Thickness-dependent Optical Properties Research Articles

Thickness-dependent optical properties in compressively strained BiFeO3/LaAlO3 films grown by pulsed laser deposition

Bismuth ferrite (BiFeO3) films with various thicknesses were epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The X-ray diffraction and Raman scattering spectra reveal that the films were highly (1 1¯ 1) oriented with the single phase. With increasing the thickness, the compressive strain decreases and the strain ratios between the film and bulk crystal are evaluated to be 1.75, 1.57, and 1. Moreover, the compressive strain induces band gap shrinkage from 2.7 to 2.65eV, while the charge transfer transition energy increases from 3.5 to 4.1eV. It could be due to the shift of O 2p states and the variation of local Fe3+ crystal field.

Thickness-dependent optical properties and nonlinear refractive index of a-Ge–Se–In thin films

The present work reports the study of linear optical properties of a-Ge20Se80−xInx (x = 0, 5, 10, 15, 20) system as a function of film thickness and at different Se/In ratio using transmission spectra. Optical gap increases with the increase in film thickness, whereas no significant change is found in the linear refractive index for all the compositions. To evaluate their potential applications for all-optical ultrafast switching devices, we report the effect of In alloying on the nonlinear refractive index and third-order susceptibility (χ(3)), evaluated from the changes of index of refraction using Wang approximations. Comparative higher values of susceptibility of the order of 10−11 (in esu) are found and show a maximum at In = 10 at.%. The largest value of susceptibility can be expected for materials with higher values. Different formulations were used to predict the nonlinear behavior of Ge–Se–In system. The addition of In to Ge20Se80−xInx glass leads to an increase in the nonlinear refractive index, almost 500 times that of fused silica. A comparison of our results shows a good agreement with values available in the literature. High nonlinearity of Ge–Se–In glasses makes them suitable for optical generation and Raman amplification.

Thickness Dependent Optical Properties of Thermally Evaporated SnS Thin Films

Thin films of tin sulphide (SnS) were prepared on glass substrates using the thermal evaporation method and the optical dispersion parameters investigated. The results obtained on the dispersion parameters and other optical constants indicated a strong thickness dependency. The results indicated that the refractive index data obeyed the single oscillator model. The dispersion energy (Ed), optical spectra moments (M-1) and (M-3) and the dielectric constant (ɛ) were all found to decrease with increase of film thicknesses. However, the oscillator energy (Eo) and angular frequency (ωp) are independent of film thickness. These results are useful tool for further understanding the electronic structure for this novel material.

Optical metrology of Ni and NiSi thin films used in the self-aligned silicidation process

The thickness-dependent optical properties of nickel metal and nickel monosilicide (NiSi) thin films, used for self-aligned silicidation process, were characterized using spectroscopic ellipsometry. The thickness-dependent complex dielectric function of nickel metal films is shown to be correlated with the change in Drude free electron relaxation time. The change in relaxation time can be traced to the change in grain boundary (GB) reflection coefficient and grain size. A resistivity based model was used as the complementary method to the thickness-dependent optical model to trace the change in GB reflection coefficient and grain size. After silicidation, the complex dielectric function of NiSi films exhibit non-Drude behavior due to superimposition of interband absorptions arising at lower frequencies. The Optical models of the complete film stack were refined using x-ray photoelectron spectroscopy, Rutherford backscattered spectroscopy, and x-ray reflectivity (XRR).

Analysis of Thickness-Dependent Optical Properties in a One-Dimensional Superconducting Photonic Crystal

The thickness-dependent optical properties in a one-dimensional superconducting photonic crystal consisting of alternating superconductor and dielectric layers are theoretically investigated by using the transfer matrix method in a stratified structure. Based on the calculated transmittance spectrum, the photonic bandgaps, passbands, and the bandedges are analyzed as functions of the thicknesses of the constituents. It is shown that the band shift is primarily dominated by the thickness change of the dielectric layer and the band enhancement can be mainly controlled by the thickness change in the superconducting layer. By choosing a suitable thickness for the superconducting or dielectric material, the higher second and third passbands can be forced to merge as a single wider second passband.

