RAS Spectra Research Articles

1 ML Wetting Layer upon Ga(As)Sb Quantum Dot (QD) Formation on GaAs Substrate Monitored with Reflectance Anisotropy Spectroscopy (RAS)

III/V semiconductor quantum dots (QD) are in the focus of optoelectronics research for about 25 years now. Most of the work has been done on InAs QD on GaAs substrate. But, e.g., Ga(As)Sb (antimonide) QD on GaAs substrate/buffer have also gained attention for the last 12 years. There is a scientific dispute on whether there is a wetting layer before antimonide QD formation, as commonly expected for Stransky-Krastanov growth, or not. Usually ex situ photoluminescence (PL) and atomic force microscope (AFM) measurements are performed to resolve similar issues. In this contribution, we show that reflectance anisotropy/difference spectroscopy (RAS/RDS) can be used for the same purpose as an in situ, real-time monitoring technique. It can be employed not only to identify QD growth via a distinct RAS spectrum, but also to get information on the existence of a wetting layer and its thickness. The data suggest that for antimonide QD growth the wetting layer has a thickness of 1 ML (one monolayer) only.

Microscopic optical anisotropy of exciton-polaritons in a GaAs-based semiconductor microcavity

Exciton-polaritons in semiconductor microcavities are ideal for the study of the exciton-light interaction and its dependence on light polarization. In this work, we report on the optical response and the dependence on polarization of a polariton microcavity using microreflectance anisotropy spectroscopy ($\ensuremath{\mu}$-RAS) with a spatial resolution of $10.0\ifmmode\times\else\texttimes\fi{}10.0\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$. We have found that, in contrast to optical reflection, the $\ensuremath{\mu}$-RAS spectra are quite inhomogeneous along the microcavity surface. We demonstrate the existence of microscopic local domains with differences in optical anisotropy of up to $20%$ within $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. These variations are independent of the detuning between the optical and excitonic resonances, which in our sample is close to 0 meV. The $\ensuremath{\mu}$-RAS line shape can be understood by using a model based on the anisotropic strain fields induced at the interfaces of the microcavity. The model agrees quite well with the experimental results and allows us to quantify the split of the energy levels of the exciton-polariton branches induced by the local break of symmetry at the microcavity interfaces.

Optical detection of graphene nanoribbons synthesized on stepped SiC surfaces

Graphene nanoribbons (GNRs) are nanostructures considered to be promising building blocks for the realization of graphene-based devices. The optical properties of GNRs are hard to determine due to their nanoscopic dimensions. Reflectance Anisotropy Spectroscopy/Reflectance Difference Spectroscopy (RAS/RDS) is a powerful optical tool to characterize highly anisotropic structures. RAS/RDS has shown to be very useful to measure the optical response of materials including semiconductor heterostructures. The technique is non-destructive and can be used in air or in vacuum conditions. Considering the highly anisotropic geometry of the GNRs, the RAS/RDS becomes a quite convenient technique to characterize the optical properties of GNRs and in general to study the dependence on the thickness of the optical properties of graphene. The GNRs used in the present work were synthesized on 6H-SiC stepped substrates and annealed in air to obtain quasi-free-standing bilayer graphene (widths: 240 nm, 210, and 120 nm). For this system, the isolation of the optical signal coming from the GNRs in the RAS spectra is not an easy task due to the fact that both GNRs and the 6H-SiC stepped substrate are highly anisotropic. To study and characterize the GNRs, we present and discuss an experimental approach to isolate the RAS signal coming from the GNRs. We also have performed nano-RAS measurements by using a near-field scanning optical microscopy technique (nanometric resolution) that supports our method. We show that RAS and nano-RAS are powerful complementary optical probes that can be used to characterize GNRs and also properties such as the visual transparency of one-, two-, or few-layer thick graphene.

