Analysis Of Spectroscopic Ellipsometry Data Research Articles

ZnS Films Deposited by ALD for Solar Cell Applications

Alternative deposition methods and materials are of interest for the fabrication of thin film solar cells since they offer potential enhancements for either low cost, high speed or high efficiency but also because they can help in better understanding the underlying physical and chemical processes that could lead to the next generation of solar cells. In this study, we will present new results on the deposition of ZnS by atomic layer deposition (ALD) as an alternate to CdS deposited by chemical bath deposition. More specifically, in-situ real time and ex-situ measurements by spectroscopic ellipsometry will be performed, allowing for the analysis of the growth processes as a function of deposition parameters. These measurements also allow for a parameterization of the dielectric functions of ZnS and the evolution of its grain size and band gap as a function of thickness. These measurements will then be correlated with ex-situ measurements, such as XRD, AFM and T&R, which is expected to validate the model used for spectroscopic ellipsometry data analysis.

Accurate Determination of the Index of Refraction of Polymer Blend Films by Spectroscopic Ellipsometry

To model the performance of a bulk-heterojunction solar cell, it is necessary to obtain information about the index of refraction of the blend layer, which is typically determined by spectroscopic ellipsometry measurements. The optical functions of poly(3-hexylthiophene)−[6,6]-phenyl C61-butyric acid methyl ester (P3HT−PCBM) blend films have been extensively studied. However, there is a large variation of the reported optical functions in the literature. Because of this fact, as well as the widespread use of P3HT−PCBM films in organic photovoltaics, we have selected this material system as an example and performed a detailed analysis of spectroscopic ellipsometry data. We illustrate the occurrence of multiple solutions and the importance of a dedicated methodology to reach a satisfactory unique solution. The proposed methodology involves the following steps: (1) multisample analysis; (2) independent thickness and surface characterization; (3) use of the adequate optical description of substrate; (4) thickness estimation from transparent range using Cauchy model; (5) fitting n and k in the entire range with fixed thickness; verify result is physically meaningful; (6) optimization of the parameters to be fitted; (7) repeating steps 5 and 6 with and without EMA layer to account for the surface roughness; (8) finally, and only if no satisfactory fit could be obtained from previous steps, attempts to introduce anisotropy, graded layers, or other nonideal models should follow.

Effect of High-temperature Annealing on Evaporated Silicon Oxide Films: A Spectroscopic Ellipsometry Study

Silicon oxide films, vacuum evaporated and annealed in Ar atmosphere at a temperature of 1000 and 1100ºC have been studied by multiple-angle spectroscopic ellipsometry (SE) in the spectral range of 280-820 nm. The thickness, complex refractive index and composition of the films have been determined from the SE data analysis, performed with multiple-layer optical models and applying the Bruggeman effective medium approximation theory. The SE results have revealed that during annealing at 1000oC the Si clusters with volume fraction of ~ 14 % crystallize in the substoichiometric SiO1.61 matrix, while annealing at 1100oC transforms the oxide structure to a fully stoichiometric SiO2 with embedded nc-Si clusters with enhanced volume fraction of 22%. The TEM micrographs visualized the Si crystallites, which have random distribution in the amorphous oxide matrix and an average size of 3 nm.

Dielectric function representation by B‐splines

AbstractAccurate dielectric function values are essential for spectroscopic ellipsometry data analysis by traditional optical model‐based analysis techniques. In this paper, we show that B‐spline basis functions offer many advantages for param‐ eterizing dielectric functions. A Kramers–Kronig consistent B‐spline formulation, based on the standard B‐spline recursion relation, is derived. B‐spline representations of typical semiconductor and metal dielectric functions are also presented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and dielectric function of two- and single-domain ZnO epitaxial films

