Bruggeman Effective Medium Approximation Model Research Articles

Optical Properties of Reactive RF Magnetron Sputtered Polycrystalline Cu3N Thin Films Determined by UV/Visible/NIR Spectroscopic Ellipsometry: An Eco-Friendly Solar Light Absorber

Copper nitride (Cu3N), a metastable poly-crystalline semiconductor material with reasonably high stability at room temperature, is receiving much attention as a very promising next-generation, earth-abundant, thin film solar light absorber. Its non-toxicity, on the other hand, makes it a very attractive eco-friendly (greener from an environmental standpoint) semiconducting material. In the present investigation, Cu3N thin films were successfully grown by employing reactive radio-frequency magnetron sputtering at room temperature with an RF-power of 50 W, total working gas pressure of 0.5Pa, and partial nitrogen pressures of 0.8 and 1.0, respectively, onto glass substrates. We investigated how argon affected the optical properties of the thin films of Cu3N, with the aim of achieving a low-cost solar light absorber material with the essential characteristics that are needed to replace the more common silicon that is currently in present solar cells. Variable angle spectroscopic ellipsometry measurements were taken at three different angles, 50∘, 60∘, and 70∘, to determine the two ellipsometric parameters psi, ψ, and delta, Δ. The bulk planar Cu3N layer was characterized by a one-dimensional graded index model together with the combination of a Tauc–Lorentz oscillator, while a Bruggeman effective medium approximation model with a 50% air void was adopted in order to account for the existing surface roughness layer. In addition, the optical properties, such as the energy band gap, refractive index, extinction coefficient, and absorption coefficient, were all accurately found to highlight the true potential of this particular material as a solar light absorber within a photovoltaic device. The direct and indirect band gap energies were precisely computed, and it was found that they fell within the useful energy ranges of 2.14–2.25 eV and 1.45–1.71 eV, respectively. The atomic structure, morphology, and chemical composition of the Cu3N thin films were analyzed using X-ray diffraction, atomic force microscopy, and energy-dispersive X-ray spectroscopy, respectively. The Cu3N thin layer thickness, profile texture, and surface topography of the Cu3N material were characterized using scanning electron microscopy.

Characterization of amorphous carbon films from 5 nm to 200 nm on single-side polished a-plane sapphire substrates by spectroscopic ellipsometry

In this work, a series of amorphous carbon films were deposited on a-plane sapphire substrates by magnetron sputtering with deposition time from 15 min to 8 h, in order to investigate the thickness and optical properties in the process of growth in a non-destructive way. They were characterized by using Mueller matrix spectroscopic ellipsometry together with topography profilometry and Raman spectroscopy. Two models of a Bruggeman effective medium approximation model and a single Cody-Lorentz oscillator model have been proposed to fit films thickness and optical constants from Ultraviolet (UV) to visible (210 nm–800 nm), and Transmission Electron Microscope (TEM) has been used to verify the proposed model for thickness fitting results. The optical constants of the amorphous carbon film have been determined by fitting together all measurements in samples deposited for 2 h or more, with the film thickness being the only sample-independent parameter. The results show that the thickness from 5 nm to 200 nm can be characterized in a nondestructive way although there is a relatively large thickness error compared with the Transmission Electron Microscope results for thin films (d < 20 nm) when the deposition time is less than 2 h because of the nonuniform deposition in the beginning. The relative error between the TEM and Spectroscopic Ellipsometry results can be reduced to 1% after 4 h sample. That means spectroscopic ellipsometry can still provide an indicator for the trend of thickness growth.

Exploring the Optical and Morphological Properties of Ag and Ag/TiO₂ Nanocomposites Grown by Supersonic Cluster Beam Deposition.

Nanocomposite systems and nanoparticle (NP) films are crucial for many applications and research fields. The structure-properties correlation raises complex questions due to the collective structure of these systems, often granular and porous, a crucial factor impacting their effectiveness and performance. In this framework, we investigate the optical and morphological properties of Ag nanoparticles (NPs) films and of Ag NPs/TiO2 porous matrix films, one-step grown by supersonic cluster beam deposition. Morphology and structure of the Ag NPs film and of the Ag/TiO2 (Ag/Ti 50-50) nanocomposite are related to the optical properties of the film employing spectroscopic ellipsometry (SE). We employ a simple Bruggeman effective medium approximation model, corrected by finite size effects of the nano-objects in the film structure to gather information on the structure and morphology of the nanocomposites, in particular porosity and average NPs size for the Ag/TiO2 NP film. Our results suggest that SE is a simple, quick and effective method to measure porosity of nanoscale films and systems, where standard methods for measuring pore sizes might not be applicable.

