Papers by Keyword: Spectroscopic Ellipsometry (SE)

Abstract: Features of the optical properties of nanostructured samples of cupric oxide CuO prepared by shock wave loading (SWL) and by high pressure torsion (HPT) methods have been investigated by means of spectroscopic ellipsometry in the range of 0.5-5.0 eV. The results for nanostructured samples are discussed in comparison with the measurement data of the CuO single crystal. Dispersion of the components of the complex permittivity in nanostructured CuO samples is significantly different from that for the single crystal. The optical spectral density of the nanostructured CuO is redistributed from the energy region above the fundamental absorption edge to the region below the edge. The shift is unexpected strong for samples prepared by the HPT method. The intensity of the optical spectra is suppressed in the range above the fundamental absorption edge and increases in low-energy one. It has been shown that the energy positions of the main electronic transitions in nanostructured samples on the whole are the same as in the single crystal; at the same time, intensity of the low-energy transitions increases. The possible causes of this increase and resolution of the fine absorption structure of the nanostructured CuO are discussed.

Abstract: Nitride materials are critical for a range of applications, including UV-visible light emitting diodes (LEDs). Advancing the performance, reliability and synthesis of AlGaN/GaN and InGaN/GaN heterojunction devices requires a systematic methodology enabling characterization of key metric like alloy composition, thickness and quality possibly in real time. This contribution reports on the real time characterization of the plasma assisted molecular beam epitaxy of AlGaN/GaN and InGaN/GaN heterostructures. Spectroscopic ellipsometry real time monitoring has revealed a number of key process and material iusses, such as the roughening of the GaN templates depending on plasma exposure during the substrate cleaning step, the composition of the alloy and the growth mode. Parameters like the plasma conditions, the surface temperature and the atomic flow ratio are investigated to understand the interplay process-material composition-structure-optical properties.

Abstract: GeC thin films have been prepared by reactive pulsed laser ablation technique. Methane pressure (PCH4) was varied from 0 to 75 milli torrs (mT). Optical analysis of all the samples was performed by spectroscopic ellipsometry (SE). The optical constants i.e. refractive index (n), extinction coefficient (k), absorption coefficient (α) and thickness of deposited film(s) were obtained by modeling and simulations of ellipsometric data. It was found that deposition parameter (change in pressure of methane) has a profound effect on the properties of the deposited films. To support our results of optical analysis, other important diagnostic techniques like atomic force microscopy (AFM), Fourier Transform Infrared Spectroscopy (FTIR) etc. were employed.

Abstract: Studies of oblique deposited nanostructured SiOx films by ellipsometric and IR spectroscopies are presented, as additional information is obtained by scanning electron microscopy (SEM). The films were deposited onto Si substrates under a 75° incidence vapor by vacuum evaporation of SiO and were annealed in Ar at 950oC. The thickness and composition of the films were estimated from the ellipsometric data analysis applying the Bruggeman effective approximation theory. Three-layer optical model described satisfactory the annealed film structure and verified the formation of nanocrystalline Si clusters. SEM micrographs showed that evaporated films consisted of silicon oxide pillars separated by air space and tilted at an angle of ~57o to Si substrate surface. The thin silica pillars were most probably free from Si-SiO2 interface leading to absence of strong absorption on LO vibrations in the IR spectrum. The estimated porosity factor was ~62 %. By annealing, the film oxidized to SiO2 but it remained columnar with a porosity factor of ~47 %.

Abstract: Thin SiOx films deposited by reactive r.f. magnetron sputtering of Si at partial pressure ratios R between oxygen and argon in the range 15%-0.03% are studied. X-ray photoelectron spectroscopy and Variable angle spectroscopic ellipsometry prove enrichment with Si of the layers deposited at R < 0.5 %. Ellipsometric data give information about the refractive index and extinction coefficient of the films. Atomic Force Microscopy results show that for all samples high temperature annealing at 1000oC leads to a decrease of the surface roughness.

Abstract: Ultra-thin Al films were deposited with different deposition times on silicon wafer and copper grid by dc magnetron sputtering. The sputtering power of 200 watt and Ar flow rate of 20 sccm were used to prepare the films. The deposition times were 40, 120 and 240 second, respectively. The deposited Al films were, then, left in the air under the humidity of 60% for 20 days. The crystal structure of ultra-thin Al films deposited on silicon wafer and copper grid were investigated by glazing x-ray diffraction (GXRD) and transmission electron microscopy (TEM), respectively. The XRD results show that after the ultra thin Al films were exposed to the air, the Al was oxidized and the Al2O3 was formed at room temperature. In addition, Al deposited for 120 and 240 second can form polycrystalline of -Al2O3 with preferred orientations of (110) and (311) planes. The TEM images show that the particle size of -Al2O3 was about 8.5 nm for deposited time of 120 second. Moreover, the spectroscopic ellipsometry (SE) data and simulation model of Bruggemann effective medium approximation (BEMA) was used to determine the volume fraction of Al2O3.

Abstract: We have characterized 4H-SiC–oxide interfaces fabricated by thermal oxidation of SiC using spectroscopic ellipsometry in the wide spectral range from visible to deep UV region. It was found that there exists an interface layer, around 1 nm in thickness, regardless of the oxide thickness from 15 nm to 40 nm. The optical constants of the interface layer have similar spectral dependence to those of SiC, though the absolute value of the refractive indices is 0.5–1 larger than that of SiC. We have discussed the structure of the interface layer based on the oxidation mechanism of SiC, like the Si-emission model.

Abstract: This work is to study the diffusion of Al into amorphous silicon (a-Si) thin film at the elevated temperature by in-situ Spectroscopic Ellipsometry (SE). The sputtered a-Si film 60 nm thick on an optically opaque Al (100 nm) layer on silicon wafer was heated in a temperature controlled heating sample stage from room temperature to 300°C and slowly cooled down to room temperature while the dynamic SE data were measured. It was found that the ∆ and Ψ spectra began to change quickly at 200°C until the temperature reached 250°C, then continue to changed very slowly until 300°C. No significant change could be observed while the sample was cooling down to room temperature. The full spectral SE measurements were also taken at every 50°C steps and used to model the diffusion of Al into the top a-Si film. The interface layer due to diffusion was modeled by Bruggeman Effective Medium Approximation (EMA) theory as the mixture of Al and Si.

Abstract: The titanium anodizations in 0.1 M sulfuric acid solution at different treated potentials and the effects of UV-irradiation on the anodized films were investigated by spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The results showed that the thickness of anodized films decreased in air within 280 min over 500 mV vs. Ag/AgCl. The features of parameters tanΨ and cosΔ in SE were in accord with the polarization curve and the thickness of anodized films, respectively. UV-irradiation could greatly decrease the thickness of the anodized films and provably assisted the reconstruction of anodized titanium films from an amorphous and porous structure to a compact structure. Regardless of UV-irradiation, the cosΔ and tanΨ were still in good agreements with the thickness of anodized films and the polarization curve, respectively. It was also found that UV-irradiation could decrease the surface roughness and make a wider atomically flat terrace on the anodized titanium surface.

Abstract: Reflectometry technique has been successfully applied to investigate the correlation between the porosity and optical property (refractive index) of the ordered mesoporous thin film deposited on silicon wafer substrates. The measured optical spectra were simulated by the Effective Medium approximation model. The reflectometry technique has been found to be appropriate for the measurement of thickness of thin films as well as thick layer films. The mesoporous silica films prepared from tri-block copolymer (F-127) as a surfactant and polypropylene oxide as a swelling agent were subsequently exposed to the ammonia vapors to enhance thermal stability and shrinkage minimization of the film that results in increased film thickness.