E1 Critical Point Research Articles

Second‐harmonic spectroscopy of Si nanocrystals embedded in silica

AbstractExtending previous second‐harmonic generation (SHG) single‐wavelength studies [1] of Si nanocrystals (NCs) embedded in fused silica, we implement spectroscopic cross‐polarized two‐beam SHG (XP2‐SHG) by employing an optical parametric amplifier as a source of widely tunable (1.6 ≤ Eph ≤ 2.4 eV) femtosecond pulses.Subsequent isolation of the SH signal produced by the NCs yields the oscillator strength and phase of their nonlinear response. We obtain a broad resonance between bulk Si E1 and E2 critical points. Possible explanations for the spectroscopic result are discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of quantum confinement and quantum size effects

AbstractRecently, the change in optical properties of ultra‐thin silicon on insulator films was shown to be due to quantum confinement. The E1 critical point shifted to higher energies with a 1/L2 dependence, and the shift was observed room temperature. Yet, quantum confinement effects in thin metal have yet to be observed optically. This leads to a number of questions that we raise and for which we provide preliminary answers. Shifts less than kT in energy can be observed optically when the final state is spatially and temporally localized. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of biaxial stressed silicon by spectroscopic ellipsometry and synchrotron x-ray scattering

Spectroscopic ellipsometry (SE) and synchrotron x-ray scattering have been applied to the non-destructive characterization of strained silicon (ε-Si). SE data from 250 nm to 500 nm were acquired from three epitaxial ε-Si samples grown on silicon germanium (SiGe) virtual substrates by chemical vapor deposition. Additional structural and compositional data were collected using x-ray scattering. The SE data were fitted by a two-step procedure involving the use of a parametric dielectric function model that is especially suitable for crystalline semiconductors. By monitoring the shift of the E1 critical point in the fitted dielectric function spectrum, the tensile strain of ε-Si can be determined from the known elastic constants and deformation potentials of Si. Strain values obtained are generally in agreement with the UV Raman measurements. The structural characteristic of the sample with the highest germanium concentration in the virtual substrate was further studied by reciprocal space mapping and atomic force microscopy.

Characterization of Biaxial Strained-Silicon by Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) and synchrotron x-ray scattering have been applied to the non-destructive characterization of biaxial tensile strained silicon (ξ-Si). SE data from 250nm - 500nm were acquired from three epitaxial ξ-Si samples grown on silicon germanium (SiGe) virtual substrates by chemical vapor deposition (CVD). The SE data was fitted by a two-step procedure using a parametric dielectric function model for crystalline semiconductors and structural data obtained separately from X-ray scattering. By monitoring the shift of the E1 critical point in the fitted dielectric function spectrum, the tensile strain of ξ-Si can be determined from known elastic constants and deformation potentials of Si. Strain values obtained are generally in agreement with Raman measurements. The structural characteristic of the sample with the highest germanium concentration in the virtual substrate was further studied by reciprocal space mapping (RSM) and atomic force microscopy.

Light Emission from Porous Silicon Photoetched in Aqueous Alkali Salt Solutions

An interband-transition model was applied to explain the emission mechanism in porous silicon (PSi) fabricated by photoetching in aqueous HF and salt (NaF and KF) solutions. The HF-formed samples show a yellow photoluminescence band at ∼2 eV. The salt-formed samples, on the other hand, show an ultraviolet (UV) emission peak at ∼3.3 eV with a spectral width of ∼0.1 eV, together with a broad emission band at ∼2.7 eV. The broad emission bands at ∼2 and ∼2.7 eV can be explained by the quantum-mechanical confinement effect, i.e., a relaxation of the momentum conservation at and above the indirect-absorption edge (supra-EgID emission). This effect may also lead to a change in the E1 critical point (CP) from the two-dimensional M0 to the zero-dimensional CP with decreasing nanocrystalline size. The change in the E1-CP dimensionality makes possible an emission in the UV region with a narrow spectral width.

