Reflectance Anisotropy Spectroscopy Research Articles

Using surface and interface optics to probe the capping, with amorphous Si, of Au atomchains grown on vicinal Si(111)

The distinct optical signatures of aligned single and double Au atom chain structures,grown on vicinal Si(111) substrates, have been identified using reflectance anisotropyspectroscopy (RAS). Deposition of 0.04 monolayers (ML) of amorphous Si (a-Si) atroom temperature perturbs the anisotropic optical response of the double chainstructure. By one third of a monolayer, no significant optical anisotropy associatedwith the chains remains. No anisotropic response re-emerges at higher coverages,up to 4.6 nm (14.5 ML) where there is recent evidence that the crystal structureof the double chain phase is maintained under the cap. The RAS results showthat the anisotropic properties of the phase are quenched by a-Si adsorption,even though the crystal structure of the capped phase appears to be preserved.

Azimuth dependent reflection anisotropy of oriented thin films

A simplified mathematical description of reflection anisotropy spectroscopy (RAS) measurements is presented. The RAS signals of a nanometer film with arbitrarily aligned dielectric axes are investigated. While RAS spectra are found to be relatively insensitive to tilting of the dielectric axes out of the surface plane, the variation of RAS signals with sample azimuthal orientation angle, θs, reveals a distinct sin θs effect, superimposed on the previously observed sin 2θs and sin 4θs terms, which provides a measure of the tilt angle.

Adsorption structure of cyclopentene onInP(001)(2×4)

We have studied the interface formation between cyclopentene and (2x4) reconstructed InP (001) surfaces by soft x-ray photoemission spectroscopy, reflectance anisotropy spectroscopy (RAS), and ab initio theory. After preparation of an uncontaminated (2 x 4) reconstruction under ultrahigh vacuum conditions, the surface was exposed to cyclopentene as monitored by RAS. The changes in the In 4d, P 2p, and C 1s core-level emission lines upon molecule adsorption indicate a covalent bonding of cyclopentene to the topmost atoms of the surface at two different bonding sites. Based on these results, a structure model is suggested, which is supported by ab initio calculations of the total-energy, the RAS signature, and the In 4d and P 2p core-level shifts. Our results suggest that the cyclopentene adsorption is a two-step process: first cyclopentene adsorbs on the mixed dimer and second the changes in the surface structure enable the additional adsorption on the second layer In-In surface bond.

Optical anisotropy readout in solid-state porphyrins for the detection of volatile compounds

The controlled adsorption of molecules of volatile compounds on Langmuir–Schäfer films of tetraphenylporphyrins produces a modification of the film optical anisotropy, as revealed by reflectance anisotropy spectroscopy (RAS). These experiments allow a better understanding of the molecular package occurring in the film as well as of its alteration upon the interaction with volatile compounds, helping in the comprehension of the mechanisms responsible for the binding of molecules. The results recommend RAS as a potential transducer technique for chemical sensing.

Reflection anisotropy spectroscopy of the oxidized diamond (001) surface

We report reflection anisotropy spectroscopy (RAS) measurements of the oxidized(001) surface of a type IIb natural diamond. These measurements were madepossible due to recent developments in diamond surface preparation. We compareRAS results from the hydrogenated, clean and oxidized C(001) surface anddemonstrate that RAS is sensitive to the structural transition of the surface from the2 × 1 reconstruction of theclean surface to the 1 × 1 reconstruction of the oxidized surface.

Adsorption configurations of hydrocarbon ring molecules on GaAs(001)‐c(4 × 4)

AbstractThe understanding of self–assembly and bonding mechanisms of organic molecules on semiconductor surfaces represents a central research aspect in the investigation of novel organic/inorganic interfaces and their technological applicability. Here, we investigated the adsorption and bond formation of cyclopentene and 1,4‐cyclohexadiene on a GaAs(001)‐ c(4 × 4) surface in order to clarify the influence of the number of intra‐molecular C=C double bonds on the respective adsorption sites. For a determination of the adsorption configuration, the interfaces were characterized electronically and optically by synchrotron based X‐ray photoelectron spectroscopy (SXPS), low energy electron diffraction (LEED) and reflectance anisotropy spectroscopy (RAS). The results reveal significantly different adsorption configurations for the two molecules. While cyclopentene bonds with a single covalent bond to the surface, 1,4‐cyclohexadiene adsorbs onto the surface by the formation of multiple covalent bonds, e.g. bridge bonds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reflectance anisotropy spectroscopy: A probe to explore organic epitaxial growth

Reflectance anisotropy spectroscopy (RAS) is demonstrated to be particularly suitable for studying the deposition of organic epitaxial layers in ultrahigh vacuum by organic molecular beam epitaxy, thanks to its high sensitivity and applicability in situ. In the case of α-quaterthiophene, both homoepitaxy and heteroepitaxy have been monitored, demonstrating the crystallinity of the films up to tens of monolayers and the epitaxial relation to the substrate. In both cases, optical RAS data are compared to the results of ex situ characterization of the same samples by atomic force microscopy.

