Variable Angle Spectroscopic Ellipsometry Research Articles

Temperature-Dependent Raman Scattering and Correlative Investigation of AlN Crystals Prepared Using a Physical Vapor Transport (PVT) Method

Ultrawide bandgap (UWBG) AlN c- and m-face crystals have been prepared using the physical vapor transport (PVT) method and studied penetratively using temperature-dependent (TD) Raman scattering (RS) measurements under both visible (457 nm) and DUV (266 nm) excitations in 80–870 K, plus correlative atomic force microscopy (AFM) and variable-angle (VA) spectroscopic ellipsometry (SE). VASE identified their band gap energy as 6.2 eV, indicating excellent AlN characteristics and revealing Urbach energy levels of about 85 meV. Raman analyses revealed the residual tensile stress. TDRS shows that the E2(high) phonon lifetime decayed gradually in the 80–600 K range. Temperature has the greater influence on the stress of m-face grown AlN crystal. The influence of low temperature on the E2(high) phonon lifetime of m-plane AlN crystal is greater than that of the high-temperature region. By way of the LO-phonon and plasma coupling (LOPC), simulations of A1(LO) modes and carrier concentrations along different faces and depths in AlN crystals are determined. These unique and significant findings provide useful references for the AlN crystal growth and deepen our understanding on the UWBG AlN materials.

Determination of Optical and Structural Parameters of Thin Films with Differently Rough Boundaries

The optical characterization of non-absorbing, homogeneous, isotropic polymer-like thin films with correlated, differently rough boundaries is essential in optimizing their performance in various applications. A central aim of this study is to derive the general formulae necessary for the characterization of such films. The applicability of this theory is illustrated through the characterization of a polymer-like thin film deposited by plasma-enhanced chemical vapor deposition onto a silicon substrate with a randomly rough surface, focusing on the analysis of its rough boundaries over a wide range of spatial frequencies. The method is based on processing experimental data obtained using variable-angle spectroscopic ellipsometry and spectroscopic reflectometry. The transition layer is considered at the lower boundary of the polymer-like thin film. The spectral dependencies of the optical constants of the polymer-like thin film and the transition layer are determined using the Campi–Coriasso dispersion model. The reflectance data are processed using a combination of Rayleigh–Rice theory and scalar diffraction theory in the near-infrared and visible spectral ranges, while scalar diffraction theory is used for the processing of reflectance data within the ultraviolet range. Rayleigh–Rice theory alone is sufficient for the processing of the ellipsometric data across the entire spectral range. We accurately determine the thicknesses of the polymer-like thin film and the transition layer, as well as the roughness parameters of both boundaries, with the root mean square (rms) values cross-validated using atomic force microscopy. Notably, the rms values derived from optical measurements and atomic force microscopy show excellent agreement. These findings confirm the reliability of the optical method for the detailed characterization of thin films with differently rough boundaries, supporting the applicability of the proposed method in high-precision film analysis.

Non‐Fouling Multi‐Azide Polyoxazoline Brush‐co‐Polymers for Sensing Applications

Abstract One of the key parameters of an artificial biosensor is a high signal‐to‐noise ratio. This is achieved by limiting non‐specific interactions while simultaneously maximizing the targeted specific interaction. Here, it is combined non‐fouling characteristics of poly(2‐methyl‐2‐oxazoline) (PMOXA) coatings with an abundance of azide groups to create a multi‐azide containing poly(2‐methyl‐2‐oxazoline‐co‐2‐(3‐azidopropyl)‐2‐oxazoline) (PMCA) that can participate in bioorthogonal strain‐promoted azide‐alkyne cycloaddition (SPAAC) for functionalization. This functional polymer is made surface‐active using the PAcrAm™ technology to obtain well‐defined spontaneously adsorbed monolayers on gold surfaces. The resistance to non‐specific interactions is tested against full human serum (HS), analyzed via variable angle spectroscopic ellipsometry (VASE), and compared to equivalent coatings based on PMOXA and azido‐poly(ethylene glycol) (PEG‐N3). The specific interactions are investigated via VASE and quartz crystal microbalance with dissipation (QCM‐D) by immobilization of dibenzocyclooctyne‐PEG4‐biotin conjugate (DBCO‐biotin) and streptavidin. The new PMCA‐based coating shows superior resistance to non‐specific protein adhesion than equivalent coatings based on commercially available PEG‐N3 and significantly increases capacity for SPAAC. A proof of principle assay (biotin‐streptavidin/biotin‐BSA/anti‐BSA) shows improved binding for the new PMCA polymer compared with single azide PEG.

