Non-stoichiometric Silicon Nitride Research Articles

Non-stoichiometric silicon nitride for future gravitational wave detectors

Silicon nitride thin films were deposited at room temperature employing a custom ion beam deposition (IBD) system. The stoichiometry of these films was tuned by controlling the nitrogen gas flow through the ion source and a process gas ring. A correlation is established between the process parameters, such as ion beam voltage and ion current, and the optical and mechanical properties of the films based on post-deposition heat treatment. The results show that with increasing heat treatment temperature, the mechanical loss of these materials as well as their optical absorption decreases producing films with an extinction coefficient as low as k=6.2(±0.5)×10−7 at 1064 nm for samples annealed at 900 ∘C. This presents the lowest value for IBD SiN x within the context of gravitational wave detector applications. The mechanical loss of the films was measured to be ϕ=2.1(±0.6)×10−4 once annealed post deposition to 900 ∘C.

Design and preparation of high-transmittance broadband antireflection coatings with tailored refractive indices deposited by PECVD

The design and preparation of conventional optical films are mainly limited by the refractive indices of available materials. Here, a simple-operation antireflection coatings (ARCs) design method was developed based on the tailored refractive index characteristics of thin-film materials. High-transmittance, broadband (400–800 nm), three-layer single- and double-sided ARCs with tailored refractive indices and transmittances of 99.2% and 98.5%, respectively, were designed. The sensitivity of the ARCs system was simulated, which predicts the variable range of the transmittance affected by the random manufacturing errors in the refractive index (±0.5%) and film thickness (±2.5%) of each layer. The ARCs consisted of nonstoichiometric silicon oxynitride and silicon nitride films formed on glass substrates by PECVD techniques. The crystallite state, relative atomic concentration, and Si-centered tetrahedral phase Si–Si4-(ν+η)OνNη of each layer were determined. The morphology images of top and cross-section views and elemental mapping of ARCs were displayed. The designed and measured transmittances were compared; the average transmittance deviations of the single- and double-sided amorphous ARCs over the visible were 0.1% and 0.4%, respectively, which show good agreement. Furthermore, the reproducibility, repeatability, and stability of PECVD were evaluated. These results are detail discussed and are expected to serve as a reference for the preparation of optical films formed via PECVD.

Simultaneous Dangling Bond and Zincophilic Site Engineering of SiNx Protective Coatings toward Stable Zinc Anodes

Metallic Zn is an appealing anode for aqueous Zn-ion batteries, but it suffers from corrosion and Zn dendrites. In this work, we develop a nonstoichiometric silicon nitride (SiNx) film with specific defect sites, i.e., Si dangling bonds (Si DBs), to modify the surface property and thus to enhance the stability of Zn anodes. The Si DBs, together with zincophilic N sites with strong Zn affinity, are able to uniformly distribute Zn2+ ions on the anodes, enabling an even Zn deposition. Meanwhile, the electrochemically inert SiNx films can also suppress the side reactions to achieve a high Coulombic efficiency. As a result, the SiNx@Zn symmetrical battery possesses a long lifespan of more than 4600 h at 1 mA cm–2. Our work highlights the importance of defect engineering of protective layers in inhibiting the uncontrollable growth of Zn dendrites for constructing high-performance Zn anodes.

The Optical and Thermo-Optical Properties of Non-Stoichiometric Silicon Nitride Layers Obtained by the PECVD Method with Varying Levels of Nitrogen Content.

In this paper, we investigated the optical and thermo-optical properties of a-SiNx:H layers obtained using the PECVD technique. SiNx:H layers with different refractive indices were obtained from silane and ammonia as precursor gases. Surface morphology and chemical composition studies were investigated using atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and energy dispersive spectrometry methods. Spectroscopic ellipsometry was used to determine the optical indexes, thicknesses and optical bandgap of the films. The main purpose was to identify the thermo-optical characteristics of layers with different refractive indexes. Thermo-optical studies were performed to determine the temperature hysteresis of optical parameters. These measurements showed that after annealing up to 300 °C and subsequent cooling, the value of optical parameters returned to the initial values.

