Dielectric Function Of Thin Films Research Articles

Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis

Thin films of Ni1−xZnxO with different composition (x=0, 0.01, 0.05 and 0.10) were successfully fabricated on glass substrates using spray pyrolysis technique. The as-prepared films were further annealed at 623K in room atmosphere for 2h. The evolution of their structural, morphological, electrical and optical properties with annealing temperature was investigated. The result reveals that the annealing of the films leads to improved surface morphology and better crystallinity. The prepared films displayed increase in conductivity followed by decrease in band gap with increase in doping concentration. However, the effect is more significant in case of annealed films, where the average transparency shows an increase of about 10% over the as prepared thin films. The red-shift of the optical band gap is due to the deep states in the band gap. The increase in density of states has been confirmed by variable range hoping (VRH) mechanism. The activation energy was found to be decreased when Zn concentration increased. Added to this the dielectric function of thin films and their spectral variation has been tentatively discussed.

Electronic and optical properties of titanium nitride bulk and surfaces from first principles calculations

Prediction of the frequency-dependent dielectric function of thin films poses computational challenges, and at the same time experimental characterization by spectroscopic ellipsometry remains difficult to interpret because of changes in stoichiometry and surface morphology, temperature, thickness of the film, or substrate. In this work, we report calculations for titanium nitride (TiN), a promising material for plasmonic applications because of less loss and other practical advantages compared to noble metals. We investigated structural, electronic, and optical properties of stoichiometric bulk TiN, as well as of the TiN(100), TiN(110), and TiN(111) outermost surfaces. Density functional theory (DFT) and many-body GW methods (Green's (G) function-based approximation with screened Coulomb interaction (W)) were used, ranging from G0W0, GW0 to partially self-consistent sc-GW0, as well as the GW-BSE (Bethe-Salpeter equation) and time-dependent DFT (TDDFT) methods for prediction of the optical properties. Structural parameters and the band structure for bulk TiN were shown to be consistent with previous work. Calculated dielectric functions, plasma frequencies, reflectivity, and the electron energy loss spectrum demonstrated consistency with experiment at the GW0-BSE level. Deviations from experimental data are expected due to varying experimental conditions. Comparison of our results to spectroscopic ellipsometry data for realistic nanostructures has shown that although TDDFT may provide a computationally feasible level of theory in evaluation of the dielectric function, application is subject to validation with GW-BSE calculations.

Screening, Friedel oscillations, and low-temperature conductivity in topological insulator thin films

In thin topological insulator (TI) films, the top and bottom surfaces are coupled by tunneling, which restores backscattering and strongly affects screening. We calculate the dielectric function in the random phase approximation obtaining a closed-form result. Unlike independent TI surfaces, the dielectric function of thin films exhibits a valley as a function of wave number $q$ and tunneling, as well as a cusp at $q=2{k}_{F}$, with ${k}_{F}$ the Fermi wave vector. As a result of the cusp, Friedel oscillations decay with distance $r$ as $sin(2{k}_{F}r)/{(2{k}_{F}r)}^{2}$. We determine the longitudinal conductivity $\ensuremath{\sigma}$ in the first Born approximation at low temperatures where screened impurities provide the dominant scattering mechanism. At high electron densities ${n}_{e}$, $\ensuremath{\sigma}\ensuremath{\propto}{n}_{e}$, while at low densities $\ensuremath{\sigma}\ensuremath{\propto}{n}_{e}^{3/2}$.

Thin films with high surface roughness: thickness and dielectric function analysis using spectroscopic ellipsometry

An optical surface roughness model is presented, which allows a reliable determination of the dielectric function of thin films with high surface roughnesses of more than 10 nm peak to valley distance by means of spectroscopic ellipsometry. Starting from histogram evaluation of atomic force microscopy (AFM) topography measurements a specific roughness layer (RL) model was developed for an organic thin film grown in vacuum which is well suited as an example. Theoretical description based on counting statistics allows generalizing the RL model developed to be used for all non-conducting materials. Finally, a direct input of root mean square (RMS) values found by AFM measurements into the proposed model is presented, which is important for complex ellipsometric evaluation models where a reduction of the amount of unknown parameters can be crucial. Exemplarily, the evaluation of a N,N’-dimethoxyethyl-3,4,9,10-perylene-tetracarboxylic-diimide (DiMethoxyethyl-PTCDI) film is presented, which exhibits a very high surface roughness, i.e. showing no homogeneous film at all.

