Absorption Coefficient Of Thin Films Research Articles

Performance-enhanced fiber optic humidity sensors based on SiO2/porous PMMA coatings.

Employing functional and structured thin films on fiber optic sensors has tremendously improved capabilities in humidity sensing applications. In this paper, we demonstrate fabrication of a fiber optic evanescent wave humidity sensor based on S i O 2/porous polymethyl methacrylate (PMMA) thin films. With the exposure to moisture, S i O 2/porous PMMA thin films absorb water molecules. The refractive index and absorption coefficient of thin films change with ambient humidity, resulting in modulation of the light intensity transmitted in fiber. A good linearity is determined between the logarithm of output light intensity and relative humidity (RH). An optimal average sensitivity of 188.3 lux/%RH is achieved with an increase of 11.7 fold in the RH range of 5% to 95%. The response and recovery times are 8s and 23s, respectively. Furthermore, the sensor exhibits low hysteresis, and excellent stability and repeatability.

Nonlinear optical absorption in thin halide perovskite films under femtosecond excitation at wavelengths of 1064 and 532 nm

The nonlinear optical responses of perovskite films CsPbI2Br, MAPbI3 + 1% PVC, MA0.15FA0.75Cs0.1PbI2.85Br0.15, and MA0.15FA0.75Cs0.1PbI3, irradiated by femtosecond laser pulses with wavelengths of 1064 and 532 nm, are investigated using z-scan measurements. Nonlinear absorption is found to occur in perovskite films irradiated at a wavelength of 1064 nm; the nonlinear absorption coefficient in thin films (45 – 65 nm) greatly exceeds that in thick ones (120 – 350 nm): 524 – 928 and 38 – 50 cm GW−1, respectively. It is shown that the nonlinear absorption saturation intensity for perovskite films depends on the pump pulse duration and increases with pulse shortening. It is also established that the nonlinear absorption coefficient of organometallic perovskite films exceeds that of a film of inorganic perovskite CsPbI2Br. Bleaching of film samples during z-scanning at a wavelength of 532 nm is related to the irreversible photochemical degradation of films exposed to intense light. The kinetics of photochemical bleaching of the films is detected at a wavelength of 532 nm.

Impact of substrate nature and film thickness on physical properties of antimony trisulphide (Sb2S3) thin films for multifunctional device applications

Sb 2 S 3 thin films were deposited with various thickness on glass and flexible polymer Polyetherimide (PEI) substrates by simple chemical bath deposition. X-ray diffraction patterns reveal the formation of polycrystalline films with orthorhombic structure. The Sb 2 S 3 thin films grown on the PEI substrate are rougher. The enhancement of roughness for Sb 2 S 3 /PEI thin films leads to improve their hydrophobic character. Furthermore, they show high values of contact angle ( CA ) around 162°. The absorption coefficient of thin films is determined to be higher than 10 4 cm −1 . The band-gap energy has been found to be between 1.6 and 2.1 eV as a function of thickness which related to the variation of structural defects. Thus, the growth of these films significantly depends on the nature of the substrate and its thickness. The obtained results show that all Sb 2 S 3 on glass and PEI thin films have potential applications in the self-cleaning windows and solar cells. • Substrate nature affects the growth orientation, microstructural and surface morphology of Sb 2 S 3 thin films. • PEI flexible substrate leads the obtainment of Sb 2 S 3 super-hydrophobic surfaces. • The physical properties of Sb 2 S 3 thin films are very sensitive to the film thickness.

Optical Properties of Ge<sub>23.5</sub>Pb<sub>20</sub>S<sub>56.5</sub> Glassy System

The spectra of refractive index and absorption coefficient in thin films of Ge23.5Pb20S56.5 glassy system were calculated on the base of analysis of transmission spectra. The refractive index has values between 2.0 and 2.6 in the spectral range between 500 nm and 1200 nm. The existence of indirect allowed transitions with energy Eg = 1.70 ± 0.01 eV and phonon energy of wph= 0.11 eV at room temperature was found. For values of the absorption coefficient a < 103см –1 the fundamental absorption edge is described by exponential function known as Urbach-rule. The static structure disorder parameter wo, is equal to 0.06 eV.

