G. Ferblantier

Properties of silicon nanoparticles embedded in SiNx deposited by microwave-PECVD

In this work, silicon-rich silicon nitride (SRN) layers were deposited on a silicon wafer by microwave-assisted plasma-enhanced chemical vapor deposition (MW-PECVD) using NH(3) and SiH(4) as precursor gases. The Si excess in the as-deposited layers as determined by the Rutherford backscattering technique was controlled by varying the precursor gas ratio. We were able to produce silicon nanoparticles (Si-nps) in the silicon nitride (SiN(x)) layers upon thermal annealing at high temperature. Energy-filtered TEM (EFTEM), complemented by photoluminescence measurements, were used to identify the experimental parameters in order to reach a high density of well-separated Si-nps (3 nm). Our results show that the MW-PECVD method is a suitable deposition tool for the formation of Si-nps in thin SRN layers.

Effect of annealing treatments on photoluminescence and charge storage mechanism in silicon-rich SiNx:H films

In this study, a wide range of a-SiNx:H films with an excess of silicon (20 to 50%) were prepared with an electron-cyclotron resonance plasma-enhanced chemical vapor deposition system under the flows of NH3 and SiH4. The silicon-rich a-SiNx:H films (SRSN) were sandwiched between a bottom thermal SiO2 and a top Si3N4 layer, and subsequently annealed within the temperature range of 500-1100°C in N2 to study the effect of annealing temperature on light-emitting and charge storage properties. A strong visible photoluminescence (PL) at room temperature has been observed for the as-deposited SRSN films as well as for films annealed up to 1100°C. The possible origins of the PL are briefly discussed. The authors have succeeded in the formation of amorphous Si quantum dots with an average size of about 3 to 3.6 nm by varying excess amount of Si and annealing temperature. Electrical properties have been investigated on Al/Si3N4/SRSN/SiO2/Si structures by capacitance-voltage and conductance-voltage analysis techniques. A significant memory window of 4.45 V was obtained at a low operating voltage of ± 8 V for the sample containing 25% excess silicon and annealed at 1000°C, indicating its utility in low-power memory devices.

Spectroscopic ellipsometry investigation of the optical properties of nanostructured Si/SiNx films

In this paper we use spectroscopic ellipsometry to investigate the optical properties of silicon quantum dots (Si-QDs) embedded in silicon-rich silicon nitride (SRSN) films over the (1.5–5.9 eV) photon energy range. These films were elaborated by microwave plasma enhanced chemical vapor deposition for various gas flow ratio [R=(NH3)/(SiH4)]. We have analyzed the ellipsometric data using various modeling methods: Forouhi–Bloomer (FB) and Tauc–Lorentz (TL) dispersion formulas and wavelength-by-wavelength (λ-by-λ) numerical inversion. This comparative study allowed us to analyze in depth the influence of ammonia flow on the physical properties of SRSN films and Si-QDs. FB and TL models and λ-by-λ inversion have revealed that the imaginary dielectric function of the Si-QDs, is characterized by a single broad peak similar to that of amorphous silicon. The fitted band gap (Eg) and the other optical properties were found to depend on the ammonia flow. We also determined that Eg obtained by FB model seems to be l...

The structural and optical properties of SiO2/Si rich SiNx multilayers containing Si-ncs

This work reports on the structural and optical properties of multilayers composed of silicon dioxide (SiO2) and silicon rich silicon nitride (SRN) films. These nanometer scale layers have been alternately deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) on quartz and silicon (Si) substrates. The samples have then been annealed at high temperature in order to obtain a crystallization of the Si atoms present in excess in the SRN films. The formation of crystalline Si has been witnessed by high resolution transmission electron microscopy (HREM) and micro-Raman measurements. Estimation of the Si-nanocrystal (Si-nc) sizes was possible by correlating the Ramans confinement model, the photoluminescence measurements and HREM imaging. The results clearly show that the band-gap of the Si-ncs formed can be controlled by this multilayer approach.