Machine learning ellipsometry as a sensitive diagnostic tool to study reproductive biology in Zika virus infected murine models

Machine-learning spectroscopy is a recent scientific and fast-growing area where specific machine-learning solutions are used to process spectroscopic data beyond traditional physical modeling. Here, several machine learning models are optimized and trained for processing highly sensitive broadband variable angle spectroscopic ellipsometry measurements as a new tool in reproductive biology. This new method is applied to classify semen samples obtained from male offspring from ZIKV-infected and uninfected murine models. The method is label-free, cost-effective, simple, and reaches excellent performance. The combination of spectroscopic depolarization degree data with a well-optimized and trained SVM model resulted in the excellent performance of AUROC = 0.99, accuracy = 92%, specificity = 87% and sensibility = 97%. Optical modeling of ellipsometry spectra with Kramers–Kronigconsistent B-splines that takes care of incoherent reflections in the samples allowed for the extraction of the average semen sample refraction index for each class, revealing small differences that were attributed to the observed semen head defects, protamination failure, and DNA fragmentation.

Uniaxial Molecular Alignment in Thin Films Composed of Discrete NDI‐T2 Oligomers Investigated by Variable‐Angle Spectroscopic and Imaging Ellipsometry

The understanding of the optical characteristics and the influence of thermally induced crystallization is vital to evaluate newly synthesized organic semiconductors for their potential applications. Variable‐angle spectroscopic ellipsometry (VASE) is employed to determine the complex dielectric function of two recently developed discrete oligomers based on naphthalene diimide (NDI) and bithiophene (T2), applying an anisotropic optical model. It is shown that the as‐cast films are optically near‐isotropic, while the annealed films exhibit pronounced anisotropies with respect to the orientation of the π−π* transition. Differences between the two distinct molecules are observed and discussed. The degree of optical anisotropy is quantified using the average molecular orientation angle φ, associated with the orientation of the π−π* transition dipole, and the related orientation order parameter S. The molecules are shown to form crystallites of several microns in size upon annealing. Imaging ellipsometry is thus additionally employed to study their optical anisotropy on a microscopic scale, and the agreement with results from macroscopic measurements is evaluated.

Optical Properties of Graphene Nanoplatelets on Amorphous Germanium Substrates.

In this work, the integration of graphene nanoplatelets (GNPs) with amorphous germanium (Ge) substrates is explored. The optical properties were characterized using Variable-Angle Spectroscopic Ellipsometry (VASE). The findings of this study reveal a strong interaction between GNPs and amorphous germanium, indicated by a significant optical absorption. This interaction suggests a change in the electronic structure of the GNPs, implying that amorphous germanium could enhance their effectiveness in devices such as optical sensors, photodetectors, and solar cells. Herein, the use of amorphous germanium as a substrate for GNPs, which notably increases their refractive index and extinction coefficient, is introduced for the first time. By exploring this unique material combination, this study provides new insights into the interaction between GNPs and amorphous substrates, paving the way for the develop of high-performance, scalable optoelectronic devices with enhanced efficiency.

Remote plasma-enhanced chemical vapor deposition of GeSn on Si (100), Si (111), sapphire, and fused silica substrates

