Showing papers by "Ludvik Martinu published in 2005"

Correlation between the sp2-phase nanostructure and the physical properties of unhydrogenated carbon nitride

Abstract: This paper consists of an investigation of the structural arrangement of the sp2 phase in amorphous unhydrogenated carbon nitride (a‐CNx) films and its effect on their physical properties. The a‐CNx films (0.16 1020∕cm3) and the EPR linewidth (of a few gausses) along with a decrease in nitrogen content. Visible Raman measurements indicate that these effects are accompanied by an increase in the degree of disorder of the sp2 phase, as inferred from the broadening and downshift of the G Raman band, and a reduction of the CN triple bond signal. The a...

Optical properties and microstructure of plasma deposited Ta2O5 and Nb2O5 films

Abstract: Advanced optical filter applications require an appropriate control of the optical constants, as well as of other suitable film properties such as mechanical performance, thermal and environmental stability, absence of refractive index inhomogeneities, and others. In the present work we studied the characteristics of two high index optical materials, namely amorphous tantalum pentoxide (Ta2O5) and niobium pentoxide (Nb2O5) prepared by plasma enhanced chemical vapor deposition, using penta-ethoxy tantalum Ta(OC2H5)5 and penta-ethoxy niobium, Nb(OC2H5)5, precursors. We particularly investigated the effect of energetic conditions on the film growth by using different modes of plasma excitation, namely rf, microwave, and dual-mode microwave/radio frequency discharges. Under sufficient ion bombardment, controlled by the rf-induced negative substrate bias, the dense Ta2O5 and Nb2O5 films exhibited a refractive index of 2.16 and 2.26 (at 550nm), respectively, while the extinction coefficient was below 10−5, as d...

Growth of vacuum evaporated ultraporous silicon studied with spectroscopic ellipsometry and scanning electron microscopy

Abstract: Using a combination of variable-angle spectroscopic ellipsometry and scanning electron microscopy, we investigated the scaling behavior of uniaxially anisotropic, ultraporous silicon manufactured with glancing angle deposition. We found that both the diameter of the nanocolumns and the spacing between them increase with film thickness according to a power-law relationship consistent with self-affine fractal growth. An ellipsometric model is proposed to fit the optical properties of the anisotropic silicon films employing an effective medium approximation mixture of Tauc-Lorentz oscillator and void. This study shows that the optical response of silicon films made at glancing incidence differs significantly from that of amorphous silicon prepared by other methods due to highly oriented nanocolumn formation and power-law scaling.

Influence of the microstructure on the optical characteristics of SrTiO3 thin films

Abstract: The influence of the microstructure of strontium-titanate-oxide (SrTiO3 or STO) thin films on their optical properties was investigated through an extensive characterization. The STO films have been deposited on silicon substrates by reactive pulsed laser deposition. The effect of the oxygen deposition pressure on the crystalline quality of the films was systematically studied by x-ray diffraction and scanning electron microscopy. Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and secondary ion mass spectrometry were used to determine the atomic density and depth concentration profiles of the various species forming the film. The refractive index and extinction coefficient were obtained using variable angle spectroscopic ellipsometry. Based on this full characterization, it is demonstrated that the optical characteristics of the films are directly correlated to their microstructural properties. In particular, the refractive index increases with film density, while losses decrease. In addition, the interface between STO and Si is characterized by an interdiffusion layer. As the deposition pressure is enhanced, the width of this layer significantly increases, inducing localized inhomogeneity of the refractive index.