Comprehensive Environmental Testing of Optical Properties in Thin Films

Environmental characterization of optical and structural properties of thin films continues to be a challenging task. To understand the failure mechanism in high temperature thin film applications, it is crucial to understand how material properties change with temperature. An accurate knowledge of the variation of the dielectric function of thin films and its relation to compositional and microstructural changes could help to prevent failures. This article presents an environmental in-situ characterization methodology that combines the study of the optical constants in an environmental chamber by spectroscopic ellipsometry, with compositional depth profile analysis using ion beam analysis techniques and a structure analysis by Raman spectroscopy. The main novelty of this methodology is that all analytical techniques are carried out sequentially in a multi-chamber cluster tool without sample exposure to undefined atmospheres. Carbon-titanium metal thin film had been studied following the described characterization methodology.

Pulsed laser deposition with simultaneous in situ real-time monitoring of optical spectroscopic ellipsometry and reflection high-energy electron diffraction

We present a pulsed laser deposition system that can monitor growth by simultaneously using in situ optical spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The RHEED precisely monitors the number of thin-film layers and surface structure during the deposition, and the SE measures the optical spectra of the samples simultaneously. The thin-film thickness information obtained from RHEED facilitates the SE modeling process, which allows extracting the in situ optical spectra, i.e., the dielectric functions of thin-films during growth. The in situ dielectric functions contain indispensable information about the electronic structure of thin-films. We demonstrate the performance of this system by growing LaMnO3+δ (LMO) thin-films on SrTiO3 (001) substrates. By using in situ SE and RHEED simultaneously, we show that real-time thickness and dielectric functions of the LMO thin-films can be effectively extracted. The simultaneous monitoring of both optical SE and RHEED offers important clues to understand the growth mechanism of atomic-scale thin-films.

The Dielectric Function of Silver Nanoparticle Langmuir Monolayers Compressed through the Metal Insulator Transition

Algorithms and experimental techniques for extracting the complex dielectric function of thin films (monolayers and bilayers) of quantum dots are developed. The algorithms are based on a combination of the so-called Newton−Raphson method, used in conjunction with a Kramers−Kronig analysis, and are used to analyze normal-incidence reflectance and transmission measurements of organically passivated silver quantum dot Langmuir monolayers. Single, unambiguous solutions to the dielectric function were determined for several particle monolayers and at various stages of compression. A transition of the quantum dot superlattice from the insulating to a metallic state was observed. When a monolayer of 6-nm-diameter Ag nanocrystals is compressed to an interparticle separation distance of ∼8−9 Å, a negative-valued featureless component of the real part of the dielectric function is detected. This indicates the onset of Drude-like behavior that is characteristic of a metallic film. The impact of superlattice disorder on this Drude response is also investigated.

Structure investigation of BN films grown by ion-beam-assisted deposition by means of polarised IR and Raman spectroscopy

We present an optical investigation, by means of polarised infrared (IR) spectroscopy and Raman scattering, of the microstructure and crystallinity of mixed films of hexagonal and cubic boron nitride (h-BN and c-BN, respectively). The films were deposited on an unheated silicon substrate by the ion-beam-assisted deposition method (IBAD) at low energy (400–500 eV). The deposition temperature, due to the ion bombardment, was in the range 200–250°C at the end of the deposition process. Different film types were grown on a silicon substrate of dimensions 75 mm×15 mm by changing the ion (nitrogen+argon) to atom (thermal boron) arrival ratio, ϕion/ϕB, in the range 0.69–3. Polarised IR reflectivity (PIRR) spectra were acquired at different positions on the BN film (different arrival ratios ϕion/ϕB) and show an important upwards shift of transverse optical (TO) and longitudinal optical (LO) phonons of the twofold degenerated mode E1u of the sp2 phase at the transition zone from sp2 to sp3 phases. Several processes can shift the IR phonon peaks, including the degree of crystallinity, film thickness, film stoichiometry and intrinsic stress. The micro-Raman results and the full-width at half-maximum values of TO phonons of the E1u mode show that the BN film has a similar crystallinity in all regions. The effect of the film thickness was shown by using a microstructure-dependent model for the IR anisotropic effective dielectric function of thin films. In order to show the influence of the film stoichiometry in the E1u(TO) peak positions, a series of samples was deposited at 100% of nitrogen by changing the arrival flux ϕN/ϕB in the interval 0.75–2.5. It have been observed that, in this range of flux ratio, the E1u(TO) phonon shift is negligible in comparison with the shift observed in the PIRR measurements. This results suggest that this E1u(TO) phonon shift is due to the intrinsic stress. If we consider the findings of Friedmann et al. [J. Appl. Phys. 76 (1994) 3088], who suggested that nucleation of the cubic phase occurs as a result of extremely high stress, and those of Medlin et al. [J. Appl. Phys. 70 (1996) 3567], who showed a direct route to c-BN formation via a transformation of h-BN into rhombohedral BN (r-BN), we can conclude that, in our case, the shift observed is due to an intrinsic high-stress phase. Then, a structural modification of h-BN into r-BN phase might be involved as a precursor for nucleation of the cubic phase.

