Spectroscopic Ellipsometry Measurements Research Articles

Identifying orientation-dependent optical properties of single-crystalline β-Ga2O3 films

We explore the effect of crystallographic anisotropy on the optical properties of high-quality β-Ga2O3 thin films by conducting experimental measurements and theoretical simulations. High resolution x-ray diffraction measurements confirm the high-quality and the single crystalline quality, ruling out the effect of defects for all films. Raman spectra reveal the presence of anisotropy, as evident from a stronger Bg(2) mode related to GaIO4 chain libration in (100) orientation. Conversely, a stronger Ag(10) mode corresponding to tetrahedral bonds is evident in the (010) orientation, while it is suppressed in the (100) orientation. Low-temperature photoluminescence spectra indicate the presence of intrinsic impurity-related emissions in the ultraviolet and blue regions for all films. An anisotropic bandgap is observed, wherein the lowest bandgap value is related to the (010) oriented sample. Spectroscopic ellipsometry measurements confirm different refractive indices and extinction coefficient values depending on crystallographic orientation, which are in good agreement with the theoretical values obtained by density functional theory, confirming the anisotropic characteristics. The theoretical calculations of charge density show that the strength of covalent bonding depends on the β-Ga2O3 orientation, while experimental findings demonstrate that as the covalent bonding character increases, the film bandgap and the refractive index decrease. The anisotropy of β-Ga2O3 with respect to the crystal orientation leads to variations in the extinction coefficient, refractive index, and bandgap energy.

Phase-Controlled Synthesis and Phase-Change Properties of Colloidal Cu-Ge-Te Nanoparticles.

Phase-change memory (PCM) technology has recently attracted a vivid interest for neuromorphic applications, in-memory computing, and photonic integration due to the tunable refractive index and electrical conductivity between the amorphous and crystalline material states. Despite this, it is increasingly challenging to scale down the device dimensions of conventionally sputtered PCM memory arrays, restricting the implementation of PCM technology in mass applications such as consumer electronics. Here, we report the synthesis and structural study of sub-10 nm Cu-Ge-Te (CGT) nanoparticles as suitable candidates for low-cost and ultrasmall PCM devices. We show that our synthesis approach can accurately control the structure of the CGT colloids, such as composition-tuned CGT amorphous nanoparticles as well as crystalline CGT nanoparticles with trigonal α-GeTe and tetragonal Cu2GeTe3 phases. In situ characterization techniques such as high-temperature X-ray diffraction and X-ray absorption spectroscopy reveal that Cu doping in GeTe improves the thermal properties and amorphous phase stability of the nanoparticles, in addition to nanoscale effects, which enhance the nonvolatility characteristics of CGT nanoparticles even further. Moreover, we demonstrate the thin-film fabrication of CGT nanoparticles and characterize their optical properties with spectroscopic ellipsometry measurements. We reveal that CGT nanoparticle thin films exhibit a negative reflectivity change and have good reflectivity contrast in the near-IR spectrum. Our work promotes the possibility to use PCM in nanoparticle form for applications such as electro-optical switching devices, metalenses, reflectivity displays, and phase-change IR devices.

Determining the Characteristics of Ultrathin Polymer Films: A Spectroscopic Ellipsometry Approach.

Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.

Impact of Temperature Optimization of ITO Thin Film on Tandem Solar Cell Efficiency.

This study examined the impact of temperature optimization on indium tin oxide (ITO) films in monolithic HJT/perovskite tandem solar cells. ITO films were deposited using magnetron sputtering at temperatures ranging from room temperature (25 °C) to 250 °C. The sputtering target was ITO, with a mass ratio of In2O3 to SnO2 of 90% to 10%. The effects of temperature on the ITO film were analyzed using X-ray diffraction (XRD), spectroscopic ellipsometry, and sheet resistance measurements. Results showed that all ITO films exhibited a polycrystalline morphology, with diffraction peaks corresponding to planes (211), (222), (400), (440), and (622), indicating a cubic bixbyite crystal structure. The light transmittance exceeded 80%, and the sheet resistance was 75.1 Ω/sq for ITO deposited at 200 °C. The optical bandgap of deposited ITO films ranged between 3.90 eV and 3.93 eV. Structural and morphological characterization of the perovskite solar cell was performed using XRD and FE-SEM. Tandem solar cell performance was evaluated by analyzing current density-voltage characteristics under simulated sunlight. By optimizing the ITO deposition temperature, the tandem cell achieved a power conversion efficiency (PCE) of 16.74%, resulting in enhanced tandem cell efficiency.