The differences between two- and single-domain ZnO epitaxial films, grown by reactive pulsed magnetron sputtering, have been studied with respect to their texture development and x-ray coherence length behavior at various substrate temperatures and oxygen partial pressures. The film in-plane ordering depends on the surface pretreatment of the sapphire substrate. After pretreatment in an oxygen radio-frequency plasma, single-domain films form even at a substrate temperature of 100°C in a wide range of oxygen pressures, and at a growth rate up to 1.2nm∕s. The single-domain films show a linear dependence of the x-ray coherence length on the substrate temperature, while a steplike dependence is characteristic of the two-domain films. The ZnO complex dielectric function was obtained using a parametrized semiconductor oscillator model for spectroscopic ellipsometry data analysis. For the films grown at 550°C, the band gap of 3.29±0.01eV is independent of the type of in-plane ordering and variation of the texture. The oscillator broadening correlates with the width of (0002) diffraction peak rocking curve. Both parameters increase at high oxygen pressure and low substrate temperature, which is attributed to a higher defect (dislocation) density.

Optical characterization of In xGa 1− xN alloys

InGaN layers were grown by molecular beam epitaxy (MBE) either directly on (0 0 0 1) sapphire substrates or on GaN-template layers deposited by metal-organic vapor-phase epitaxy (MOVPE). We combined spectroscopic ellipsometry (SE), Raman spectroscopy (RS), photoluminescence (PL) and atomic force microscopy (AFM) measurements to investigate optical properties, microstructure, vibrational and mechanical properties of the InGaN/GaN/sapphire layers. The analysis of SE data was done using a parametric dielectric function model, established by in situ and ex situ measurements. A dielectric function database, optical band gap, the microstructure and the alloy composition of the layers were derived. The variation of the InGaN band gap with the In content ( x) in the 0 < x ≤ 0.14 range was found to follow the linear law E g = 3.44–4.5 x. The purity and the stability of the GaN and InGaN crystalline phase were investigated by RS.

Photo-oxidation and the Absence of Photodarkening in Ge2Sb2Te5 Phase Change Material

AbstractGe2Sb2Te5 is under intense investigation for phase-change memory devices, including rewriteable DVDs where optical illumination is used to switch between the glassy and crystalline states. We investigate the influence of optical irradiation on amorphous phase. Many chalcogenides display photo-oxidation, photodarkening or photo-bleaching, but little has been reported on the Ge-Sb-Te system. Using spectroscopic ellipsometry (SE) and secondary ion mass spectrometry, we determine that the samples have a strong tendency to photo-oxidize; if this is not accounted for, then the analysis of SE data appears to show photodarkening. Other authors recently reported photodarkening in nonstoichiometric GexSb20-xTe80 [Pamukchieva et al., Proc. SPIE 5581, 608 (2004); Pamukchieva et al., J. Optoelectron. Adv. Mater 7, 1277 (2005)], but our analysis suggests that the changes were also the result of photo-oxidation. The oxide has lower value of (n, k) than Ge2Sb2Te5, and can be etched by hydrofluoric acid or water. The photo-oxidation is presumably the result of free carrier generation in the Ge2Sb2Te5. Our observation of negligible photodarkening is consistent with previous works that found less photodarkening in tellurides compared with selenides or sulfides, and that an increase in the mean coordination number, here by Ge addition, further reduces the photodarkening effect.

Real-Time Growth Control of Ge–Sb–Te Multilayer Film as Optical Recording Media by In Situ Ellipsometry

Using an in situ ellipsometer, we monitored the thin film growth curve of optical recording media in real time. Utilizing the complex refractive indices of Ge2Sb2Te5 and ZnS–SiO2 obtained from the analysis of spectroscopic ellipsometry data, we calculated the growth curves of ellipsometric constants vs thickness. By comparing the calculated growth curve of ellipsometric constants with the measured one, and by analyzing the effect of the density variation of the Ge2Sb2Te5 recording layer on ellipsometric constants, we precisely monitored the growth rate of Ge2Sb2Te5 and ZnS–SiO2 films, respectively, and eventually controlled the whole growth process of a multilayer sample made of Ge2Sb2Te5 and ZnS–SiO2 films. The accurate thickness control using in situ ellipsometry was verified through the analysis of the deposited multilayer sample using an ex situ spectroscopic ellipsometer.

