A detailed analysis based on first-principles calculations with self-energy corrections is combined with photoemission spectroscopy to determine the origin of features observed in reflectance anisotropy spectroscopy ͑RAS͒ at semiconductor surfaces. Using the InP(001)(2ϫ4) surface as a model case we obtain quantitative agreement between slab calculations and low-temperature RAS measurements. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the absorption processes causing the anisotropy signal take place in the uppermost few atomic layers of the substrate. RAPID COMMUNICATIONS R16 336PRB 61 W. G. SCHMIDT et al. RAPID COMMUNICATIONS R16 338PRB 61 W. G. SCHMIDT et al.
We have investigated the microscopic structure and optical anisotropy of (2ϫ4) reconstructed GaP͑001͒ surfaces. Optical and electron spectroscopy from GaP͑001͒ surfaces prepared in ultrahigh-vacuum conditions were combined with first-principles calculations of the energetics and reflectance anisotropy. Symmetry, composition and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, photoemission spectroscopy and reflectance anisotropy spectroscopy. In contrast to most earlier reports, we find that the stable Ga-rich surface corresponds to a (2ϫ4) reconstruction. No (4ϫ2) reconstruction could be observed, independent of the preparation method. Depending on the Ga coverage, however, two distinct line shapes in the reflection anisotropy spectra occur, indicating the existence of at least two different surface phases with (2ϫ4) periodicity. This agrees with our total-energy calculations: Four (2ϫ4) structural models may be stable depending on the chemical potentials of the surface constituents. All considered (4 ϫ2) structures, however, are unstable. Based on the comparison between calculated reflectance anisotropy spectra and measured data we suggest mixed GaP dimers on top of the Ga-terminated substrate as groundstate geometry for the cation-rich phase of GaP͑001͒(2ϫ4). Our results indicate the formation of P dimers at the surface for the more anion-rich phase of GaP͑001͒(2ϫ4). ͓S0163-1829͑99͒02828-3͔
We have investigated the optical anisotropy of GaP͑001͒ and InP͑001͒ surfaces. The samples were prepared by homoepitaxial metalorganic vapor phase epitaxy growth and either directly transferred into ultrahigh vacuum ͑UHV͒ or in situ capped and, after transfer, decapped in UHV by thermal desorption of a P/As capping layer. Symmetry, composition, and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflectance anisotropy spectroscopy. We observe (2ϫ1)/(2ϫ2)-like reconstructions for the very P-rich and (2ϫ4) reconstructions for the more cation-rich surfaces. No (4ϫ2) reconstruction could be prepared, independent of the preparation method. A comparison of the reflectance anisotropy between GaP͑001͒ and InP͑001͒ surfaces shows similar line shapes for the very cation-rich (2ϫ4) surfaces. For less cation-rich surfaces, however, we observe distinct differences between the spectra of the two systems. In both cases, different line shapes in the reflection anisotropy spectra occur for the (2ϫ4) periodicity, suggesting the existence of different (2ϫ4) geometries. The experimental results are discussed on the background of atomic structures, total energies and reflectance anisotropy spectra obtained ab initio from density-functional theory local-density approximation calculations.
Static and dynamic Fresnel zone lenses were fabricated in quartz glass by means of microstructuring techniques. Two types of on-axis and off-axis lenses with different focal lengths and of different apertures were designed to operate at wavelengths of 1.52 mu m and 633 nm. The blazed profile of the on- axis and off-axis lenses was approximated by up to 16 and up to four discrete levels respectively. Dynamic, that is electrically switchable, lenses have been realized by filling the structured surface with a liquid crystal. The optical properties of the lenses, such as the focal spot sizes and the diffraction efficiencies, were investigated. Further the switching behaviour of the dynamic lenses was studied. The design and fabrication of the static and dynamic, on- and off-axis Fresnel zone lenses as well as their optical and switching properties will be presented
The optical properties of indium islands on GaAs(001) surfaces have been studied by reflectance anisotropy spectroscopy as a function of metal coverage. A large optical anisotropy is observed, which shows an oscillatory behavior and scales with the island size: mean island sizes determined by scanning electron microscope correspond to the wavelengths where extremes in the optical anisotropy arise. We explain this behavior by surface plasmon resonances of the island structure which induce a huge optical anisotropy related to the anisotropic shape and distribution of the In islands. Model calculations of the reflectance anisotropy spectroscopy signal based on a layer system where the island film is represented by an effective medium consisting of ellipsoidal metal particles in a vacuum matrix reproduce the main oscillation and support our conclusion
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