Sensitive detection of low doses beta particles using quartz crystal oscillators

The demand for developing a cheap and novel ionizing radiation detection technique has recently increased. This work aims to establish a new detection method for beta particles based on a quartz tuning fork (QTF). QTF coated and uncoated with 40 nm of aluminum (Al) were used to evaluate the system under different conditions. All samples were exposed to different doses of beta using a 90Sr beta source. The results confirmed that a clear frequency shift was observed during beta irradiation. Once the radiation source was removed, the frequency returned to normal behavior. This effect is more pronounced with the Al-coated sample due to more tensile stress caused by thermal heating induced by beta radiation. Before and after beta irradiation, the optical properties of SiO2 wafers (quartz) were also studied using spectroscopic ellipsometry (SE) measurements. The SE results were largely consistent with the QTF resonance frequency shift results. In conclusion, experiment results proved the probability of using QTF as sensitive, accurate, and easy-to-use sensing for low-dose beta radiation.

Tracking optical properties of VOx films to optimize polycrystalline VO2 fabrication

In-situ real-time spectroscopic ellipsometry (RTSE) measurements are performed on a growing amorphous vanadium oxide (a-VOx) film to determine the complex dielectric function (ε = ε1 + iε2) spectra, structure, and oxygen content (x) with depth during deposition. VOx with x ∼2 is annealed to produce polycrystalline VO2 and characterized by near-infrared to ultraviolet (0.75–5.9 eV) spectroscopic ellipsometry during heating and cooling from room temperature (RT) to 343 K to RT to track hysteresis effects in the semiconducting-to-metal transition (SMT). Spectra in ε are measured for as-deposited and annealed films prepared with varying deposition parameters to determine the as-deposited film x and verify the SMT. Temperature-dependent IR extended spectroscopic ellipsometry is performed from 0.06 to 0.74 eV to determine the IR ε spectra of polycrystalline VO2 across the SMT. As-deposited amorphous VOx films with x from 1.89 ≤ x ≤ 2.14 transition to polycrystalline VO2 after annealing and exhibit the SMT when heated from RT to 343 K which are verified by substantially increased ε2 magnitude at ≥343 K. By identifying x of a-VOx from ε spectra, a shortened path of qualifying a-VOx films which will crystallize to phase change VO2 is developed.

Epitaxial growth and stoichiometry control of pyrochlore Nd2Ru2O7 thin films

We report on the epitaxial growth of pyrochlore NRO thin films utilizing a reactive off-axis sputtering technique. The growth process employed Ru and Nd metal targets with a repetitive substrate temperature sequence. X-ray diffraction, magnetic susceptibility, and spectroscopic ellipsometry measurements confirm that the structural, magnetic, and electronic properties of near-stoichiometric NRO films match those of the bulk material. Our spin-polarized density functional calculations based on Nd2Ru2O7 structural parameters accurately describe the optical spectra and assign Hubbard bands to the interband transitions observed above the optical band gap of 0.2 eV. By utilizing the technique's capability to adjust the degree of ruthenium deficiency, we investigated Nd2Ru2O7 films across a wide range of stoichiometric variations. Decreasing the Ru/Nd ratio results in lattice expansion, an increase in the optical band gap, and the suppression of Ru 4d intersite optical transitions. Additionally, this adjustment facilitates the elimination of minor inclusions of a ferromagnetic NdOx impurity phase, influencing the magnetic properties of stoichiometric films at low temperatures. The successful growth of Nd2Ru2O7 films opens up promising opportunities for designing and exploring strain- and light-induced states in pyrochlore ruthenates. Published by the American Physical Society 2024

Effect of Low-Temperature Oxygen Plasma Treatment of Titanium Alloy Surface on Tannic Acid Coating Deposition.