Optical properties of pyrolytic Silicon Nitride SiNx enriched with silicon

Non-stoichiometric silicon nitride SiNx, enriched with silicon, is a promising material for the non-volatile resistive memory development. The current studies devoted to investigation of the optical properties of SiNx synthesized in a low-pressure reactor at 800oC at different ratios of dichlorosilane (SiH2Cl2) to ammonia (NH3). It was found that for films synthesized at SiH2Cl2/NH3=1/1, 1/2 and 1/3, the corresponding bandgap values are 3.83, 4.17 and 4.40 eV. At the same time, the corresponding values of the parameter x, found according to the theoretical dependence of the bandgap value on x for SiNx calculated from the first principles, are 1.26, 1.30 and 1.32. Thus, by increasing the SiH2Cl2/NH3 ratio, it is possible to create non-stoichiometric SiNx films with a controlled silicon enrichment degree with high uniformity of chemical composition and thickness. Keywords: silicon nitride, memristor, absorption coefficient, ellipsometry, quantum chemical modeling.

Оптические свойства пиролитического нитрида кремния SiN-=SUB=-x-=/SUB=-, обогащённого кремнием

Non-stoichiometric silicon nitride SiNx, enriched with silicon, is a promising material for the non-volatile resistive memory development. The current studies devoted to investigation of the optical properties of SiNx synthesized in a low-pressure reactor at 800 oC at different ratios of dichlorosilane (SiH2Cl2) to ammonia (NH3). It was found that for films synthesized at SiH2Cl2/NH3 ratio =1/1, 1/2 and 1/3, the corresponding bandgap values are 3.83, 4.17 and 4.40 eV. At the same time, the corresponding values of the parameter x, found according to the theoretical dependence of the bandgap value on x for SiNx calculated from the first principles, are 1.26, 1.30 and 1.32. Thus, by increasing the SiH2Cl2/NH3 ratio, it is possible to create non-stoichiometric SiNx films with a controlled silicon enrichment degree with high uniformity of chemical composition and thickness.

Photoluminescence and excitation energy transfer in non-stoichiometric silicon nitride

Photoluminescence of non-stoichiometric silicon nitride films has been studied at excitation range 3.35–5.16 eV. Several bands with energies 3.15, 3.4, 3.62, 3.8, and 3.95 eV have been observed in the PL spectra. The bands at 3.62, 3.8, 3.95 eV assigned to exciton annihilation. Emission at energies 3.15 and 3.4 eV shifts in red spectrum region with excitation energy decreasing. The shift is explained by photo-induced neutralization of charged defect states and the emission can be associated with radiative recombination of charge carriers localized on donor-acceptor pairs. Energy-resolved photoluminescence method reveals the excitation energy transfer channels. It has been found that excitons participate in process of energy transfer to donor-acceptor pairs, providing 3.4 eV band position invariant to the excitation energy.

Charge transport mechanism in the forming-free memristor based on silicon nitride

Nonstoichiometric silicon nitride SiNx is a promising material for developing a new generation of high-speed, reliable flash memory device based on the resistive effect. The advantage of silicon nitride over other dielectrics is its compatibility with the silicon technology. In the present work, a silicon nitride-based memristor deposited by the plasma-enhanced chemical vapor deposition method was studied. To develop a memristor based on silicon nitride, it is necessary to understand the charge transport mechanisms in all states. In the present work, it was established that the charge transport in high-resistance states is not described by the Frenkel effect model of Coulomb isolated trap ionization, Hill–Adachi model of overlapping Coulomb potentials, Makram–Ebeid and Lannoo model of multiphonon isolated trap ionization, Nasyrov–Gritsenko model of phonon-assisted tunneling between traps, Shklovskii–Efros percolation model, Schottky model and the thermally assisted tunneling mechanisms. It is established that, in the initial state, low-resistance state, intermediate-resistance state and high-resistance state, the charge transport in the forming-free SiNx-based memristor is described by the space charge limited current model. The trap parameters responsible for the charge transport in various memristor states are determined.

