Hydrogenated Silicon Nitride Thin Films Research Articles

Structural and optical study of amorphous hydrogenated silicon nitride thin film as antireflection coating on solar cell

The structural, optical and electrical aspects of as deposited as well as annealed amorphous hydrogenated silicon nitride(a-SiNx: H) thin films of different stoichiometry, grown on silicon wafer by radio frequency- plasma enhanced chemical vapor deposition (RF-PECVD) are thoroughly investigated. Photoluminescence (PL) measurement verify that Si rich a-SiNx: H (SRSN) film of refractive index 2.68, gives good PL as compared to near stoichiometric film and support the presence of Silicon quantum dots (Si QDs) embedded in a-SiNx: H matrix. Detailed structural analysis by high resolution transmission electron microscopy (HRTEM) revealed that as deposited SRSN thin film contains amorphous Silicon quantum dots (a-Si QDs) which are grown by phase separation of SRSN film during the synthesis process. These SRSN thin films of different thicknesses have been deposited at the surface of Silicon solar cell as anti-reflection coatings (ARCs). By reflectance measurement, it is observed that the ARC contains a-Si QDs in a-SiNx: H matrix, more effectively minimize the reflection of incident light across the wavelengths ranging from 300 to 800 nm. With a-Si QDs/ a-SiNx: H ARC, we demonstrate an increase in short circuit current density, open circuit voltage and conversion efficiency by 4.34 mA/cm2, 0.01V and 0.21% (absolute) respectively and indicate its utility in improving the performance of Si solar cells. Copyright © 2017 VBRI Press.

Silicon Nitride and Hydrogenated Silicon Nitride Thin Films: A Review of Fabrication Methods and Applications.

Silicon nitride (SiNx) and hydrogenated silicon nitride (SiNx:H) thin films enjoy widespread scientific interest across multiple application fields. Exceptional combination of optical, mechanical, and thermal properties allows for their utilization in several industries, from solar and semiconductor to coated glass production. The wide bandgap (~5.2 eV) of thin films allows for its optoelectronic application, while the SiNx layers could act as passivation antireflective layers or as a host matrix for silicon nano-inclusions (Si-ni) for solar cell devices. In addition, high water-impermeability of SiNx makes it a potential candidate for barrier layers of organic light emission diodes (OLEDs). This work presents a review of the state-of-the-art process techniques and applications of SiNx and SiNx:H thin films. We focus on the trends and latest achievements of various deposition processes of recent years. Historically, different kinds of chemical vapor deposition (CVD), such as plasma enhanced (PE-CVD) or hot wire (HW-CVD), as well as electron cyclotron resonance (ECR), are the most common deposition methods, while physical vapor deposition (PVD), which is primarily sputtering, is also widely used. Besides these fabrication methods, atomic layer deposition (ALD) is an emerging technology due to the fact that it is able to control the deposition at the atomic level and provide extremely thin SiNx layers. The application of these three deposition methods is compared, while special attention is paid to the effect of the fabrication method on the properties of SiNx thin films, particularly the optical, mechanical, and thermal properties.

Structural and gas barrier properties of hydrogenated silicon nitride thin films prepared by roll-to-roll microwave plasma-enhanced chemical vapor deposition

Hydrogenated amorphous silicon nitride films (a-SiNx:H) have attracted attention as barrier layers to prevent the diffusion of water vapor and other gases into electronic devices. Herein, the chemical composition and physical properties of a-SiNx:H films prepared using a roll-to-roll microwave plasma-enhanced chemical vapor deposition (MW-PECVD) process at low temperatures and variable NH3/SiH4 flow-rate ratios (R) were analyzed. This study was performed to identify the factors affecting the gas permeability of these films as represented by their water vapor transmission rate (WVTR) values. The films fabricated under SiH4-rich conditions (R < 2.75) were oxidized during storage in air to produce silicon oxynitride (SiOxNy), whose properties differed from those of silicon nitride (SiNx). It was found that gas barrier properties were affected by both the film and the Si–H bond densities. Under optimal conditions, the WVTR of a 100-nm-thick SiNx single layer was as low as 7 × 10−3 g m−2 d−1, and the Si–H bond density was 6.71 × 1021 cm−3. The compressive and tensile stresses of the SiNx:H films depended on the values of R, with excellent mechanical stability and moisture barrier properties having been obtained for R = 2.75–3.00.