In this study, the effect of low-temperature oxygen plasma treatment with various powers of a titanium alloy surface on the structural and morphological properties of a substrate and the deposition of a tannic acid coating was investigated. The surface characteristics of the titanium alloy were evaluated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle measurements. Following this, the tannic acid coatings were deposited on the titanium alloy substrates and the structural and morphological properties of the tannic acid coatings deposited were subject to characterization by XPS, SEM, and spectroscopic ellipsometry (SE) measurements. The results show that the low-temperature oxygen plasma treatment of titanium alloys leads to the formation of titanium dioxides that contain -OH groups on the surface being accompanied by a reduction in carbon, which imparts hydrophilicity to the titanium substrate, and the effect increases with the applied plasma power. The performed titanium alloy substrate modification translates into the quality of the deposited tannic acid coating standing out by higher uniformity of the coating, lower number of defects indicating delamination or incomplete bonding of the coating with the substrate, lower number of cracks, thinner cracks, and higher thickness of the tannic acid coatings compared to the non-treated titanium alloy substrate. A similar effect is observed as the applied plasma power increases.

A-Si/SiO2 nanolaminates for tuning the complex refractive index and band gap in optical interference coatings.

A-S i/S i O 2 nanolaminates are deposited by magnetron sputtering and show a decreasing absorption when the a-Si single-layer thickness is reduced from 2.4n m to 0.7n m. Moreover, an increase of the Tauc band gap by 0.18e V is measured. Experimental Tauc band gaps are compared to calculated effective band gaps, utilizing a numerical Schrödinger solver. Further, it is demonstrated that the refractive index can be controlled by adjusting the a-Si and S i O 2 single-layer thicknesses in the nanolaminates. The nanolaminates are optically characterized by spectroscopic ellipsometry, transmittance, and reflectance measurements. Additionally, TEM images reveal uniform, well-separated layers, and EDX measurements show the silicon and oxygen distribution in the nanolaminates.

Method for bandgap interpolation of perovskite's spectral complex refractive index.

Lead halide perovskites are a promising class of materials for solar cell applications. The perovskite bandgap depends on the material composition and is highly tunable. Opto-electrical device modelling is commonly used to find the optimum perovskite bandgap that maximizes device efficiency or energy yield, either in single junction or multi-junction configuration. The first step in this calculation is the optical modelling of the spectral absorptance. This requires as input the perovskite's complex refractive index N as a function of wavelength λ. The complex refractive index consists of real part n(λ) and imaginary part k(λ). For the most commonly used perovskites, n and k curves are available from spectroscopic ellipsometry measurements, but usually only for a few discrete bandgap energies. For solar cell optimization, these curves are required for a continuous range of bandgap energies. We introduce new methods for generating the n and k curves for an arbitrary bandgap, based on interpolating measured complex refractive index data. First, different dispersion models (Cody-Lorentz, Ullrich-Lorentz and Forouhi-Bloomer) are used to fit the measured data. Then, a linear regression is applied to the fit parameters with respect to the bandgap energy. From the interpolated parameters, the refractive index curve of perovskite with any desired bandgap energy is finally reconstructed. To validate our method, we compare our results with methods from literature and then use it to simulate the absorptance of a single junction perovskite and a perovskite/silicon tandem cell. This shows that our method based on the Forouhi-Bloomer model is more accurate than existing methods in predicting the complex refractive index of perovskite for arbitrary bandgaps.

Controlling Pt nanoparticle sintering by sub-monolayer MgO ALD thin films.

Metal nanoparticle (NP) sintering is a major cause of catalyst deactivation, as NP growth reduces the surface area available for reaction. A promising route to halt sintering is to deposit a protective overcoat on the catalyst surface, followed by annealing to generate overlayer porosity for gas transport to the NPs. Yet, such a combined deposition-annealing approach lacks structural control over the cracked protection layer and the number of NP surface atoms available for reaction. Herein, we exploit the tailoring capabilities of atomic layer deposition (ALD) to deposit MgO overcoats on archetypal Pt NP catalysts with thicknesses ranging from sub-monolayers to nm-range thin films. Two different ALD processes are studied for the growth of MgO overcoats on Pt NPs anchored on a SiO2 support, using Mg(EtCp)2 and H2O, and Mg(TMHD)2 and O3, respectively. Spectroscopic ellipsometry and X-ray photoelectron spectroscopy measurements reveal significant growth on both SiO2 and Pt for the former process, while the latter exhibits a drastically lower growth per cycle with an initial chemical selectivity towards Pt. These differences in MgO growth characteristics have implications for the availability of uncoated Pt surface atoms at different stages of the ALD process, as probed by low energy ion scattering, and for the sintering behavior during O2 annealing, as monitored in situ with grazing incidence small angle X-ray scattering (in situ GISAXS). The Mg(TMHD)2-O3 ALD process enables exquisite coverage control allowing a balance between physically blocking the Pt surface to prevent sintering and keeping Pt surface atoms free for reaction. This approach avoids the need for post-annealing, hence also safeguarding the structural integrity of the as-deposited overcoat.

