We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on Al_2O_3(0001) and AlN/Al_2O_3(0001) using plasma-assisted molecular beam epitaxy. Spectroscopic ellipsometry, spanning the far-infrared to ultraviolet range (0.04 - 5.5 eV), is conducted at room temperature to determine the optical constants n and k of the films. Spectral fits reveal two interband transitions at approximately 0.35 and 0.60 eV. In the infrared range, the ellipsometry data also reveals a pronounced Reststrahlen band stemming from transversal and longitudinal optical phonons at approximately 403 and 629 cm^-1, respectively. The relative static and high-frequency permittivities are estimated to be about 39 and 15, respectively. A Born effective charge of approximately 2.7, extracted from the far-infrared region, indicates that CrN is partially ionic.

In this work, we have used in situ spectroscopic ellipsometry to investigate the thickness-dependent dielectric functions of PtSe2, a transition metal dichalcogenide material that exhibits interesting optical and electrical properties. Continuous ellipsometry measurements were obtained while a ∼25 nm-thick PtSe2 film was grown using molecular beam epitaxy. By using Kramers–Kronig-consistent oscillators to represent the dielectric functions, we fit the ellipsometry spectra to obtain the dielectric function and the thickness of PtSe2 during its entire growth cycle. The dielectric function changes significantly with thickness, where the centroid of three oscillators used to represent the band-to-band transitions of PtSe2 redshifts as the thickness increases, reaching a saturation value as the film approaches ∼20 nm. Furthermore, the Drude contribution also changes with thickness. More significantly, we can decipher the sample composition as a function of time by using an effective medium approximation-model to represent the film.

In-situ spectroscopic ellipsometry (SE) was used to analyze monolayer and few-layer samples of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. MoS2 and WS2 film growth times ranged from 5 - 40 min to achieve a range of film coalescence and surface coverage from less than a monolayer to considerable bilayer and additional layers. Post-growth measurements using atomic force microscopy, photoluminescence and Raman spectroscopy were used to assess the evolution of surface morphology and film properties with growth time to determine the areal coverage of monolayer, bilayer, multilayer and void regions. The room temperature SE spectra were modeled using an effective medium approximation to obtain the monolayer and bilayer dielectric functions of MoS2 and WS2. Using these distinct dielectric functions, changes in ellipsometry parameters associated with partial to full coverage of monolayers and bilayers were simulated. Such simulations will enable improved control of the layer-by-layer deposition of MoS2 and WS2 by monitoring in-situ spectroscopic ellipsometry.

Using spectroscopic ellipsometry, we studied the optical properties of four Pb1-xCdxTe (0 <= x <= 0.20) films. The Pb1-xCdxTe films, deposited on silicon substrates using an electron-beam deposition technique, show a rock-salt structure, with their lattice constants decreasing as a function of Cd concentration. Ellipsometry measurements, which covered a wide spectral range between 0.1 eV to 4.1 eV, determined the index of refraction and the extinction coefficient of the films. As the Cd concentration increases from 0 % to 20 % in the Pb1-xCdxTe films, the index of refraction decreases by similar to 10 %, across the entire energy range. Similarly, there seems to be a decrease in the extinction coefficient with the increase of Cd concentration. An oscillator model, depicting the optical functions of each Pb1-xCdxTe film, allowed us to obtain the band gap of each film which blue-shifts as the Cd concentration is increased. Besides the fundamental band gap, we recovered the higher-order electronic transitions that occur in the Brillouin zone of the Pb1-xCdxTe lattice.

Intermolecular interactions govern the optical and electronic properties of organic semiconductor thin films. Optimizing the function of molecular devices such as organic solar cells and light emitting diodes relies on understanding structural effects in a heterogeneous film environment. Because organic thin films are typically more disordered than inorganic crystalline semiconductors, advanced techniques are often needed to sufficiently characterize these materials. However, polarized infrared reflectance measurements are a benchtop method that can provide well-resolved vibrational spectra with specificity in their molecular orientation. By using infrared reflectance-absorbance spectroscopy (IRRAS) we relate the effects of intermolecular packing geometry on thin film vibrational spectra of perylene diimide (PDI), a prototypical molecular semiconductor. We analyze a suite of PDIs with differing sidechain substitutions, which adopt distinct crystal packing arrangements in thin films, by comparison of the reflectance spectra to isotropic KBr mixtures. We observe differences in the degree of Davydov splitting as a function of displacement along the short and long molecular axes and the π-π stacking distance that are evident in either the symmetric or anti-symmetric CO stretching frequencies of the solid. The four PDIs also display changes in relative intensity between the two modes that indicate different average molecular orientations.