Optical and physicochemical properties of hydrogenated silicon nitride thin films: Effect of the thermal annealing

ABSTRACTThe effect of thermal annealing on the optical and physicochemical properties of hydrogenated silicon nitride films was studied. These films were deposited by plasma-enhanced chemical vapor deposition from a mixture of silane, ammonia, and nitrogen. Subsequently, the films were annealed at various temperatures ranging from 400°C to 1000°C. The properties of the films were studied using ellipsometry and Fourier transform infrared spectroscopy. The Maxwell Garnet model considers the silicon nitride material as heterogeneous with three distinct phases: silicon, stoichiometric silicon nitride, and hydrogen. Based on the ellipsometric analysis, the annealing treatment leads to reduce the volume fraction of both hydrogen and silicon. As a result, the stoichiometry parameter significantly increases from 1.24 to 1.32 making it closer to the stoichiometric silicon nitride one. According to the infrared data, a noticeable decrease in the total hydrogen concentration in the films was obtained with respect to the annealing temperature.

Opto-Structural Properties of Silicon Nitride Thin Films Deposited by ECR-PECVD

Amorphous hydrogenated silicon nitride thin films a-SiNx:H (abbreviated later by SiNx) were deposited by Electron Cyclotron Resonance plasma enhanced chemical vapor deposition method (ECR-PECVD). By changing ratio of gas flow (R = NH3/SiH4) in the reactor chamber different stoichiometric layers x = [N]/[Si] ([N] and [Si] atomic concentrations) are successfully deposited. Part of the obtained films has subsequently undergone rapid thermal annealing RTA (800°C/1 s) using halogen lamps. Optical and structural characterizations are then achieved by spectroscopic ellipsometry (SE), ion beam analysis and infrared absorption techniques. The SE measurements show that the tuning character of their refractive index n(λ) with stoichiometry x and their non-absorption properties in the range of 250 - 850 nm expect for Si-rich SiNx films in the ultraviolet UV range. The stoichiometry x and its depth profile are determined by Rutherford backscattering spectrometry (RBS) while the hydrogen profile (atomic concentration) is determined by Elastic Recoil Detection Analysis (ERDA). Vibrational characteristics of the Si-N, Si-H and N-H chemical bonds in the silicon nitride matrix are investigated by infrared absorption. An atomic hydrogen fraction ranging from 12% to 22% uniformly distributed as evaluated by ERDA is depending inversely on the stoichiometry x ranging from 0.34 to 1.46 as evaluated by RBS for the studied SiNx films. The hydrogen loss after RTA process and its out-diffusion depend strongly on the chemical structure of the films and less on the initial hydrogen concentration. A large hydrogen loss was noted for non-thermally stable Si-rich SiNx films. Rich nitrogen films are less sensitive to rapid thermal process.

Characterization of silicon nitride thin films deposited by hot-wire CVD at low gas flow rates

We examined the chemical, structural, mechanical and optical properties of amorphous hydrogenated silicon nitride thin films deposited by hot-wire chemical vapour deposition using SiH4, NH3 and H2 gases at total flow rates below 33sccm. Time of flight secondary ion mass spectroscopy reveal that the film surfaces consist of predominantly Si with hydrogenated SixNyOz species. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy corroborate on the N/Si ratio. Electron energy loss spectroscopy discloses that the thickness of the nitrogen rich oxidized interface between the SiNx films and the c-Si substrate decrease with an enhancing NH3 flow rate. By varying the NH3 flow rate, dense SiNx films can be realized with hydrogen content between 16 and 9at.%, a refractive index between 3.5 and 1.9 and optical band gap ranging from 2 to 4.5eV. The SiNx film stress is compressive for N/Si<0.4 and tensile for higher N/Si>0.55. Mechanisms relating the HWCVD conditions and the film structure and properties are proposed.