A Revolutionary Approach to Study the Hydrogen Electrode Formation on Different Noble Metals Exposed to Monolayer to Sub-Monolayers Range of Water Using Scanning Kelvin Probe

The inadequacy of modern surface analytical tools to probe the electrochemical double layer (EDL) which governs the catalytic reactions at the interface hampers its in-depth investigation. The reason for this is that the EDL is usually buried under the bulk electrolyte. Though great progress was made in computational modeling the EDL [1,2], at the experimental level similar progress is necessary. One approach is to study the EDL of electrodes covered by electrolyte of nano-scale thickness. Till date, such studies of electrodes covered by electrolyte in the monolayer to sub-monolayer range are scarce, although they were at the center of the birth of electrochemical surface science [3]. Recently the “hydrogen electrode in the dry” concept which was established on palladium surface under dry nitrogen conditions could be a novel approach to understand that facet of the electrode interface activities [4]. The amount of water in this case is confined to just 1-2 monolayers to adsorbed water or even below. Similarly, this approach was successful applied for studying the current-potential relationships for oxygen reduction reaction exposed to different amounts of water ranging from micro meter to monolayers of thickness [5]. In this study, this concept was explored to other noble metal systems such as Pd, Au, Pt, and Ir to understand the hydrogen electrode formation.The present study focuses on hydrogen electrode formation on palladium, iridium, gold, and platinum exposed to inert conditions carried out by scanning Kelvin probe method (see fig) at different hydrogen activities. The activities of hydrogen on the surface was defined by electrochemically (entry side from a=0 to a=0.2) cathodic hydrogen charging method. Firstly, the hydrogen activity of 0.2 was defined on the palladium side by increasing the cathodic potential stepwise by potentiostatic hydrogen charging and measuring the corresponding potential on the exit side by Kelvin probe. This potential on palladium in both wet (>95% rh) and dry (<0.1% rh) conditions follows a 1:1 correlation with the applied entry potential, in accordance with the Nernst equation. Iridium samples show 1:1 in humid conditions, but not in dry conditions, i.e. in presence of a much thinner layer of adsorbed water molecules. At intermediate partial pressures of water, a linear response with slope lower than 1 was observed, that increased with increasing humidity. Whereas, gold samples show no correlation in both wet and dry conditions. Additionally, hydrogen electrode formation on emersed surface was checked to understand the effect of an already existing EDL on the formation of the hydrogen electrode. Also, ellipsometry spectroscopy measurements were carried out to determine the water layer thickness at different partial pressures of water. Overall, the current experimental approach of measuring the potential of these different “dry” electrodes gives more insights about hydrogen electrode formation on noble metals at different hydrogen activities, effect of adsorbed anions, and the effect of different amounts of adsorbed water.

Ambipolar surface conduction in oxygen sub-stoichiometric molybdenum oxide films

The surface electric conduction in amorphous and crystallized molybdenum oxide films was studied as a function of electronic structure by current–voltage and simultaneous spectroscopic ellipsometry measurements on structures of the kind Al/Molybdenum oxide (MoOx)/Al, at temperatures up to 400 °C and in ambient air. At room temperature, both amorphous and crystalline MoOx samples were found to be sub-stoichiometric in oxygen. The random distribution of oxygen vacancies and the imperfect atomic ordering induced the creation of an intermediate band (IB) located near the valence band and of individual electronic gap states. At temperatures below 300 °C, the conduction was found to exhibit ambipolar character in which electrons and holes participated, the former moving in the conduction band and the latter in the IB and though gap states. Above 300 °C, due to samples gradual oxidation and improvement of atomic ordering (samples crystallization), the density of states in the IB and the gap gradually decreased. The above in their turn resulted in the gradual suppression of the ambipolar character of the conduction, which at 400 °C was completely suppressed and became similar to that of ordinary n-type semiconductor. The above phenomena were found to be reversible, so as the semiconducting MoOx samples were returning to room temperature the ambipolarity of the conduction was gradually re-appearing giving rise to an unusual phenomenon of “metallic” temperature variation of electrical resistance when electrons were injected.