Thickness-dependent optical properties of ZnO thin films

Spectroscopic ellipsometry was employed to study the thickness dependence of the optical properties of ZnO thin films. ZnO thin films with a nominal thickness of 100, 200, and 400 nm were deposited by filtered cathodic vacuum arc (FCVA) method. The optical band gap showed a slight blue shift with respect to the bulk value with increasingly thicker ZnO films deposited on Si, while no shift with thickness could be observed for ZnO films deposited on a SiO2/Si substrate, indicating a possible effect of the SiO2 buffer layer on the film optical properties. The Urbach tail parameter E0 increased as the film thickness is increased, indicating a decrease in the structural disorder with increasing film thickness.

Thickness-dependent optical properties of La0.7Sr0.3MnO3 thin films

We report on a systematic study of the thickness dependence of the optical properties of La0.7Sr0.3MnO3 thin films epitaxially grown on a LaAlO3 substrate. The x-ray powder-diffraction data indicate that the c-axis lattice constant is enhanced with decreasing the film thickness due to the compressive strain in the film plane produced by lattice mismatch. Magnetization curves show a decrease of the Curie temperature (TC) for decreasing thickness of films. Optical reflectance and transmittance measurements provide evidence that the position of Mn-O stretching mode shifts toward low frequency and the energy of the charge-transfer transition between O 2p and Mn 3d states increases with the decrease of film thickness. Most importantly, an analysis of the small-polaron absorption in the mid-infrared region shows that the polaron binding energy increases with decreasing the film thickness, suggesting that the strain dependence of TC mainly results from the strain-induced electron–phonon coupling.

Comment on “Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films” [J. Appl. Phys. 90, 1067 (2001)

A recent article suggested that “paracrystalline” silicon could be detected in very thin hydrogenated amorphous silicon films by using spectroscopic ellipsometry. We show that the important features of the ellipsometry data can be reproduced using a simple optical model of amorphous Si:H, with no assumptions about unusual structures or thickness-dependent optical properties.

Thickness-dependent changes in the optical properties of PPV- and PF-based polymer light emitting diodes

We explore the thickness-dependent optical properties of single layer polymer light emitting diodes for two materials, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-ethenylene-2,5-dioctyloxy-1,4-phenylene-ethenylene] (MEH-DOO-PPV) and poly(2,7-(9,9-bis(2-ethylhexyl))fluorene)-2,7-bis(4-methylphenyl)phenylamine (PF with 2% endcap). We compare experimental electroluminescence spectra and radiance values as a function of emissive layer thickness with simulations utilizing dipole methods within a transfer-matrix formalism. The technique is then extended to explore how simulated results depend on the assumed location of emission within the polymer layer. We show that thickness-dependent optical properties of these devices are dominated by interference effects.

Thickness dependent optical properties of CdI 2 films

Structural, compositional, morphological and optical studies were carried out on thermally evaporated CdI 2 films as a function of film thickness. The stoichiometric films of hexagonal structure show a wavy thickness dependence of lattice parameter c and a thickness independent a. The optical absorption data show a best fit for a direct type of inter-band transition near the absorption edge yielding a direct optical energy gap of 3.6 eV. Part of the optical data was fitted to an indirect type of transition to determine the indirect optical energy gap of about 3 eV. Both energy gaps show thickness dependences, which can be explained qualitatively by a thickness dependence of the grain size through decreasing grain boundary barrier height with grain size. Similarly, the variation of the refractive index with thickness can be explained by the variation of c with thickness.

Localized plasmon excitation in metal nanoclusters as a tool to study thickness-dependent optical properties of copper phthalocyanine ultrathin films

Thin film sandwich samples have been prepared of copper phthalocyanine ultrathin solid films with incorporated metal (silver, indium) nanoclusters, surrounded by an amorphous silicon environment. The samples were investigated by transmission electron microscopy in both lateral and cross-sectional geometries. In view of the optical properties, we observed a gradual blue wavelength shift of the localized metal cluster plasmon excitation for about 300 nm accompanying an equivalent copper phthalocyanine thickness increase from `zero' to a threshold thickness of about 4 nm. We attribute this behaviour to the formation of bulk-like optical properties of the copper phthalocyanine film, which is completed at the observed equivalent threshold thickness.