Reflectance Anisotropy Spectra Research Articles

Fourier Transform Infrared Reflection Anisotropy Spectroscopy of Semiconductor Crystals and Structures: Development and Application in the Mid-Infrared.

A new method of reflection anisotropy spectroscopy (RAS) with increased mid-IR efficiency owing to the use of a Fourier transform infrared (FT-IR) spectrometer has been developed. An optical setup was implemented using a photoelastic modulator (PEM) to modulate the direction of linear polarization of the probe beam originating from the Michelson interferometer. An original measurement algorithm was proposed to eliminate the influence of spectral inhomogeneity of the PEM efficiency on the obtained spectra using appropriate calibration. It was shown that to preserve the sign of the RAS signal, it is necessary to use a specialized procedure for phase correction of the interferogram registered by the FT-IR spectrometer. In the visible range, good agreement was confirmed between the obtained reflection anisotropy (RA) spectra of a semiconductor crystal and the results of independent measurements using a conventional diffraction-grating spectrometer-based setup. The RA spectrum of a III-V semiconductor heterostructure in the mid-infrared range (λ up to 8 µm) is demonstrated. Application of the developed FT-IR RAS method to layered black phosphorus has enabled characterization of anisotropic interband transitions in this graphene-like semiconductor crystal.

Eumelanin Adsorption on Silicon: Optical Properties of Si(001)-Adsorbed Eumelanin Tetrameric Protomolecules

We predict the optical properties (in particular optical absorbance and reflectance anisotropy spectra (RAS)) of the silicon(001) surface organically functionalized by the adsorption of chosen tetr...

An algorithm for the in situ analysis of optical reflectance anisotropy spectra

We report on a computer algorithm for the in situ analysis of reflectance anisotropy (RA) spectra in a time frame compatible with the epitaxial growth of cubic semiconductors. This algorithm allows for the in situ acquisition of RA spectra and their decomposition into two components whose amplitude depends on the As coverage of the semiconductor surface. One of such components is associated with the surface orthorhombic strain due to the surface reconstruction and has an amplitude that strongly depends with surface reconstruction and thus As coverage. This fact opens the possibility of using reflectance anisotropy spectroscopy (RAS) as an optical probe to characterize the As surface coverage in real time. To demonstrate the performance of the algorithm we report on RA measurements carried out during the homoepitaxial growth of GaAs (001). We show that the algorithm is capable of analyzing a set of 500 RA spectra in a time span of about 10 s. This allows for a range of applications for the developed algorithm, including the surface characterization and fine tuning of the substrate stoichiometry just before epitaxial growth, during the growth of the buffer layer.

On the origin of reflectance-anisotropy oscillations during GaAs (0 0 1) homoepitaxy

We report on the first spectroscopic study of reflectance-anisotropy (RA) oscillations during molecular beam epitaxy (MBE) GaAs homoepitaxy. Real-time RA spectra measured during epitaxial growth were carried out with a recently developed rapid RA multichannel spectrometer with 100 ms per spectrum acquisition time. An analysis of the time-resolved RA spectra shows that RA oscillations are mostly due to the periodic modulation of the surface orthorhombic strain associated to surface reconstruction. Results reported here demonstrate the power of real-time RA spectroscopy as a probe for the study of epitaxial growth processes. In particular, given its sub monolayer surface-strain sensitivity, RA spectroscopy results a very convenient tool to study epitaxial growth mechanisms in real-time with sub monolayer resolution. This capability allows for real-time RA spectroscopy to be used as a probe for the in situ, real-time control of epitaxial growth, with the additional advantage of operating in higher pressure systems such as CVD, where RHEED monitoring cannot be implemented.

Optical Properties of Free and Si(001)‐Adsorbed Pyrimidinic Nucleobases

In this work, we predict and analyze the optical spectra of pyrimidinic uracil‐like nucleobases thymine (THY), uracil (URA), and 5‐fluorouracil (5‐FU) and their reflection anisotropy spectra (RAS) upon adsorption on the silicon (001) surface. First‐principles results based on plane‐wave density functional theory show chemically sensitive features in gas phase optical absorption spectra that redshift/blueshift according to the orbitals involved in the corresponding transition. In the RAS, a characteristic lineshape is found, typical of the energetically favored “dimer bridge” configuration, and remarkably similar for all the investigated Si(001):X systems (X = THY, URA, 5‐FU). We show that molecular tilting and breaking of the glide plane symmetry have a negligible effect on the optical spectra, despite their influence on the surface bandstructure. Contrarily to gas phase spectra, chemically sensitive RAS features only appear above 4.5 eV, and can be recognized as molecular contributions consistent with gas phase optical absorption results whereas substrate effects dominate at lower energies.