Low temperature SiNx:H films deposited by inductively coupled plasma for solar cell applications

Amorphous hydrogenated silicon nitride thin films with different chemical compositions (SiNx:H) have been synthesized by using low frequency inductively coupled plasma of Si+N2+H2 at a low temperature of 100°C. The bonding configurations, bond density, hydrogen content, and chemical composition, as well as the refractive index are intensively investigated by a variety of characterization tools. Silicon nitride based antireflection layer on alkaline-textured silicon surface reduces the reflectivity to about 4%. As-deposited silicon nitride films exhibit an excellent passivation effect on p- and n-type Si. The surface recombination velocity is reduced to 36cm/s on n-type silicon with resistivity of 2–3Ωcm. The passivation effect originates from the H-related chemical passivation and fixed charge related field passivation. The growth mechanism of SiNx:H from the precursor gas of H2 diluted mixture of silane and nitrogen is also discussed.

Hydrogenated Silicon-Nitride Thin Films as Antireflection and Passivation Coatings for Multicrystalline Silicon Solar Cells

Hydrogenated Silicon-Nitride Thin Films as Antireflection and Passivation Coatings for Multicrystalline Silicon Solar Cells

Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiNx:H alloys prepared by reactive evaporation

A wide range of amorphous hydrogenated silicon nitride thin films with an excess of silicon was prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions. A strong visible photoluminescence at room temperature was observed for the as-deposited films as well as for films annealed up to 1100°C. The chemical composition and the structure of the films were investigated using x-ray photoelectron, thermal desorption, and Raman spectroscopies, infrared absorption measurements, grazing incidence x-ray diffraction experiments, and transmission electron microscopy. Two luminescence mechanisms were identified for the films depending on the annealing temperature. For annealing temperatures below 650°C, the films are made of amorphous silicon-rich phases mixed with nitrogen-rich phases. These inhomogeneities in the chemical composition, coupled with the evolution of the photoluminescence energies and intensities with the hydrogen content, suggest that the emission is due to the recombination process of the photogenerated carriers within the band-tail states. For temperatures higher than 800°C, a phase separation occurs and the films could be described as silicon nanoclusters embedded in an insulating amorphous silicon nitride matrix. The clusters are amorphous, and then crystallized when the annealing temperature is high enough. The correlation between the clusters sizes and the photoluminescence results suggests that the emission observed after annealing treatments at temperature higher than 900°C is due to the quantum confinement of the carriers inside the silicon clusters. By carefully choosing the preparation and the annealing conditions, it is possible to tune the photoluminescence energy in the visible range.

Optical properties of amorphous hydrogenated silicon nitride thin films

This paper presents a study of the optical properties of amorphous hydrogenated silicon nitride (a-Si3N4:H) films under deposition temperature influence. The films were deposited by low-frequency plasma-enhanced chemical vapor deposition at temperatures of 200, 300, and 400°C and 0.6-Torr pressure. The mixture gases were silane (SiH4), ammonia (NH3), and hydrogen (H2). The optical properties of the films samples were obtained by means of the transmittance spectra and from spectroellipsometry measurements. Then, the optical parameters of the films were determined using the Swanepoel, Cauchy, and Sellmeier models. The refractive index dispersion curves were well fitted with both the Cauchy and the Sellmeier theoretical model. The equivalence between the parameters that characterize the two models is established.

Improvement of the photoluminescence properties in a-SiNx films by introduction of hydrogen

Photoluminescence properties of amorphous hydrogenated silicon nitride thin films with various compositions are presented. The as-deposited samples prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions exhibit visible photoluminescence at room temperature without any annealing treatment. The evolution of the photoluminescence properties with increasing nitrogen concentration in the films is correlated to structural investigations performed with Fourier-transform infrared spectroscopy and optical characterization obtained from transmission measurements in the ultraviolet-visible-near-infrared range. It is shown that the introduction of hydrogen is of prime importance to improve the photoluminescence intensity of the films.

Optical absorption in amorphous hydrogenated silicon nitride thin films deposited by the electron cyclotron resonance plasma method and subjected to rapid thermal annealing

We have analyzed the influence of rapid thermal annealing (from 300 to 1050 °C) on the optical properties of a-SiN x :H. Three compositions were investigated: x = 0.97, x = 1.43 and x = 1.55. Two different behaviors are observed depending on whether the as-grown nitrogen to silicon ratio of the samples is above or below the percolation threshold ( x = 1.1) of Si Si bonds, in the matrix of silicon nitride. The samples with x = 1.43 and 1.55 experience an increase of the Tauc coefficient ( B) and a decrease of the Urbach parameter ( E 0 ) at low annealing temperatures, while at high temperatures the trend is inverted. On the contrary, the samples with x = 0.97 show a slight and continuous increase of B and a similar decrease of E 0 . The different behavior of the films with x < 1.1 is explained by the percolation of the Si Si bonds, which maintains the order of the structure at high annealing temperatures, preventing the inversion of the trends of B and E 0 .

