C. Pickering

Atmospheric impregnation of porous silicon at room temperature

Microporous and mesoporous Si layers contain a very large surface area that affects both their optical and electrical properties. Secondary ion mass spectroscopy (SIMS) analysis is used for the first time to simultaneously monitor all the major impurities on that surface. SIMS data on a microporous layer demonstrate that its chemical composition changes dramatically with time during ambient air exposure. Similar trends are observed for mesoporous layers. Extended storage in air at room temperature converts the hydride surface of freshly anodized layers to that of a contaminated native oxide. Characterization techniques need to take the metastability of the hydride surface into account since the structural, optical, and electrical properties of porous Si can consequently change with time upon exposure to ambient air. Low‐temperature photoluminescence and spectroscopic ellipsometry data on freshly anodized and ‘‘aged’’ microporous and mesoporous layers are chosen to illustrate typical changes in optical pro...

Optical properties of porous silicon films

Abstract The optical constants of porous silicon layers formed in wafers with resistivities of 0.01–25 Omega cm and densities of 20%–75% were determined by ellipsometry in the wavelength range 0.28–0.7 microm and by reflectance and transmission from 0.6 to 2.5 microm. The IR refractive index was found to decrease as the density was reduced and an effective medium model was used to obtain density values from the optical data. Good agreement was obtained with gravimetric densities for 0.01 Omega cm samples on the assumption of a mixture of crystalline silicon and air, but material of higher resistivity showed apparently reduced optical densities because of the presence of some oxidized silicon. The optical measurements indicate that the material is a chemical mixture (SiO x ) at high oxygen concentrations. e 1 and e 2 spectra showed that 0.01 Omega cm samples had better crystallinity than higher resistivity samples and this was confirmed by the observation of amorphous-like bands in photoluminescence spectra of the latter material.

Spectroscopic ellipsometry characterization of strained and relaxed Si1-xGex epitaxial layers

Spectroscopic ellipsometry has been used to study thick, relaxed and thin, strained epilayers of Si1−xGex on Si in the range 0.1<x<0.25. Dielectric functions of relaxed Si0.87Ge0.13 and Si0.8Ge0.2 have been obtained and long‐wavelength absorption coefficient values, required for interference fringe fitting, shown to be higher than measured previously. The dielectric function of strained Si0.78Ge0.22 has been measured for the first time and the effects of strain on the critical points shown to be consistent with deformation potential theory. An interpolation procedure has been developed for the fitting of layer composition and thickness, and excellent agreement with conventional techniques obtained for a series of uncapped single epilayers. The surface roughness of Si1−xGex epilayers has been studied as a function of time and deposition temperature and shown to play an important role in the modeling. The application of the technique to the characterization of buried strained layers is discussed.

Dielectric function spectra of strained and relaxed Si1−xGex alloys (x=0–0.25)

Dielectric function spectra for strained and relaxed Si1−xGex alloys with x∼0.13 and 0.20 are presented in numerical form. The effect of strain is shown to cause a modification of the spectra in the E1 critical point region, resulting in a decrease in refractive index at 1.96 eV, amounting to 0.06 at x=0.22. The spectral dependence of the refractive index is presented for a series of strained layers. An overview is given of spectral databases and the single‐wavelength ellipsometry data available in the literature.

Dielectric functions and critical points of strained InxGa1−xAs on GaAs

Dielectric function spectra of strained InxGa1−xAs (x≤0.25) epilayers on GaAs are presented for the first time, together with spectra of relaxed layers of the same compositions. Critical point energies, obtained by line‐shape fitting to second‐derivative spectroscopic ellipsometry (SE) data, show an increase in the E1, E1+Δ1 splitting with strain, in agreement with theory using GaAs deformation potentials. SE is shown to be capable of determining layer thickness, composition, and strain in this alloy system.

Spectroscopic ellipsometry characterisation of light-emitting porous silicon structures

Large differences are observed in the (n) spectra of porous Si produced in n + ,p + , p - substrates. Loss of c-Si structure is explained by a reduction in effective crystallite size due to quantum wire formation and does not require inclusion of an amorphous phase. Critical point energy shifts are attributed to confinement and stress effects. Spectroscopic ellipsometry is used to obtain porosity and detect porosity gradients.

Spectroscopic ellipsometry of Si1−xGex epilayers of arbitrary composition 0≤x≤0.255

Critical point (CP) transition energies have been calculated for strained Si1−xGex (0≤x≤0.255) between 2.5 and 3.5 eV from Lorentzian fits to the second differential of reference dielectric function spectra. E1 and E’0 transition energies are similar to those of the relaxed alloy. Comparison with deformation potential theory shows E1+Δ1 to be coincident with E’0 due to a strain‐induced up shift in the former’s transition energy. The reference spectra and CP transition energies are used in an interpolation procedure to analyze spectroscopic ellipsometry spectra of both uncapped and buried layers of strained Si1−xGex. Compositions and thicknesses are obtained in good agreement with alternative techniques.

In-situ dual-wavelength and ex-situ spectroscopic ellipsometry studies of strained SiGe epitaxial layers and multi-quantum well structures

In-situ dual-wavelength ellipsometry and ex-situ spectroscopic ellipsometry have been used to study strained Si 1-x Ge x /Si multilayers. Reference dielectric function spectra of strained Si 1-x Ge x with 0.06<x<0.29 have been obtained for the first time and an interpolation procedure based on the strain dependence of the dominant critical point energies developed. Good agreement with composition and thickness values from corroborative techniques was obtained when the effects of strain were taken into account

Spectroscopic ellipsometry of epitaxial Si {100} surfaces

The dielectric spectra of Si {111} and Si {100} orientations are shown to be equivalent using ex situ spectroscopic ellipsometry on clean epitaxial surfaces. The peak values for the real and imaginary parts (er, i) of the dielectric function exceed those previously reported, values of ei (at 4.25 eV)≥47 being obtained. Surface features with lateral scales of ≊0.5–2 μm, do not affect the dielectric spectra significantly. The high dielectric function peaks indicate that the nanometer lateral‐scale roughness on these epitaxial surfaces is very small.

The surface roughness and optical properties of high quality Si epitaxial layers

Abstract New recently published Si dielectric function data collected from specially prepared epitaxial Si are compared with literature values. Atomic force and optical microscopy studies of the epitaxial surfaces revealed them to be smooth on the fine scales to which ellipsometry is sensitive. The use of these new dielectric function data is demonstrated to lead to significantly improved fitting of ellipsometric data collected from a thermal oxide on Si structure.