An accurate knowledge of the optical constants (refractive index n and extinction coefficient k) of ScN is crucial for understanding the optical properties of this binary nitride semiconductor as well as for its use in optoelectronic applications. Using spectroscopic ellipsometry in a spectral range from far infrared to far ultraviolet (0.045-8.5 eV), we determine n and k of ScN layers grown on Al_2O_3(0001) substrates by plasma-assisted molecular beam epitaxy. Fits of ellipsometry data return the energies of four oscillators representing critical points in the band structure of ScN, namely, 2.03, 3.89, 5.33, and 6.95 eV. As the infrared range is dominated by free carriers, the vibrational properties of the layers are examined by Raman spectroscopy. Despite the rocksalt structure of ScN, several first-order phonon modes are observed, suggesting a high density of point defects consistent with the high electron density deduced from Hall measurements. Finally, photoluminescence measurements reveal an emission band slightly above the lowest direct bandgap. We attribute the redshift of the peak emission energy from 2.3 to 2.2 eV with increasing layer thickness to a reduction of the O concentration in the layers.

We used spectroscopic ellipsometry to study the dielectric function of an azobenzene polymer called Poly (disperse Red 1 methacrylate) (PDRM) as it underwent a photoisomerization process. The polymer films were fabricated by dissolving PDRM in tetrahydrofuran, and by spin coating films on silicon substrates. After fabri-cating the films, we used UV-VIS spectroscopic ellipsometry to determine the index of refraction, extinction coefficient and the thickness of each film. Using a heating cell coupled to the ellipsometer, we obtained tem-perature dependent ellipsometry data from 34 degrees C to 180 degrees C, spanning a spectral region between 300 nm and 1500 nm. Using the raw data, we determined that the glass transition temperature of PDRM was 85 degrees C +/- 3 degrees C. Employing a three-oscillator model, we deduced the temperature dependent dielectric functions of PDRM films. Subsequently, the films were photoisomerized at two polarizations using a visible laser. For the horizontal po-larization, there is nearly a 15% a change in the extinction coefficient as the PDRM films undergo a photo-isomerization process.

Multilayer heterostructures of the topological insulator Bi2Se3 and conventional semiconductor In2Se3, as well as solid solution layers of (Bi1-xInx)(2)Se-3 layers were grown by molecular beam epitaxy and analyzed in-operando using spectroscopic ellipsometry (SE). SE spectra were obtained after the deposition of each layer to deter-mine the respective dielectric functions and thicknesses of each layer. In contrast to ex-situ SE, where uncertainty in the dielectric function and thicknesses of individual layer impose limitations to extract a correct model for the dielectric function of such multilayer heterostructures from a single set of SE data, the step-by-step in-situ SE data recover more precise dielectric functions for Bi2Se3, In2Se3, (Bi0.7In (0.3))(2)Se-3 and the cap-layer. The optical models developed for multilayer structures can decipher minute perturbations in layers as the growth progresses. Our models show that a ~ 7 nm Bi2Se3 layer grown next to a sapphire substrate seems to disappear as the structure is annealed at 600 C. Finally, when the dielectric functions were represented as a collection of Kramers-Kronig-consistent oscillators, in-situ SE predicted the quality of films; the weighted-average broadening parameter for oscillators used for Bi2Se3 films grown on (Bi0.7In (0.3))(2)Se-3 layer is-20% smaller compared to Bi2Se3 films directly grown on sapphire, confirming that the former film is of better quality, and providing a direct metric to quantify film quality and defect concentration. These conclusions were corroborated by transport data.