Electroreflectance study of silicon nanocrystals fabricated from n-type silicon substrate

Electroreflectance (ER) spectroscopy has been performed on Si nanocrystals (NCs) fabricated from n-type crystalline Si substrates. The Si NCs were fabricated by anodization with varying anodizing current densities. Two ER features were observed at 1.1–3.2 eV, which were interpreted as due to optical transitions in confined electron–hole pairs. The transition energies reveal that the mean crystal size has a maximum against the increased anodizing current. Furthermore, extra ER features were weakly observed at 3.4 eV corresponding to the E1(E0′) critical points, and the lifetime broadening induced by surface roughening is discussed as an internal origin responsible for the weaken ER features.

Optical properties of molecular-beam-epitaxy-grown InGaMnAs thin films

The authors have determined the dielectric functions of a series of molecular-beam-epitaxy-grown (In0.5Ga0.5)1−xMnxAs thin films deposited on InP substrates. Two variable angle spectroscopic ellipsometers, covering both the IR and the UV range (0.2–30μm), were used to obtain optical spectra for each of the samples. Using a standard inversion technique, the experimental data were modeled to obtain the dielectric function for each of the quaternary samples. By using a parametric semiconductor model, they deduced the critical point parameters corresponding to the electronic transitions in the Brillouin zone. Their analysis indicates that in this particular quaternary system, while the critical point associated with the fundamental gap, E0, blueshifts as a function of Mn concentration, the E1 critical point shows a redshift with respect to the Mn concentration.

Dielectric function of disorder in high-fluence helium-implanted silicon

Dielectric function of disorder in single-crystalline silicon (c-Si) implanted by He with energy of 40keV and fluences from 1×1016 to 1×1017cm−2 were determined around the E1 and E2 critical points (CPs) by spectroscopic ellipsometry. The implanted material was modeled by an effective medium composition of c-Si and damaged Si. The dielectric function of damaged Si was calculated using the model dielectric function of Adachi to fit the E1 and E2 CP parameters of the MDF. The penetration depth of light in the photon energy range of 3–5eV is less than 100nm, which allows a simple layer structure of (surface oxide)/(surface amorphous layer)/(c-Si+damaged Si as a substrate). The oscillator energies and strengths decrease, while the broadening parameters increase with increasing fluence. Rutherford backscattering spectrometry was used for cross-checking of the surface disorder.

Spectroscopic ellipsometry characterization of ultrathin silicon-on-insulator films

Spectroscopic ellipsometry is used to characterize the optical response of a series of ultrathin silicon-on-insulator (SOI) films with thickness ranging from 50to2nm. Due to the potential barrier formed by the buried oxide layer and the interface with air, the electrons in the top silicon layer of the thinner samples experience dimensional confinement effects. Transition energies were extracted for the E1 and E2 critical points using reciprocal space analysis. This method offers improved accuracy over conventional regression-based techniques, such as line shape fitting, by eliminating error due to correlation among multiple fit parameters. It is shown that dimensional confinement increases the E1 transition energy of ∼3.375eV for bulk silicon to 3.38, 3.393, 3.42, and 3.45eV for the ∼10, ∼7, ∼4, and ∼2nm SOI films, respectively. This trend resembles the 1∕L2 law predicted by quantum mechanical considerations for a confined particle. The dependence of these critical point transitions is investigated as a function of film thickness L. Additionally, the E2 critical point transition energy is found to be much less affected by quantum confinement, contrary with theoretical predictions.

Second-harmonic generation from Si∕SiO2∕Hf(1−x)SixO2 structures

Optical second-harmonic generation (SHG) is used to characterize thin films of Hf(1−x)SixO2 (x=0, 0.3, 0.45, 0.65, and 1) deposited by atomic layer epitaxy at 375°C on Si substrates with 11Å SiO2 buffer layers. Reflected SHG intensity measured at room temperature increases monotonically with Hf content for as-deposited samples and varies strongly with the temperature (600–1000°C) of postdeposition rapid anneals in NH3. Spectroscopic analysis shows that the variable SHG component peaks at SH photon energy of 3.37eV—the bulk silicon E1 critical point energy—a clear signature of electric-field-induced second-harmonic generation in the bulk Si space-charge region. The results suggest that SHG is a sensitive, potentially in situ, probe of internal electric fields attributable to composition- and annealing-dependent fixed charge in the oxide layers.