Optics of Inhomogeneous Thin Films with Defects: Application to Optical Characterization

This review paper is devoted to optics of inhomogeneous thin films exhibiting defects consisting in transition layers, overlayers, thickness nonuniformity, boundary roughness and uniaxial anisotropy. The theoretical approaches enabling the inclusion of these defects into formulae expressing the optical quantities of these inhomogeneous thin films are summarized. These approaches are based on the recursive and matrix formalisms for the transition layers and overlayers, averaging of the elements of the Mueller matrix using local thickness distribution or polynomial formulation for the thickness nonuniformity, scalar diffraction theory and Rayleigh-Rice theory or their combination for boundary roughness and Yeh matrix formalism for uniaxial anisotropy. The theoretical results are illustrated using selected examples of the optical characterization of the inhomogeneous polymer-like thin films exhibiting the combination of the transition layers and thickness nonuniformity and inhomogeneous thin films of nonstoichiometric silicon nitride with the combination of boundary roughness and uniaxial anisotropy. This characterization is realized by variable angle spectroscopic ellipsometry and spectroscopic reflectometry. It is shown that using these optical techniques, the complete optical characterization of the mentioned thin films can be performed. Thus, it is presented that the values of all the parameters characterizing these films can be determined.

Characteristics of Silicon Nitride Deposited Thin Films on IT Glass by RF Magnetron Sputtering Process

Silicon nitride thin films are deposited by RF (13.57 MHz) magnetron sputtering process using a Si (99.999 %) target and with different ratios of Ar/N2 sputtering gas mixture. Corning G type glass is used as substrate. The vacuum atmosphere, RF source power, deposit time and temperature of substrate of the sputtering process are maintained consistently at 2 ~ 3 × 10−3 torr, 30 sccm, 100 watt, 20 min. and room temperature, respectively. Cross sectional views and surface morphology of the deposited thin films are observed by field emission scanning electron microscope, atomic force microscope and X-ray photoelectron spectroscopy. The hardness values are determined by nano-indentation measurement. The thickness of the deposited films is approximately within the range of 88 nm ~ 200 nm. As the amount of N2 gas in the Ar:N2 gas mixture increases, the thickness of the films decreases. AFM observation reveals that film deposited at high Ar:N2 gas ratio and large amount of N2 gas has a very irregular surface morphology, even though it has a low RMS value. The hardness value of the deposited films made with ratio of Ar:N2=9:1 display the highest value. The XPS spectrum indicates that the deposited film is assigned to non-stoichiometric silicon nitride and the transmittance of the glass with deposited SiO2-SixNy thin film is satisfactory at 97 %.

Optical characterization of inhomogeneous thin films containing transition layers using the combined method of spectroscopic ellipsometry and spectroscopic reflectometry based on multiple-beam interference model

This paper presents the results of the optical characterization of inhomogeneous thin films of polymer-like SiOxCyHz and non-stoichiometric silicon nitride SiNx. An efficient method combining variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at the near-normal incidence based on the multiple-beam interference model is utilized for this optical characterization. The multiple-beam interference model allows us to quickly evaluate the values of ellipsometric parameters and reflectance of the inhomogeneous thin films, which exhibit general profiles of their optical constants. The spectral dependencies of the optical constants of the inhomogeneous SiOxCyHz and SiNx thin films are determined using the Campi–Coriasso dispersion model. The profiles of the optical constants of these films can also be determined. Furthermore, the transition layers at the lower boundaries of the characterized films are also taken into account. Spectral dependencies of the optical constants of these transition layers are also determined using the Campi–Coriasso dispersion model.

Atomic and Electronic Structures of a-SiNx:H

The atomic structure and the electronic spectrum of a-SiNx:H films, which are grown by plasmachemical deposition with varied ammonia and monosilane flow rates, are studied. As a result of varied flow rates, stoichiometric parameter x is changed over wide limits, from 0.73 to 1.33. The films are analyzed by structural and optical techniques to determine stoichiometric parameter x and its influence on the valence band top and the band gap in the density of states. The experimental and calculated electronic structure parameters of a-SiNx are compared, and good agreement between them is achieved over wide film composition (parameter x) limits. The experimental data obtained can be used to simulate the electron transport characteristics in nonstoichiometric silicon nitride films, which is important for creating memristors based on them.