Investigation on optical and physico-chemical properties of LPCVD SiOxNythin films

In this work, the correlation between BEMA model and FTIR analysis was employed to investigate the chemical composition of silicon oxynitride (SiOxNy) films deposited by low pressure chemical vapour deposition (LPCVD) technique at temperature of 850 °C from nitrous oxide N2O, ammonia NH3 and dichlorosilane SiH2Cl2. Different stoichiometries were obtained for different values of relative gas flow ratio NH3/N2O while keeping the SiH2Cl2 flow constant. The optical properties were studied using spectroscopic ellipsometry. Apart from films thickness, their refractive index as well as their SiO2 and Si3N4 volume fractions were deduced using the Bruggeman effective medium approximation (BEMA) model. It was found that the refractive index increases from 1.45 to 1.60 with increasing nitrogen incorporation. The Fourier Transform Infrared spectroscopy was carried out to study the evolution of chemical bonding of LPCVD SiOxNy films. In order to improve the qualitative analysis of their Si-N and Si-O vibrational modes, a correlation between Fourier transform infrared and spectroscopic ellipsometry measurements was established. A shift of infrared peaks position with the increase of relative gas flow ratio is observed. Furthermore, the calculated areas of absorption bands were used to estimate the SiOxNy stoichiometry. This quantitative analysis was proved with an adequate method in the literature. We found a decrease of x values from 1.94 to 1.26 and an increase of y from 0.04 to 0.49, when the NH3/N2O gas flow ratio increases. This behavior was explained by the increase of nitrogen content as well as the decrease of oxygen content in the SiOxNy film.

Spectroscopic ellipsometry study of Si nanocrystals embedded in a SiOx matrix: Modeling and optical characterization

In this work, we report on a spectroscopic ellipsometry study of thin films of silicon nanocrystals embedded in a SiOx matrix that is performed to better understand how substoichiometric SiOx species affect the optical properties of these systems. The silicon nanocrystals are fabricated in a SiOx matrix by thermal annealing of magnetron-sputtered thin films in a wide range of x values (0<x<2) in order to produce every possible substoichiometric SiOx species in various fractions. A generic optical model to describe the thin film content with various SiOx volumetric fractions is developed and used to analyze the data obtained from the spectroscopic ellipsometry. Tauc-Lorentz and Bruggeman effective medium approximation models are employed to determine the ellipsometric angles (Ψ and Δ) and the complex dielectric function. We show that the optical properties of the nc-Si embedded in a SiOx matrix are highly dependent on the value of x and on the volumetric fractions of the substoichiometric SiOx species present in the system.

Dependence of optical properties on the thickness of amorphous Ge30Se70 thin films

In order to calculate the optical properties of evaporated Ge30Se70 thin films and their relation to thickness, amorphous Ge30Se70 thin films of different thicknesses were deposited by thermal evaporation at a base pressure of 7.5×10−6Torr at room temperature. The optical transmission and reflection spectra of all films were measured in the wavelength range of 0.2–2.5μm. Efficient parameterization of the spectral dependence of the optical constants of amorphous Ge–Se thin films was obtained by applying a suitable dielectric function model. The O’Leary, Johnson and Lim (OJL) model, based on joint density of states functions, was used to analyze the optical spectra. The best fit was obtained by configuring the film as two layers, the top layer consisting of bulk Ge30Se70 material embedded in air medium containing different volumes of voids. Therefore, the OJL model coupled with the Bruggeman effective-medium approximation model were used to determine the optical constants of the Ge30Se70 thin films. The photon energy dependence of the dielectric function, ɛ=ɛr−iɛi of the investigated films is presented. The film thickness, absorption coefficient α, refractive index n, extinction coefficient k, static refractive index n(0) and optical band gap Eg were deduced. We found that increasing the film thickness increased the direct optical energy gap and decreased the refractive index.

