Surface Dielectric Anisotropy Research Articles

Optical reflectance anisotropy of Al(110): Experiment andab initiocalculation

The reflectance anisotropy of a clean Al(110)-vacuum interface has been measured experimentally, and compared with the results of an ab-initio self-consistent full-potential linear augmented plane-wave calculation. Good agreement is found in the interband regime, while a discrepancy in the Drude regime is consistent with anisotropy in the intraband contribution. The calculated surface dielectric anisotropy and reflectance anisotropy of Al(110) are not significantly affected by surface relaxation, suggesting that optical transitions between surface states and surface resonances make only a minor contribution to the surface dielectric response. The measured reflectance anisotropy of Al(110) is found to be only weakly affected by oxidation and by the deposition of Ni overlayers, providing further evidence in support of this conclusion.

Ab initiocalculation of depth-resolved optical anisotropy of the Cu(110) surface

We calculate the surface dielectric anisotropy (SDA) and the reflectance anisotropy of the relaxed Cu-(110) surface using the ab initio self-consistent full potential linear-muffin-tin orbital method within the local-density approximation. Besides interpreting the spectra in terms of transitions involving surface and/or bulk states, we carry out a depth depending scanning of them, from surface to bulk, with the resolution of one atomic layer. We show that the SDA mainly arises in the vacuum region above the surface and in the first atomic layer.

Surface-induced d-band anisotropy on Cu( [formula omitted

Reflectance difference spectroscopy (RDS) was used to study the surface-induced anisotropy of the electronic band structure on Cu(1 1 0) in the energy range between 3 and 5.5 eV. The spectral features in this energy range can be well reproduced by approximating surface dielectric anisotropy by the first derivative of the known bulk dielectric function. In contrast to the conventional ‘derivative model’, however, the different sensitivity to the relevant Cu d-bands to the surface anisotropy has to be respected. The results can be rationalized in a tight-binding picture relating the shift of the surface projected d-bands to the different atomic densities along the in-plane [0 0 1] and [ 1 ̄ 1 0 ] directions, respectively. As a confirmation, the temperature dependence of the RDS features was measured and found to be in good agreement with the thermal shifts of the associated optical transitions.

Optical properties of the Au(110) surface

We have used reflectance anisotropy spectroscopy to determine the surface dielectric anisotropy of Au(110) under ultrahigh-vacuum conditions. Additional angle-resolved photoemission spectroscopy measurements were carried out to correlate surface optical with surface electronic properties. The surface dielectric anisotropy mainly results from single-particle excitations involving the near surface bulk d and $\mathrm{sp}$ bands and free-electron contributions in the spectral region below $\ensuremath{\sim}2.4 \mathrm{eV}.$ Surface-state contributions to the surface optical properties of Au(110) are discussed.

Optical response of reconstructed GaP(001) surfaces

GaP(001) surfaces were found to reconstruct with (2\ifmmode\times\else\texttimes\fi{}1) or (2\ifmmode\times\else\texttimes\fi{}4) symmetry under surface conditions corresponding to a high or low V/III surface stoichiometry ratio, respectively. These surface reconstructions, identified by reflection high-energy electron diffraction, have been prepared in a chemical beam epitaxy (CBE) system by varying the sample temperature. Reflectance anisotropy spectra (RAS) of these surfaces have been taken under both CBE and metal-organic vapor phase epitaxy conditions. Three different phases of (2\ifmmode\times\else\texttimes\fi{}4) symmetry have been distinguished according to their characteristic RAS response in agreement with recent theoretical predictions [A.M. Frisch, W.G. Schmidt, J. Bernholc, M. Pristovsek, N. Esser, and W. Richter, Phys. Rev B 60, 2488 (1999)]. For the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction a line-shape analysis of the RAS signatures was performed and their temperature shifts have been compared to the respective shifts of the bulk critical points. These experiments indicate the GaP(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface dielectric anisotropy originating from transitions between bulk states modified by the surface due to band-folding effects and anisotropic shifts of the ${E}_{1}$ and ${E}_{0}^{\ensuremath{'}}$ critical point energies.

Optical Properties of Cu-(110) Surface

ABSTRACTWe present the first ab initio calculations of the optical properties of the (110) clean surface of Cu for which the Anisotropic Surface Reflectivity (RAS) has been recently measured. This technique exploits the anisotropy of the reflectivity along the directions [110], [001] on the surface. Calculations are performed with a Full Potential Linear Muffin Tin Orbital method, within LDA, in the slab geometry. The comparison of our calculated Surface Dielectric Anisotropy (SDA) ω[1110] – [001] with the one obtained from the RAS spectrum is almost satisfactory for a slab of 11 atomic layers.

Substrate dependence of adlayer optical response in reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) offers extreme surface sensitivity and can be used to study gas-solid and solid-liquid interfaces. Although the technique has already made a considerable contribution to in situ monitoring of the epitaxial growth of semiconductors, its impact has thus far been limited by difficulties in interpreting experimental results. In particular, RAS spectra are related to both Δε2′ and Δε2″, the real and imaginary parts of the surface dielectric anisotropy, with weighting factors determined by the complex dielectric function of the substrate. Here RAS calculations are performed for a variety of substrates assuming a model overlayer dielectric anisotropy. Three distinct regimes of behavior are observed: RAS spectra can resemble either Δε2′, Δε2″ or −Δε2″ (or some combination of the three) depending on the substrate and the photon energy. It is therefore crucial to properly account for these substrate effects if RAS is to be used to determine the azimuthal orientation of adsorbates or to study the symmetries of surface states, for example. Similar considerations also apply to the related technique of surface differential reflectance. The Fresnel-based theoretical method used here and the substrate optical functions presented should serve as a useful database for future work as RAS is applied to new systems.

