Optical simulation of external quantum efficiency spectra of CuIn_(1-x)Ga_xSe_2 solar cells from spectroscopic ellipsometry inputs

Applications of in-situ and ex-situ spectroscopic ellipsometry （SE） are presented for the development of parametric expressions that define the real and imaginary parts （ε1, ε2） of the complex dielectric function spectra of thin film solar cell components. These spectra can then be utilized to analyze the structure of complete thin film solar cells. Optical and structural/compositional models of complete solar cells developed through least squares regression analysis of the SE data acquired for the complete cells enable simulations of external quantum efficiency （EQE） without the need for variable parameters. Such simulations can be compared directly with EQE measurements. From these comparisons, it becomes possible to understand in detail the origins of optical and electronic gains and losses in thin film photovoltaics （PC） technologies and, as a result, the underlying performance limitations. In fact, optical losses that occur when above-bandgap photons are not absorbed in the active layers can be distinguished from electronic losses when electron-hole pairs generated in the active layers are not collected. This overall methodology has been applied to copper indium-gallium diselenide （Culn1-xGaxSe2; CIGS） solar cells, a key commercialized thin film PV technology. CIGS solar cells with both standard thickness （〉2 μm） and thin （〈1 μm） absorber layers are studied by applying SE to obtain inputs for EQE simulations and enabling comparisons of simulated and measured EQE spectra. SE data analysis is challenging for CIGS material components and solar cells because of the need to develop an appropriate （ε1, ε2） database for the CIGS alloys and to extract absorber layer Ga profiles for accurate structural/compositional models. For cells with standard thickness absorbers, excellent agreement is found between the simulated and measured EQE, the latter under the assumption of 100% collection from the active layers, which include the CIGS bulk and CIGS/CdS heterojunction interface layers. For cells with thin absorbers, however, an observed difference between the simulated and measured EQE can be attributed to losses via carrier recombination within a- 0.15 μm thickness of CIGS adjacent to the Mo back contact. By introducing a carrier collection probability profile into the simulation, much closer agreement is obtained between the simulated and measured EQE. In addition to the single spot capability demonstrated in this study, ex-situ SE can be applied as well to generate high resolution maps of thin film multilayer structure, component layer properties and their profiles, as well as short-circuit current density predictions. Such mapping is possible due to the high measurement speed of 〈1 s per （ , 4） spectra achievable by the multichannel ellipsometer.

Mixed polarization in determining the film thickness of a silicon sphere by spectroscopic ellipsometry

The effect of a spherical shape on the measurement result of spectroscopic ellipsometry （SE） is analyzed, and a method to eliminate this effect is proposed. Based on the simulation result of the SE measurement on a silicon sphere by ray tracking, we find that the sphere makes the parallel incident beam of the SE be divergent after reflection, and the measurement error of the SE caused by this phenomenon is explained by the mixed polarization theory. By settling an aperture in front of the detector of the SE, we can almost eliminate the error. For the silicon sphere with a diameter of 94 mm used in the Avogadro project, the thickness error of the oxide layer caused by the spherical shape can be reduced from 0.73 nm to 0.04 nm by using the proposed method. The principle of the method and the results of the experimental verification are presented.

ELLIPSOMETRY MEASUREMENT ON ANISOTROPIC FILM AND CRYSTAL

In this paper, a method of light intensity mode ellipsometry for optical constant measurement on anisotropic films and crystals is studied. Softwares for this method have also been developed Based on the theories and correspond softwares, experiments are carried out carefully for anisotropic film and bulk samples with optical axis parallel to their surface as well as optical axis perpendicular to the surface. Moreover, the discrepancy between measured data and calculated results are also analyzed. The measurement system developed in this paper is powerful to measure optical parameter of anisotropic film and bulk samples.

Optical study of Ba(Mn_xTi_(1-x)O_3) thin films by spectroscopic ellipsometry

Stoichiometric Ba（MnxTi（1-x）O3） （BMT） thin films with various values of x were deposited on Si（111） substrates by the sol-gel technique. The influence of Mn content on the optical properties was studied by spectroscopic ellipsometry （SE） in the UV–Vis–NIR region. By fitting the measured ellipsometric parameter （Ψ and Δ） with a four-phase model （air/BMT＋voids/BMT/Si（111））, the key optical constants of the thin films have been obtained. It was found that the refractive index n and the extinction coefficient k increase with increasing Mn content due to the increase in the packing density. Furthermore, a strong dependence of the optical band gap Eg on Mn/Ti ratios in the deposited films was observed, and it was inferred that the energy level of conduction bands decreases with increasing Mn content.

