Values Of Complex Refractive Index Research Articles

Characteristics of Absorbing Aerosols in Mexico City: A Study of Morphology and Columnar Microphysical Properties

This paper presents an analysis of the morphology and columnar microphysical properties of atmospheric aerosols in Mexico City (MC) for the period 2022–2023. The morphological study focused on the structure description of soot particles and tar balls (TB). By transmission electron microscope (TEM) and scanning electrode microscope (SEM), voluminous soot aggregates mixed with TBs were observed. The chemistry shows that both soot and TBs are mostly carbonaceous species with well-defined morphologies. On the other hand, the columnar aerosol microphysical properties recovered from AERONET show that the particles have a bimodal aerosol size distribution (ASD) with two modes: fine and coarse. The ASD remains constant without showing significant seasonal changes, only with some variability for coarse particles. The aerosol optical depth (AOD) value is significantly high, typical of urban areas. The real (n) and imaginary (k) parts of the complex refractive index (CRI) were obtained from the photometric measurements. The CRI values show seasonal variations, with spring being the season with the highest values for n, while the highest values for k were measured in winter.

Optical modelling of TCO based FTO/TiO2 multilayer thin films and simulation in hydrogenated amorphous silicon solar cell

Hydrogenated Amorphous silicon (a:Si:H) has low amounts of defects making it attractive for photovoltaic applications. To improve power conversion efficiency (PCE) of a:Si:H solar cells, this study investigated the effect of introducing FTO/TiO2 multilayer thin films into its structure to serve as antireflection coating. The multilayer thin films were characterized and optimized by optical simulations using a computer program, GLSIM (glazing simulator). The program was written in FORTRAN and implemented in MATLAB. The multi-Fresnel equations were employed to create the GLSIM program. Then using the program, together with the pairs of real and imaginary values of complex refractive index, n and k respectively, the transmittance and reflectance data of FTO/TiO2 multilayer thin films on glass substrate were computed. The optimized FTO/TiO2 multilayer thin films were then incorporated into silicon solar cell with structure glass/FTO/TiO2/n-a-Si:H/i-a-Si:H/p-a:Si:H/P+-BSF and characterized using SCAPS-1D software. The effect of varying layer thickness on the solar cell performance was also investigated. The optimized solar cell had a thickness of 100 nm, 50 nm, 900 nm, 100 nm, 10μm and 5μm for FTO, TiO2, n-a-Si:H, i-a-Si:H, p-a-Si:H and P+-BSF respectively. The device output performance were 37.96 mA/cm2, 1.34 V, 56.37% and 28.72% for Jsc, Voc, FF and η respectively showing a remarkable improvement in the solar cell performance. These results show potential of fabricating an improved hydrogenated silicon solar cell.

Evaluating the accuracy of absorbing aerosol optical properties measured using single particle cavity ring-down spectroscopy

Single particle cavity ring-down spectroscopy (CRDS) allows direct and continuous measurements of the extinction cross-sections for levitated micron-scale aerosol particles of constant or evolving composition. Our recent interests have concerned single particle CRDS measurements for light-absorbing particles. These measurements are made on single, spherical particles evaporating over the radius range 1500–700 nm that are levitated within a 405-nm cavity ring-down spectrometer using a linear electrodynamic quadrupole trap. Here, we quantify the accuracy and precision of our cavity ring-down time measurements and the consequences of particle motion within our electrodynamic trap. Next, we estimate the uncertainty in our particle size retrievals from angularly resolved elastic light scattering measurements for particles with a range of absorption strengths. Finally, we assess the accuracy of complex refractive index values retrieved from simultaneous ring-down time and particle size measurements. The assessments account for the impacts of shot and detector noise in the measured ring-down times, the driven particle motion within the electrodynamic trap, and particle absorption strength (for imaginary refractive indices in the range ∼0.0–0.1). The real and imaginary refractive indices are retrieved to an accuracy better than 0.005 and 0.002, respectively, for almost all absorption strengths studied, and for particles of constant or evolving composition. Although particles confined in our electrodynamic quadrupole trap undergo considerable levels of driven motion (with oscillation amplitudes of several tens of micrometers), the contribution of this motion to uncertainty in retrieved refractive indices is negligible compared to contributions from particle sizing errors.