Reflectance anisotropy studies of 5×2‐Au structures grown on Si(111) surfaces with different step formations

AbstractReflection anisotropy spectroscopy (RAS) studies are reported of Si(111)‐5×2‐Au structures grown on substrates with different offcuts and step densities. Such substrates produce a predominant domain on the (111) terrace that would otherwise produce a negligible RAS signal, due to domain averaging. Changes in the RAS spectra with offcut angle are observed in the 1 to 3 eV region, where Au‐induced structures appear. Results from offcuts towards [$ {\bar 1}{\bar 1} $2] of 2°, 3°, 9.5°, and ‐4° are presented. The variation in the RAS spectra is consistent with different populations of single‐ and double‐chain gold structures formed in the region of the steps. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In‐situ monitoring of MOVPE growth with reflectance anisotropy spectroscopy in an industrial used multi wafer reactor

We investigated structures grown in an industrial used multi wafer MOVPE reactor, in order to understand the influence of the growth parameters on the RAS spectra. We investigated the effect of p- and n-type doping on the RAS spectra of different materials important for optoelectronic devices such as AlGaAs and InGaAlP. Furthermore, the effect of ordering in GaInP layers on the RAS spectra has been studied. Growth parameters such as substrate misorientation, growth temperature and doping level have been varied in order to study different degrees of ordering in AlGaInP. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of oriented polypeptides on Au(111) surface depends on the secondary structure controlled by peptide length

We synthesized three different lengths of poly(L-lysine) containing an -SH group at the terminal (PLL(n)-SH, n (polymerization degree) = 4, 10, 30) and adsorbed them on an Au(111) surface. To analyze the formation process and the structure of self-assembled monolayers (SAMs), we used atomic force microscopy (AFM) and Fourier transform infrared reflection absorption spectra (FT-IR RAS). At the initial stage of SAM growth, formation of nanosize domains was confirmed by AFM imaging. The alpha-helical PLL(30)-SH exhibited a well-defined SAM structure after adsorption reached equilibrium. The alpha-helical PLL(30)-SH was almost perpendicular to the gold surface and exhibited interesting molecular packing due to the secondary structure of PLL(30)-SH and the underlying Au(111) array. The tilt angle of the helix axis from the substrate normal was estimated to be about 50 degrees (AFM) and 44 degrees (FT-IR RAS) respectively. On the other hand, PLL(4)-SH and PLL(10)-SH formed beta-sheet-type SAMs on the Au(111) surface based on the structure determined by FT-IR RAS spectrum.

Optical anisotropy of nanostructured noble metal surfaces

The reflection anisotropy spectra of the noble metal (110) surfaces are discussed in terms of surface modified bulk transitions within the derivative model and derivative model parameters for a range of variously structured surfaces are presented. The utility and consistency of the derivative model in explaining noble metal spectra is demonstarted. RAS spectra of ion bombarded noble metal surfaces are explained in terms of roughness induced uncertainty in the electronic wavevectors in the surface region. Spectra of nanostructured Cu(110) surfaces are presented and discussed. Contrasting spectral evolution during annealing for the (110) and (771) surfaces is attributed to differing step densities. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Calculation of reflectance anisotropy for semiconductor surface exploration

AbstractReflectance anisotropy spectroscopy (RAS) is exquisitely sensitive to probe surfaces, with many potential applications in determining surface geometries or monitoring material growth. Thanks to recent computational and methodological progress it has now become possible to calculate surface optical spectra accurately and with true predictive power. Here I review briefly the simulation of RAS spectra and discuss recent methodological advances, which allow for the modelling of self‐energy, excitonic and local‐field effects in large and complex systems. Numerical results for semiconductor surfaces in comparison to measured data are used to illustrate the potential and limits of the different levels of theory. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray photoemission spectroscopy and Fourier transform infrared studies of dye molecule doped conducting polymer films

Conducting polymer polythiophene (PT) films incorporated with dye molecule brilliant green (BG) were prepared by electrochemical doping and diffuse injection methods. The charge transfer and interaction between the doped molecule and PT polymer chains were investigated by analyzing the core-level energies and spectral profiles of the atomic components. Vibrational states of the doped dye molecule and the polymer backbone were also investigated by Fourier transform infrared (FTIR) RAS and FTIR transmission methods in order to examine the dopant configuration into the polymer chains in the hybrid films. In the case of BG doped films, X-ray photoemission spectroscopy (XPS) core-level analysis of S2p split indicates simultaneous doping of BG cation and HSO4− in the hybrid films prepared by electrochemically reduced and dye diffuse injection methods. The synthesized film samples were categorized into two types by the difference of S2p core-level energy shift. In the cationlike doped sample, the energy shift of the lower peak of S2p suggests the charge transfer between BG molecules and polymer chains creating a n type conducting polymer state. The orientation and configuration of the dopant BG molecule is discussed by assignment of the origin of the peak at 2000cm−1 which appeared only in the FTIR transmission spectrum of the BG-doped sample not observed in the FTIR RAS spectrum. The three possible cases of the origin of the peak are presented: Benzene out-of-plane mode, -NCS, and CCN-. According to those interpretations, three possible models of the BG configuration are suggested.