Control of silicon nanoparticle size embedded in silicon oxynitride dielectric matrix

In this study, silicon rich silicon oxynitride layers containing more than 15% nitrogen were deposited by electron cyclotron resonance assisted plasma enhanced vapor deposition in order to form silicon nanoparticles after a high temperature thermal annealing. The effect of the flows of the precursor gases on the composition and the structural properties of the layers was assessed by Rutherford backscattering spectroscopy, elastic recoil detection analysis, and infrared spectroscopic measurements. The morphological and crystallinity properties were investigated by energy filtered transmission electron microscopy and Raman spectroscopy. We show that the excess of silicon in the silicon oxynitride layer controls the silicon nanoparticles size. On the other hand, the crystalline fraction of particles is found to be strongly correlated to the nanoparticle size. Finally, the photoluminescence measurements show that it is also possible to tune the photoluminescence peak position between 400 and 800 nm and its intensity by changing the silicon excess in the silicon rich silicon oxynitride matrix.

Ellipsometric demonstration of the existence of a strong correlation between size distribution and optical responses of silicon nanoclusters in a nitride matrix

We report on ellipsometric study of the influence of size distribution (SD) of silicon nanoclusters (Si-ncs) in a nitride matrix on their optical responses. The latter are extracted by using a modified Maxwell-Garnett theory that explicitly takes into account inhomogeneous broadening of the Si-ncs. The effects of the size dispersion (σ) and shape (Gaussian or lognormal) of the SD are investigated. It is evidenced that the value of σ may substantially change the whole line shapes of the Si-ncs dielectric functions. We also show that a Gaussian SD may lead to somewhat different properties as compared to the lognormal one.

Determination of the optical properties and size dispersion of Si nanoparticles within a dielectric matrix by spectroscopic ellipsometry

We report on a comparative study between dielectric functions of Si nanoparticles (Si-NPs) obtained from Bruggeman effective medium approximation (BEMA), Maxwell-Garnett (MG), and a modified Maxwell-Garnett (MMG) models. Unlike BEMA and MG, a size-distribution dependent dielectric function of Si-NPs is considered in the introduced MMG model. We show that the standard deviation sigma of a size distribution can be evaluated by analyzing the imaginary part of the dielectric functions of Si-NPs extracted from BEMA and MMG. In order to demonstrate this, several samples composed of Si-NPs embedded in silicon-rich silicon nitride are investigated by spectroscopic ellipsometry over the photon energy range varying between 2 and 4 eV. Assuming a lognormal size distribution of the Si nanoparticles, it is evidenced that the parameter sigma ranges between 1.15 and 1.35. The values of size dispersion deduced by this methodology are in good agreement with TEM observations.

Formation of silicon nanoparticles from high temperature annealed silicon rich silicon oxynitride films

Silicon rich silicon oxynitride layers were deposited by ECR-PECVD in order to form silicon nanoparticles upon high thermal annealing at 1100°C. The effect of the gas precursor type and flows on the atomic composition and the structural properties was assessed by RBS and ERDA analysis as well as by Raman spectroscopy. The morphological and crystalline properties of the resulting nanoparticles were investigated by TEM analysis. We have found that the silicon nanoparticules average size and the crystalline fraction depend strongly on the silicon excess in the SiN and SiON layer.

Si Nanocrystals Embedded in a Silicon Oxynitride Matrix

We investigated the morphological and structural change in silicon nanostructures embedded in the silicon oxynitride matrix. The study has been carried out on thin films thermally annealed at high temperature, after deposition at 400°C by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapour Deposition (ECR- PECVD), under different deposition parameters. Our study evidenced the existence of a well defined threshold for the silicon content in the film (around 47%), to get Si nano-crystallization in the silicon oxynitride matrix. Both Si nano-crystals and Si nano-columns have been observed by TEM analysis in two samples having a similar Si content but deposited under different conditions.

Spectroscopic ellipsometric analysis of silicon-rich silicon nitride layers for PV applications

Silicon-rich silicon nitride (SRN) layers are of great interest for the 3rd generation solar cells. They can be used as luminescent converters to enhance photovoltaic (PV) conversion in the blue spectrum or, as high band-gap materials in an all Si tandem cells. Silicon nanoparticules (Si-nps) of controlled size and density embedded in the silicon nitride (SiN) matrix as a light converter film are investigated for the first application. For the tandem cell application, a superlattice composed of nano-sized SiN layers containing Si-nps is used. The aim of this work is to use spectroscopic ellipsometry to study the optical (band gap) and structural (size, density) properties of Si-nps embedded in SRN layers. The extracted size and bandgap of Si-nps from SE modeling were verified with high resolution TEM and room temperature photoluminescence (PL).