The optical and structural properties of AlN thin films characterized by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) was used to characterize AlN thin films deposited on SiO2/Si substrates by r.f. magnetron sputtering. Refractive index and thickness of each layer were extracted from the SE data analysis employing a multi-layer model and effective medium approximation. Non-destructive characterization of multi-layer films, with total thickness up to 2.5 μm, was successfully carried out, despite of the strong correlation between the film parameters such as thickness and surface roughness. The refractive indices of the AlN films were in the range of 1.98–2.15, which are typical values for polycrystalline AlN. The good agreement between the results of SE and those from X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM), regarding crystallinity, rms roughness, and thickness of the films, confirms that spectroscopic ellipsometry is a useful tool to extract the information on the film quality and uniformity.

In situ and real-time ellipsometry monitoring of submicron titanium nitride/titanium silicide electronic devices

We fabricated Al/TiN x /(Ti/Si)×15/Si(100) submicron devices by magnetron sputtering. Spectroscopic Ellipsometry (SE) was used for in situ characterization of TiN x , Ti and Si layers. The intermixing of Ti/Si layers and the phase transformations of TiSi during annealing were monitored by Kinetic Ellipsometry (KE). The metallic behavior of the TiN x layers was studied by analyzing their dielectric function in the IR region, measured by Fourier Transform InfraRed Spectroscopic Ellipsometry (FTIRSE), and by the unscreened plasma energy ( ℏω pu) calculated by SE data analysis.

Recent progress in thin film growth analysis by multichannel spectroscopic ellipsometry

In this article we review recent advances in instrumentation for multichannel spectroscopic ellipsometry (SE) and its applications in real time analysis of thin film growth. An established approach for multichannel SE utilizes the rotating polarizer configuration to provide 50- to 100-point spectra in the ellipsometric angles and the polarized reflectance for photon energies ranging from 1.5 to 4.5 eV, with a minimum acquisition time of 15 ms. In a recent advance, the upper photon energy limit in rotating-polarizer multichannel SE has been extended to 6.5 eV for the study of wide bandgap thin films. In addition, multichannel SE in the rotating-compensator configuration has also been developed recently. This new instrument configuration provides spectra in the four parameters that define the Stokes vector of the light beam reflected from the film surface, with a minimum acquisition time of 25 ms. The additional information in the Stokes vector allows one to assess sample non-idealities that lead to depolarization of the incident beam upon reflection. Real time SE data analysis procedures have been developed to handle a variety of problems in thin film deposition including (i) nucleation and bulk layer growth on smooth and rough substrates, (ii) nucleation and bulk layer growth on rough substrates accompanied by modification of the substrates, and (iii) graded layer growth with rough surfaces. Illustrative examples from our studies of amorphous semiconductor materials and devices demonstrate these capabilities.

SPUTTERED AMORPHOUS CARBON FILMS: MICRO-STRUCTURE, DENSITY, OPTICAL AND MECHANICAL PROPERTIES

Amorphous carbon (a-C) films, single and multilayered, were deposited by rf magnetron sputtering, from graphite, on c-Si substrates. In-situ and real-time Spectroscopic Ellipsometry (SE) was used to determine the optimum deposition conditions where the formation of controllable sp 3 sties can be achieved. The growth mechanisms of sputtered a-C films and their properties (optical, compositional, density and mechanical) were studied using a variety of novel characterization techniques. SE data analysis provides: (i) the sp 3 and sp 2 volume fractions and (ii) the optical properties, that are combined well with the density, the internal stresses and the elastic properties of ultra-thin to thick a-C films. In sputtering, the applied substrate bais voltage Vb (up to -200V) controls the energy (30≤E≤230 eV) of the Ar+ ions bombarding the growing film surface, affecting all film properties and responses and classifying the films in three types. The film developed with E≈30 eV (rich in sp 2 sites) and 30&lt;E≤130 eV (rich in sp 3 sites) are defined by type I and II, respectively. Type III is defined by films developed with E&gt;130 eV, where the formation of a new and dense carbon phase is deteched, exhibiting a semi-metallic optical behavior. The experimental results show: a) in films of type I and II the stress, density, hardness and elastic modulus are directly related with the sp 3 content and described well with the so far proposed models on the formation mechanism of tetrahedral carbon, and b) the density and the elastic properties, in type III films, depend not only on the sp 3 content but also on the new carbon phase. a-C films deposited by sputtering exhibit unique properties that can be tailored to meet specific application requirements. For example, the combination of single a-C films of type I and II in developing multilayered films resulted to the growth of stable, thick and highly sp 3 bonded films with improved elastic properties.