Study of the spectral dependences of the absorption coefficient in thin films of hydrogenated silicon by the method of constant photocurrent with modulated excitation

Study of the spectral dependences of the absorption coefficient in thin films of hydrogenated silicon by the method of constant photocurrent with modulated excitation

A method to extract absorption coefficient of thin films from transmission spectra of the films on thick substrates

In this paper we present a method that allows extraction of the absorption coefficient of a thin film from transmittance spectrum of the film on a silicon substrate. The method essentially removes all “optical effects,” such as interference fringes, reflectance losses, substrate absorption, etc. The method requires that the refractive index of the film is known at one wavelength and that the thickness of the film is approximately known, both of which are generally available from ellipsometric measurements. As a by-product of the procedure, the method also extracts optical constants of the film over the entire spectral range of interest and provides improved values of thickness and refractive index over those provided by ellipsometry.

Optical properties of amorphous thin film of Se-Te-Ag system prepared by using thermal evaporation technique

AbstractOptical reflection and transmission spectra of Se80−xTe20Agx (where x=0, 5, 10 and 15, molar percent) chalcogenide thin films have been obtained in the range 300–1 200 nm at room temperature. Glassy samples are prepared by melt quenching technique and thin films are obtained by using vacuum evaporation technique on glass transition at room temperature. The present paper reports the effect of Ag contents on various optical parameters such as optical band gap, refractive index, extinction coefficient, absorption coefficient. Reflection spectra are used for calculating these optical parameters. The band gap Eg is determined from the plot of (αhν)1/2 versus hν. Real and imaginary parts of the dielectric constant changes slightly with the variation of Ag contents in different samples. The film exhibits indirect band gap which increases with increasing Ag content. The observed behavior is explained on the basis of Davis and Mott model for the density of states. The absorption coefficient of thin films exhibits linear dependence on photon energy. Other optical parameters are calculated by using theory of reflectivity. All the optical parameters change with the variation of the film composition. Computerized spectrophotometer JASCO-UV/VIS/NIR is used to determine optical constants.

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.

Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells

The spectral dependence of the optical absorption coefficient in thin films of hydrogenated microcrystalline silicon is measured over nine orders of magnitude in the subgap, defect-connected region, and in the above-the-band gap region. Transmittance, reflectance, and constant photocurrent method measurements are combined with Fourier-transform photocurrent spectroscopy (FTPS). Results are analyzed and interpreted as due to electron transitions from defects or interband electron transitions, all having direct relevance to the thin-film microcrystalline silicon solar cell performance. FTPS is a fast and sensitive quantitative method for quality assessment of microcrystalline silicon absorber in solar cells and can be used for quality monitoring in solar cell production.

Evaluation of Mirage Effect Spectrometry for Optical Absorption Depth Profiling of Photodegradation in Thin Poly(Vinylchloride) Films

Mirage effect (photothermal deflection) spectrometry was examined as a tool for the quantitative depth profiling of the optical absorption coefficient in thin films of poly(vinylchloride) (PVC) photolyzed under ultraviolet (UV) light. Profiles of the photoproduct of dehydrochlorination reactions were induced by controlled UV photolysis in test films on the scale of 10–100 μm, with total absorbance in the range 0.3–0.5 a.u. at 465 nm. Photolyzed test samples were prepared as laminate assemblies containing 10–25 individual layers fused together under pressure, according to a methodology we previously developed. These assemblies were mechanically separated into individual layers after photolysis and spectrophotometrically analyzed. Photothermal depth reconstructions of the optical absorption coefficient were made in these assemblies via inverse problem theory applied to the experimental photothermal deflection response, with the destructive layer-by-layer spectrophotometric analysis performed in parallel as a reference measurement. Optical absorption coefficient profiles recovered by the mirage effect were in agreement with the reference spectrophotometric data to within a root-mean-square error of better than 15% of full scale, with a nearly quantitative recovery of the peak absorption, depth, and initial slope of the film's absorption profile.

