J.-T. Zettler, J. Rumberg, K. Ploska, K. Stahrenberg, M. Pristovsek, W. Richter, M. Wassermeier, P. Schützendübe,
J. Behrend, and L. Däweritz
physica status solidi (a) 152 (1995) 35
GaAs(001) surfaces in both metalorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied under As stabilisation as well as during growth by reflectance anisotropy spectroscopy (RAS). During MBE growth reflection high energy electron diffraction (RHEED) measurements are performed simultaneously. As a consequence of the oscillation density of steps during island growth, time resolved RAS measurements show oscillations with monolayer periodicity in both epitaxial systems. In order to separate morphology related and surface reconstruction domain related contributions to RAS oscillations an anisotropic effective medium model is applied. In both MBE and MOVPE domains of reduced As coverage close to the island boundaries are found to be responsible for RAS oscillations. In MBE this typically results in oscillations between spectra belonging to (2x4) reconstructions of different mean As dimer density. In MOVPE the surface oscillates between states with an increased and decreased cocentration of Ga dimers while c(4x4)-like As dimers are still present.          [zurück]
 

L. Däweritz, H. Kostial, M. Ramsteiner, R. Klann, P. Schützendübe, K. Stahrenberg, J. Behrend, R. Hey,
M. Maier, and K. Ploog
physica status solidi (b) 194 (1996) 127
For Si d-like doping of GaAs lateral ordering processes and segregation in growth direction have been investigated in real time by monitoring long-range and short-range ordering effects using reflection high-energy electron diffraction (RHEED) and reflectance anisotropy spectroscopy (RAS). When Si is supplied in pulses to GaAs(001) distinct ordering processes occur which are promoted by misorientation steps on the vicinal surface. The incorporation of Si atoms on GaAs sites in a single plane can be completed to a high degree. The depth profile of the electron concentration is closely related to Si segregation during GaAs overgrowth. In the case of a doping layer of extremely high coverage it shows a minimum at the centre of the doping spike. For samples grown on vicinal GaAs(001) surfaces under conditions favourable for wire-like Si incorporation a considerable enhancement of the exciton photoluminescence (PL) intensityy and decay time has been found. [zurück]
 

M. Wassermeier, J. Behrend, J.-T. Zettler, K. Stahrenberg, K.-H. Ploog
Appl. Surf. Sci. 107 (1996) 48
We have studied GaAs(001) surface reconstructions prepared by molecular beam epitaxy in-situ by spectroscopic ellipsometry (SE) and reflectance difference spectroscopy (RDS or RAS). The simultaneous measurement of the dielectric function in SE allow us to identify surface and bulk related contributions to the RDS signal and to extract the optical anisotropy De=e_-110-e₁₁₀. For the (4x2), the c(4x4), and the (2x4) reconstructions we find resonances in Im(De) at 2.25 eV, 2.6 eV and 2.85 eV at T=80°C. The resonance at 2.85 eV corresponds at growth temperatures (T=600°C) to the 2.6 eV feature so far attributed to electronic transitions of the As dimers and coincides with the E₁ bulk transition. Measurements as a function of temperature addtionally reveal that they both share the same temperature dependence. This close relationship between the As dimer and the bulk optical response will be dicussed. [zurück]
 

M. Wassermeier, J. Behrend, K.-H. Ploog, J.-T. Zettler, K. Stahrenberg, and W. Richter
Physical Review B 53 (1996) 13542
GaAs(001) surface reconstructions prepared by molecular-beam epitaxy were studied in situ by spectroscopic ellipsometry (SE) and reflectance difference spectroscopy (or reflectance anisotropy spectroscopy) as a function of temperature. Simultaneous measurements of the dielectric function by SE and the reflectance difference Dr/r allow us to identify surface and bulk-related contributions to the optical anisotropy De=e_-110-e₁₁₀. For the (4x2), the c(4x4), and the (2x4) reconstructions we find resonances in Im(De) at 2.9 eV, 2.6 eV and 2.25 eV at T=80°C. Although the  resonance at 2.9 eV coincides with the E₁ bulk transition and also shows the same temperature dependence, they can be distinguished, because the surface contribution changes the sign when switching from the (2x4) to the c(2x2) reconstruction and the bulk contribution remains the same. [zurück]
 

V. Fernandez, D. Pahlke, N. Esser, K. Stahrenberg, O. Hunderi, A. M. Bradshaw, W. Richter
Surface Science 377-379 (1997) 388
We report measurements of the azimuthal anisotropy in the normal incidence reflectance of clean and adsorbate-covered single crystal Ag(110) surfaces. Reflectance anisotropy spectra were taken between 1.5 and 5.5 eV photon energy. Previous studies in air had indicated a very large anisotropy in the region around the surface plasmon energy. On the clean surface this feature is strongly reduced compared to that observed on a contaminated surface. By oxygen dosage the (1x1) surface structure is modified to (4x1) and (2x1) as displayed by LEED. The simultaneously observed optical anisotropy shows a correlation with oxygen coverage and restores to a large extent the anisotropy observed in air. [zurück]
 

L. Däweritz, K. Stahrenberg, P. Schützendübe, J.-T. Zettler, W. Richter, K.-H. Ploog
J. Cryst. Growth 175/176 (1997) 310-316
The Si incorporation on GaAs(001) has been studied in real time by reflection high-energy electron diffraction (RHEED) and reflectance anisotropy spectroscopy (RAS) at conditions of enhanced adatom mobility. In the RAS spectra, besides resonances due to As and Ga dimers on the GaAs surface also features due to Si dimers in single and double layers as well as As dimers adsorbed on them are identified. The incorporation process is very complex due to Si-induced As desorption, combined incorporation of Si and As, and readsorption of As on (Si, Ga). At conditions of high adatom mobility (low As₄ pressure) a single Si layer is nearly completed before Si incorporation in a second layer takes place, whereas for reduced adatom mobility (high As₄ pressure) islanding with Si incorporation in a double layer is found in early growth stages. [zurück]
 

K. Stahrenberg, T. Herrmann, N. Esser, J. Sahm, W. Richter, S. V. Hoffmann, and Ph. Hofmann
Physical Review B 58 (1998)
The (110) surface of an Ag crystal was investigated by reflectance anisotropy spectroscopy (RAS) and angle-resolved photoemission spectroscopy (ARUPS). A strong resonance in the optical spectra of the clean surface is assigned to a surface state transition at the Y-bar-point of the surface Brillouin zone. This resonance is absent on the oxygen covered surface. The accompanying photoemission spectra show the corresponding occupied surface state on the clean surface as well as its disappearance with oxygen coverage. [zurück]