GeSn films were simultaneously deposited on Si (100), Si (111), c-plane sapphire (Al2O3), and fused silica substrates to investigate the impact of the substrate on the resulting GeSn film. The electronic, structural, and optical properties of these films were characterized by temperature-dependent Hall-effect measurements, x-ray diffractometry, secondary ion mass spectrometry, and variable angle spectroscopic ellipsometry. All films were polycrystalline with varying degrees of texturing. The film on Si (100) contained only GeSn (100) grains, 40.4 nm in diameter. The film deposited on Si (111) contained primarily GeSn (111) grains, 36.4 nm in diameter. Both films deposited on silicon substrates were fully relaxed. The layer deposited on Al2O3 contained primarily GeSn (111) grains, 41.3 nm in diameter. The film deposited on fused silica was not textured, and the average grain size was 35.0 nm. All films contained ∼5.6 at. % Sn throughout the layer, except for the film deposited on Al2O3, which contained 7.5% Sn. The films deposited on Si (111), Al2O3, and fused silica exhibit p-type conduction over the entire temperature range, 10–325 K, while the layer deposited on the Si (100) substrate shows a mixed conduction transition from p-type at low temperature to n-type above 220 K. From ∼175 to 260 K, both holes and electrons contribute to conduction. Texturing of the GeSn film on Si (100) was the only characteristic that set this film apart from the other three films, suggesting that something related to GeSn (100) crystal orientation causes this transition from p- to n-type conduction.

Optical Properties of Europium-Doped Silicon-Based Thin Films

The integration of a silicon-based light emitter into existing CMOS technology has long been intriguing due to its optoelectronic compatibility with microelectronics [1]. However, the indirect band gap nature of bulk silicon has hindered its effectiveness as a light emitter. In addressing this limitation, rare earth ions have emerged as significantly interesting candidates owing to their unique optical and electronic properties. Rare earth-doped silicon structures have earned special attention as they exhibit sharp light emission in different spectral regions [2] . This notable feature is attributed to the effective excitation of rare earth ions within the host matrix. Efficiently excited rare earth ions can produce visible emissions ranging from infrared to ultraviolet, presenting possibilities in diverse applications such as solid-state lighting, displays, lasers, photovoltaics, and optical communication. The incorporation of rare earth-doped silicon structures presents a promising solution to overcome the challenges posed by silicon's intrinsic properties, thereby paving the way for improved performance across a diverse range of optoelectronic applications. Europium is a attractive rare earth material with two optically active states, Eu2+ and Eu3+, enabling it to generate a diverse range of color emissions extending from blue to red depending on the surrounding matrix [3].In this study, we investigated the optical properties and compositions of europium (Eu)-doped thin films including silicon oxide, silicon oxynitride, and silicon carbonitride. For this purpose, thin films were fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) with in-situ magnetron sputtering on p-type 3" Si (100) substrates. In-situ Eu doping was performed by a radio frequency (RF) magnetron sputtering gun, using a 0.25 in. thick, 2 in. (diameter) 99.9% pure Eu sputtering target. Precursor gases, including silane (diluted in 90% argon), oxygen (diluted in 90% argon), nitrogen (diluted in 90% argon), and ethane were utilized. Annealing was performed on the as-deposited films over a broad temperature range, from 600° to 1100°C, in a nitrogen (N2) environment. Rutherford backscattering spectrometry (RBS) was performed to determine the atomic concentration of the film constituents. Variable angle spectroscopic ellipsometry (VASE) analysis was conducted to investigate the optical properties of the films. Room temperature photoluminescence (PL) experiments were performed using a laser diode excitation source operating at a wavelength of 375nm. Notably, bright visible emission was observed in some of the thin films. Finally, we discuss the influence of the atomic concentration of Eu and the annealing temperature on the emission properties observed in the photoluminescence experiments.[1] F. Azmi, Y. Gao, Z. Khatami, and P. Mascher, “ Tunable emission from Eu:SiOxNy thin films prepared by integrated magnetron sputtering and plasma enhanced chemical vapor deposition ,” J. Vac. Sci. Technol. A, vol. 40, no. 4, p. 043402, 2022, doi: 10.1116/6.0001761.[2] A. Brik et al., “Annealing Effects on Structural Characteristics of Europium Doped Silicon-Rich Silicon Nitride,” Silicon, vol. 14, no. 14, pp. 8417–8425, 2022, doi: 10.1007/s12633-021-01636-w.[3] D. Li, X. Zhang, L. Jin, and D. Yang, “Structure and luminescence evolution of annealed Europium-doped silicon oxides films,” Opt. Express, vol. 18, no. 26, p. 27191, 2010, doi: 10.1364/oe.18.027191.