On the AlAs/GaAs (001) interface dielectric anisotropy

Reflectance anisotropy spectroscopy (RAS/RDS) so far has been mostly used for surface studies on III–V semiconductors. In the work reported here it was applied to the GaAs–AlAs (001) interface for measuring the interface dielectric anisotropy (IDA) by in-situ experiments performed in molecular beam epitaxy (MBE). From the measured RAS spectra of very thin AlAs overlayers (1–40 monolayers) on GaAs(001) the different contribution of surface, bulk and interface were separated. For the analysis, a three-layer model was used consisting of a single AlAs layer sandwiched between a well-separated and anisotropic interface layer and a surface layer. The upper limit for the ‘thickness’ of the surface and interface layers could be determined. Finally, an improved analytical formula for the deconvolution of surface and interface anisotropies is derived.

Observation of Single Helical Peptide Molecule Incorporated into Alkanethiol Self-Assembled Monolayer on Gold by Scanning Tunneling Microscopy

The bicomponent self-assembled monolayers (SAMs) were formed by incorporation of helical peptides into the alkanethiol monolayer, which was prepared in advance on gold substrate, and were characterized by Fourier transform infrared reflection-absorption spectroscopy (FTIR RAS) and scanning tunneling microscopy (STM). FTIR RAS spectra showed that the helical peptides were incorporated into the defects in the alkanethiol SAM without disturbing the film structure. A single helical peptide molecule was observed clearly in the bicomponent SAMs by STM. Several helical peptides associated together in a parallel arrangement to form a helix-bundle structure. Observation of the long molecule of ∼4 nm length standing vertically on the surface indicated that electron transfer occurred over a long distance through the helical peptide under the STM conditions.

Gallium‐rich reconstructions on GaAs(001)

AbstractGa‐rich reconstructions on GaAs(001) surfaces were prepared by annealing and Ga dosing of Molecular Beam Epitaxy grown samples and analyzed in‐situ by Reflectance Anisotropy Spectroscopy and Reflection High‐Energy Electron Diffraction. Annealing or dosing gallium above about 800 K invariably results in a (4 × 2)/c(8 × 2) reconstruction. Lowering the temperature or annealing below 800 K results in a (2 × 6)/(3 × 6) reconstruction. By dosing the (2 × 6)/(3 × 6) reconstruction with more than 0.2 monolayer of gallium, it transforms into a (4 × 6) reconstruction. The observed translational symmetries and measured RAS spectra are compared with results of first‐principles calculations. None of the (2 × 6) structures proposed in the literature is energetically stable. The RAS spectrum calculated for the ζ(4 × 2) model resembles reasonably the data measured for the (4 × 2) surface. The RAS spectra calculated for (2 × 6) symmetries indicate that mixed Ga‐As dimers likely are a structural element of the corresponding surface reconstructions. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Local-order of chemically-prepared GaAs( [formula omitted]) surfaces

In the present work HCl–propanol treated and vacuum annealed GaAs(1 0 0) surfaces were studied by means of soft X-ray photoemission and reflectance anisotropy spectroscopies (SXPS, RAS). On the as-treated surface, As 3d and Ga 3d core level spectra (CLs) are found to be similar to the ones observed on As-capped GaAs(1 0 0) surfaces after annealing at 350 °C, which desorbs most of the overlayer. RAS measurements of the as-treated surface and after annealing up to 350 °C show spectra similar to those detected on the as-grown surfaces with c(4×4) reconstruction. Anneals in the 380–470 °C temperature range lead to the appearance of a RAS line at 3 eV which is characteristic of As dimers. This finding coincides with the presence of a surface component in the As 3d core level spectrum with a chemical shift of −0.5 eV, which we attribute to As-dimers. Further annealing at 540 °C yields in RAS spectra an intense line at 2.2 eV due to Ga dimers, while the Ga 3d CLs do not exhibit any appreciable change.

