Surface-induced Optical Anisotropy Research Articles

Surface-induced optical anisotropy of inhomogeneous media

We demonstrate that the presence of interfaces induces anisotropy in the optical properties of thin inhomogeneous layers. Several mechanisms are discussed that can control the properties of this surface-induced anisotropy. We found that the effective refractive indices for s- and p-polarized fields are different and depend on the thickness of the layer, concentration and optical properties of inclusions in the layer, and the angle of incidence.

RAS在线监测AlGaAs的MOCVD生长

The MOCVD growth of AlxGa1-xAs for application in high-power laser diodes was studied by using time resolved reflectance anisotropy spectroscopy (RAS) and normalized reflectance (NR) were studied. Multi-layer AlxGa1-xAs structures with different Al compisition were grown on GaAs (001) substrates. The most suitable photon energy for monitoring the growth process was investigated. The NR and RAS signals at photon energy near the fundamental band gap showed an oscillatory behavior during the growth. The different contribution of surface-induced optical anisotropy and interface-induced optical anisotropy could be distinguished in situ by RAS transient spectra. The intensities of the RAS and NR signals were strongly dependent on the aluminium composition.

Advances in multichannel ellipsometric techniques for in-situ and real-time characterization of thin films

Recent advances in the design of multichannel ellipsometers based on rotating-element principles have significantly increased their power for applications involving in-situ, real-time analysis of thin films and surfaces. These advances include: (i) extensions in the upper spectral limit beyond the typical value of 4.5 eV, and (ii) increases in the number of extracted parameters beyond the usual two, namely the ellipsometric angles ( ψ, Δ). An extended spectral range is enabled, for example, by use of a tandem light source configuration and a grating spectrograph having two stages of order-sorting filters mounted over the window of the linear array detector. With an upper spectral limit of 6.5 eV, such ellipsometers are particularly useful in growth analysis for wide band gap thin films. The larger parameter set is enabled by use of dual or single rotating compensators for polarization state modulation and/or analysis, replacing the traditional rotating polarizer. For the single rotating-compensator instrument in the PSC rA configuration, one can extract the Stokes vector of the reflected beam, and thus its degree of polarization. This in turn can provide information on sample thickness variations over the beam cross-section and can suppress the effects of multiply-scattered light so that non-uniform and macroscopically rough surfaces can be analyzed. For the dual rotating compensator in the PC r1SC r2A configuration, one can extract the entire Mueller matrix of the sample and thus the characteristics of anisotropic surfaces and thin films. Applications of these advances include studies of thickness evolution of the phase for boron nitride and silicon film growth, and the nature and origin of surface-induced optical anisotropy of (110) silicon.

Surface-induced optical anisotropy of Si and Ge

We compare surface-induced optical anisotropy spectra measured by reflectance difference/anisotropy spectroscopy of Si and Ge. Our previous work on Si has shown that we can roughly distinguish two types of contributions to the optical response of surfaces: direct contributions involving surface state transitions and indirect contributions in which the surface modifies the bulk response. We demonstrate here that this view is valid for Ge as well. For Si the indirect surface contributions exhibit three extreme, basic line shapes, which are all related to the bulk dielectric function εb or nanostructured material. We demonstrate that this line shape analysis can also be applied to Ge.

Analysis of high-index Si(001) surfaces by reflectance difference spectroscopy

We report surface-induced optical anisotropy spectra of high-index Si(115), (114), and (113) surfaces obtained using reflectance difference spectroscopy. Air-oxidized surfaces show sharp derivative-type features that are step-induced and located near the critical point energies of bulk Si, consistent with those of lower-index Si(001) surfaces. Clean reconstructed surfaces are characterized by a broad feature near 3 eV that tends to decrease in amplitude upon H exposure and a step-induced structure near the (E0′,E1) transition of bulk Si. In contrast, H exposure of Ge-covered surfaces tends to sharpen and enhance lower-energy structures. The derivative-type features located near the bulk critical point energies of Si can be described in terms of electronic states localized by the finite penetration depth of light.