Error Corrected Sub-Monolayer Ellipsometry for Measurement of Biomolecular Interactions

Already in 1946 Alexandre Rothen from the Rockefeller Institute for Medical Research, New York published the use of ellipsometry for the measurement of antigen-antibody interactions. And he found some effects that could not be explained by the assumption that the interaction of the molecules is only of chemical nature. 55 years later we started research on antibody-antigen interaction and found similar results. To make sure that these measurements are not produced by measurement artifacts, each component of our measurement technique is error analyzed and error corrected if necessary. With such type of error corrected instrumentation we found, that there must be an interaction between antigens and antibodies that is based on longitudinal electromagnetic waves, which are able to work through thin 7 nm membranes. A similar interaction is found for the virus-antigen interaction. Our measurement results are in contrast to the assumption that the antigen-antibody and antigen-virus interaction is only of chemical nature.

Rotating polarizer, compensator, and analyzer ellipsometry

In this paper we propose theoretically a set of ellipsometric configurations using a rotating polarizer, compensator, and analyzer at a speed ratio of N1ω：N2ω：N3ω. Different ellipsometric configurations can be obtained by giving different integral values to N1, N2, and N3. All configurations are applied to bulk c-Si and GaAs to calculate the real and imaginary parts of the refractive index of the samples. The accuracies of all ellipsometric configurations are investigated in the presence of a hypothetical noise and with small misalignments of the optical elements. Moreover, the uncertainties in the ellipsometric parameters as functions of the uncertainties of the Fourier coefficients are studied. The comparison among different configurations reveals that the rotating compensator–analyzer configuration corresponds to the minimum error in the calculated optical parameters.

Identification of Optical Transitions by Spectroscopic Ellipsometry (SE) on CuIn₃Se₅Bulk Compounds

Bulk materials were synthesized by the Bridgman technique using the elements Cu, In and Se. These samples were characterized by Energy Dispersive Spectrometry (EDS) to determine the elemental composition, as well as by X-ray diffraction for structure, hot point probe method for type of conductivity, Optical response (Photoconductivity) and Photoluminescence (PL) to determine the band gap value and Spectroscopic Ellipsometry to find energy levels above the gap in the band scheme at room-temperature. They show a nearly perfect stoechiometry and present a p-type conductivity. CuIn3Se5 either has a Stannite structure, an Ordered Defect Chalcopyrite structure (ODC), or an Ordered Vacancy Chalcopyrite structure (OVC). The gap energy obtained for the different samples was 1.23 eV. Energy levels above the gap in the band scheme were determinate by measuring the dielectric function at room temperature for energies lying between 1.5 and 5.5 eV. Many transitions were observed above the gap for different samples. Spectroscopic Ellipsometry gave evidence for the interpretation of the choice of gap values which were compatible with that obtained from solar spectrum.

Terahertz Bessel Beam Applied to Thickness Measurement of Ellipsometry Methods

This paper introduces a Terahertz (THz) ellipsometer thickness measurement method based on Bessel beams. The ellipsometry method is used to measure the thickness of film in the THz band. And the thickness of film could be measured in the range of hundreds of microns which the measurement of film thickness by ellipsometer is usually only a few hundred nm in visible light. In addition, the photon energy of the THz-wave is very low and has little damage to the film. The THz Bessel beam has the characteristics of self-healing and diffraction-free. It can make the film thickness measurement within the diffraction-free distance, which is conducive to the flexibility of the ellipsometer system. We use a multi-frequency method to measure film with two-dimensional finite different time domain (FDTD) to numerical simulation analysis of light intensity successfully.

