Combination Of Spectroscopic Ellipsometry Research Articles

Optical properties and London dispersion interaction of amorphous and crystallineSiO2determined by vacuum ultraviolet spectroscopy and spectroscopic ellipsometry

The interband optical properties of crystalline (quartz) and amorphous ${\mathrm{SiO}}_{2}$ in the vacuum ultraviolet (VUV) region have been investigated using combined spectroscopic ellipsometry and VUV spectroscopy. Over the range of 1.5--42 eV the optical properties exhibit similar exciton and interband transitions, with crystalline ${\mathrm{SiO}}_{2}$ exhibiting larger transition strengths and index of refraction. Crystalline ${\mathrm{SiO}}_{2}$ has more sharp features in the interband transition strength spectrum than amorphous ${\mathrm{SiO}}_{2}$, the energy of the absorption edge for crystalline ${\mathrm{SiO}}_{2}$ is about 1 eV higher than that for amorphous ${\mathrm{SiO}}_{2}$, and the direct band-gap energies for $X$-cut and $Z$-cut quartz are 8.30 and 8.29 eV within the absorption coefficient range $2\text{--}20\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In crystalline ${\mathrm{SiO}}_{2}$ we report different interband transition peaks at 16.2, 20.1, 21, 22.6, and 27.5 eV, which are in addition to those lower energy transitions previously reported at 10.4, 11.6, 14, and 17.1 eV. We find the bulk plasmon energy in $X$- and $Z$-cut crystalline quartz and amorphous ${\mathrm{SiO}}_{2}$ to be at 24.6, 25.2, and 23.7 eV, respectively. The oscillator strength $(f)$ sum rules of the interband transitions for crystalline ${\mathrm{SiO}}_{2}$ is 10--10.8 electrons per formula unit for transition energies up to 45 eV. These differences in the electronic structure and optical properties, and the physical densities of crystalline and amorphous ${\mathrm{SiO}}_{2}$, can be attributed to differences in the intermediate-range order (IRO) and long-range order (LRO) of the different forms of ${\mathrm{SiO}}_{2}$. The intimate relationship between the electronic structure and optical properties and the London dispersion interaction has attracted increased interest recently, and the role of amorphous silica and other structural glass formers as a fluid in high-temperature wetting and materials processes means a detailed knowledge of the optical properties and London dispersion interaction in ${\mathrm{SiO}}_{2}$ is important. Hamaker constants for the London dispersion interaction of the configuration of two layers of $c\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$ or $a\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$ separated by an interlayer film have been determined, using full spectral methods, from the interband transition strength. The London dispersion interaction is appreciably larger in $c\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$ than $a\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$ due to the increased physical density, index of refraction, transition strengths, and oscillator strengths in quartz.

A-Si/SiO x Bragg-reflectors on micro-structured InP

a-Si/SiOx Bragg-reflectors for the wavelength region from 500 nm to 830 nm with a low number of pairs were grown on cylindrical half-pipes on InP substrates using plasma-enhanced chemical vapor deposition with regard to applications for the confinement of the optical modes in micro-resonators. The optical properties of the Bragg-reflectors were determined by the new method of detection-focal spatially resolved spectroscopic ellipsometry in combination with the confocal micro-reflection technique.

Hydrogen-plasma etching of hydrogenated amorphous silicon: a study by a combination of spectroscopic ellipsometry and trap-limited diffusion model

The kinetics of etching hydrogenated amorphous silicon by a hydrogen plasma has been studied by in-situ spectroscopic ellipsometry measurements. The formation of a hydrogen-rich sublayer is clearly emphasized. Its thickness increases from 7 to 27 nm when the temperature during the hydrogen-plasma treatment is raised from 100 to 250°C. This effect is interpreted by solving the differential equation for trap-limited hydrogen diffusion through a mobile surface. By assigning the thickness of this sublayer to the mean diffusion distance of hydrogen we determined values of the effective diffusion coefficient of hydrogen higher than 10−14 cm2s−1 with an activation energy of 0.22 eV. The density of hydrogen traps is found to decrease from 7.3 × 1018 to 4.5 × 1017 cm−3 as the temperature of the hydrogen treatment increases from 100 to 250°C with an activation energy of 0.43 eV. This effect is interpreted by a thermal equilibrium involving hydrogen transitions between shallow states and hydrogen-trapping sites.