Surface-induced optical anisotropy of the (001) and (113) silicon surfaces

The surface-induced optical anisotropy of clean and hydrogenated (001) and (113) surfaces of silicon has been calculated. The equilibrium atomic configurations of the surface atomic structures were determined from fully converged self-consistent total-energy calculations within the local density approximation. The optical functions of the silicon surfaces have been obtained beyond the density-functional theory with ab initio pseudopotentials and by the semiempirical tight-binding method. The reflectance anisotropy (RA) spectroscopy signal of the clean (001) and (113) surfaces in the region around 3.0 eV originates from surface to bulk optical transitions. On the hydrogenated (113) silicon surface a strong surface induced RA response is obtained in the region of the ${E}_{1}$ and ${E}_{2}$ bulk critical points, where the line shape corresponds to the first derivative of the bulk dielectric function. The calculated RA spectrum of the hydrogenated (113) silicon surface in the regions of the ${E}_{1}$ and ${E}_{2}$ structures is more intense in relation to the (001) surface. These results will be discussed in comparison with experimental data.

Surface-induced optical anisotropy of oxidized, clean, and hydrogenated vicinal Si(001) surfaces

We report surface-induced optical anisotropy (SIOA) spectra obtained by reflectance-difference spectroscopy (RDS) of several vicinal Si(001) surfaces. Chemically (RCA) cleaned surfaces show sharp, step-induced, derivative-like features that increase with vicinal angle. The RD signal for these surfaces can be modeled as an anisotropy that is localized to the surface and can be described in terms of a dichroic bandgap. The spectra of clean reconstructed surfaces show both a broad structure near 3 eV, which appears to involve dimer dangling bond transitions, and a second feature near 4.3 eV that occurs near the E 2 peak of bulk Si. The difference between spectra of the 10° and 6° vicinal samples yields a resultant signal similar to that of Si(113). We also report the RD signal for the monohydride and dihydride phases for the hydrogenated surfaces.

Hydrogenated and oxidized vicinal Si(001) surfaces investigated by reflectance-difference spectroscopy

We have investigated the effects of hydrogenation and oxidation on the surface-induced optical anisotropy (SIOA) of vicinal Si(001) as measured by reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). By considering several vicinal Si(001) surfaces we better isolate terrace from step-induced contributions. For double-domain surfaces etched by HF or oxidized by H 2O 2, the RD response has an energy-derivative-like shape and can be characterized by three main features, the first near 3.4 eV and a second and third around 4.3 eV. These energies are near those of the ( E′ 0, E 1) and E 2 interband critical points of bulk silicon. The magnitudes of these peaks vary strongly with offcut angle suggesting that there is a strong step-induced contribution to the SIOA. Two of these features are affected by a change of the surface species, which indicates that the species bonded to the steps are changing. The RD response obtained by exposing atomic H to a double-domain surface hydrogenated by HF is similar to that obtained by exposing a clean single-domain surface to atomic hydrogen.

Interpretation of surface-induced optical anisotropy of clean, hydrogenated, and oxidized vicinal silicon surfaces investigated by reflectance-difference spectroscopy

Using reflectance-difference spectroscopy, we determine surface-induced optical anisotropy (SIOA) spectra of clean, hydrogenated, and oxidized (113) and vicinal (001) Si surfaces to obtain a better understanding of the origin of the optical response of surfaces and interfaces. Hydrogenation was performed either by etching in dilute HF or by exposing clean surfaces to atomic hydrogen. Hydrogenated and oxidized vicinal surfaces show energy-derivativelike spectra that roughly scale with offcut angle, indicating step-induced behavior, and exhibiting features near 3.4 and 4.2 eV, the threshold energies of the (E1, E0′) and E2 interband critical points of bulk Si, respectively. The appearance of derivativelike line shapes indicates that bulk threshold energies become dichroic near the surface due to the surface-induced modification of the potential, as supported by model calculations. However, direct integration yields dielectric functions somewhat different from bulk values, indicating that the surface affects the near-surface dielectric function. SIOA spectra for clean surfaces are qualitatively different, exhibiting dielectric-functionlike line shapes that appear to originate primarily from terraces, although indications of smaller derivativelike contributions are also found.

Surface-Induced Optical Anisotropies of Single-Domain (2 x 1) Reconstructed (001) Si and Ge Surfaces.