Temperature-dependent terahertz dielectric modulation of a high-performance magneto-optic CeF3 crystal

We present experimental observations of the high-performance magneto-optic (MO) CeF3 crystal using terahertz time-domain spectroscopy (THz-TDS) in the temperature range 40–300 K for the first time. The complex refractive index strongly depends on temperature, which increases with increasing temperature. Intriguingly, the changes of complex refractive index values reach more than 0.1 between 40 K and 300 K, which is larger than terbium scandium aluminum garnet (TSAG) crystal. The weak loss tangent indicates that the crystal has a high potential as a high-dielectric-constant photovoltaic device in the terahertz domain.

Optical and physical properties, time-period, and severity of dust activities as a function of source for the main dust sources of the Middle East

To achieve long-time and comparable aerosol properties with the maximum geographical coverage possible in the Middle East, four AERONET sites (Kuwait university, Solar village, IASBS, and Mezaira) are selected. Aerosol optical depth (AOD) at 870 nm, Ångström exponent (α) for two wavelengths, 440 and 870 nm, real and imaginary parts of aerosol refractive index at 870 nm (mr, mi), and aerosol effective radius are studied for these sites during 2010–2017. Daily-mean values of aerosol properties (mr≥ 1.51, mi≤ 0.013, and α ≤ 0.88), show that dust particles are the dominant aerosol type in the atmosphere of the Middle East area. It has been seen that dusty days (daily-mean AOD(870) ≥ 0.50, and α ≤ 0.50), assign a notable percentage (14%–19%) of all days of data for all sites with the exception of the IASBS site. Based on the monthly-averaged MODIS-Terra Deep Blue AOD at 550 nm, it is seen four main dust sources in the Middle East: Mesopotamian area in Iraq and East Syria, northern part of the Arabian Peninsula, Elrob Elkhali desert at southern part of the Arabian Peninsula, and sources of East Arabian Peninsula located in Oman country. All sources of dust are active during dry months in spring and summer seasons. The monthly-averaged wind field data of ERA-interim ECMWF model reveals that dust events in northern part of the Arabian Peninsula are mainly originated from North Arabian Peninsula (Tigris-Euphrates basin) sources in spring (summer). In East Arabian Peninsula (Mezaira site), dusty conditions have continued until late summer and it is related to dust activities of sources, located at southern (Elrob Elkhali desert) and eastern part (Oman country) of the Arabian Peninsula. Dust recorded in Iran plateau (IASBS site), can be originated from sources of Tigris-Euphrates basin via westerly wind at higher altitudes. Also, it seems that dust particles originated from Tigris-Euphrates basin, have smaller sizes and higher values of complex refractive index and therefore it is essential to consider source functionality of dust particles optical and physical properties for each sub-region of the Middle East.

Comparing aerosol refractive indices retrieved from full distribution and size- and mass-selected measurements

Refractive index retrievals (also termed inverse Mie methods or optical closure) have seen considerable use as a method to extract the refractive index of aerosol particles from measured optical properties. Retrievals of an aerosol refractive index use one of two primary methods: 1) measurements of the extinction, absorption and/or scattering cross-sections or efficiencies of size- (and mass-) selected particles for mass-mobility refractive index retrievals (MM-RIR) or 2) measurements of aerosol size distributions and a combination of the extinction, absorption and/or scattering coefficients for full distribution refractive index retrievals (FD-RIR). These two methods were compared in this study using pure and mixtures of ammonium sulfate (AS) and nigrosin aerosol, which constitute a non-absorbing and absorbing material, respectively. The results indicate that the retrieved complex refractive index values are correlated to the amount of nigrosin in the aerosol but can be highly variable with differences in the real and imaginary components that range between –0.002 and 0.216 and –0.013 and 0.086; the average and standard deviation of the differences are 0.046 ± 0.046 and 0.023 ± 0.033, respectively. Forward calculation of the optical properties yielded average absolute values of the relative deviation of ≈ 15% and ≈ 26% for FD-RIR data using the MM-RIR values and contrariwise. The range of retrieved refractive indices were used to calculate the normalized global average aerosol radiative forcing of a model accumulation mode remote continental aerosol. Deviations using the refractive indices of the pure materials range from 9% to 32% for AS and 27% to 45% for nigrosin. For mixtures of nigrosin and AS, deviations were all > 100% and not always able to capture the correct direction of the forcing; i.e., positive versus negative.