Optical constants of germanium antimony telluride (GST) in amorphous, crystalline, and intermediate states

The optical constants of germanium antimony telluride (GST), measured by spectroscopic ellipsometry (SE), for the spectral range of 350-30,000 nm are presented. Thin films of GST with composition Ge2Sb2Te5 are prepared by sputtering. As-deposited samples are amorphous, and when heated above the phase transition temperature near 150 °C, films undergo an amorphous to face-centered cubic crystalline phase transition. The optical constants and thicknesses of amorphous and crystalline GST films are determined from multi-angle SE measurements, applying a general oscillator model in both cases. Then, in order to evaluate the optical constants at intermediate states throughout the phase transition, GST films are heated in situ on a temperature stage, and single-angle SE measurements are carried out at discrete temperature steps in a range from 120–158 °C. It is shown that ellipsometric data for partially crystallized states can be fit by treating the GST as an effective medium consisting of its amorphous and crystalline states. Its optical constants, fractional crystallinity, and thickness can be determined at intermediate crystallization states throughout the phase transition. As a practical demonstration of the usefulness of this method, samples are held at fixed temperatures near the transition temperature, and SE is performed periodically. The fraction of crystallinity is determined as a function of time, and an activation energy for the amorphous to crystalline phase transition is determined.

Refractive indices and extinction coefficients of p-type doped Germanium wafers for photovoltaic and thermophotovoltaic devices

Our work is primarily driven by the absence of optical parameters for p-type Ge wafers, adaptable to any specific doping value, and, at the same time, able to cover any application from infrared to ultraviolet regime. To address this, we have determined the complex refractive indices of p-type commercial Ge wafers through a wide spectral range (200–25000 nm) employing spectroscopic ellipsometry and transmittance measurements. The doping levels in these wafers vary between 1015 and 1018 cm−3, which are most commonly used in photovoltaic, thermophotovoltaic and optoelectronic applications. Our data fitting procedure resulted in a dielectric function that successfully reproduces not only the critical points associated with interband transitions above the bandgap, but also absorption features below the bandgap connected to intraband processes. Consequently, we obtained the complex refraction indices of these Ge wafers as a function of dopant concentration, and these have been corroborated through experimental reflectance and transmittance measurements. Additionally, we achieved a successful validation of these refractive indices by simulating the external quantum efficiency of Ge single-junction solar cell with two different thickness fabricated on a Ge wafer with a different doping level than those analyzed in this study.

Study of electronic, thermoelectric, and optical response of zinc oxynitride thin films

In this research work, we have used density functional theory (DFT) and experimental approaches to explore the impact of nitrogen substitution on the electronic, thermoelectric, optical and structural aspects of Zn Ox=1−y Ny compounds with varying dopant content. The ZnO band gap was shown to progressively decrease with the successive increases in N concentration in theoretical and experimental estimated results. The thermoelectric properties of pristine and N-doped ZnO compositions were analyzed by employing Boltztrap measurements. The structural, morphological, and optical properties of sputtered thin films were studied extensively. X-ray diffraction studies exhibited the monophasic hexagonal wurtzite structure without any secondary phase. Field emission scanning electron microscopy was performed to examine the surface morphologies of the prepared thin films, which depicted that the higher dopant content caused grain agglomeration. Optical parameters were extracted using spectroscopic ellipsometry measurements and the optical band gap was inferred using Tauc plot. Importantly, the sample having maximum dopant content revealed tunable ENZ response in the NIR region. It is anticipated from computed simulation and experimental outcomes that the electronic, optical, and thermoelectric behavior of ZnO compounds could be enhanced by ensuing nitrogen inclusion. These engineered materials would be well-suited for optoelectronic applications within the visible and near-infrared energy regions as well as in thermoelectric applications.

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.