Structural and optical properties of epitaxial ScxAl1−xN coherently grown on GaN bulk substrates by sputtering method

Three scandium aluminum nitride (ScAlN) thin films with different Sc compositions of 6%, 10%, and 14% were heteroepitaxially grown on n-type GaN bulk substrates by a low-temperature sputtering method. Atomically flat and smooth surfaces were observed by atomic force microscopy. The ScAlN films were coherently grown on GaN, and the c-axis lattice constants increased with increase in the Sc composition, confirmed by x-ray diffraction. The refractive index and the extinction coefficient of ScAlN were extracted by variable angle spectroscopic ellipsometry. The refractive index slightly increased and the extinction coefficient showed red shift with increase in the Sc composition. The optical bandgap of the ScAlN films was also extracted, which slightly shrunk with increase in the Sc composition.

Effects of carbazolyl and diphenylamino substituents bearing methoxy groups on the performance of hole-transporting materials in OLEDs

Newly designed and synthesized derivatives of pentaphenylbenzene with methoxy-substituted carbazolyl or diphenylamino moieties were investigated to estimate their applicability as hole transport materials. Both the compounds exhibit high thermal stability. The intramolecular charge transfer is blocked for the film of the compound containing diphenylamino groups. The intermolecular charge transfer is induced in the film of carbazolyl-containing compound. The derivative of pentaphenylbenzene and diphenylamine exhibits higher hole drift mobility (2.4·10−3 cm2/V·s at the electric field of 5.5·105 V/cm) and by 0.1 eV lower ionization potential than the carbazolyl-containing compound. Both the compounds were utilized as hole-transporting materials in a series of organic light emitting diodes (OLEDs) based on of thermally activated delayed fluorescence. With the maximum values of external quantum efficiency of 25.9 % and power efficiency of 43.4 lm/W, OLEDs containing the layers of the synthesized compounds outperformed the device based on TCTA by 4 %, without the change in spectral properties. Variable angle spectroscopic ellipsometry revealed the moderate average roughness of the films of the compound deposited by the thermal vacuum evaporation technique with an arithmetic mean deviation of not more than 0.8 nm. The prominent hole transport characteristics of the compounds make them good candidates for utilization in optoelectronic devices.

Accurate band alignment of sputtered Sc2O3 on GaN for high electron mobility transistor applications

Sc2O3 is a promising gate dielectric for surface passivation in GaN-based devices. However, the interface quality and band alignment of sputtered Sc2O3 on GaN has not been fully explored. In this work, x-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry were performed to extract the discontinuities in the valence and conduction bands of the Sc2O3/GaN system. Sc2O3 films were deposited on GaN using radio frequency sputtering. The valence band offset of Sc2O3/GaN was determined to be 0.76 ± 0.1 eV using Kraut’s method. The Sc2O3 band gap of 6.03 ± 0.25 eV was measured using O 1s energy loss spectroscopy. The electron affinity measurements of GaN and Sc2O3 using XPS secondary electron cut-off spectra provided an additional degree of accuracy to the derived band line-up for the Sc2O3/GaN interface. The band alignment results were compared with literature values of band offsets determined experimentally and theoretically for differently grown Sc2O3 films on GaN.

Congenital Zika virus infection impacts on male mouse offspring's reproductive biology.

Congenital ZIKV infection promotes alarming effects on male offspring's reproductive biology. This study showed the presence of the ZIKV antigen in the testis parenchyma, decreased testosterone levels, and sperm abnormalities in male offspring born to infected mothers. Infection with ZIKV during pregnancy is associated with fetal developmental problems. Although neurological issues are being explored more in experimental studies, limited research has focused on the reproductive health consequences for offspring born to infected mothers. In this context, this study aimed to assess the impact of ZIKV infection during pregnancy on the testes and sperm of adult male offspring. Female mice were intraperitoneally inoculated with a Brazil strain of ZIKV during the 5.5th day of embryonic gestation. The offspring were evaluated 12 weeks after birth to analyze cellular and molecular changes in the testes and sperm. A novel approach combining variable-angle spectroscopic ellipsometry and machine learning modeling was also introduced for sperm sample analysis. The study revealed the presence of ZIKV protein in the testis parenchyma of adult male offspring born to infected mothers. It was shown that the testes exhibited altered steroidogenesis and inflammatory mediators, in addition to significant issues with spermiogenesis that resulted in sperm with DNA fragmentation, head defects, and protamination failure. Additionally, sperm dielectric properties and artificial intelligence showed potential for rapid identification and classification of sperm samples from infected mice. These findings provide crucial insights into the reproductive risks for men born from ZIKV-infected pregnant women.