Real time spectroscopic ellipsometric analysis of Ge film growth on Si(001) substrates

The early stage of the growth of Ge planar films on a Si(001) surface is investigated in real-time by using the spectroscopic ellipsometric technique. We demonstrate the possibility to measure the deposition rate and follow its evolution during the growth. The use of spectroscopic ellipsometry in combination with atomic force microscopy and X-ray photoelectron spectroscopy demonstrates that the deposition rate evolution is related to the stress status of the growing film through the variation of its complex dielectric constant.

Tetra point wetting of liquid K–KCl mixtures: Spectroscopic characterization of mesoscopic wetting and prewetting films

The wetting and prewetting transitions at the metal-rich K–KCl melt–sapphire interface have been investigated by spectroscopic ellipsometry in combination with normal incidence reflectivity in the spectral range 0.8⩽ℏω⩽2.2 eV at temperatures up to 730 °C. Along the coexistence curve a salt-rich liquid wetting film is observed which is identified by the spectral features of the liquid state F-center. Unusually thick wetting films are found ranging from 30 nm near 540 °C to 300 nm approaching the monotectic temperature of 751 °C. Their composition has been determined from the absorption coefficients of the F-center band and it corresponds to about 90 mole % salt. At conditions off coexistence and near the prewetting line, similar mesoscopically thick wetting films exist. Crossing the prewetting line towards metal-rich solutions, the optical properties at the interface agree with those of the nearly free electron metal. The high thickness of the prewetting films is qualitatively explainable by charging and double layer formation at the interface. The occurrence of liquid F-center-like states up to 200 K below the monotectic temperature gives evidence of a strong undercooling of the wetting films with respect to the bulk phase. These characteristics of the wetting transition in a metal–molten salt solution can be described by the tetra point wetting scenario for binary fluid mixtures.

Real-time optical characterization of heteroepitaxy by organometallic chemical vapor deposition

Heteroepitaxy of GaP on Si(100) and GaAs(100) is investigated under organometallic chemical vapor deposition conditions using combined spectroscopic ellipsometry (SE) and non-normal-incidence reflectance-difference (-anisotropy) spectroscopy. Real-time monitoring greatly assists in identifying optimum starting surfaces for heteroepitaxy since prolonged exposure to PH3 results in roughening of Si(100) and GaAs(100) surfaces, in agreement with previous work. Real-time SE data of GaP on Si indicate that under our conditions GaP and Si interpenetrate as optically identifiable materials over the first 75 Å, suggesting that either trimethylgallium or a reaction by-product can act as a catalyst for the formation of Si{111} facets.

In situ monitoring of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy

Comprehensive characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) requires a combination of thin-film, near-surface-, and surface-sensitive techniques to determine layer thicknesses and compositions, composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained non-destructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Here we describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated into a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously with a single optical path. Data are obtained in parallel from 240 to 840 nm with a high-speed 16-bit photodiode array (PDA) at a repetition rate greater than 2 Hz and a precision of ±0.0001. We provide examples of its use, and show in particular that GaP intermixes with Si during the initial stages of heteroepitaxy. Capabilities of the presented configuration and its potential for future investigations are discussed.

Morphological properties of porous-Si layers for n +-emitter applications

Porous silicon layers prepared on n + emitters are investigated by a combination of spectroscopic ellipsometry, transmission electron microscopy and sheet resistance measurements. The porous silicon is formed by an anodic surface treatment of uniformly doped emitters prepared by chemical vapour deposition and of diffused emitters. The results show a good correspondence between the thicknesses obtained with the different techniques. The porosity of the layers increases for increasing doping level and saturates at a doping above ∼5×10 19/cm 3. The formation rate shows an inverse doping dependence. The top surface layer of the porous silicon is in all cases more dense than the bulk, which is related to an initially faster formation rate.

