L. Mantese

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

Abstract 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.

Optical characterization of surfaces during epitaxial growth using RDS and GIXS

Structures of (001)GaAs surfaces during organometallic vapor phase epitaxy (OMVPE) have been studied by simultaneous measurements using reflectance difference spectroscopy (RDS) and grazing incidence X-ray scattering (GIXS). The data obtained by the two techniques are strongly correlated. We find that RDS, although not a diffraction probe, can distinguish most surface reconstructions of (001)GaAs. The information obtained by these two probes is complementary, so by combining RDS and GIXS data obtained from surfaces under static and dynamic conditions we can gain detailed information about atomic arrangements and growth mechanisms. Under static conditions, we find that RD spectra under OMVPE conditions are similar to those previously observed in ultrahigh vacuum (UHV). By simultaneously monitoring the surface reconstructions using GIXS and RDS, we have established that RDS accurately reflects the local structural configurations but is not uniquely sensitive to changes in long range order or subtle differences in symmetry. Growth oscillations were also measured simultaneously by RDS and GIXS under various conditions. The observed oscillation periods agree with each other, but the details are condition dependent, thus providing important clues concerning the surface processes involved. For example, the one-to-one correlation observed between RDS and GIXS oscillations suggests that the RDS oscillations are related to island formation.

Surface and interface effects on ellipsometric spectra of crystalline Si

We present the first systematic investigation of the differences among reference-quality ellipsometrically measured pseudodielectric function 〈e〉 spectra of crystalline Si, which are nominally used to approximate the bulk dielectric function of this material. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects, the latter representing shifts between bulk and measured critical point energies, as well as changes in excited-carrier lifetimes due to the surface. Model calculations indicate that these four effects account for nearly all differences among spectra studied, although a second-energy-derivative component appears at the E1 transition in some cases. The isotropic contribution to the surface-local-field effect is observed for the first time.

Evidence of near-surface localization of excited electronic states in crystalline Si

Surface- and interface-related spectra, obtained either directly by techniques such as reflectance-difference (-anisotropy) spectroscopy or indirectly by subtracting pseudodielectric function spectra obtained ellipsometrically on surfaces with different chemical termination, exhibit features related to energy derivatives of the bulk dielectric function. We argue that these spectra provide direct evidence that the excitations involved are localized both in space and time. These data unequivocally indicate that critical point energies obtained from above-band-gap ellipsometric or reflectrometric optical spectra are not necessarily equal to bulk values, and that surface chemical and structural termination is at least one contributing factor. Present surface-optical calculations do not include these effects, which may explain, in part, remaining discrepancies between theory and experiment.

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...

Lineshapes of surface induced optical anisotropy spectra measured by RDS/RAS

In this paper we discuss lineshapes of reflectance-difference spectroscopy (RDS/RAS) spectra of various surfaces. This work aims toward a basic understanding of the surface optical response. We find that in some cases RDS spectra are given by the bulk dielectric function or its energy derivative. In the former case screening effects such as the surface local field effect are dominant. In the latter case the surface modifies the bulk electronic structures and we find that the excited states must be localized near the surface. Even in the simplest case where the anisotropy originates at the surface the optical anisotropy in general depends on the dielectric function of the bulk as well as that of the surface.

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 eb or nanostructured material. We demonstrate that this line shape analysis can also be applied to Ge.

Systematic differences among nominal reference dielectric function spectra for crystalline Si as determined by spectroscopic ellipsometry

Abstract Results of the first systematic investigation of differences among reference-quality ellipsometrically measured pseudodielectric function 〈e〉 spectra of crystalline Si are discussed. These data are nominally used to approximate the bulk dielectric function of this material for optical modeling. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects even between spectra obtained for seemingly identical H-terminated surfaces. Model calculations indicate that these effects account for nearly all differences among spectra studied. The isotropic contribution to the surface-local-field effect is also reported.

Photon-induced localization and final-state correlation effects in optically absorbing materials

Two consequences of the absorption of light in optically absorbing materials that appear not to have been recognized previously are: (1) localization of the final electron and hole states involved in the absorption process into wave packets and (2) propagation of these wave packets with their respective group velocities. We demonstrate the existence of these phenomena by applying first-order time-dependent perturbation theory to a simple model that can be solved analytically even when correlations that are ordinarily discarded in the random phase approximation are retained. This approach provides a natural explanation of components in surface- and interface-optical spectra that are related to energy derivatives of the bulk dielectric function eb and apparent differences in nominally bulk critical point energies Eg and broadening parameters Γ depending on surface conditions.

PHOTON-INDUCED LOCALIZATION IN OPTICALLY ABSORBING MATERIALS

Abstract We show that components of surface- and interface-related optical spectra that are related to derivatives of their bulk dielectric functions are due to a dynamic photon-induced localization of the initial and final states. Localization is described by correlation effects that arise from the finite penetration depth of light in optically absorbing materials, and lead to a substantially different perspective of optical absorption than that given by conventional theory.