Modelling the dielectric function of thin films measured by spectroscopic ellipsometry: determination of microstructure and density

The dielectric function of thin-film amorphous hydrogenated silicon (a-Si:H) deposited by the glow discharge and hot wire technique has been investigated by spectroscopic ellipsometry. An improved interpretation of the ellipsometric data taking into account the influence of hydrogen incorporation into the amorphous network enables to determine the film density and the void volume fraction of the material. Thus ellipsometry provides a convenient and contactless means of characterizing the structural properties of thin films. The film density varies considerably with substrate temperature and hydrogen content depending on the individual deposition technique. A reduction of film density is mainly caused by the formation of voids in the amorphous network. The influence of the substrate temperature on the growth of dense a-Si: H films is pointed out.

Line-shape analysis of ellipsometric spectra on thin conducting polymer films

Optical properties in terms of the complex dielectric function of thin films of poly(3-hexylthiophenes) on gold substrates have been measured with spectroscopic ellipsometry in the photon energy range 1.5–3.5 eV. With line-shape analysis using a modified Lorentzian model, the broad absorption band around 2.25 eV can be resolved in three resonances, and the changes in the line-shape parameters due to degradation, heat treatment and the dependence of film thickness were studied. Degradation is observed as a loss of total oscillator strength and broadening of the central resonance in the absorption band, while heat treatment mainly results in a redistribution of oscillator strengths. The energy difference between the resonances were found to be constant when the absorption coefficient is analysed, but not if the dielectric function is analysed. As the latter quantity is more directly related to the density of states distribution, this indicates that the previously suggested vibronic model for the absorption band is incomplete.

7218. The deposition of diamond films by filament techniques: F Jansen et al,J Vac Sci Technol, A8, 1990, 2785–3790

We present an optical investigation, by means of polarised infrared (IR) spectroscopy and Raman scattering, of the microstructure and crystallinity of mixed films of hexagonal and cubic boron nitride (h-BN and c-BN, respectively). The films were deposited on an unheated silicon substrate by the ion-beam-assisted deposition method (IBAD) at low energy (400–500 eV). The deposition temperature, due to the ion bombardment, was in the range 200–250°C at the end of the deposition process. Different film types were grown on a silicon substrate of dimensions 75 mm×15 mm by changing the ion (nitrogen+argon) to atom (thermal boron) arrival ratio, ϕion/ϕB, in the range 0.69–3. Polarised IR reflectivity (PIRR) spectra were acquired at different positions on the BN film (different arrival ratios ϕion/ϕB) and show an important upwards shift of transverse optical (TO) and longitudinal optical (LO) phonons of the twofold degenerated mode E1u of the sp2 phase at the transition zone from sp2 to sp3 phases. Several processes can shift the IR phonon peaks, including the degree of crystallinity, film thickness, film stoichiometry and intrinsic stress. The micro-Raman results and the full-width at half-maximum values of TO phonons of the E1u mode show that the BN film has a similar crystallinity in all regions. The effect of the film thickness was shown by using a microstructure-dependent model for the IR anisotropic effective dielectric function of thin films. In order to show the influence of the film stoichiometry in the E1u(TO) peak positions, a series of samples was deposited at 100% of nitrogen by changing the arrival flux ϕN/ϕB in the interval 0.75–2.5. It have been observed that, in this range of flux ratio, the E1u(TO) phonon shift is negligible in comparison with the shift observed in the PIRR measurements. This results suggest that this E1u(TO) phonon shift is due to the intrinsic stress. If we consider the findings of Friedmann et al. [J. Appl. Phys. 76 (1994) 3088], who suggested that nucleation of the cubic phase occurs as a result of extremely high stress, and those of Medlin et al. [J. Appl. Phys. 70 (1996) 3567], who showed a direct route to c-BN formation via a transformation of h-BN into rhombohedral BN (r-BN), we can conclude that, in our case, the shift observed is due to an intrinsic high-stress phase. Then, a structural modification of h-BN into r-BN phase might be involved as a precursor for nucleation of the cubic phase.