Plasmonic reflectance anisotropy spectroscopy of metal nanoclusters in a dielectric multilayer. Theory

A theory of plasmonic reflectance anisotropy spectroscopy (RAS) is developed for nanocluster layer at an interface. The model of identical ellipsoidal metal particles occupying the sites of rectangular lattice is used to calculate the effective plasmonic polarizability of nanoparticles. The anisotropic local field due to optically induced dipole plasmons and their interface-conditioned images is taken into account. Within the theory, resonant reflectance anisotropy spectra recently observed for In nanoclusters on InAs surface are explained, the anisotropy being associated with the difference between frequencies of plasmons with orthogonal in-layer polarizations. The frequency difference is treated in terms of anisotropy of the particles shape or/and the layer structure, its sign being opposite for the two types of anisotropy. The plasmonic RAS is concluded to serve as a method for investigating the anisotropy of nanocluster arrays.

Optical properties of organically functionalized silicon surfaces: Uracil-like nucleobases on Si(001)

We predict UV reflectance anisotropy spectra (RAS) of the organically functionalized silicon (001) surface covered by pyrimidinic uracil-like nucleobases. First-principles results based on density functional theory show characteristic spectral features appearing in the UV range between 3 and 7 eV, besides the expected quench in the well-known two-minima RAS signal of clean Si(001). Nucleobase adsorption in the energetically favored ``dimer bridge'' configuration gives rise to a characteristic RAS line shape, common to thymine, uracil, and 5-fluorouracil. We trace back the origin of such spectral features by singling out RAS structures induced by relaxation and passivation effects on the Si surface, and those directly associated with molecular excitations. The former turn out to be the same for the three nucleobases, and are totally unaffected by molecular tilting. The sign and position of the latter RAS peaks at higher energy exhibit a moderate nucleobase dependence, and can be fully rationalized in terms of the molecular orbitals involved. The present theoretical results call for a RAS experimental study in the UV region extending up to $\ensuremath{\simeq}6--7$ eV.

Plasmonic reflectance anisotropy spectroscopy of metal nanoparticles on a semiconductor surface

A theory of plasmonic differential anisotropic reflection of light from nanoparticles located near the interface between media is developed. The model of a monolayer consisting of identical ellipsoidal metal particles occupying sites of a rectangular lattice is investigated. Effective plasmonic polarizabilities of nanoparticles in the layer are calculated self-consistently using the Green’s function technique in the quasipoint dipole approximation. The local-field effect caused by anisotropic dipole plasmons of particles in the layer and their image dipoles is taken into account. The lately observed resonant reflectance anisotropy spectra of indium nanoclusters on InAs surface are explained by the difference between frequencies of plasmons with the orthogonal polarizations in the surface plane. The difference between the plasmon frequencies is attributed to anisotropy of the particles shape or/and the layer structure; the signs of frequency difference for the two types of anisotropy being different.

Reflectance anisotropy spectroscopy of the Si(111)−(5×2)Au surface

Hybrid DFT calculations of atomic structure, electronic band structure, and reflectance anisotropy (RA) are used to correlate atomic and electronic structures and optical transitions of eight structures for the $\mathrm{Si}(111)\text{\ensuremath{-}}(5\ifmmode\times\else\texttimes\fi{}2)\mathrm{Au}$ phase with Au coverage and Si adatom site occupancy. The structure recently proposed by Kwon and Kang (KK) [S. G. Kwon and M. H. Kang, Phys. Rev. Lett. 113, 086101 (2014)] and strongly supported by surface x-ray diffraction [T. Shirasawa et al., Phys. Rev. Lett. 113, 165501 (2014)] is found to be the most stable in the Au coverage range from 0.6 to 0.8 ML. The band structure of the Si adatom covered $(5\ifmmode\times\else\texttimes\fi{}4)$ surface has a surface state whose dispersion and position agree very well with the prominent ${S}_{1}$ surface state observed in angle-resolved photoemission experiment. Reflectance anisotropy spectra in the energy range 0.5 to 5 eV for the eight surface structures show variations with Au and Si adatom coverage which can be explained in terms of filling of this surface state. The best agreement between predicted and measured RA spectra, which are sensitive to both Au atom and Si adatom coverage, is found for coverages from 0.7 to 0.75 ML Au, when the surface has regions where Si adatoms are absent and where there are sufficient adatoms to cause surface-state band filling, in equal parts. Surface formation energy calculations favor a coverage of 0.7-ML Au. The structures of the $(5\ifmmode\times\else\texttimes\fi{}2)$ KK and $(6\ifmmode\times\else\texttimes\fi{}6)\mathrm{Au}$ phases on the Si(111) surface are compared and it is shown that the repeat unit of the KK phase also occurs in the $(6\ifmmode\times\else\texttimes\fi{}6)$ structure.