We report surface-induced optical anisotropy spectra of clean $\left(2\ifmmode\times\else\texttimes\fi{}1\right)$ reconstructions of (001) Si and Ge surfaces, using oxygen exposure to separate contributions from dimers and steps. For Ge, the line shape is relatively simple and collapses uniformly with oxygen exposure. For Si, the results are more complicated and H termination must be considered as well. The clean-surface data of both Si and Ge are incompatible with a purely surface-to-surface transition model.

Calculation of surface-induced optical anisotropy for a dipolium model via d-parameter theory.

The surface-induced optical anisotropy of Si (110) for several angles of incidence is evaluated in a dipolium model using d-parameter theory. By comparison with the results found earlier in a (numerically) exact calculation, we conclude that d-parameter theory can provide helpful qualitative insights and reasonable quantitative accuracy.

In-Situ and Ex-Situ Studies of Silicon Interfaces and Nanostructures by Ellipsometry and Rds

AbstractWe report surface-induced optical anisotropy (SIOA) spectra and dielectric function data of vicinal Si(001) surfaces using reflectance-difference spectroscopy (RDS) and spectroscopic ellipsometry (SE). To find the main contributions of the optical response we took data for clean and by hydrogen, oxygen, and arsenic modified surfaces. The SIOA lineshapes, as measured by reflectance-difference (RD) spectroscopy, can be either derivative-like or similar to the dielectric function as measured by SE. The derivative-like spectra appear to be associated with step contributions while the dielectric-function-like behavior appears to arise from terrace dimers and/or changes in chemical species bonded to the steps. In addition, we present SE data of silicon nanostructures formed by electrochemical etching and find that their behavior is dominated by interface properties.

Microscopic treatment of the angular dependence of surface induced optical anisotropy.

The angular dependency of surface induced optical anisotropy (SIOA) has been calculated by means of the discrete dipole approach. Within the basic assumptions of this approach, exact results (full incorporation of local-field effects and retardation) have been obtained for the semi-infinite problem using the double-cell method. This method allows for an independent treatment of bulk and surface. For off-normal incidence, the microscopic behavior of the system has been investigated. Near the Brewster angle, the absolute value of the microscopic p-type response is larger than the s-type response. In general, the SIOA decreases for increasing angles of incidence. Only near Brewster's angle the relative p-type anisotropic reflectance difference increases dramatically, but under experimentally very unstable conditions. Further, the macroscopic anisotropic response, ${\mathit{dR}}_{\mathit{s}}$/${\mathit{R}}_{0}$, ${\mathit{dR}}_{\mathit{p}}$/${\mathit{R}}_{0}$, \ensuremath{\delta}\ensuremath{\Psi}, and \ensuremath{\delta}\ensuremath{\Delta}, has been calculated as a function of photon energy for three angles of incidence, 60\ifmmode^\circ\else\textdegree\fi{}, 70\ifmmode^\circ\else\textdegree\fi{}, and 80\ifmmode^\circ\else\textdegree\fi{}, and for the (110) surfaces of three popular semiconductors, Si, GaP, and GaAs, respectively.

Surface-induced optical anisotropy of the Si(110) surface.

The reflectance-anisotropy spectrum of a hydrogen-covered Si(110) surface is calculated taking into account anisotropic surface polarizabilities and exactly solved local-field effects. Including in the calculation the most important features of the (2\ifmmode\times\else\texttimes\fi{}1) and (5\ifmmode\times\else\texttimes\fi{}1) reconstructions, i.e., the missing chains along the [001] direction, it is clearly demonstrated that the shape of anisotropic reflectance spectra is extremely sensitive to local-field effects. The agreement between the measured spectrum of the oxidized surface and the calculated spectrum of the (5\ifmmode\times\else\texttimes\fi{}1) reconstruction is striking as far as the order of magnitude, peak positions, and relative peak intensities are concerned.

Anisotropic off-normal incidence optical reflection from GaP (110) surfaces

This article contains a theoretical study for off-normal incidence surface induced optical anisotropy (SIOA). The discrete dipole approximation was used to calculate the off-normal incidence optical response of slabs. By means of the two slab approach those results were converted to semi-infinite reflectivities. The calculated ellipsometric parameter δΔ shows large variations near the Brewster angle, but only the p-polarized reflection has a clearly increased SIOA sensitivity. So experimentally a straightforward determination of ΔRp should be preferred. Advantages have to be sought in the optical observation of surface state related phenomena at subbandgap conditions.