Spectroscopic Ellipsometry Study of the Dielectric Function of Cu(In_1–xGa_x)₃Se₅Bulk Compounds: Identification of Optical Transitions

Using Spectroscopic Ellipsometry (SE), the optical properties of Cu(In1−xGax)3Se5 bulk compounds, grown by the Bridgman method, were analyzed by varying x composition (0 ≤ x ≤ 1). Energy levels above the gap in the band scheme were determined by measuring the complex dielectric function ?at room-temperature for energies between 1.5 and 5.5 eV using a variable angle of incidence ellipsometer. The transitions values E1, E2 and E3 were observed above the gap for different samples of Cu(In1−xGax)3Se5 alloy. When a gallium atom replaces an indium atom, one assumes globally that the levels related to selenium and copper are unchanged. Conversely, the levels corresponding to the conduction band are shifted towards higher energies. Thus, the gap increases as the composition of gallium increases. Spectroscopic Ellipsometry (SE) gave evidence for the interpretation of the choice of gap values which were compatible with that obtained from solar spectrum. Several other characterization methods like Energy Dispersive Spectrometry (EDS), hot point probe method, X-ray diffraction, Photoluminescence (PL), Optical response (Photoconductivity) were presented in this paper. The Cu(In1−xGax)3Se5 have an Ordered Vacancy Chalcopyrite-type structure with lattice constants varying as a function of the x composition. The band gap energy of Cu(In1−xGax)3Se5 compounds is found to vary from 1.23 eV to 1.85 eV as a function of x.

Characterization of Nanorod Structure Using Spectroscopic Ellipsometry

We investigate the resonance modes of gold nanorods on an Indium tin oxide (ITO) coated glass substrate using spectroscopic ellipsometry. The unit cell of the structure investigated is composed of two gold nanorods with differing lengths. In such a structure, we can excite the bright resoance and the dark resonance modes. Numerical simulation of the gold nanorod on substrate was performed with the bright resonance mode at 825.0 nm and the dark resonance mode at 1107.1 nm respectively. Using spectroscopic ellipsometry we optically characterize the fabricated gold nanostructure, with the bright resonance mode at 700.0 nm and the dark resonance mode at 1350.0 nm respectively. The experimental results from ellipsometry show a good agreement with the results from simulation.

Advanced Mueller matrix ellipsometry:Instrumentation and emerging applications

Mueller matrix ellipsometry(MME)provides the 4×4 Mueller matrix of a sample under test,which determines how the state of polarization is changed as light interacts with the sample.Due to the redundant information contained in the Mueller matrix,MME has gained more and more extensive applications in the characterization of surfaces,interfaces,thin films,and nanostructures.In addition,the instrumentation of MME has also achieved great developments since its advent in the 1970 s.In this paper,we will first review the basic principle as well as the common system layouts of MME for the full Mueller matrix measurement.Then,the basic procedure of ellipsometry data analysis is reviewed.After that,some new developments in MME in our lab for different applications are introduced,including the broadband MME,the high-resolution imaging MME,and the high-speed MME.Some emerging applications of the developed MMEs are also presented.Conclusions and perspectives of the advanced ellipsometry are finally drawn and discussed.

Protein microarray biosensors based on imaging ellipsometry techniques and their applications

After years of development,biosensors based on imaging ellipsometry and biosensors based on total internal reflection imaging ellipsometry have been successfully implemented in various engineering systems.Their experimental setups,detection principles,and biological and clinical applications are briefly reviewed.

Optical properties of InN studied by spectroscopic ellipsometry

With recently developed In N epitaxy via a controlling In bilayer, spectroscopic ellipsometry（SE） measurements had been carried out on the grown In N and the measured ellipsometric spectra were fitted with the Delta Psi2 software by using a suitable model and the dispersion rule. The thickness was measured by a scanning electron microscope（SEM）. Insight into the film quality of In N and the lattice constant were gained by X-ray diffraction（XRD）. By fitting the SE, the thickness of the In N film is consistent with that obtained by SEM cross-sectional thickness measurement. The optical bandgap of InN was put forward to be 1.05 e V, which conforms to the experimental results measured by the absorption spectrum and cathodoluminescence（CL）. The refractive index and the extinction coefficient of interest were represented for InN, which is useful to design optoelectronic devices.