Infrared analysis of thin films: amorphous, hydrogenated carbon on silicon

The infrared analysis of thin films on a thick substrate is discussed using the example of plasma-deposited, amorphous, hydrogenated carbon layers (a-C:H) on silicon substrates. The framework for the optical analysis of thin films is presented. The main characteristic of thin film optics is the occurrence of interference effects due to the coherent superposition of light multiply reflected at the various internal and external interfaces of the optical system. These interference effects lead to a sinusoidal variation of the transmitted and reflected intensity. As a consequence, the Lambert-Beer law is not applicable for the determination of the absorption coefficient of thin films. Furthermore, observable changes of the transmission and reflection spectra occur in the vicinity of strong absorption bands due to the Kramers-Kronig relation. For a sound data evaluation these effects have to be included in the analysis. To be able to extract the full information contained in a measured optical thin film spectrum, an experimentally measured spectrum has to be simulated using the full formalism including the Kramers-Kronig relation. Infrared absorption spectra and the resulting k spectra in the range of the CH vibrational bands around 3000 cm-1 are presented for a variety of a-C:H layers. The shape and the total intensity of the peak are quite sensitive to the film structure. Soft, polymerlike hydrocarbon layers are characterized by a well structured, intense IR absorption band, while hard, amorphous, hydrogenated carbon layers exhibit a structureless, broad IR absorption band with relative low intensity. The k spectra of the CH vibrational bands can be considered as fingerprint for the type of a-C:H film.

Experimental and theoretical comparison between absorption, total electron yield, and fluorescence spectra of rare-earthM5edges

Besides the now well-known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state may occur which render x-ray fluorescence different from the true absorption in $3d$ transition metals at the $L$ edge and at the ${M}_{4,5}$ edges of rare earths. Special selection rules of the radiative de-excitation process play an important role there. We have measured the absorption coefficient of thin films of lanthanum, samarium, and thulium deposited on an aluminum foil, at room temperature, through the simultaneous detection of the transmission, total electron yield, and 150-eV bandwidth fluorescence yield. The latter result shows differences as compared to the other two, and exhibits polarization effects depending upon the angle between incident and outgoing photons. The resonant x-ray fluorescence spectrum is calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. Very good agreement is obtained between the theory and experiment.

Conductivity dependence on the thickness of hydrogenated, amorphous silicon-carbon films

We report experimental results on the dependence of conductivity on thickness in hydrogenated amorphous silicon and hydrogenated amorphous silicon carbon thin film alloys. The analysis of multilayer structures indicates that band bending at the vacuum-film interface accounts for most of the conductivity changes. The film-substrate interface seems to play a minor role in the conductivity experiments. The behavior of the sub-gap absorption coefficient of thin films shows that the material near the interfaces and in the bulk has the same defect density.

Determination of absorption coefficients of thin films

The equations that are usually presented as those used to determine the absorption coefficients of materials in film form based on measurements of transmission and reflection coefficients are fundamentally incorrect. These equations omit a multiplicative factor arising from the complex nature of the refractive indices of the materials. This factor enters explicitly into the relationship between the transmission and reflection coefficients for such materials and is not necessarily close to unity, although in practice this factor can be approximated by unity at least in the infrared through the optical range of wavelengths.

Photothermal deflection spectroscopy of conjugated polymers

Along with the third-order nonlinear susceptibility, χ (3), the magnitude of the optical absorption in the transparent window below the principal absorption edge is an important parameter for conjugated polymers used in active integrated optical devices. Photothermal deflection spectroscopy (PDS) is an ideal technique for determining the absorption coefficients of thin films of ‘transparent’ materials. We have used PDS to measure the optical absorption spectra of the conjugated polymers, poly(1,4-phenylene-vinylene) (and derivatives) and polydiacetylene-4BCMU, in the spectral region from 0.55 to 3 eV. We find that the shape of the absorption edge varies considerably from polymer to polymer, with polydiacetylene-4BCMU having the steepest absorption edge. The minimum absorption coefficients measured varied somewhat with sample age and quality, but were typically in the range 1–10 cm −1. In the region below 1 eV, overtones of C-H stretching modes dominate the absorption behavior. We also observe that irradiation of all of these polymers with light above ∼ 2.5 eV produces enhanced absorption below the fundamental edge. In the absence of light, these excitations decay with characteristic times of 10–1000 s and in some cases they may determine the effective IR transparency in the energy range 1.0–1.8 eV.