Influence of compositional variation on the optical and morphological properties of Ge[sbnd]Sb[sbnd]Se films for optoelectronics application

Thin films of GexSb40-xSe60 (x = 15, 20, 25, 27, 32 and 35 at.%) were deposited on quartz by vacuum thermal evaporation of the pre-synthesized parental glasses powders. By spectroscopic ellipsometry the complex refractive index values are determined in the 0.20–33 μm spectral range. The optical band gap and single oscillator energies are established as a function of the Ge content and average coordination number Z. In the (0.85–3.5) μm spectral range the GexSb40-xSe60 films possess about 80% transmission. Infrared ellipsometric and Raman studies identified SeSe and GeGe homopolar bonds, oxygen and hydrogen related impurity bonds and heteropolar GeSe and SbSe chemical bonds. Neutron- and X-ray Diffraction data coupled with Reverse Monte Carlo simulations support the optical results reproducing the GeSe and SbSe cell units and cross-linked GeGe bonds. Atomic force microscopic imaging confirmed smooth surfaces with low RMS roughness values (<2.3 nm) and continuous structure of grains of (16–22) nm diameters. All the considered material parameters show peculiarities in the compositional dependences around 27 at.% Ge, corresponding to average coordination number Z = 2.67, testifying structural phase transition at high Ge content in the GexSb40-xSe60 films.

Characteristics of brown carbon in the urban Po Valley atmosphere

Abstract. We investigate optical–microphysical–chemical properties of brown carbon (BrC) in the urban ambient atmosphere of the Po Valley. In situ ground measurements of aerosol spectral optical properties, PM1 chemical composition (HR-ToF-AMS), and particle size distributions were carried out in Bologna. BrC was identified through its wavelength dependence of light absorption at visible wavelengths, as indicated by the absorption Ångström exponent (AAE). We found that BrC occurs in particles with a narrow monomodal size distribution peaking in the droplet mode, enriched in ammonium nitrate and poor in black carbon (BC), with a strong dependance on OA-to-BC ratios, and SSA530 of 0.98 ± 0.01. We demonstrate that specific complex refractive index values (k530 = 0.017 ± 0.001) are necessary in addition to a proper particle size range to match the large AAEs measured for this BrC (AAE467 − 660 = 3.2 ± 0.9 with values up to 5.3). In terms of consistency of these findings with literature, this study i. provides experimental evidence of the size distribution of BrC associated with the formation of secondary aerosol;ii. shows that in the lower troposphere AAE increases with increasing OA-to-BC ratios rather than with increasing OA – contributing to sky radiometer retrieval techniques (e.g., AERONET);iii. extends the dependence of AAE on BC-to-OA ratios previously observed in chamber experiments to ambient aerosol dominated by wood-burning emissions. These findings are expected to bear important implications for atmospheric modeling studies and remote sensing observations as regards the parametrization and identification of BrC in the atmosphere.

Comparison of aerosol properties retrieved using GARRLiC, LIRIC, and Raman algorithms applied to multi-wavelength lidar and sun/sky-photometer data

Abstract. Aerosol particles are important and highly variable components of the terrestrial atmosphere, and they affect both air quality and climate. In order to evaluate their multiple impacts, the most important requirement is to precisely measure their characteristics. Remote sensing technologies such as lidar (light detection and ranging) and sun/sky photometers are powerful tools for determining aerosol optical and microphysical properties. In our work, we applied several methods to joint or separate lidar and sun/sky-photometer data to retrieve aerosol properties. The Raman technique and inversion with regularization use only lidar data. The LIRIC (LIdar-Radiometer Inversion Code) and recently developed GARRLiC (Generalized Aerosol Retrieval from Radiometer and Lidar Combined data) inversion methods use joint lidar and sun/sky-photometer data. This paper presents a comparison and discussion of aerosol optical properties (extinction coefficient profiles and lidar ratios) and microphysical properties (volume concentrations, complex refractive index values, and effective radius values) retrieved using the aforementioned methods. The comparison showed inconsistencies in the retrieved lidar ratios. However, other aerosol properties were found to be generally in close agreement with the AERONET (AErosol RObotic NETwork) products. In future studies, more cases should be analysed in order to clearly define the peculiarities in our results.