Plasma Polymerized Organosilicon Thin Films for Volatile Organic Compound (VOC) Detection

Plasma polymerized (PP) thin films deposited in a soft or intermediate plasma discharge from hexamethyldisiloxane (HMDSO) were developed as sensors for the detection of volatile organic compound (VOC) vapors. Energy dispersive X-ray spectroscopy (EDX) and X-ray reflectometry (XRR) were performed to determine the organosilicon films’ elemental composition and density. Spectroscopic ellipsometry measurements were carried out to determine the refractive index of the films. Quartz crystal microbalance (QCM) and ellipsometry coupled to vapor sorption were used to investigate the sorption mechanism of several VOC vapors into the films as a function of the plasma deposition conditions. The density and the refractive index of the PP-HMDSO films increased with the plasma energy due to a different chemical composition and different proportion of free volumes in the material network. The PP-HMDSO films showed different affinities towards the VOC vapors depending on the plasma discharge energy. The films elaborated in the lowest plasma energy revealed a good sensitivity towards the VOCs, especially toluene (one of the BTEX vapors), compared to the other films deposited under higher plasma energy. In addition, the selectivity between toluene and other non-BTEX VOCs such as heptane and ethanol decreased to become zero while increasing the plasma energy.

Detecting ultrathin ice on materials for optical coatings at cryogenic temperatures

The performance of optical cavities in gravitational wave detectors (GWD) is negatively affected by the growth of ice layers when operating at cryo temperatures. Loss of performance begins when the ice overlayer is only a few-nm thick. Careful planning is then required to minimize, monitor and take into account the presence of ultrathin ice on cryo-cooled optical surfaces. Here we employed spectroscopic ellipsometry (SE) to study icing on the surfaces of SiO2 and Ti:Ta2O5 thin films, two materials used in the high-reflective mirrors of current GWD. SE measurements were performed at 75 K. The data presented suggest that SE is a most convenient tool to monitor in operando the ice formation on the surfaces of GWD mirrors. Furthermore, ultrathin ice layers can affect the evaluation of the optical properties of materials at low temperatures, a valuable task for those next-generation GWD that will operate at cryogenic temperatures. The characterization of an ultrathin ice overlayer (10 nm) allowed to determine for the first time the low-temperature optical properties of Ti:Ta2O5. The same approach could be applied to determine the low-temperature optical properties of other dielectric films, thus helping to screen new materials for cryo-operated GWD mirrors.

Optical Properties of Reactive RF Magnetron Sputtered Polycrystalline Cu3N Thin Films Determined by UV/Visible/NIR Spectroscopic Ellipsometry: An Eco-Friendly Solar Light Absorber

Copper nitride (Cu3N), a metastable poly-crystalline semiconductor material with reasonably high stability at room temperature, is receiving much attention as a very promising next-generation, earth-abundant, thin film solar light absorber. Its non-toxicity, on the other hand, makes it a very attractive eco-friendly (greener from an environmental standpoint) semiconducting material. In the present investigation, Cu3N thin films were successfully grown by employing reactive radio-frequency magnetron sputtering at room temperature with an RF-power of 50 W, total working gas pressure of 0.5Pa, and partial nitrogen pressures of 0.8 and 1.0, respectively, onto glass substrates. We investigated how argon affected the optical properties of the thin films of Cu3N, with the aim of achieving a low-cost solar light absorber material with the essential characteristics that are needed to replace the more common silicon that is currently in present solar cells. Variable angle spectroscopic ellipsometry measurements were taken at three different angles, 50∘, 60∘, and 70∘, to determine the two ellipsometric parameters psi, ψ, and delta, Δ. The bulk planar Cu3N layer was characterized by a one-dimensional graded index model together with the combination of a Tauc–Lorentz oscillator, while a Bruggeman effective medium approximation model with a 50% air void was adopted in order to account for the existing surface roughness layer. In addition, the optical properties, such as the energy band gap, refractive index, extinction coefficient, and absorption coefficient, were all accurately found to highlight the true potential of this particular material as a solar light absorber within a photovoltaic device. The direct and indirect band gap energies were precisely computed, and it was found that they fell within the useful energy ranges of 2.14–2.25 eV and 1.45–1.71 eV, respectively. The atomic structure, morphology, and chemical composition of the Cu3N thin films were analyzed using X-ray diffraction, atomic force microscopy, and energy-dispersive X-ray spectroscopy, respectively. The Cu3N thin layer thickness, profile texture, and surface topography of the Cu3N material were characterized using scanning electron microscopy.