Temperature dependence of the InP(001) bulk and surface dielectric function

The bulk dielectric function, the surface reflectance anisotropy, and the surface dielectric anisotropy of InP(001) were determined from room temperature up to 875 K. Measurements were performed in-situ on as-grown samples in both a metal-organic vapour phase epitaxy and a chemical beam epitaxy (CBE) system using a rotating analyser type ellipsometer (SE) and a reflectance anisotropy spectrometer (RAS). The temperature dependence of the bulk critical points was deduced by performing a direct line shape analysis of the SE spectra. A high-temperature bulk dielectric function database for optical in-situ studies was established by applying a combined cubic spline/harmonic oscillator interpolation scheme to the measured data. Three InP(001) surface reconstructions were found by reflection high-energy electron diffraction (RHEED) measurements in the CBE system: (2 x 1) and c(4 x 4) under phosphorus-rich conditions and a (2 x 4) reconstruction under reduced phosphorus supply. Characteristic RAS spectra are assigned to these reconstructions. The temperature shift of the main features of the surface dielectric anisotropy spectra are compared to those of the bulk critical points.

Structural transformation of As-stabilized surfaces caused by Ga-deposition detected by time-resolved surface photo-absorption

Evolution of surface structures during conversion from various As-stabilized surfaces to Ga-stabilized ones is observed by surface photo-absorption. Desorption of As dimers parallel to [110] is observed during the structural change of c(4 × 4) and (2 × 4)γ. The evolution of Ga- and As-related peaks is isolated in surface dielectric anisotropy spectra, which are determined using p- and s-polarized surface photo-absorption spectra during the conversion of (2 × 4)β to a Ga-stabilized (4 × 2) surface.

Structural transformation of As-stabilized surfaces caused by Ga-deposition detected by time-resolved surface photo-absorption

Evolution of surface structures during conversion from various As-stabilized surfaces to Ga-stabilized ones is observed by surface photo-absorption. Desorption of As dimers parallel to [110] is observed during the structural change of c(4 × 4) and (2 × 4) γ. The evolution of Ga- and As-related peaks is isolated in surface dielectric anisotropy spectra, which are determined using p- and s-polarized surface photo-absorption spectra during the conversion of (2 × 4) β to a Ga-stabilized (4 × 2) surface.

Real time observation of surface dielectric responses of GaAs(001) using surface photo-absorption

Surface photo-absorption is used to observe the changes in isotropic and anisotropic surface dielectric responses caused by As desorption during surface conversion from c(4 × 4) to (2 × 4)γ of As-stabilized GaAs(001) surfaces. The anisotropic part agrees with the change in the surface dielectric anisotropy spectrum observed in reflectance-difference measurements during this surface conversion. The spectral evolutions of surface dielectric response are observed in real time with a time resolution of 0.1 s during surface conversion.

Dielectric response of As-stabilized GaAs surfaces observed by surface photo-absorption

This letter discusses changes in the isotropic and anisotropic surface dielectric response caused by surface conversion from c(4×4) to (2×4) of a GaAs(001) As-stabilized surface, observed using surface photo-absorption. The anisotropic part quantitatively reproduces the change in the surface dielectric anisotropy spectra observed in reflectance difference measurements during this surface conversion. The isotropic part can be measured only by surface photo-absorption and comparison is made with a calculation using a three-phase model consisting of vacuum/As/GaAs.

Surface dielectric functions of (2×1) and (1×2) reconstructions of (001) GaAs surfaces

Theoretical calculations of the surface dielectric function of (001) GaAs with As- and Ga-terminated (2×1) reconstructions are presented. An empirical nearest-neighbor tight-binding model with six orbitals (s,x,y,x2−y2,3z2−r2) per atom is used. The calculated dielectric functions anisotropies for GaAs (2×1) surfaces are analyzed with reference to the electronic structures. The theoretical results are in qualitative agreement with present data, and allow structures observed at 1.8, 2.6, and 4.1 eV in the surface dielectric anisotropy to be related to transitions from Ga bonding dimer to Ga dangling bond, As lone pair to As antibonding dimer, and As bonding dimer to As antibonding dimer transitions, respectively.

Optical Approaches to Real-Time Analysis and Control of Crystal Growth

ABSTRACTA variety of optical methods are now available for studying surface processes and for monitoring layer thicknesses and compositions during semiconductor crystal growth by molecular beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD), and related techniques. Spectroellipsometry (SE) and spectroreflectometry (SR), the older, primarily bulk-sensitive probes, are now augmented by new, primarily surface-sensitive probes such as reflectance-difference spectroscopy (RDS), second-harmonic generation (SHG), and laser light scattering (LLS). Examples of real-time growth studies now include SE determinations of thicknesses and compositions of AlxGa1–xAs layers on GaAs by organometallic molecular beam epitaxy (OMMBE) to 10 Å thickness scales, RDS determinations of surface dielectric anisotropy spectra of various (001) GaAs surfaces relevant to crystal growth by MBE, and LLS determinations of the evolution of surface roughness during chemical vapor deposition (CVD) growth on Si. Proven capabilities suggest new applications, particularly to growth-interrupted and metastable systems.