Thickness-Dependent Optical Effects in Infrared Reflection-Absorption Spectra of a Fairly Thick Polymer Layer

Thickness-dependent intensities and positions of absorption lines in infrared reflection-absorption spectra of thin films on reflective substrates at near-normal incidence are investigated. Two types of absorption bands in a polymer, the weak and the strong, were examined. Their optical properties were determined by the dielectric response function. The optical path of the beam was described by the coherent sum of all successively reflected beams. The thickness-dependent properties of absorption bands were examined in three typical thickness regions. At small thickness, the peak intensity oscillates around the mean value defined by the simple internal absorptance of the beam crossing the double layer. For medium thickness, the peak position swings around the original frequency and its intensity oscillations move above the simple internal absorptance. In layers within the high-thickness region, optical distortions cause large changes in line shape due to approaching the bulk reflectance. A simple analytical interpretation is possible only within the low-thickness region. The width of these thickness regions depends on the absorptivity of the considered band; for strong bands they are considerably narrower than for weak bands. The theoretically predicted effects compare well with those measured in RAS spectra of variously thick silicon resin layers on aluminium substrates.

GaAs(001): Surface Structure and Optical Properties

The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.

Reflection anisotropy spectroscopy: a new probe of metal surfaces

AbstractReflection anisotropy spectroscopy (RAS) is a non‐destructive optical probe of surfaces capable of operation within a wide range of environments. The RAS technique was developed to study semiconductor growth and more recently has been used to probe the optical properties of metal surfaces. Surface sensitivity has been demonstrated with the detection of reconstruction, adsorbate interaction and the observation of transitions involving surface‐localized electronic states. The RAS technique has non‐ultrahigh vacuum (UHV) environment capability and thus opens up the study of metal surfaces in catalytic and corrosive environments at high pressure and at the solid/liquid interface. The combination of RAS with a full range of surface science techniques applied to the study of well‐defined surfaces aids in the interpretation of RAS spectra. The focus of this article will be on well‐characterized metal surfaces prepared and studied under UHV conditions. Copyright © 2001 John Wiley & Sons, Ltd.

Study of interaction between polyimide and Cu under a high humidity condition

Polyimide (PI) films coated on Cu films were treated under a high humidity condition. FT-IR RAS measurements have clarified the interface reactions of Cu with PI, the hydrolysis of imide functional group and formation of Cu carboxylate. From the calculation of RAS spectra measured at various incident angles, the diffusion of Cu carboxylate was confirmed from interface to the surface of PI. The diffusion length of Cu carboxylate ranges about 1 μm from Cu interface. The RAS result coincides with the ATR depth profile result and TEM-EDX image. Transmission electron microscope (TEM) images show that Cu diffuses mainly into the Cu carboxylate formation under our high humidity condition.

Reflectance anisotropy spectroscopy of the growth of perylene-3,4,9,10-tetracarboxylic dianhydride on chalcogen passivated GaAs(001) surfaces

The properties of organic molecular layers grown from perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on GaAs(001) substrates were investigated using reflectance anisotropy spectroscopy/reflectance difference spectroscopy (RAS/RDS). In an attempt to grow ordered organic layers GaAs(001) surfaces were modified with sulfur prior to the evaporation of PTCDA under ultrahigh vacuum conditions. The chalcogen modification results in a gallium sulfide-like surface layer terminated by S dimers, which shows a (2×1) low-energy electron diffraction pattern. The lines shapes of the RAS spectra of S modified surfaces show sharp derivative-like features at the E1 gap and broad spectral features at higher energies likely related to E0′ and E2 gaps of bulk GaAs. For low PTCDA coverages the shape of the spectra in the energy range of the GaAs bulk features is unchanged which indicates a low interaction between substrate and organic layer. Additional features appear in the spectra for PTCDA coverages even below 0.3 nm which can be attributed to transitions between the highest occupied molecular orbital and the lowest unoccupied molecular orbital at 2.23 eV. While the sharp feature due to the E1 gap of GaAs is essentially unaffected, the optical anisotropy at higher energies is increasing strongly with increasing PTCDA layer thickness which is due to interference effects as shown by a model calculation.

Photoreactivation does not alter ras and p53 mutation spectra in ultraviolet radiation–induced corneal sarcomas of Monodelphis domestica

Photoreactivation does not alter ras and p53 mutation spectra in ultraviolet radiation–induced corneal sarcomas of Monodelphis domestica

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.