Approximate methods for the optical characterization of inhomogeneous thin films: Applications to silicon nitride films

Abstract In this paper the overview of the most important approximate methods for the optical characterization of inhomogeneous thin films is presented. The following approximate methods are introduced: Wentzel–Kramers–Brillouin–Jeffreys approximation, method based on substituting inhomogeneous thin films by multilayer systems, method based on modifying recursive approach and method utilizing multiple-beam interference model. Principles and mathematical formulations of these methods are described. A comparison of these methods is carried out from the practical point of view, ie advantages and disadvantages of individual methods are discussed. Examples of the optical characterization of three inhomogeneous thin films consisting of non-stoichiometric silicon nitride are introduced in order to illustrate efficiency and practical meaning of the presented approximate methods.

Optical Characterization of Non-Stoichiometric Silicon Nitride Films Exhibiting Combined Defects

The study was devoted to optical characterization of non-stoichiometric silicon nitride films prepared by reactive magnetron sputtering in argon-nitrogen atmosphere onto cold (unheated) substrates. It was found that these films exhibit the combination of three defects: optical inhomogeneity (refractive index profile across the films), uniaxial anisotropy with the optical axis perpendicular to the boundaries and random roughness of the upper boundaries. The influence of the uniaxial anisotropy was included into the corresponding formulae of the optical quantities using the matrix formalism and the approximation of the inhomogeneous layer by a multilayer system consisting of large number thin homogeneous layers. The random roughness was described using the scalar diffraction theory. The processing of the experimental data was performed using the multi-sample modification of the least-squares method, in which experimental data of several samples differing in thickness were processed simultaneously. The dielectric response of the silicon nitride films was modeled using the modification of the universal dispersion model, which takes into account absorption processes corresponding to valence-to-conduction band electron transitions, excitonic effects and Urbach tail. The spectroscopic reflectometric and ellipsometric measurements were supplemented by measuring the uniformity of the samples using imaging spectroscopic reflectometry.

Approximations of reflection and transmission coefficients of inhomogeneous thin films based on multiple-beam interference model

A multiple-beam interference model is used to derive approximate formulae for the reflection and transmission coefficients of inhomogeneous thin films exhibiting large gradients of refractive index profiles. It is shown that these formulae are constituted by series containing the Wentzel-Kramers-Brillouin-Jeffreys term and correction terms with increasing order corresponding to number of considered internal reflections inside the films. A numerical analysis enabling us to show the influence of a degree of inhomogeneity on spectral dependencies of reflectance and ellipsometric parameters of inhomogeneous films is performed. Advantages and disadvantages of our approach compared with other approximate approaches are discussed. The optical characterization of a selected non-stoichiometric silicon nitride film prepared by reactive magnetron sputtering onto silicon single crystal substrate is performed for illustration of using our formulae in practice.

Film materials based on a-SiNx:H with high refractive index obtained by plasma enhanced chemical vapour deposition technology

In this work, the investigation results of amorphous hydrogenated silicon nitride layers (a-SiNx:H) obtained by plasma-enhanced chemical vapour deposition technology have been presented. Chemical compositions, surface morphology and surface topography studies were performed on the layers using Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectrometry and atomic force microscopy methods. By using optical methods: spectroscopic ellipsometry and thermal spectroscopic ellipsometry; the optical parameters (refractive index, extinction coefficients), optical band gap of a-SiNx:H layers and film thickness in the spectral range of 190–1700 nm have been determined. Additionally, we carried out thermo-optical studies to determine the temperature hysteresis of optical parameters. a-SiNx:H layers exhibit similar properties as non-stoichiometric Si-rich silicon nitride layers. They exhibited amorphous structure and their main components were silicon. After heating and cooling processes values of refractive index n and extinction coefficient k return to the baseline. In our paper we present thermal dependence of a-SiNx:H films refractive index. Presented layers showed similar optical constants to silicon substrate but they exhibiting much better dielectric properties than Si. a-SiNx:H layers exhibited very high refractive indices (about 3.8). The layers, presented in this work may potentially have broad applications in optoelectronics, photovoltaics and photonics.