Cavities in helium implanted and annealed silicon characterized by spectroscopic ellipsometry

The formation of helium induced cavities in silicon during short-time annealing is analyzed by spectroscopic ellipsometry. Specimens implanted with 40 keV He+ ions to a dose of 5×1016 cm−2 are heat treated at 800 °C for times of 1–1200 s by rapid thermal annealing. Spectroscopic ellipsometry is employed to obtain quantitative information on the cavity volume depth profiles. A newly developed formula is used to model the optical multilayer depth profiles. The cavity volume is found to increase during annealing for about 300 s and to decrease for longer annealing times. Over this characteristic time a marked change in the He loss occurs, which has been reported only recently. Swelling of the helium implanted and annealed silicon is analyzed using an atomic force microscope. Step heights are consistent with the cavity volume per unit area obtained from spectroscopic ellipsometry data analysis. The number density of cavities after annealing for 600 s is calculated to be ≈1.16±0.27×1017 cm−3 and is found to be largely independent of depth in the central part of the cavity layer.

Multiple sample analysis of spectroscopic ellipsometry data of semi-transparent films

When analyzing spectroscopic ellipsometry data it can be difficult to find unique solutions if, for instance, the dielectric function of a film must be found together with the film thickness. One method to find the solutions is `multiple sample analysis'. The basic assumption for this method is that the optical properties of the unknown layers are identical in all samples. In the analysis procedure a set of samples with different film thicknesses are prepared and measured, preferably at multiple angles of incidence. Several (one for each sample) identical optical models are then defined with coupled optical properties of the unknown layers and all data are fitted simultaneously. This will give solutions for the dielectric function of the films and the film thicknesses due to parameter decoupling. The analysis can also be extended to more complex samples including more than one film thickness, surface and interface roughness, and porosity. In this study we have applied the multiple sample analysis method to find dielectric functions, film thicknesses and surface roughnesses of thin films of Ta 2O 5, ScN and CeO 2.

Spectroscopic ellipsometry data analysis: measured versus calculated quantities

Spectroscopic ellipsometry is a very powerful technique for optical characterization of thin-film and bulk materials, but the technique measures functions of complex reflection coefficients, which are usually not of interest per se. The interesting characteristics, such as film thickness, surface roughness thickness and optical functions can be determined only by modeling the near-surface region of the sample Jones matrix. However, the measured quantities are not equivalent to those determined from the modeling. Ellipsometry measurements determine elements of the sample Mueller matrix, but the usual result of modeling calculations are elements of the sample. Often this difference is academic, but if the sample depolarizes the light, it is not. Ellipsometry calculations also include methods for determining the optical functions of materials. Data for bulk materials are usually accurate for substrates, but are not appropriate for most thin films. Therefore, reasonable parameterizations are quite useful in performing spectroscopic ellipsometry data analysis. Recently, there has been an increased interest in anisotropic materials, both in thin-film and bulk form. A generalized procedure will be presented for calculating the elements of the Jones matrix for any number of layers, any one of which may or may not be uniaxial.