The Reaction of Clean Li Surfaces with Small Molecules in Ultrahigh Vacuum: II. Water

Reactions at the Li/ interface were studied at 160 to 290 K in ultrahigh vacuum by a combination of spectroscopic ellipsometry and Auger electron spectroscopy. Ice multilayers, ca. 100 ML thick, were deposited on clean Li at 160 K. The evaporation rate of water at 160 K is sufficiently slow that the ice layer remains on the surface for about 1 h. After 10 min at 160 k, a pure LiOH layer ca. 70 Aå thick is produced, and after 1 h there is evidence of a slow conversion of LiOH to in the layer, probably at the Li/LiOH interface. Raising the temperature to 240 K results in desorption of the adsorbed water and conversion of all the LiOH to a porous (60% void) layer composed mostly of (35%) with some metallic Li mixed in. Raising the temperature further to 290 K results in densification of the layer by both collapse of the voids and by diffusion of Li into the interstices of the , increasing the Li content to 27% and shrinking the film thickness to 26 Aå. Based on these results, a model for the behavior of small amounts of water in Li battery electrolyte is presented.

Application of diamond-like carbon films as hermetic coatings on optical fibres

The application of diamond-like carbon (DLC) films as hermetic coatings on optical fibres was investigated. Films with refractive indices in the range 1.7-2.0 (at λ = 675 nm) were deposited at pressures up to 5 Torr. The hermeticity of the coatings was determined using water penetration studies monitored using a combination of spectroscopic ellipsometry and atomic force microscopy. The resistance to moisture penetration was found to be better for higher refractive index films. The coatings were applied to optical fibres in a continuous in-line system as the fibre was drawn. Coatings of thicknesses in the range of 10–30 nm were achieved on quartz fibres with outer diameters of 200–1000 μm at fibre drawing speeds of up to 3 m min−1.

Modulated ellipsometric measurements and transfer-matrix calculation of the field-dependent dielectric function of a multiple quantum well.

We perform a complete nondestructive optical characterization of a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-well (MQW) sample. Modulated ellipsometry, a combination of spectroscopic ellipsometry and photomodulation spectroscopy, is used as an experimental technique. The electric field is modulated via photoexcitation of electron-hole pairs in the p-i-n structure. The field-induced modification in the dielectric function of the multiple quantum well is derived in the spectral range from the GaAs to the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As band gap. We show that the modulated ellipsometry technique is particularly well suited to characterize higher quantized transitions. The experimental results are compared to an extensive theoretical model of the MQW dielectric function, based on a transfer-matrix solution of the Schr\"odinger equation. The effective-mass dispersion of electrons and light holes are treated by the Kane model, and the excitonic transitions are introduced by oscillator functions. A detailed comparison of the experimental spectra and this theory allows a very precise determination of the sample parameters, such as well thickness, barrier thickness, and aluminum content.

Vacuum-ultraviolet radiation damage of the KCl surface—application of combined spectroscopic ellipsometry and reflectometry

A spectroscopic study (1.7 to 4.0 eV) of the surface damage layer, formed when KCl is exposed to vacuum-ultraviolet radiation, is carried out using ellipsometry and reflectometry. The results indicate the formation of a thin layer (~30–300 Å) with a high density of F-centers (~3 × 10 19 cm −3) and of aggregate centers near the surface. The F-center absorption band in the damage layer appears at 0.04 ± 0.02 eV to lower energy and with 30% larger width, relative to the bulk F-band. An approximate theoretical description of the defect-surface interaction indicates that this shift and broadening is of the correct magnitude and direction to be the result of such an interaction. However, the possibility that the apparent shift may also result from the presence of unresolved aggregate absorptions cannot be ruled out. It is also shown that a thin exponentially inhomogeneous film, such as that assumed to result from photochemical coloration of KCl, can be approximated as an “equivalent homogeneous film” for the purpose of analysis of ellipsometric data.