Structure of Si(111)-In Nanowires Determined from the Midinfrared Optical Response

The anisotropic optical response of Si(111)-(4x1)/(8x2)-In in the midinfrared, where ab initio studies predict significant changes in the band structure between competing models of this important quasi-1D system, has been measured using infrared spectroscopic ellipsometry (IRSE) and reflection anisotropy spectroscopy (RAS). Both IRSE and RAS of the (8x2) phase show that the anisotropic Drude tail of the (4x1) phase is replaced by two peaks at 0.50 and 0.72 eV, which appear in ab initio optical response calculations for the hexagon model of the (8x2) structure, but not the trimer model.

Effect of thermally induced surface defects on the optical anisotropy of Ag(110)

We show that the temperature dependence of the intensity of the $\ensuremath{\sim}1.7\text{ }\text{eV}$ peak in the reflection anisotropy spectrum of clean Ag(110) can be explained by considering the effect of thermally induced surface defects and the thermal shift in binding energy of the occupied surface state involved. Two simple models are used to quantify the defect formation energy and the length scale over which the contribution to the intensity of this reflection anisotropy spectroscopy (RAS) feature is quenched due to the presence of thermal defects. We have simulated the decrease in the RAS peak intensity with increased temperature, achieving consistency with related measurements on ion-bombarded and adsorbate-covered surfaces, and previous estimates of the defect activation barrier for the Ag(110) surface. This study demonstrates the potential of RAS as a tool for monitoring surface kinetic behavior.

The [formula omitted]-reconstructed cleavage surface of diamond: A challenging test for experiment and theory

We have recently measured for the first time–by using Reflectance Anisotropy spectroscopy- the optical gap between dangling-bond states on a single-domain diamond C(111)-2 × 1 surface. The experimental result we have obtained (1.47±0.02 eV ) represents the conclusion of the long-standing effort to determine if this surface is metallic or semiconducting. The large surface gap (even larger for possible excitonic effects) opens a new scientific question: at the moment neither state-of-the-art band calculations have succeeded in obtaining it, nor has its physical origin been explained.

Pseudomorphic growth of organic semiconductor thin films driven by incommensurate epitaxy

A stable pseudomorphic phase of α-quaterthiophene, a well known organic semiconductor, is obtained by growing films with organic molecular beam epitaxy (OMBE) on a single crystal of another organic semiconductor, namely, tetracene. The structural characteristics of the new phase are investigated by monitoring in situ the OMBE process by reflectance anisotropy spectroscopy; thus assessing that incommensurate epitaxy is in this case, the driving force for tuning the molecular packing in organic molecular films and in turn, their solid state properties.

Reflectance-anisotropy spectroscopy and surface differential reflectance spectra at the Si(100) surface: Combined experimental and theoretical study

We present reflectance-anisotropy spectroscopy (RAS) and surface differential reflectivity (SDR) experiments carried out on well-characterized single-domain nominal silicon (100) surface at room temperature. The results are compared with first-principles calculations which include many-body effects, namely, self-energy and excitonic effects. These strongly modify the line shape of the optical spectra. For a set of the five measurements, the RAS of clean, monohydride, and dyhidride surfaces, and the corresponding SDRs, good agreement between theory and experiment is obtained only when, in the calculation, the electron-hole interaction is accounted for.

Determination of the structure of adenine monolayers adsorbed at Au(110)/electrolyte interfaces using reflection anisotropy spectroscopy

Reflection anisotropy spectroscopy (RAS) has been used to show that at saturation coverage adenine adsorbs on the Au(110)/electrolyte interface in a base-stacking configuration with the plane of the bases orientated vertically on the surface and with the long axis of the molecules parallel to the [110] direction. Changes in the RAS observed from adsorbed adenine as a result of changes in the potential applied to the Au(110) electrode could arise from slight changes in the orientation of the molecules in the vertical plane.