Spectroscopic Ellipsometry and Raman Studies on Sputtered TiO<sub>2</sub> Thin Films

Titanium dioxide films were deposited using DC magnetron sputtering technique onto silicon substrates at ambient temperature and at an oxygen partial pressure of 7×10 –5 mbar and sputtering pressure (Ar + O2) of 1×10–3 mbar. The composition of the films, analyzed by Auger Electron Spectroscopy (AES), revealed the stoichiometry with an O and Ti ratio of 2.08. The optical constants of the as-deposited TiO2 thin film were determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric-function spectra reveal distinct structures at energies of the E1, E1+D1 and E2 critical points due to interband transitions. The Dielectric constant values were found to be substantially lower than those for the bulk TiO2. The dielectric related optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity are presented and analyzed. The deposited films were calcinated at 673 and 773 K. The influence of post-deposition calcination on the Raman scattering of the films was studied. The existence of Raman active modes A1g, B1g and Eg corresponding to the Raman shifts are reported in this paper. The improvement of crystallinity of the TiO2 films as shown by the Raman scattering studies has also been reported.

Si3H8 Based Epitaxy of Biaxially Stressed Silicon Films Doped with Carbon and Arsenic for Cmos Applications

AbstractArsenic doped Si:C films were grown epitaxially using Silcore® (Si3H8), methylsilane and arsine as source gases with growth rates up to 90 nm/min at 550°C. We observed a strong dependence of C and As incorporation on growth rate, caused by a growth rate dependent surface segregation behavior of C and As. High-resolution x-ray diffraction measurements of the epi layers reveal a perpendicular lattice constant as low as 5.323 Å. This corresponds to a biaxial stress of more than 2 GPa. Grazing exit (224) x-ray diffraction reciprocal space maps show the full tetragonal strain expected from the C content, suggesting that no relaxation of the films has occurred. Spectroscopic ellipsometry (SE) spectra were acquired in the 0.74 to 6.6 eV photon energy range. For intrinsic Si:C alloys, the E1 critical point shows the expected blueshift due to the alloying. For Si:C:As, a strong broadening of the E1 and E2 transitions exists, due to scattering of Bloch electrons in the crystal by As atoms. Additionally, free carrier effects lead to a decrease (increase) of the real (imaginary) part of the dielectric function in the near-infrared, consistent with a free carrier concentration in excess of 5×1020 cm-3. Transmission electron microscopy (TEM) of As doped Si:C films confirmed single-crystal epitaxial growth.

Ellipsometric study of the electronic structure ofGa1−xMnxAsand low-temperatureGaAs

We have measured the optical constants of GaMnAs from 0.62 eV to 6 eV, using spectroscopic ellipsometry. The second derivatives of the dielectric function are examined through a critical point analysis. The E1 critical point shifts to higher energies with increased doping of Mn, while all other critical points appear unaffected. The evolution of the critical points results from the interplay between band gap renormalization from ionized impurities and sp-d hybridization of the Mn induced impurity band and GaAs valence and conductions bands.

Spectroscopic ellipsometry studies on polycrystalline Cd0.9Zn0.1Te thin films

The pseudodielectric-function spectra, e(E) = e1(E) + ie2(E), of polycrystalline Cd0.9Zn0.1Te thin films in the 1.7–5.5 eV photon energy range were measured by spectroscopic ellipsometry. The measured dielectric function spectra reveal distinct structures at energies of the E1, E1 + Δ1 and E2 critical points due to interband transitions. The vacuum evaporated Cd0.9Zn0.1Te thin films exhibited zinc-blende structure with predominant (111) orientation. The rms roughness of the film evaluated by an ex-situ atomic force microscopy is 3.7 nm. Dielectric related optical constants, determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the film were also determined by using optical transmittance measurements, and results were discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and structural properties of SixSnyGe1−x−y alloys

Single-phase SixSnyGe1−x−y alloys (x⩽0.25,y⩽0.11) were grown on Si using chemical vapor deposition. First principles simulations predict that these materials are thermodynamically accessible and yield lattice constants as a function of Si/Sn concentrations in good agreement with experiment. An empirical model derived from experimental SixGe1−x and SnyGe1−y binary data also provides a quantitative description of the composition dependence of the lattice parameters. Spectroscopic ellipsometry of selected samples yields dielectric functions indicating a band structure consistent with highly crystalline semiconductor materials of diamond symmetry. Incorporation of Si into SnyGe1−y leads to an additional reduction of the E2 critical point, as expected based on the E2 values of Si and Ge.