IR‐Spectroscopic Data of Thin Insulating Films on Semiconductors. New Methods of Interpretation and Analysis

AbstractOn the basis of the Fresnel theory special IR optical equations for the description of the transmission (T), reflection (R), and attenuated total reflection (ATR) of thin films on semiconductors are derived. It is shown that these relations (quadratic approximations of film thickness) lead to a theoretical explanation for the detection and identification of transverse and longitudinal phonon‐polaritons by means of oblique incidence of polarized light realized in different experimental modes (T, R, ATR). A comparison of calculated model spectra of SiO2 with corresponding experimental spectra shows good quantitative agreement. Furthermore, in this framework a very effective method of determination of the dielectric functions of thin films from their IR spectra is developed and applied to an Al2O3‐film on silicon.

Thin films of poly(3-hexylthiophene) studied with spectroscopic ellipsometry

We have used spectroscopic ellipsometry in the photon energy range 1.5–2.5 eV to determine the dielectric function of thin films (3–30nm) of undoped poly(3-hexylthiophene) prepared with a spinning technique on gold substrates. We have observed characteristic absorption peaks in the energy range 2–2.5 eV. By means of second derivative lineshape fitting, we demonstrate that it is possible to quantitatively extract values on model parameters like peak energies, linewidths and oscillator strengths. We also show how light-induced degradation changes the polymer as reflected in the dielectric response of the polymer.

Optical Properties of Thin Layers of Bovine Serum Albumin, γ-Globulin, and Hemoglobin*

We have used spectroscopic ellipsometry to determine the dielectric functions of thin films of γ-globulin, bovine serum albumin, and hemoglobin in the visible and near-uv photon energy range. We show that both thickness and dielectric function can be resolved for monomolecular films adsorbed on substrates with relatively low polarizability. The data which we consider to be closest to the intrinsic dielectric response of the protein films were obtained on HgTe and HgCdTe substrates. Less resolution was obtained on silicon substrates. For a density-deficient film, we were able to model the dielectric response with effective medium theories, and the void fraction could be determined.

A method of measuring the dielectric functions of thin films by the excitation of SPW in multi-layer system

A method of measuring the dielectric functions of thin films by the excitation of SPW in multi-layer system

A method of measuring the dielectric functions of thin films by the excitation of SPW in multi-layer system

The attenuated-total-reflection method (ATR) is used to measure the dielectric constant of a thin layer in a prism-metal-dielectric-air system. The addition of the dielectric layer on the metal produces a shift in the dispersion curve and a change in the intensity of the ATR signals compared with the results for the three-layer prism-metal-air system. Both the shift of the dispersion curve and the intensity ratio are found to be linear functions of the thickness of the added layer. The ATR method, therefore, is considered to be a simple and accurate measuring technique of the dielectric functions.

Unambiguous determination of thickness and dielectric function of thin films by spectroscopic ellipsometry

We present a spectroscopic method of determining both thickness and dielectric functions of thin films on previously characterized substrates. The method requires that spectroscopic ellipsometric data be taken over an energy range where the substrate has a sharp optical structure, e.g. a critical point. These data are used to determine a “pseudodielectric” function for the film, which necessarily depends on the assumed film thickness. We develop a general first-order theory which shows that the correct thickness and dielectric function of the film are those for which the substrate feature vanishes in the calculated pseudodielectric function. The theory is also used to investigate sensitivity and applicability. We show that sensitivity can be enhanced by differentiating the pseudodielectric function with respect to energy. The method is demonstrated with several numerical and experimental examples, which illustrate that monolayer thickness accuracy can be achieved.