Approach to Quantitative Detection of CD146 with a Label-free Protein Biosensor Based on Imaging Ellipsometry

CD146 glycoprotein, belonging to the immunoglobulin gene superfamily, plays a unique role in heterophilic cell-cell interactions as a calcium ions independent cell adhesion

CD146 detection with real-time total internal reflection imaging ellipsometry

Adhesion molecule CD146 (100-130kDa) belongs to the immunoglobulin super family and it is originally identified as a biomarker for melanoma. Recently, CD146 is found as

Advances in Phase Detection Technique of Surface Plasmon Resonance Biosensor

Surface plasmon resonance (SPR) sensing is an optical method based on evanescent wave.SPR biosensor can detect interaction of label-free biomolecules in real-time.With further development,it can become a research instrument in proteomics.SPR biosensor can be divided intensity measurement and phase measurement,and the latter possesses higher sensitivity than the former one.This paper attempts to summarize the SPR phase detection theory,discuss the major developments,compare the merits and deficiencies of various methods,and look forward to future prospects.

Spectroscopic ellipsometric study of the optical properties of Ag_2O film prepared by direct-current magnetron reactive sputtering

The Ag2O film, as-dcposited by direct-current magnetron reactive sputtering at a substrate temperature of 150 ℃, clearly shows a preferential orientation （111）, and is capable of lowering the threshold value of the thermal decomposition temperature to about 200℃, which is helpful to its application in optical and magneto-optical storage. This paper fits its optical constants in terms of a general oscillator model by using measured ellipsometric parameters. The fitted oscillator energy 2.487 eV is close to the optical direct interband transition energy value of the Ag2O film determined by Tauc equation; whereas, the fitted oscillator energy 4.249 eV is far from the fitted plasma oscillator energy 4.756 eV by single-oscillator energy. The photoluminescence spectrum centred at about 2.31 eV indicates a direct-energy gap photoluminescence mechanism of the Ag2O film.

Study on electrical defects level in single layer two-dimensional Ta2O5

Two-dimensional atomic-layered material is a recent research focus, and single layer Ta205 used as gate dielectric in field-effect transistors is obtained via assemblies of Ta205 nanosheets. However, the electrical performance is seriously affected by electronic defects existing in Ta2Os. Therefore, spectroscopic ellipsometry is used to calculate the transition energies and corresponding probabilities for two different charged oxygen vacancies, whose existence is revealed by x-ray photoelectron spectroscopy analysis. Spectroscopic ellipsometry fitting also calculates the thickness of single layer Ta2Os, exhibiting good agreement with atomic force microscopy measurement. Nondestructive and noncontact spectroscopic ellipsometry is appropriate for detecting the electrical defects level of single layer Ta2O5.

Ellipsometric configurations using a phase retarder and a rotating polarizer and analyzer at any speed ratio

In this paper, we propose an ellipsometer using a phase retarder and rotating polarizer and analyzer at a speed ratio I：N. Different ellipsometric configurations are presented by assuming N = 1, 2, and 3. Moreover, two values of the offset angle of the retarder are considered for each ellipsometric configuration. The Mueller formalism is employed to extract the Stokes parameters, from which the intensity received by the detector is obtained. The optical properties of c-Si are calculated using all configurations. A comparison between different configurations is carried out considering the effect of the noise on the results and the uncertainties in the ellipsometric parameters as functions of the uncertainties of the Fourier coefficients. It is found that the alignment of the phase retarder has a crucial impact on the results and the ellipsometric configuration with speed ratio 1：1 is preferred over the other configurations.

Thin Film Characterization Using Rotating Polarizer Analyzer Ellipsometer with a Speed Ratio 1:3

In a recent previous work, we proposed a rotating polarizer-analyzer ellipsometer (RPAE) in which the two elements are rotating synchronously in the same direction with a speed ratio 1:3. We applied this technique to bulk samples. In this work, we present theoretically the characterization of 100 nm SiO2 thin film using this spectroscopic RPAE. We assume a structure consisting of air (ambient)/SiO2 (thin film)/c-Si (substrate). The ellipsometric parameters ψ and Δ are calculated when a clean signal is received by the detector and when a hypothetical noise is imposed on this signal. The film thickness and the optical constants of the film are calculated for the noisy signal in the spectrum range 200 - 800 nm. The results are compared with the proposed thickness and with the accepted values for SiO2 optical constants.