Optical birefringence is a fundamental optical property of crystals widely used for filtering and beam splitting of photons. Birefringent crystals concurrently possess the property of linear dichroism (LD), which allows asymmetric propagation or attenuation of light with two different polarizations. This property of LD has been widely studied from small molecules to polymers and crystals but has rarely been engineered on demand. Here we use the newly discovered spin-charge coupling in the van der Waals antiferromagnetic insulator FePS3 to induce large in-plane optical anisotropy and consequently LD. We report that the LD in this antiferromagnetic insulator is tunable both spectrally and in terms of its magnitude as a function of the cavity coupling. We demonstrate near-unity LD in the visible–near-infrared range in cavity-coupled FePS3 crystals and derive its dispersion as a function of the cavity length and FePS3 thickness. Our results hold wide implications for the use of cavity-tuned LD as a diagnostic probe for strongly correlated quantum materials and offer new opportunities for miniaturized, on-chip beamsplitters and tunable filters. Researchers use spin-charge coupling and FePS3 crystals to induce large in-plane optical anisotropy and near-unity linear dichroism in the visible–near-infrared range.

In-operando spectroscopic ellipsometry (SE) was used to measure the optical response of Bi2Se3 films grown on sapphire substrates in a molecular beam epitaxy (MBE) reactor during cool-down from a growth temperature of 225 degrees C. A temperature dependent dielectric model was refined for the topological insulator Bi2Se3 by taking SE spectra at different temperatures and fitting the amplitude ratio and phase difference of orthogonally polarized light using Bi2Se3 films with varying thickness. In-operando SE demonstrated here enabled determining the dielectric function of substrate and growing film unobscured by surface or interface reactions. Its sensitivity to sample temperature and film thickness variations allows determining growth temperature, absolute film thickness, and growth rate in real time, rendering it a reliable and universal approach for a direct comparison of growth conditions between different growth campaigns, thus offering the potential to improve reproducibility of the growth conditions for Bi2Se3 based films and heterostructures.

Using spectroscopic ellipsometry, the dielectric functions of a series of topological insulators, including Bi2Te3, Bi2Se3, and their ternary alloys, were determined. The ellipsometry measurements were obtained using an IR-spectroscopic ellipsometer, spanning a spectral range between 2000 and 35 000 nm. A standard inversion technique was used to model the ellipsometry spectra, which produced the dielectric functions of each of the topological insulator films. These dielectric functions were analyzed further to obtain characteristics such as their bandgap, carrier concentration, and effective mass. Specifically, Kramers–Kronig consistent oscillators were used to represent the Drude contribution as well as the energy gap. The authors found that the bandgap of Bi2(Te1−xSex)3 ternary alloys are larger than their binary constituents (i.e., Bi2Te3 and Bi2Se3). By exploring the temperature dependence of the bandgap, the authors deduced the electron–phonon coupling parameters for the Bi2(Te1−xSex)3 system, using the Bose–Einstein occupation distributions. Compared with Bi2Te3, the authors found that the electron–phonon coupling parameters of the ternary alloys of Bi2(Te1−xSex)3 are smaller.

This paper describes the creation of mesoporous inorganic films based on the plasma processing of ligand-capped nanocrystals. We use nanorods of HfO2 as a model system and report an extensive characterization of the chemistry, structure, mechanical properties, and reactivity to show that (i) the aspect ratio of the nanorods regulates the pore size and pore volume of the films in a predictable manner and yields an increase in porosity over spherical nanocrystals of up to 60%, (ii) the modulus (>25 GPa) and hardness (>1.1 GPa) are sufficient to tolerate chemical-mechanical planarization, and (iii) the catalytic activity can be finely controlled by the choice of ligands, which regulate the surface chemistry and water adsorption in the final product. This approach is an attractive route to create in two simple and scalable steps crack-free inorganic mesoporous films for applications in catalysis, energy storage, energy harvesting, and more.

Using a combination of reflectivity and ellipsometry, we determined the far-infrared dielectric functions of molecular beam epitaxy-grown Hg1-xCdxSe thin films between 85 cm(-1) and 8, 000 cm(-1). Spectroscopic ellipsometry, performed between 400 cm(-1) and 8000 cm(-1), recovered the dielectric function and the thickness of each film. Ellipsometry results were then used to model the reflectivity data allowing us to obtain absolute reflectance values and map the dielectric function from reflectivity between 85 cm(-1) and 8, 000 cm(-1), and to obtain the absorption due to free electrons, phonons, and band electrons. Specifically, our models find two transverse optical modes for Hg1-xCdxSe, where the HgSe-like mode blue-shifts and the CdTe-like mode red-shifts with increasing Cd concentration.