Research highlights

Etching or Stabilization of GaAs(001) under Alkali and Halogen Adsorption
 

The advanced modifications of epitaxial growth techniques such as migration enhanced and atomic layer epitaxies allow one to grow semiconductor structures in which the interface smoothness and the thickness of layers are controlled with the ultimate precision of one monolayer. Along with atomic-layer growth, for modern nanotechnology, it is important to develop techniques of atomic-layer (“digital”) etching, which consist of layer-by-layer removal of a semiconductor with monolayer resolution. Here it is shown experimentally by photoemission and by ab initio calculations that adsorption of electropositive cesium on the As-rich surface of GaAs(001) and, in a symmetric fashion, adsorption of electronegative iodine on the Ga-rich surface, induce a decrease of the surface stability, thus facilitating surface etching.

Conversely, Cs adsorption on the Garich surface and I adsorption on the As-rich surface lead to an increased surface stability. Etching occurs when adsorption-induced charge transfer weakens the backbonds of the top arsenic atoms for the case of Cs on the As-rich B2(2×4) surface and the lateral bonds in the topmost surface layer for I on the Ga-rich Chi(4×2) surface. The possibilities of reversible transitions between the two reconstructed surfaces and of atomic layer etching with monolayer precision are demonstrated. Presented in: Etching or Stabilization of GaAs(001) under Alkali and Halogen Adsorption, O.E. Tereshenko et al, The Journal of Physical Chemistry C, 116,8535-8540 (2012)

 

Optical constants of Sr thin films
 

Extinction coefficient of Sr compared with literature data

The optical constants n and k of strontium (Sr) films were obtained in situ in the 6–1220−eV range from transmittance measurements performed at room temperature. Strontium films were deposited onto 5nm thick carbon films supported onto different grids. The refractive index n was obtained with KK analysis over an extended spectral range. Experimental data show that Sr is a promising candidate for transmittance filters and reflective multilayers in the EUV range.

Presented in: Transmittance and optical constants of Sr films in the 6–1220 eV spectral range, Luis Rodrıguez-de Marcos, Juan I. Larruquert, Josè A. Aznarez, Manuela Vidal-Dasilva, Sergio Garcıa-Cortes, Josè A.Mendez, Luca Poletto, Fabio Frassetto, A. Marco Malvezzi, Daniele Bajoni, Angelo Giglia, Nicola Mahne and Stefano Nannarone, J. Appl. Phys., 111353-1-7 (2012)

 

 

Photoemission and X-ray Absorption Study of the Interface between 3,4-Ethylenedioxythiophene-Related Derivatives and Gold
 

In molecular electronics, the interface between the organic semiconductors (either molecular or polymeric films) and metal electrodes is critical for the electronic level alignment and for the determination of charge injection barriers. Here is studied with valence band photoemission and near edge X-ray absorption fine structure (NEXAFS) spectroscopy the chemisorption of 3,4-ethylenedioxythiophene (EDOT) derivatives on Au(111) single crystal and polycrystalline surfaces. The interpretation of the valence band spectra is carried out by comparing with DFT calculations of the densities of states of different possible reaction products that can be formed at the interface: valence band features that are associated to EDOT products are identified together with new structures that are ascribed to reactive dissociation products at the interface.

In particular the presence of unsubstituted thiophene/oligothiophene molecules is observed, together with possible alkyl chains. Simulation of the NEXAFS spectra of EDOT and thiophene and comparison with literature data on thiophene and oligothiophene films are used for the interpretation of the X-ray absorption spectra of bi-EDOT, TET on Au, and the TET polymer and PEDOT at different electro-polymerization times. NEXAFS indicates that the thiophene rings tend to adopt a planar configuration at the interface. Published in: Photoemission and X-ray Absorption Study of the Interface between 3,4-Ethylenedioxythiophene-Related Derivatives and Gold, L. Pasquali et al., The Journal of Physical Chemistry C, 116, 15010-15018 (2012)

 

 

Extreme ultraviolet multilayer for the FERMI@Elettra free electron laser beam transport system

 

A new kind of multilayer interferential coating has been developed for the beam transport system of FERMI@Elettra Free Electron Laser. The system is based on an aperiodic capping layer on the top of a Pd/B4C multilayer structure optimized for high reflectance at 6.67 nm. This system allows to combine the high reflectance of the buried ML with the filter properties of the capping layer, useful in rejecting the fundamental harmonic when the goal is to select only the third FEL harmonic.

The structure has been deposited and also characterized along one year in order to investigate its temporal stability.  Reflectance measurements have been done and repeated after one year at the BEAR beamline, and revealed no measurable performance changes, suggesting good temporal stability. Presented in: Extreme ultraviolet multilayer for the FERMI@Elettra free electron laser beam transport system, A.J. Corso et al., Optics Express, 20, 7 (2012)