Characterization of Ferroelectric Thin Film/SiO2/Si Structure by Photoreflectance

SrBi2Ta2O9(SBT)/SiO2/n-Si stacked structures have been characterized by photoreflectance spectroscopy (PRS). The spectrum near E1 critical point energy is enhanced by deposition of SBT film. Annealing effect of the spectral intensity corresponds to that of (115) peak in XRD pattern of SBT film. Moreover, the PRS intensity increases when SBT film is poled by negative voltage. These spectral changes depend on the Si surface potential increase induced by remanent polarizations in the SBT film. The Si surface potential increase is 0.05 eV for −3 V poling. Critical point energy was estimated by peak positions of the PRS and large tensile stress is increased by the poling. It has been found that the spectral intensity closely corresponds to the ferroelectricity of the SBT film.

Optical constants of DC magnetron sputtered titanium dioxide thin films measured by spectroscopic ellipsometry

AbstractOptical constants of DC magnetron sputtered TiO2 thin film have been determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric‐function spectra reveal distinct structures at energies of the E1, E1 + Δ1 and E2 critical points are due to interband transitions. The root mean square roughness of the magnetron sputtered TiO2 thin films evaluated by ex‐situ atomic force microscopy is 5.8 nm. The Dielectric constant values were found to be substantially lower than those for the bulk TiO2. The dielectric related Optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using the optical transmittance measurements and the results were discussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical constants of cubic GaN/GaAs(001): Experiment and modeling

The optical constants ε(E)=ε1(E)+iε2(E) of unintentionally doped cubic GaN grown on GaAs(001) have been measured at 300 K using spectral ellipsometry in the range of 1.5–5.0 eV. The ε(E) spectra display a structure associated with the critical point at E0 (direct gap) and some contribution mainly coming from the E1 critical point. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden–Muñoz model dielectric function [M. Muñoz et al., J. Appl. Phys. 92, 5878 (2002)]. This model is based on the electronic energy-band structure near critical points plus excitonic and band-to-band Coulomb-enhancement effects at E0, E0+Δ0 and the E1, E1+Δ1, doublet. In addition to evaluating the energy of the E0 critical point, the binding energy (R1) of the two-dimensional exciton related to the E1 critical point was estimated using the effective mass/k⋅p theory. The line shape of the imaginary part of the cubic-GaN dielectric function shows excitonic effects at room temperature not withstanding that the exciton was not resolved.

Electroreflectance of hexagonal gallium nitride at the fundamental and E1 spectral regions

We have measured the room-temperature electroreflectance (ER) of hexagonal gallium nitride. Our measurements were obtained at the fundamental optical gap and at the E1 critical point regions. The measurements were performed using front contact ER. In the fundamental region, the ER spectra were found to be of excitonic nature and in the low field regime. In the E1 region, the ER spectra were also in the low field regime, but were fitted with two-dimensional critical points.

Raman scattering, photoluminescence and spectroscopic ellipsometry studies on polycrystalline Cd0.96Zn0.04Te thin films

Cd0.96Zn0.04Te thin films are deposited onto well-cleaned glass substrates (Corning 7059) kept at room temperature by vacuum evaporation. Rutherford backscattering spectrometry and X-ray diffraction techniques are used to determine the thickness, composition and crystalline structure and grain size of the films, respectively. The films exhibited zinc blende structure with predominant (111) orientation. The surface morphology of the films is studied by scanning electron microscopy and atomic force microscopy. The rms roughness of the as-deposited films is 3.7 nm. The Raman spectra of the films show longitudinal- and transverse-optical (LO, TO) modes, which arise from CdTe- and ZnTe-like vibrations. The photoluminescence spectrum of the films shows intense emission due to free and bound exciton recombination and no emission associated with crystal imperfection and deeper impurity levels. The pseudodielectric-function spectra, ε(E)=ε1(E)+iε2(E), of the films are measured by spectroscopic ellipsometry at room temperature. The measured dielectric function spectra reveal distinct structures at energies of the E1, E1+Δ1 and E2 critical points.