Design and characterization of a novel single-particle polar nephelometer

ABSTRACTA new polar nephelometer (PN) has been developed to measure simultaneously the scattering angular distributions from 11.7° to 168.3° for individual particles in planes parallel and perpendicular to the polarization of the incident laser beam. Each detection plane had 21 silicon photodiode detectors to detect scattered light at a rate of 100 Hz. Laboratory experiments to validate the performance of the instrument were conducted using nearly mono-disperse spherical particles (polystyrene latex [PSL] and nigrosine) and nonspherical particles (sodium chloride [NaCl] and soot). The observed scattering angular distributions for individual PSL particles were in good agreement with the results of simulations based on Mie theory. Complex refractive index values for nigrosine particles were determined by comparing the observed scattering angular distributions with the results of simulations. Clear differences between the measured scattering angular distributions and the results of simulations based on Mie t...

Changes in column aerosol optical properties during extreme haze-fog episodes in January 2013 over urban Beijing

Several dense haze-fog (HF) episodes were occurred in the North China Plain (NCP), especially over Beijing in January 2013 characterized by a long duration, a large influential region, and an extremely high PM2.5 values (>500 μg m−3). In this study, we present the characteristics of aerosol optical properties and radiative forcing using Cimel sun-sky radiometer measurements during HF and no haze-fog (NHF) episodes occurred over Beijing during 1–31 January, 2013. The respective maximum values of daily mean aerosol optical depth at 440 nm (AOD440) were observed to be 1.21, 1.43, 1.52, and 2.21 occurred on 12, 14 19, and 28 January. It was found that the Ångström exponent (AE) values were almost higher than 1.0 during all the days with its maximum on 26 January (1.53), suggests the dominance of fine-mode particles. The maximum (minimum) aerosol volume size distributions occurred during dense HF (NHF) days with larger particle volumes of fine-mode. The single scattering albedo, asymmetry parameter, and complex refractive index values during HF events suggest the abundance of fine-mode particles from anthropogenic (absorbing) activities mixed with scattering dust particles. The average shortwave direct aerosol radiative forcing (DARF) values at the bottom-of-atmosphere (BOA) during HF and NHF days were estimated to be 112.29 ± 42.18 W m−2 and −58.61 ± 13.09 W m−2, while at the top-of-atmosphere (TOA) the forcing values were −45.78 ± 22.17 W m−2 and −18.64 ± 5.84 W m−2, with the corresponding heating rate of 1.61 ± 0.48 K day−1 and 1.12 ± 0.31 K day−1, respectively. The DARF values retrieved from the AERONET were in good agreement with the SBDART computed both at the TOA (r = 0.95) and the BOA (r = 0.97) over Beijing in January 2013.

Optical Characterization and Modeling of Coatings Intended as High Temperature Solar Selective Absorbers

SiCNH thin coatings synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) are considered as high temperature ceramic materials for solar selective multilayer absorbers. As indicated by ellipsometry and spectrophotometry measurements, their optical characteristics make them suitable as solar absorbing and antireflective coatings in multilayer stacks and/or ceramic- metal composites (cermets). These measurements gave access to spectral values of complex refractive index N = n + ik, reflectance R and transmittance T of the coatings. R and T were also modeled from N, based on Fresnel equations, and a good agreement was found between simulated and measured reflectance in the visible region. A transfer matrix method was then applied to predict the optical response (R, T) of virtual multilayer stacks including SiCNH materials. W-SiCNH cermets were also considered as absorptive layers in these stacks and their refractive indices were calculated using Bruggeman effective medium approximation theory. Solar absorptance αS and thermal emittance ɛ(T) were estimated from simulated reflectance. Layer thicknesses and cermet composition were optimized using CODE software and a MATLAB in-house code, so as to maximize solar-heat conversion efficiency which relied on high αS (up to 95%) and low ɛ (down to 6% at 500°C). The corresponding global yield which could be attained in typical Fresnel and central tower solar plants using these selective coatings was also calculated and compared to current technology.