Disentangling Thermal from Electronic Contributions in the Spectral Response of Photoexcited Perovskite Materials.

Disentangling electronic and thermal effects in photoexcited perovskite materials is crucial for photovoltaic and optoelectronic applications but remains a challenge due to their intertwined nature in both the time and energy domains. In this study, we employed temperature-dependent variable-angle spectroscopic ellipsometry, density functional theory calculations, and broadband transient absorption spectroscopy spanning the visible to mid-to-deep-ultraviolet (UV) ranges on MAPbBr3 thin films. The use of deep-UV detection opens a new spectral window that enables the exploration of high-energy excitations at various symmetry points within the Brillouin zone, facilitating an understanding of the ultrafast responses of the UV bands and the underlying mechanisms governing them. Our investigation reveals that the photoinduced spectral features remarkably resemble those generated by pure lattice heating, and we disentangle the relative thermal and electronic contributions and their evolutions at different delay times using combinations of decay-associated spectra and temperature-induced differential absorption. The results demonstrate that the photoinduced transients possess a significant thermal origin and cannot be attributed solely to electronic effects. Following photoexcitation, as carriers (electrons and holes) transfer their energy to the lattice, the thermal contribution increases from ∼15% at 1 ps to ∼55% at 500 ps and subsequently decreases to ∼35-50% at 1 ns. These findings elucidate the intricate energy exchange between charge carriers and the lattice in photoexcited perovskite materials and provide insights into the limited utilization efficiency of photogenerated charge carriers.

Variable Angle Spectroscopic Ellipsometry Characterization of DMOAP-Functionalized Graphene Oxide Films

In our research, we investigated the optical properties of Graphene Oxide (GO) films functionalized with N,N-Dimethyl-N-octadecyl(3-aminopropyl)trimethoxysilyl chloride (DMOAP) using Variable Angle Spectroscopic Ellipsometry (VASE). We found that after the functionalization process, there was a notable increase in the thickness of the GO films. Additionally, there were observable changes in the Lorentz oscillator parameters, signaling modifications in the electronic transitions and structural properties of the films. Our results indicate that the functionalization with DMOAP not only impacts the electronic conjugation within the GO structures but also enhances the optical conductivity of the films, which is a finding of significant importance for potential applications in electronics, photonics, and materials science.

Dual-functional poly(2-methyl-2-oxazoline)/poly(2-(dimethylamine)ethyl methacrylate) mixed brushes with switchable protein adsorption and antibacterial properties