Determination of bone mineral volume fraction using impedance analysis and Bruggeman model

Abstract Measurements by impedance spectroscopy and Bruggeman effective medium approximation model were employed in order to determine the mineral volume fraction of dry bone. This approach assumes that two or more phases are present into the composite: the matrix (environment) and the other ones are inclusion phases. A fragment of femur diaphysis dense bone from a young pig was investigated in its dehydrated state. Measuring the dielectric properties of bone and its main components (hydroxyapatite and collagen) and using the Bruggeman approach, the mineral volume filling factor was determined. The computed volume fraction of the mineral volume fraction was confirmed by a histogram test analysis based on the SEM microstructures. In spite of its simplicity, the method provides a good approximation for the bone mineral volume fraction. The method which uses impedance spectroscopy and EMA modeling can be further developed by considering the conductive components of the bone tissue as a non-invasive in situ impedance technique for bone composition evaluation and monitoring.

Characterization of porous low-k films using variable angle spectroscopic ellipsometry

Variable angle spectroscopic ellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-k films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%–4% more porosity at the interface compared with the bulk for both films. This work shows that VASE™, a nondestructive measurement tool, can be used to characterize single- and multigraded layer porous films quickly and effectively.

Ellipsometric study of the adsorption of bovine serum albumin into porous silicon

The method of spectroscopic ellipsometry has been applied to study in situ the adsorptionof bovine serum albumin (BSA). The porosity and amount of adsorbed BSA weredetermined by fitting the ellipsometric data to the Bruggeman effective mediumapproximation model. The presence of intermediate adsorbed layers of polyelectrolytes wasfound to increase protein adsorption.

Ellipsometric study of silicon nanocrystal optical constants

Samples of silicon nanocrystals on various substrates were prepared by cluster beam deposition of silicon nanoparticles, obtained by laser-induced pyrolysis of silane in a flow reactor. Using optical ellipsometry, the optical properties (refractive index and extinction coefficient) of the as-prepared silicon nanocrystal layers were determined in the wavelength range from 240 to 700 nm. Two dispersion models were used to describe the silicon nanocrystal optical properties: the Bruggeman effective medium approximation model and the Tauc–Lorentz model. The study showed that while a simple Bruggeman effective medium approximation model could not completely account for the silicon nanocrystal dispersion behavior, the optical response of the silicon nanocrystal layers could be satisfactorily described by a Tauc–Lorentz model. The present study also showed that, as for porous silicon, the silicon nanocrystal optical indexes significantly deviate from those of bulk crystalline and amorphous silicon. It confirms the special behavior of silicon under its nanoscale form.

Roughness and chemistry of silicon and polysilicon surfaces etched in high-density plasma: XPS, AFM and ellipsometry analysis

Surface chemistry and morphology of polysilicon thin films etched in a high-density fluorocarbon plasma under various conditions are studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry. XPS reveals the presence of a fluorocarbon layer, which composition and thickness depend on the plasma conditions. Ellipsometry measurements show the need to consider the superficial roughness. Surface roughness and morphology obtained by AFM are used to define geometric models suitable to represent the top layer when processing the ellipsometry data. Results are discussed and compared to that given by the Bruggeman effective medium approximation (BEMA). The BEMA model always agrees with the geometric model, which is the closest to the observed surface morphology. However, if a good agreement is obtained between surface roughness and top layer thickness for unetched or weakly damaged samples, a discrepancy is observed for the etched samples. Formation of a non-transparent fluorocarbon layer on these sample is put forward to explain this behaviour.

Morphology Control Of The Electrochromic Effect In Tungsten Oxide Thin Films

AbstractElectrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of scanning electron microscopy, spectroscopic ellipsometry, and electrochromic coloration experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with a previous report which found that tungsten oxide thin films are dominated by a hierarchy of dense columnar regions and less dense void regions. This morphology not only dominates the structure of tungsten oxide thin films but also strongly controls the electrochromic properties. From spectroscopic ellipsometry data and Bruggeman effective medium approximation models presented here, it seems reasonable that a cermet model of absorption can accurately describe the tungsten oxide preparationelectrochromic property relationship.