The Structure of Adenine Adsorbed at Sub-Saturation Coverage at Au(110)/Electrolyte Interfaces

It is demonstrated using Reflection anisotropy spectroscopy (RAS) that at sub-saturation coverage adenine adsorbs on the Au(110)/electrolyte interface in a base-stacking configuration with the plane of the bases orientated vertically on the surface and with the long axis of the molecules parallel to the [1-10] direction. This orientation is the same as that determined for saturation coverage. We also show that RAS can be used to determine the adenine coverage of the Au(110) surface. [DOI: 10.1380/ejssnt.2009.225]

MOVPE growth of InGaAs/GaAsP-MQWs for high-power laser diodes studied by reflectance anisotropy spectroscopy

We have intensively studied the growth and strain behaviour of In x Ga 1− x As multi-quantum wells (MQWs) for application in high-power laser diodes. Their growth in metal-organic vapour-phase epitaxy (MOVPE) was monitored by reflectance anisotropy spectroscopy (RAS). The influence of indium content variation in the InGaAs quantum wells and strain compensation using GaAsP layers on the reflectance anisotropy for single and multi-quantum wells has been studied. A clear dependence of the RAS signal on the indium and phosphorus content has been found. Furthermore, the beginning of strain relaxation during growth can be monitored by a difference in the RAS signals of the GaAs layers below and above the MQW structure. With optimised growth conditions complete laser structures have been grown. Laser diodes with very small far-field angles and different numbers of quantum wells were processed into broad-area lasers, which show output powers up to 60 W in pulse mode.

Azimuthal dependent reflection anisotropy spectroscopy of Ag(110) near the plasmon resonance energy

The reflection anisotropy (RA) of Ag(110) has been investigated near 3.9 eV as a function of azimuthal angle θ using a photoelastically modulated spectrometer. At 3.9 eV the RA signal was small and varied as sin 4θ. At photon energies away from 3.9 eV the signal increased and varied as cos 2θ. Jones vector modeling of the system showed that in addition to the commonly observed cos 2θ dependence, which disappears when the reflection is isotropic, there is a sin 4θ dependence that occurs when the underlying dielectric function is anisotropic; in cubic materials this term is small but for other materials it may be very large.

In situ verification of single-domain III-V on Si(100) growth via metal-organic vapor phase epitaxy

Reflectance anisotropy spectroscopy (RAS) was used in situ for the quantification of antiphase domains on surfaces of thin GaP films deposited onto Si(100) by metal-organic vapor phase epitaxy (MOVPE). The preparation of a single-domain GaP∕Si(100) surface was determined via the analysis of RAS peak intensities in reference to the well-known P-rich surface reconstruction of homoepitaxially grown GaP(100). Both preprocessed Si(100) substrates and MOVPE as-grown GaP∕Si(100) films were also characterized ex situ by atomic force microscopy to identify the formation of mono- and diatomic surface steps and to analyze of the domain distribution, respectively.

Optical anisotropy induced by ion bombardment of Ag(001)

Grazing incidence ion bombardment results in the formation of nanoripples that induce an anisotropic optical reflection The evolution of the reflectance anisotropy has been monitored in situ with reflectance anisotropy spectroscopy. The Rayleigh-Rice theory (RRT) has been used to analyze the optical spectra quantitatively and provides the evolution of the average ripple period and root-mean-squared surface roughness. After an incipient phase, both the increase in the periodicity and the roughness vary roughly with the square root of the sputter time. Additional high-resolution low-energy electron diffraction (HR-LEED) measurements have been performed to characterize details of the average structure created by ion bombardment.

Comparative Sb and As segregation at the InP on GaAsSb interface

We study the formation of the InP on GaAsSb interface grown by molecular beam epitaxy at 450 °C. Using angle resolved x-ray photoemission spectroscopy (XPS), we show that Sb strongly segregates whereas As does not, leading to a Sb-rich InP surface. Similarly, XPS spectra recorded on air-exposed samples reveal oxidized Sb but no oxidized As. We perform a quantitative analysis and determine a Sb segregation coefficient very near to 1. This result is in good agreement with previous reflectance anisotropy spectroscopy observations on the same interface.

In situ passivation of GaAs surface with aluminum oxide with MOVPE

In situ passivation of GaAs surface subsequent to the growth in a metalorganic vapor phase epitaxy (MOVPE) reactor has been made possible using trimethylaluminum (TMAl). The adsorption layer on GaAs, presumably consisting of aluminum and the decomposition product of TMAl, was oxidized upon exposure to air to form thin AlO x layer. TMAl supply of only 0.5 monolayer completely prevented the oxidation of As on the surface, as confirmed by XPS. The passivation layer mostly prevented the oxidation of As upon O 2 annealing for 5 min at 250 °C. For the successful passivation, complete desorption of excess As on the GaAs surface was essential prior to the injection of TMAl. Otherwise, AlAs layer was formed and arsenic oxide was inevitably formed. The optimum length of H 2 purge to desorb As was determined to be 2 min with in situ surface monitoring using reflectance anisotropy spectroscopy (RAS). This passivation method, combined with the succeeding deposition of Al 2O 3 as a gate dielectric in a different reactor, provides the GaAs/gate interface without As-oxide. The method is applicable to the MOVPE growth of electron channel layers containing As for III–V metal–insulator–semiconductor field effect transistors (MISFETs).