Excitation spectra of Bi/Si(001) interfaces

Present article deals with the theoretical and experimental investigation of optical properties of Bi/Si(001) interfaces in a wide spectral range (1–4 eV). Theoretical excitation spectra of vicinal Si(001) surface, Bi/Si(001)–0.5 monolayer and Bi/Si(001)–1 monolayer interfaces are calculated by time‐dependent density functional theory with the hybrid Becke and Lee, Yang and Parr pseudopotential. Calculations and experimental studies of the absorption spectrum of the samples revealed the blue shift of the optical absorption edge with increasing of the bismuth covering degree. Features of the experimentally obtained reflectance anisotropy spectra (RAS) and surface differential reflectance spectra (SDRS) concerned with changing of the silicon surface reconstruction from 2 × 1 to 1 × 1 with the increasing of the bismuth covering degree from 0.5 to 1 monolayer. It is shown that the correlation between the theoretically calculated and experimental data makes it possible to identify the electronic transitions in the surface region.

Effects of local field and inherent strain in reflectance anisotropy spectra of AIIIBV semiconductors with naturally oxidized surfaces

Reflectance anisotropy (RA) spectra of naturally oxidized (001) surfaces of GaAs and InAs crystals are measured for photon energies from 1.5 up to 5.5 eV. The differential high-accuracy RA spectra reveal features substantially different from those caused by either a reconstruction of clean surface or a built-in near-surface electric field. Models of atomic structure with anisotropic transition layers of excess arsenic atoms specific for GaAs(001)/oxide and InAs(001)/oxide interfaces are proposed. In conformity with these models, a general theory of reflectance anisotropy is developed for semiconductor/oxide interfaces within the Green's function technique. The theory takes into account the combined effect of local field due to interface dipoles and of intrinsic near-surface strain of the crystal. Measured RA spectra are analyzed in the model of valence-bond dipoles occupying a rectangular lattice in a multilayer medium. Comparing the measured and calculated spectra, we conclude that RA spectra of oxidized GaAs(001) and InAs(001) surfaces are simultaneously influenced by interface and near-surface anisotropies. The former is responsible for the broad-band spectral features which are associated with polarizability of the valence bonds attached to As atoms at the crystal/oxide interface. The near-surface anisotropy is due to inherent uniaxial straining the near-surface region of crystal. The effect of strain on RA spectra is experimentally and theoretically substantiated for GaAs crystal wafers undergone a uniaxial applied stress. Basically, this work results in the following. It establishes the physical nature of different levels of RA spectra observed in a majority of papers, but never analyzed. It demonstrates how the studied features of RA spectra could be applied for optical characterization of strained interfaces and atomic layers.

Optical properties of surfaces with supercellab initiocalculations: Local-field effects

Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects. Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using the ``effective-medium theory'' with vacuum and also in terms of interaction between replicas, as the supercell approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) $2\ifmmode\times\else\texttimes\fi{}1$ surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to nonlinear optics for surfaces.

A multichannel reflectance anisotropy spectrometer for epitaxial growth monitoring

We report on a reflectance anisotropy (RA) spectrometer capable of measuring reflectance spectra on the 100 ms time-scale and sensitivity in the upper 10−4 range. A multichannel lock-in amplifier was used to acquire 32 wavelengths RA spectra covering the 2.25–3.85 eV photon energy range, where the E1 and transitions of GaAs and other technologically relevant III–V semiconductor are located. The RA spectra recorded during the first stages of the GaAs homoepitaxial deposition are presented for the first 0.38 monolayers of growth, showing significative changes in the lineshape with low noise. Thanks to the capabilities of this instrument, it is possible to observe in detail, in terms of the evolution of RA spectra, the processes carried out during the migration of surface reconstruction between two stable phases present in the homoepitaxial growth of GaAs.