Herein, binary mixed brushes consisting of poly(2-methyl-2-oxazoline) (PMOXA) and poly(2-(dimethylamine)ethyl methacrylate) (PDMAEMA) with different chain lengths were fabricated by successive grafting of NH2-terminated PMOXA and SH-terminated PDMAEMA onto polydopamine-anchored substrates. The mixed-brush coating was characterized by variable-angle spectroscopic ellipsometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, zeta potential measurements, water contact angle, and atomic force microscopy. The mixed brushes showed tunable surface charge, wettability, and surface roughness, depending on the degree of PDMAEMA swelling under varying pH and ionic strength (I). Then the adsorption behaviors of pepsin, bovine serum albumin (BSA), γ-globulin, and lysozyme, four very different proteins with regard to isoelectric point, on the mixed brushes coating were studied by using fluorescence microscopy and surface plasmon resonance. When the chain length of PDMAEMA was about twice as long as PMOXA, the mixed brushes not only had high adsorption capacity for pepsin, BSA, and γ-globulin but also had a desorption efficiency of 86.9%, 87.1%, and 93.5%, respectively. It is explained that electrostatic attraction between the protonated PDMAEMA and positively charged acidic proteins (pepsin and BSA, whose isoelectric points were below the pKa of PDMAEMA) would drive the intensive adsorption (at pH 3, I = 10−3 mol·L−1 for pepsin, and pH 5, I = 10−5 mol·L−1 for BSA), while desorption was dominated by the hydrophilic PMOXA when PDMAEMA was shrinking (at pH 7, I = 10−1 mol·L−1 for pepsin, and pH 9, I = 10−1 mol·L−1 for BSA). Furthermore, the isoelectric precipitation led to the adsorption of neutral protein (γ-globulin, whose isoelectric point was near the pKa of PDMAEMA) at pH 7, I = 10−5 mol·L−1, while electrostatic repulsion and antifouling PMOXA triggered the desorption of γ-globulin at pH 3, I = 10−1 mol·L−1. However, alkaline protein (lysozyme, whose isoelectric point was higher than the pKa of PDMAEMA) exhibited slight adsorption on PMOXA/PDMAEMA mixed brushes under test conditions, regardless of whether PMOXA or PDMAEMA occupied the outermost layer. The antibacterial property of the mixed brushes against Escherichia coli was investigated. PMOXA/PDMAEMA mixed brushes showed significant bactericidal activity at pH 3, I = 10−3 mol·L−1, while the rinse of pH 9, I = 10−1 mol·L−1 solution could remove most of the residual bacteria. This work not only enables controlled adsorption of proteins with different isoelectric points but also ensures that the surface of the coating is minimized from bacterial contamination.

Optical Constants and Optical Anisotropy of Ultrathin Gold Films

Continuous and homogeneous thin gold films are fabricated on Si–SiO2 substrates using an additional seed layer based on Cu oxide or oxidized Cu. The optical properties of these gold films with thicknesses from 3 to 50 nm are studied with the help of variable angle spectroscopic ellipsometry. The optical constants and the dielectric functions of thin Au films in the wavelength range of 240–1700 nm are extracted by fitting measured ellipsometric data using the Fresnel isotropic and anisotropic models. By applying the Drude approximation to the infrared range, values of the plasma frequency ωp and relaxation time τ are obtained as a function of film thickness. It is found that plasma frequency ωp of the gold films is mostly constant in the thickness range of 3–50 nm with an average being close to the bulk value ωp ≈ 8.45 eV. The relaxation time decreases dramatically from the bulk values of τ ≈ 14 fs to τ ≈ 2 fs for gold films of 3 nm in an agreement with the confinement effect. It is established that the thinnest gold films (≈3 nm) are described better by an anisotropic layer and the underlying reasons for such optical anisotropy are discussed.

Evolution of the characterization of horizontally aligned organic light‐emitting molecules and their recent progress of molecular design

AbstractThis review article surveys the organic small molecular materials, light‐emitting for most of them, being studied for the molecular alignment or orientation in thin films. Following a chronicle order, three practical characterization techniques, variable angle spectroscopic ellipsometry (VASE), angle‐dependent photoluminescence (ADPL), and grazing incidence wide‐angle x‐ray scattering (GIWAXS) have been used to probe phosphorescence, thermally activated delayed fluorescence (TADF) materials, as well as those of non‐phosphorescence and non‐TADF materials.

Optical and Structural Properties of Europium-Doped Silicon Oxide Fabricated Using Integrated Sputtering and Chemical Vapour Deposition

Europium (Eu) doped silicon oxide (SiOx) thin films containing Eu concentrations of 0.2 to 6.4 at% were fabricated using a hybrid deposition system combining a magnetron sputtering gun serving as the doping source with electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD). The influence of annealing conditions on the structural and luminescence properties was thoroughly studied. The optical properties of the films were investigated by performing variable angle spectroscopic ellipsometry (VASE) and photoluminescence (PL) measurements. The Eu-related emission was found to be highly dependent on the deposition parameters and annealing conditions. Eu2+ and Eu3+ emissions, which are attributed to blue and red light emissions, respectively, were observed. The structural properties were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses, and the formation of nanoclusters was confirmed. At annealing temperatures of 900 °C and beyond, EuxSiyOz crystals were formed, and Eu ions were optically activated. As the light emissions of these thin films are in the blue and red wavelength range, they are promising candidates to be used as greenhouse covers and transparent solar cells.