Hydrogenated silicon nitride thin films deposited between 50 and 250 °C using nitrogen/silane mixtures with helium dilution

Silicon nitride thin films, deposited by plasma enhanced chemical vapor deposition at temperatures between 250 and 50 °C from SiH4, N2 and He, were characterized using transmission infrared spectroscopy, ellipsometry, wet etch rate, and current-voltage analysis. At 250 °C using SiH4/N2/He flow ratios of 1/150/75, films with refractive index=1.80 and H concentrations &amp;lt;20%, distributed equally in Si-H and N-H units were obtained. The concentration of hydrogen and its distribution in N-H and Si-H bonds are sensitive to process temperature, suggesting that thermally driven N incorporation reactions are important during growth. Inert gas dilution allows films to be formed at &amp;lt;100 °C, with bonded hydrogen configurations similar to films deposited at higher temperatures. Current versus voltage traces of as-deposited films show charge trapping, which can be reduced by extended low temperature anneals. These results show that chemical composition can be controlled in low temperature silicon nitride deposition. This processing approach may be useful for encapsulation or for barrier layer formation on low temperature organic electronic devices or flexible transparent plastic substrates.

Control of stability and current-voltage characteristics in amorphous hydrogenated silicon nitride thin film diodes

Current transport and instability mechanisms in thin film diodes with nonstoichiometric silicon nitride (a-SiNx:H) semiconducting layers have been investigated. In common with amorphous silicon thin film transistors the electrical characteristics of these diodes have been found to drift during use. We found that the initial current-voltage characteristics are related to the choice of interfacial treatments. This is explained by trapping of electrons at interface defect states and by tunneling of electrons into the conduction band via these states. We have also found a relationship between the initial characteristics of the diodes and the rate of drift due to electrical stressing. A threshold exists below whose drift is independent of the current-voltage characteristics and above which there is a strong dependence. This dependence of drift on current-voltage characteristic is consistent with field enhanced defect creation in the a-SiNx:H layer.

Charge trapping centers in N-rich silicon nitride thin films

We have examined the behavior of the Si dangling-bond center in regard to charge trapping in N-rich amorphous hydrogenated silicon nitride thin films. The effects of multiple electron and hole injections were monitored by electron paramagnetic resonance. These results continue to support a model in which the Si dangling bond is a negative U defect in N-rich nitrides, and that a change in charge state of pre-existing diamagnetic positively and negatively charged sites is responsible for the memory properties of silicon nitride thin films.

Optically Induced Paramagnetism in Amorphous Hydrogenated Silicon Nitride Thin Films

ABSTRACTThe creation mechanisms of Si and N dangling bond defect centers in amorphous hydrogenated silicon nitride thin films by ultra-violet (UV) illumination are investigated. The creation efficiency and density of Si centers in the N-rich films are independent of illumination temperature, strongly suggesting that the creation mechanism of the spins is electronic in nature, i.e., a charge transfer mechanism. However, our results suggest that the creation of the Si dangling bond in the Si-rich films are different. Last, we find that the creation of the N dangling-bond in N-rich films can be fit to a stretched exponential time dependence, which is characteristic of dispersive charge transport.

Electrically neutral nitrogen dangling-bond defects in amorphous hydrogenated silicon nitride thin films

We have investigated the effects of different post-deposition temperature anneals and N-H concentrations, on the generation of ultraviolet (UV)-induced two-coordinated nitrogen dangling bonds in plasma-enhanced chemical vapor deposited (PECVD) silicon nitride films using electron spin resonance (ESR). It is shown that the nitrogen dangling bond is activated by post-deposition anneals with temperatures as low as 500 °C in N-rich PECVD nitride films followed by subsequent UV broadband illumination. It also appears that there is a possible correlation between the initial N-H concentration in the films and the concentration of generated nitrogen dangling bonds following the anneal/UV sequence. We also report the charge state associated with the nitrogen dangling bond using a combination of ESR and capacitance versus voltage measurements; these measurements suggest that this two-coordinated defect is electrically neutral when paramagnetic.