Determination of optical constants of solgel-derived inhomogeneous TiO_2 thin films by spectroscopic ellipsometry and transmission spectroscopy

Amorphous and nanocrystalline TiO(2) thin films coated on a vitreous silica substrate by a solgel dip coating method are investigated for optical properties by spectroscopic ellipsometry (SE) together with transmission spectroscopy. A method of analysis of SE data to determine the degree of inhomogeneity of TiO(2) films has also been presented. Instead of the refractive index, the volume fraction of void has been assumed to vary along the thickness of the films and an excellent agreement between the experimental and calculated data of SE below the fundamental band gap has been obtained. The transmission spectrum of these samples is inverted to obtain the extinction coefficient k spectrum in the wavelength range of 300-1600 nm by using the refractive indices and parameters of structure determined by SE. The nonzero extinction coefficient below the fundamental band-gap energy (3.2 eV) has been obtained for the nanocrystalline TiO(2) and shows the presence of optical scattering in the film.

Characterization of Interface Layer of Silicon on Sapphire Using Spectroscopic Ellipsometry

Epitaxial silicon on sapphire (SOS) grown using a commercial process was characterized by means of spectroscopic ellipsometry (SE). The SE data of a thin Si layer of SOS were analyzed based on the air/SiO2/Si/Al2O3 structure, in which the Si layer was analyzed using the established optical properties of bulk-Si and model dielectric functions (MDFs) of Si, and also with two layers, the bottom layer with MDFs and the top layer with bulk-Si. The best fit of the SE data with two layers shows that the optical properties of the Si layer in the early stage of growth are different from those of bulk-Si due to the effect of lattice mismatch between Al2O3 and Si. The thickness of the interface layer estimated from SE data analysis is around 20 nm, in reasonable agreement with the previous reports.

Spectroscopic ellipsometry of thin films on transparent substrates: A formalism for data interpretation

By reviewing the operational principles of modern rotating analyzer ellipsometry, we propose general analytical formalism which permits the analysis of spectroscopic ellipsometry data for thin films on transparent substrates. The inclusion of incoherent reflection from the backsurface of an optically thick substrate changes the usual definitions of Δ and ψ. Experimental data on both glass and ZnO-coated glass are modeled to confirm the validity of this new formalism. We also simulated the ellipsometry spectra of amorphous silicon on glass to show the potential for in situ studies. The results show that the accuracy of ellipsometry data can be improved by including this incoherent backsurface reflection in the measurement, as opposed to roughening the back of the substrate.

Characterization of sputter deposited inconel/carbon x-ray multilayers

The reflectivity of x-ray multilayers made of new pairs of materials was theoretically calculated at a wavelength of 45 Å. Carbon and inconel were then selected for ‘‘low index’’ and ‘‘high index’’ layers, respectively, in the multilayer system. Single layers, bilayers, and multilayers of carbon and inconel were characterized by grazing x-ray reflectometry (GXR), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and spectroscopic ellipsometry (SE). From GXR data at 1.54 Å refractive indices and film thicknesses of inconel and carbon layers were obtained. From an analysis of SE data, the dielectric function and the thickness as of inconel and carbon thin films were deduced. XPS and AES measurements revealed the presence of carbides at the interface between carbon and inconel. GXR analysis was performed on ten-layer C/inconel multilayers. The GXR interference pattern was found to broaden after annealing the samples at 600 °C.

Optical properties and structure of microcrystalline hydrogenated silicon prepared by radio-frequency magnetron sputtering

We have studied the optical properties of hydrogenated microcrystalline silicon films prepared by rf magnetron sputtering. Spectroscopic ellipsometry (SE) has been used to measure the dielectric spectra in the 1.66–5.6-eV range. Films produced with substrate temperature higher than 200 °C and hydrogen partial pressure ratio equal to 0.5 exhibited a microcrystalline structure. Distinct differences were observed between such films as amorphous and microcrystalline films in their dielectric function spectra and other optical quantities. An energy shift and a broadening of the E1 and E2 transitions for the microcrystalline samples were calculated from the analysis of SE data. It was observed that the rf power and the substrate temperature were the primary factors controlling the microcrystallite formation. A strong density dependence of microcrystallinity was observed, for all the optical parameters investigated. The effect of the hydrogen concentration and the hydrogen-bonding configuration on the optical properties is also discussed.