Extraction of complex refractive index of absorbing films from ellipsometry measurement

Numerical extraction of complex refractive index of an unknown absorbing layer inside a multilayer sample from ellipsometry measurement is discussed. The approach of point by point extraction considering all points of spectroscopic data as independent data points is investigated. This problem has typically multiple solutions and the standard method consisting in fitting calculated to experimental point is likely to converge to a wrong solution if a precise guess value is not given. An alternate method is proposed, based on the determination of contours of the ellipsometric function, to provide all solutions in an as extended as wanted range of complex refractive index values. The method is tested through different kinds of sample examples. Errors relative to any of the parameters used in the sample model are calculated and discussed. This method should be helpful in many practical cases of ellipsometry data interpretation.

On time dynamics of the complex refractive index and particle microstructure according to data of spectronephelometer measurements in mixed-composition smokes

The specific features in the dynamics of the complex refractive index (CRI) are studied for three particle fractions during aging of mixed-composition wood smokes, generated due to simultaneous particle emission from two sources in the regimes of pyrolysis and flaming combustion, for two days. The initial stage is characterized by the generation of smoke in which the imaginary part of the particle refractive index χ decreases, on average, by a factor of 600 with the growth of particle size. When smoke is aged for a long time, χ decreases (by a factor of 1.2) for ultrafine black carbon particles, simultaneously increasing for moderately (by a factor of 1.5) and coarsely (by a factor of 4.7) dispersed particles. This suggests that the dynamics of microphysical smoke composition, at all stages of its existence, is mainly determined by the coagulationdriven penetration of black carbon from the nanometer-size range toward the region of larger particle sizes. Numerical experiment showed that the inverse problem during aging of mixed-composition smokes can be solved correctly only using CRI values strictly corresponding to a given time instant. Attempts to solve the inverse problem using some time invariable CRI values lead to a several-fold increase in the discrepancy between the measured and retrieved optical characteristics versus the 6–10% discrepancy for the actual CRI values. Testing of the three-fraction method for solving the inverse problem with the help of modeled aerosol with a known CRI (ethylene glycol) showed that our approach ensured high-accuracy retrieval of the true CRI for the optically active particles in a moderately dispersed size range. Using the three-fraction method decreases the discrepancy, on average, by a factor of two versus the traditional one-fraction method (assuming that particles are homogeneous in the nature).

Polarisation properties of comet NEAT C/2001 Q4

Comets exhibit high (up to 25 %) amount of optical polarization when they are observed through ground based or space telescopes. These polarizations are caused due to the scattering of cometary dust. The observed linear polarisation of comets is generally a function of the wavelength of incident light (λ), the scattering angle (θ), the geometrical shape and size of the particle and the composition of dust particles in terms of the complex values of the refractive index. The scattering properties of cometary dust will help to know the nature of cometary dust. In the present work, the observed linear polarization data of Comet NEAT are studied through simulations using Ballistic Particle-Cluster Aggregate (BPCA) and Ballistic Cluster-Cluster Aggregate (BCCA). Using Superposition T-matrix code, the best-fitting values of complex refractive indices are calculated which can well fit the observed polarization data of Comet NEAT C/2001 Q4. The best fitting values of complex refractive indices coming out from the present analysis correspond to mixture of both silicates and organics.

Aggregate dust model to study the polarization properties of comet C/1996 B2 Hyakutake

The observed linear polarization data of comet Hyakutake are studied at wavelengths λ = 0.365 μm, λ = 0.485 μm and 0.684μm through simulations using Ballistic Particle–Cluster Aggregate and Ballistic Cluster–Cluster Aggregate aggregates of 128 spherical monomers. We first found that the size parameter of the monomer, x ∼ 1.56–1.70, turned out to be the most suitable which provides the best fits to the observed dust scattering properties at three wavelengths: λ = 0.365 μm, 0.485 μm and 0.684 μm. Thus, the effective radius of the aggregate (r) lies in the range 0.45 μm ≤ r ≤ 0.49 μm at λ = 0.365 μm; 0.60 μm ≤ r ≤ 0.66 μm at λ = 0.485 μm and 0.88 μm ≤ r ≤ 0.94 μm at λ = 0.684 μm. Now using superposition t-matrix code and the power-law size distribution, n(r) ∼ r−3, the best-fitting values of complex refractive indices are calculated for the observed polarization data at the above three wavelengths. The best-fitting complex refractive indices (n, k) are found to be (1.745, 0.095) at λ = 0.365 μm, (1.743, 0.100) at λ = 0.485 μm and (1.695, 0.100) at λ = 0.684 μm. The refractive indices derived from the present analysis correspond to a mixture of both silicates and organics, which are in good agreement with the in situ measurement of comets by different spacecraft.

