationalized by examining the electronic structure of a model Si micro-cluster defect, and of the oxygen substitutional impurity. Specifically, this article describes the influence of oxygen and aluminum impurities on the number of states available for electron capture in silicon nitride as trapping layer.

Native defects in hexagonalβ-Si3N4studied using density functional theory calculations

A comprehensive study of single native point defects in hexagonal silicon nitride ($\ensuremath{\beta}$-Si${}_{3}$${\mathrm{N}}_{4}$) has been carried out based on density functional calculations of formation energies. Both nitrogen- and silicon-rich native defect centers form donor and acceptor states in the band gap of $\ensuremath{\beta}$-Si${}_{3}$${\mathrm{N}}_{4}$, confirming their amphoteric behavior. Silicon dangling bonds resulting from structural nitrogen vacancies (${V}_{\mathrm{N}}$) are the most abundant native defects, in particular, in their acceptor state (${V}_{\mathrm{N}}^{\ensuremath{-}}$ and ${\mathrm{V}}_{\mathrm{N}}^{3\ensuremath{-}}$). Hydrogenation promotes the appearance of Si dangling bond defects in the neutral state consistent with the observation of paramagnetic centers in the gap of amorphous silicon nitrides by electron spin resonance. This explains the utility of silicon nitride as charge trapping layer in nonvolatile memories.

Synthesis of Cubic Silicon Nitride in a Cylindrical Recovery Capsule

Cubic silicon nitride was synthesized from a mixture of hexagonal silicon nitride and copper powder in a cylindrical recovery capsule. It was shown that the synthesis occurred only in the region of the Mach reflection of shock waves near the axis of the capsule. A method of loading the material with ejection of the sample from the capsule into an airtight container was developed. As a result of loading, up to 20% of the starting hexagonal silicon nitride transforms to the cubic phase.

Structural characterization of buried nitride layers formed by nitrogen ion implantation in silicon

The synthesis of buried silicon nitride insulating layers was carried out by SIMNI (separation by implanted nitrogen) process using implantation of 140keV nitrogen (14N+) ions at fluence of 1.0×1017, 2.5×1017 and 5.0×1017cm−2 into 〈111〉 single crystal silicon substrates held at elevated temperature (410°C). The structures of ion-beam synthesized buried silicon nitride layers were studied by X-ray diffraction (XRD) technique. The XRD studies reveal the formation of hexagonal silicon nitride (Si3N4) structure at all fluences. The concentration of the silicon nitride phase was found to be dependent on the ion fluence. The intensity and full width at half maximum (FWHM) of XRD peak were found to increase with increase in ion fluence. The Raman spectra for samples implanted with different ion fluences show crystalline silicon (c-Si) substrate peak at wavenumber 520cm−1. The intensity of the silicon peak was found to decrease with increase in ion fluence.

Molecular modeling of ionic liquid tribology: Semi-empirical bonding and molecular structure

The interactions between selected ionic liquids and either an aluminum oxide or a silicon nitride surface are modeled in this report using semi-empirical methods. The ionic liquids include a series of seven cationic imidazolium derivatives with PF 6 − as the anion. The tribological properties of these ionic liquids and their interactions with aluminum and Al–steel alloys are modeled using a smooth aluminum oxygen surface. The ionic liquids are allowed to form a complex with this surface and the enthalpies of complex formation are seen to correlate with the tribological properties of the seven ionic liquids. The seven ionic liquids are also complexed with a hexagonal silicon nitride surface and their tribological properties are predicted on the basis of complex formation enthalpies.

A comparison of experimental and calculated electron-energy loss near-edge structure of carbon, and the nitrides of boron, carbon and silicon using multiple scattering theory

In this paper we present a comparison of experimental K-shell near-edge structure for a variety of materials with edges calculated using multiple scattering theory. The material systems studied include diamond and graphite, cubic and hexagonal boron nitride, hexagonal silicon nitride (β-Si3N4) and the proposed structure for hexagonal carbon nitride (β-C3N4). We found that multiple scattering theory successfully predicts the majority of features in the near-edge structure of these materials. Discrepancies between experiment and theory can be accounted for by the non-self-consistent nature of potentials employed in the calculations. This work shows that multiple scattering calculations could be employed to help interpret features in the near-edge structure of carbon-based materials and in light element binary alloys.

The investigation of properties of silicon nitride films obtained by RPECVD from hexamethyldisilazane

The silicon nitride films were obtained by remote plasma enhance chemical vapor deposition (RPECVD) using hexamethyldisilazane or its mixture with ammonia in the range 373–773 K. The correlations between the chemical composition, deposition rates, optical, electrical and structural properties and the growth conditions were established. It was found that the formation of two polycrystalline hexagonal phases SiC and Si 3N 4 was realized by using pure hexamethyldisilazane as precursor. The ammonia addition in gas mixture leaded to change of the chemical composition and structure of silicon nitride films, namely, the disappearance of carbon-bonding and SiC formation, and the order of hexagonal silicon nitride.

Crystal Structures of Silicon Nitride

VASSILIOU and Wilde1 have recently reported a hexagonal silicon nitride, the X-ray diffraction pattern of which differs from that of the ‘orthorhombic’ silicon nitride extracted from silicon steels by Leslie, Carroll and Fisher2. After preliminary evidence of two different nitrides3, Turkdogan, Bills and Tippett4 have now definitely established the existence of two forms of silicon nitride, designated as α and β, with the same chemical compositions (Si3N4) and the same measured densities (3.19 ± 0.01 gm./ml.). Even more recently, the same two phases, I and II, as observed by Vassiliou and Wilde have been claimed by Popper and Ruddlesden5 to be orthorhombic and rhombohedral respectively.