Optical anisotropy of cyclopentene terminated GaAs(001) surfaces

Up to now most of the experimental work regarding the adsorption of organic molecules has been concerned with silicon. Here we study the interface formation on a III–V-semiconductor, GaAs(001). We show that reflectance anisotropy spectroscopy (RAS) is a sensitive technique for investigating the interface formation between organic molecules and semiconductor surfaces. With RAS it is possible to determine the surface reconstruction and the structural changes at the interface during the deposition of organic molecules. These changes and the underlying adsorption process are discussed here for the adsorption of cyclopentene on GaAs(001)c(4×4), (2×4) and (4×2). PACS 61.66.Hq; 72.80.Le; 34.50.Dy; 68.47.Fg     

Magnetic hyperfine interaction near Fe(100) surfaces in ultrahigh vacuum

The DCEMS technique in UHV has been used to measure for the first time the temperature dependence (31 to 295 K) of the magnetic hyperfine field B_HF averaged over several atomic layers near the surface of α-⁵⁷Fe(100) thin films covered by residual gas adatoms. The measured B_HF(T) curve closely follows that of bulk α-Fe. The surface magnetic moment of a residual gas coated Fe(100) surface in UHV appears to be not significantly modified as compared to the bulk.     

Atomic structures of gallium-rich GaAs(001)-4×2 and GaAs(001)-4×6 surfaces

Scanning tunneling microscopy is applied for the first time to an atomic-resolution investigation of the 4×2 and 4×6 phases on a gallium-rich GaAs(001) surface obtained by molecular-beam epitaxy and migration-enhanced epitaxy. A unified structural model is proposed with consideration of the results of experiments and first-principles calculations of the total energy. In this model the 4×2 phase consists of two Ga dimers in the top layer and a Ga dimer in the third layer, and the 4×6 phase is matched to periodically arranged Ga clusters at the corners of a 4×6 unit cell on top of the 4×2 phase. Zh. éksp. Teor. Fiz. 111, 1858–1868 (May 1997)     

Atomic structure of InSb(001) and GaAs(001) surfaces imaged with noncontact atomic force microscopy

Noncontact atomic force microscopy (NC-AFM) has been used to study the c(8x2) InSb(001) and the c(8x2) GaAs(001) surfaces prepared by sputter cleaning and annealing. Atomically resolved tip-surface interaction maps display different characteristic patterns depending on the tip front atom type. It is shown that representative AFM maps can be interpreted consistently with the most recent structural model of A(III)B(V)(001) surface, as corresponding to the A(III) sublattice, to the B(V) sublattice, or to the combination of both sublattices.     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

(4×2) and (2×4) reconstructions of GaAs and InP(001) surfaces

Received: 21 March 1997/Accepted: 12 August 1997     

Large-area wafer bonding of GaAs using hydrogen and ultrahigh vacuum atmospheres

Uniform direct or fusion wafer bonding of GaAs wafers up to 4 inch in diameter was achieved by means of two methods: (i) pre-heating, bonding at elevated temperatures and post-annealing in a H₂ atmosphere (gas environmental hot bonding) and (ii) bonding inside an UHV apparatus at temperatures as low as 150 °C after cleaning with atomic hydrogen. Both methods yield atomically abrupt interfaces as shown by cross-sectional TEM and by imaging the screw-dislocation network formed at low angles of twist between the wafers. At large twist angles, additional "step" dislocations arising from bonding across surface steps could be clearly imaged. The problem of occasionally occuring microvoids, probably arising due to insufficient pre-cleaning or at excessive post-annealing, is addressed. Both bonding procedures neither need mechanical loading of the wafers nor channel-patterning of the surfaces.     

In-situ optical spectroscopy and electronic properties of pyrrole sub-monolayers on Ga-rich GaAs(001)

We report on the characterization of sub-monolayers of pyrrole adsorbed on Ga-rich GaAs(001) surfaces. The interfaces were characterized by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and reflectance anisotropy spectroscopy (RAS) in a spectral range between 1.5 and 8 eV. The adsorption of pyrrole on Ga-rich GaAs(001) modifies the RAS spectrum of the clean GaAs surface significantly at the surface transitions at 2.2 and 3.5 eV indicating a chemisorption of the molecules. By the help of transients at these surface transitions during the adsorption process, we were able to prepare different molecular coverages from a sub-monolayer up to a complete molecular layer. The different coverages of pyrrole were imaged by STM and electronically characterized by STS. The measurements reveal that the adsorbed molecules electronically insulate the surface and indicate the formation of new interface states around −3.5 and +4.2 eV. The RAS measurements in the UV region show new anisotropies in the spectral range of the optical transitions of the adsorbed pyrrole molecules. Our measurements demonstrate the potential of optical and electronic spectroscopy methods for the characterization of atomically thin molecular layers on semiconductor surfaces allowing a direct access to the properties of single adsorbed molecules.     

AES and PYS studies of the polar GaAs(111) as surface after thermal cleaning in ultrahigh vacuum

Auger Electron Spectroscopy (AES) and Photoemission Yield Spectroscopy (PYS) have been used in the investigations of elemental composition and electronic properties of the polar GaAs-(111) As surface after thermal cleaning by electron bombardment heating at 770 K in an ultrahigh vacuum of 10⁻⁷ Pa. The surface concentration of As was 0.01 ML which corresponds to the (1 × 1) and weak (3 × 3) atomic structure, whereas the work function and absolute band bending were 4.05 ± 0.02 eV and −0.21 ± 0.04 eV, respectively. Moreover, two filled electronic surface state bands localized in the band gap below the Fermi level EF and in the upper part of the valence band were observed which have been described as the dangling-bond surface state and back-bond surface state bands, respectively.     

Electron loss spectroscopy of clean and oxygen covered GaAs(110) surfaces

Electron energy loss spectra of clean and oxygen covered GaAs(110) surfaces have been measured with a four grid retarding field analyser. Loss spectra of clean cleaved p- and n-type surfaces are slightly different and different states of adsorption for the oxygen on the two surfaces are found. The loss peaks which are common in the spectra obtained from clean surfaces of both types of material have been interpreted in terms of bulk and surface excitations. The data associated with the bulk excitations are in good agreement with previous optical and electron transmission data while loss peaks at 11.5 and 18.5 eV are interpreted as the surface plasma loss and a surface state transition respectively. For n-type material extra loss peaks were observed. In the case of oxygen adsorption on these surfaces new loss peaks were found at 13.5, 17.2 and 28.1 eV in both spectra and are assumed to be characteristic of the oxygen. Further, for n-type material an extra peak occurs at 8.2 eV.