Indium-zinc-oxide thin films produced by low-cost chemical solution deposition: Tuning the microstructure, optical and electrical properties with the processing conditions

Indium-zinc-oxide (IZO) films were prepared by spin coating an ethanol-ethylene-glycol precursor solution with a Zn/(In + Zn) ratio of 0.36 on glass. The effects of temperature on the structure, microstructure, electrical, and optical properties of the IZO thin films were investigated by thermal analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, electron and atomic-force microscopy, X-ray photoelectron spectroscopy and variable-angle spectroscopic ellipsometry. The prepared IZO thin films heated at 500, 600, and 700 °C in air were transparent, without long-range ordering, and with an RMS surface roughness of less than 1 nm. The lowest electrical resistivity at room temperature, 0.0069 Ωcm, was observed for the 115-nm-thick IZO thin film heated at 600 °C in air and subsequently post-annealed in Ar/H2. The thin film exhibited a microstructure characterized by grains typically 20 nm in size and had no organic residues. This film exhibits uniaxial optical anisotropy due to its ultra-thin lamellae with a high electron density. The ordinary refractive index was fitted as a Tauc-Lorentz-Urbach function, which is typical of an indirect absorption edge occurring in amorphous semiconductor materials. The principal absorption peak with an onset at about 2.8 eV and a Tauc gap energy of ∼2.6 eV is similar to those observed for In2O3. The described process of chemical solution deposition and subsequent curing is promising for the low-cost fabrication of IZO thin films for transparent electronics, and can be used to tune the structure and microstructure of IZO thin films, as well as their electrical and optical properties.

Optical Properties of Sputtered Iron Sulfide for Photonic Device Applications

Iron Pyrite is an earth-abundant, non-toxic, low-cost material commonly referred to as fool’s gold due to its golden-yellow appearance. It belongs to the family of transition metal dichalcogenides (TMDs) and gained fame as a potential solar material during the 70s and 80s. The reason was its suitable band gap and excellent absorption coefficient. However, the performance of pyrite never achieved a beneficial place among other materials and remained a mysterious path of research so far. With advanced technology and better characterization techniques and equipment available, the focus is again shifting towards exploring pyrite as a potential photovoltaic material which can compete with other materials, such as silicon, primarily in terms of cost. However, the true nature of pyrite and its application in photovoltaics cannot be adequately defined without looking deeply into its optical properties and interaction with light [1-3].Here in this work, we explored the photosensitivity of pyrite thin films at different thicknesses. These thin films were fabricated using an RF-powered sputtering method using a 99.9% FeS2 target. P-doped silicon and fused silica were used as substrates. The pressure and RF power were kept at 03 mTorrs and 70 watts, respectively, and the procedure took place at room temperature with an argon gas flow rate of ~24 cm3/min. The thickness was controlled by varying the sputtering time from 20 to 90 mins. As-prepared pyrite films were characterized through variable angle spectroscopic ellipsometry (VASE) and UV-vis spectrophotometer. An optical model was first developed for VASE in which the thin pyrite film showed light-absorbing characteristics (B-spline). Based on that model, the film thickness was determined. It was observed that the thickness of pyrite increased linearly with time, having grown to ~100 nm after 90 minutes of sputtering.UV-vis spectrophotometry was used to collect the transmission, reflectance and absorbance data for the films deposited onto a fused silica substrate. It was observed that the films showed very high absorbance in the UV range, and the absorbance tended to increase with the increase in thickness. The direct band gap was then calculated using absorbance data and building Tauc’s plot, which was observed to be around 2.25 eV.In conclusion, owing to its suitable properties and optical characteristics, iron pyrite opens many paths to scientists and researchers to explore its utilization not only in photovoltaics but also in other opto-electronic fields. However, further studies are also required to deeply understand its mechanism and capability. Nonetheless, high absorption of light in UV and visible range at very thin layers, suitable band gap and easy accessibility could be breakthrough parameters in near future for pyrite.