Spectral dependence of the absorption coefficient of thin films of non-stoichiometric Cu 2− xTe

A transmission and reflection spectra of polycrystalline thin films (from about 500 to about 3000 Å) of Cu 2− x Te compounds, where 0 < × ⩽ 0.25 with steps in x of 0.05, have been studied at room temperature in the wavelength region from 0.295 to 2.5 μm. A new technique is described to determine either the composition or the thickness of the film of copper telluride. In this method reflectance-transmittance data for the Cu 2− x Te film are needed in the small wavelength region 0.5−1.0 μm. The position and the value of the reflectance minima and the transmittance maxima uniquely determine the film composition and its thickness respectively. The absorption coefficient α of the different compositions of Cu 2− x Te have been determined for at least three different thicknesses. For all the compositions, direct and indirect transitions exist. The value of the energy gap corresponding to the direct transitions strongly depends on the value of x, increasing as x increases. On the contrary, the indirect energy gap corresponding to the indirect transitions slightly decrease as x increases. The frequency dependence of the optical constants n, k, ε′ = n 2 − k 2, ε″ = 2 nk was also given. It was found that the value of the high frequency optical constants, as estimated from extrapolation of the curves, did not depend on the value of x The theoretical calculations of the direct energy gaps of all compositions using the simple relation E g( x, T) = 1.233 + 3.374 x - 3.1 × 10 −4 T(1−2 x) agree fairly well with the experimental values.

Subgap Absorption in Conjugated Polymers

ABSTRACTAlong with χ(3), the magnitude of the optical absorption in the transparent window below the principal absorption edge is an important parameter which will ultimately determine the utility of conjugated polymers in active integrated optical devices. With an absorptance sensitivity of &lt; 10-5, Photothermal Deflection Spectroscopy (PDS) is ideal for determining the absorption coefficients of thin films of “transparent” materials. We have used PDS to measure the optical absorption spectra of the conjugated polymers poly[1,4-phenylenevinylene] (and derivitives) and polydiacetylene-4BCMU in the spectral region from 0.55 eV to 3 eV. Our spectra show that the shape of the absorption edge varies considerably from polymer to polymer, with polydiacetylene-4BCMU having the steepest absorption edge. The minimum absorption coefficients measured varied somewhat with sample age and quality, but were typically in the range 1 cm-1 to 10 cm-1. In the region below 1 eV, overtones of C-H stretching modes were observed, indicating that further improvements in transparency in this spectral region might be achieved via deuteration or fluorination.

Evaluation of Si Thin Films by Photothermal Deflection Spectroscopy (PDS)

This paper reports a theoretical and experimental study of a photothermal deflection spectroscopy (PDS) method for thin films. Multiple reflection of the incident light is taken into account in the theoretical analysis, using a simplified one-dimensional model. An experiment was performed to estimate the optical absorption coefficients α of a-Si:H and µc-Si:H films. It is possible to obtain the difference in α between these Si films very accurately even for αL<10-2. Thus PDS is very effective for measuring the small absorption coefficients of thin films.

In-gap optical absorption of amorphous Si3N4

The optical absorption coefficient of thin films of chemically vapor deposited amorphous Si3N4 has been measured for photon energies ranging from 1.7 to 3.9 eV using photothermal deflection spectroscopy. From these data it can be inferred that these films contain between 10 and 100 ppm in-gap electronic states, a number consistent with estimates of the trapped charge density in metal-nitride-oxide-semiconductor devices. While other investigators have inferred the existence of well defined trap energies from electrical data, the present measurements yield only broad, rather featureless absorption spectra.

Thin-film absorption coefficients by attenuated-total-reflection spectroscopy

The application of attenuated-total-reflection spectroscopy to the measurement of the absorption coefficient of thin films is presented. For low absorption the sensitivity of ATR is discussed in terms of the concept of an effective thickness. Both the case in which the refractive index of the film is higher and the case in which it is lower than that of the ATR trapezoid are considered. Experimental ATR data for antireflection-coating materials for laser windows is analyzed and compared with calorimetric data.