Impedance spectroscopy and optical characterization of polymethyl methacrylate/indium tin oxide nanocomposites with three-dimensional Voronoi microstructures

This study describes the optical and ac electrical characterization of polymethyl methacrylate (PMMA)/indium tin oxide (ITO) nanocomposites containing a Voronoi-type segregated network microstructure. Optical transmission microscopy, scanning electron microscopy (SEM), and impedance spectroscopy were used to investigate the microstructure and electrical properties of the composites. The complex refractive index values of the composites were measured by using a modified version of the internal reflection intensity analysis (IRIA) method. SEM images of the composites reveal that the microstructure consists of polyhedral-shaped PMMA particles, where the ITO nanoparticles become concentrated along the edges of the PMMA to form a percolated network. Electrical measurements as a function of filler content indicate that the percolation threshold is &amp;lt;0.50 vol % ITO. The in-plane and through-plane refractive indices obtained by the modified IRIA method suggest that the microstructure is isotropic. The estimated extinction coefficients increase with higher ITO content, which is expected for a percolating system. A microstructural model, which takes into account the accumulation of the ITO nanoparticles at the edges of the polyhedron-shaped PMMA particles, is proposed that can predict the percolation threshold in composites that have Voronoi-type segregated network microstructures.

Aggregate dust model to describe polarization properties of Comet Hale-Bopp

In this work, the observed linear-polarization data of Comet Hale–Bopp are studied at wavelengths λ= 0.485 and 0.684 μm through simulations using Ballistic Particle–Cluster Aggregate and Ballistic Cluster–Cluster Aggregate. Using superposition t-matrix code, the best-fitting values of complex refractive indices are calculated which can well match the observed polarization data at the above two wavelengths. It is found from our analysis that there is not a single value of the monomer radius (am) that can simultaneously fit the observed polarization data at two wavelengths 0.485 and 0.684 μm. However, for different am in the two wavelengths, corresponding to similar size parameter of the monomer (x≈ 1.56), better fit can be achieved. The best-fitting complex refractive indices (n, k) coming out from this analysis are found to be (1.778, 0.059) at λ= 0.485 μm when am= 0.12 μm and (1.755, 0.080) at λ= 0.684 μm when am= 0.17 μm. The above values of refractive index show silicate behaviour as the values are close to amorphous olivine. Thus, it can be concluded that the silicate composition can best match the observed polarization data of Comet Hale–Bopp at λ= 0.485 and 0.684 μm. Finally, the polarization properties of Comet Hale–Bopp are compared with Comet Levy 1990XX.

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long‐term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between −1.6% and −2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Ångström's parameters α and β measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high‐altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of α as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated.

Effect of Si nanoparticles embedded in SiOx on optical properties of the films studied by spectroscopic ellipsometry and photoluminescence spectroscopy

The optical properties of silicon oxide (SiOx) films, vacuum evaporated and thermally annealed in argon atmosphere at 700°C and 1000°C, have been studied by spectral ellipsometry and photoluminescence (PL) measurements. The thickness, complex refractive index values and composition of the films, as well as the size of the formed Si nanocrystallites have been elucidated from the spectral ellipsometric measurements performed in the range of 280–820nm. The ellipsometric data was analysed by the Bruggeman effective-medium approximation. It has been shown that annealing at 700°C leads to coagulation of Si atoms into amorphous clusters, while at 1000°C the Si clusters start to crystallize and form nanocrystallites with average size of 2.2nm. The PL emission from the films annealed at 700°C is blue-shifted and 5–10 times more intense in comparison with the films annealed at 1000°C. This behaviour is correlated with the films structure, i.e., amorphous Si nanoclusters formed in the oxide matrix at 700°C yield higher PL intensity than Si nanoclusters crystallized at 1000°C.