A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si bulk wafer

A new materials group to implement dense wavelength division multiplexing (DWDM) in Si photonics is proposed. A large thermo-optic (TO) coefficient of Si malfunctions multiplexer/demultiplexer (MUX/DEMUX) on a chip under thermal fluctuation, and thus DWDM implementation, has been one of the most challenging targets in Si photonics. The present study specifies an optical materials group for DWDM by a systematic survey of their TO coefficients and refractive indices. The group is classified as mid-index contrast optics (MiDex) materials, and non-stoichiometric silicon nitride (SiNx) is chosen to demonstrate its significant thermal stability. The TO coefficient of non-stoichiometric SiNx is precisely measured in the temperature range 24–76 °C using the SiNx rings prepared by two methods: chemical vapor deposition (CVD) and physical vapor deposition (PVD). The CVD-SiNx ring reveals nearly the same TO coefficient reported for stoichiometric CVD-Si3N4, while the value for the PVD-SiNx ring is slightly higher. Both SiNx rings lock their resonance frequencies within 100 GHz in this temperature range. Since CVD-SiNx needs a high temperature annealing to reduce N–H bond absorption, it is concluded that PVD-SiNx is suited as a MiDex material introduced in the CMOS back-end-of-line. Further stabilization is required, considering the crosstalk between two channels; a ‘silicone’ polymer is employed to compensate for the temperature fluctuation using its negative TO coefficient, called athermalization. This demonstrates that the resonance of these SiNx rings is locked within 50 GHz at the same temperature range in the wavelength range 1460–1620 nm (the so-called S, C, and L bands in optical fiber communication networks). A further survey on the MiDex materials strongly suggests that Al2O3, Ga2O3 Ta2O5, HfO2 and their alloys should provide even more stable platforms for DWDM implementation in MiDex photonics. It is discussed that the MiDex photonics will find various applications such as medical and environmental sensing and in-vehicle data-communication.

Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge

CMOS platforms operating at the telecommunications wavelength either reside within the highly dissipative two-photon regime in silicon-based optical devices, or possess small nonlinearities. Bandgap engineering of non-stoichiometric silicon nitride using state-of-the-art fabrication techniques has led to our development of USRN (ultra-silicon-rich nitride) in the form of Si7N3, that possesses a high Kerr nonlinearity (2.8 × 10−13 cm2 W−1), an order of magnitude larger than that in stoichiometric silicon nitride. Here we experimentally demonstrate high-gain optical parametric amplification using USRN, which is compositionally tailored such that the 1,550 nm wavelength resides above the two-photon absorption edge, while still possessing large nonlinearities. Optical parametric gain of 42.5 dB, as well as cascaded four-wave mixing with gain down to the third idler is observed and attributed to the high photon efficiency achieved through operating above the two-photon absorption edge, representing one of the largest optical parametric gains to date on a CMOS platform.

IR spectra of ICPCVD SiNx thin films for MEMS structures

Optical properties of non-stoichiometric silicon nitride (SiNx) films for thermo sensitive membranes of microelectromechanical systems (MEMS) and microoptomechanical systems (MOMS) has been studied applying infrared (IR) spectroscopy. For the structures SiNx/Si and (thin metal layer)/SiNx/Si transmission and reflection spectra in the region of wave numbers of 500-7000 cm-1 has been investigated. For the investigated structures analysis of optical properties observed in the IR spectra both in the form of selective absorption bands and interference modulation of a baseline was conducted.

Visible Photoluminescence of Non-Stoichiometric Silicon Nitride Films: The Effect of Annealing Temperature and Atmosphere

The radiative properties of non-stoichiometric silicon nitride SiNx fi lms produced by plasma-enhanced chemical vapor deposition were studied. Intense room-temperature photoluminescence (PL) in the visible region was recorded after annealing for both Si-rich (x = 1.13) and N-rich (x = 1.5) silicon nitride fi lms. The position of the PL band maximum depended signifi cantly on the fi lm composition. The PL band maxima for Si-rich and N-rich SiNx fi lms were detected in the red (660 nm) and blue (450 nm) spectral regions, respectively. The effect of the annealing atmosphere on PL of the SiNx fi lms was studied. It was shown that the PL intensity depended not only on the annealing temperature but also on the annealing atmosphere. The observed features of the non-stoichiometric SiNx PL spectra were explained in terms of defect states in the SiNx band gap.