Plasmonic anisotropy of In nanocluster arrays on InAs(001) surface observed by differential reflectance spectroscopy

Reflectance anisotropy (RA) spectroscopy is applied to study indium nanocluster arrays formed on InAs(001) crystal surface by electrochemical treating. The measured RA spectra exhibit a resonant feature of negative sign whose scale ~10−1 exceeds by two orders of magnitude the scale of the related spectra conventionally observed for semiconductor surfaces. Plasmonic origin of the resonant feature is unambiguously established both experimentally and theoretically. The RA spectra of In nanoclusters covering InAs(001) surface are interpreted in terms of coupled dipole plasmons belonging to ellipsoidal metal nanoparticles. From the experimental RA spectra, an effective plasmonic anisotropy is estimated within a lattice model of indium nanoparticle coverage. It is concluded that the in-surface semi-axes of ellipsoids can differ by a few percent, as well as the periods of rectangular lattice occupied by nanoparticles.

Reflectance anisotropy of the anatase Tio2(0 0 1)-(4 × 1) surface

The dielectric functions of bulk anatase TiO2 and its (4 × 1) reconstructed (0 0 1) surface were calculated using a hybrid density functional theory method. The bulk dielectric function is compared to data obtained from synchrotron radiation reflectivity measurements in the energy range 0–20 eV and to dielectric functions from two Bethe–Salpeter calculations. There is agreement between predictions of the two theoretical methods, except at the absorption threshold, where there is some shift of spectral weight to lower energy in the Bethe–Salpeter calculations. All features observed in the dielectric function derived from synchrotron reflectivity data are reproduced in calculations. There are some differences in relative peak intensities for dielectric functions derived from experiment and theory. A dispersionless surface state is found which is localised on the topmost oxygen ion in the reconstructed surface. The reflectance anisotropy spectrum of the TiO2(0 0 1)-(4 × 1) surface shows strong features in the energy range to 12 eV but does not show any features caused by sub-bandgap surface state transitions.

Reflectance anisotropy spectroscopy of clean and Sb covered Ge(001) surfaces and comparison with clean Si(001) surfaces

Reflectance anisotropy (RA) spectra of clean, dimerized Si and Ge (001) surfaces with (2×1), p(2×2), and c(4×2) surface unit cells and Sb covered Ge(001) with a (2×1) unit cell are calculated using a local orbital hybrid density functional theory approach. The weight of Fock exchange in the DFT Hamiltonian is chosen so that bulk dielectric function critical point transition energies and dielectric constants for Si and Ge agree with experimental values. The RA spectrum calculated for the c(4×2) phase of Si(001) is in excellent agreement with experiment. RA spectra calculated for Ge(001) clean surface phases show two prominent minima below 2 eV. The RA spectrum of the clean Ge(001)–(2×1) surface was measured at room temperature down to 0.4 eV and found to be in agreement with currently available experimental data above 1.5 eV. These RA spectra show a single minimum in the range up to 2 eV, but do not show a second minimum where another minimum is predicted by calculation. Wavefunction amplitudes at the Γ‾ point of the surface Brillouin zone for the two occupied surface states of Si(001)’c(4×2).

Materials for light-induced water splitting: In situ controlled surface preparation of GaPN epilayers grown lattice-matched on Si(100)

Energy storage is a key challenge in solar-driven renewable energy conversion. We promote a photochemical diode based on dilute nitride GaPN grown lattice-matched on Si(100), which could reach both high photovoltaic efficiencies and evolve hydrogen directly without external bias. Homoepitaxial GaP(100) surface preparation was shown to have a significant impact on the semiconductor-water interface formation. Here, we grow a thin, pseudomorphic GaP nucleation buffer on almost single-domain Si(100) prior to GaPN growth and compare the GaP0.98N0.02/Si(100) surface preparation to established P- and Ga-rich surfaces of GaP/Si(100). We apply reflection anisotropy spectroscopy to study the surface preparation of GaP0.98N0.02 in situ in vapor phase epitaxy ambient and benchmark the signals to low energy electron diffraction, photoelectron spectroscopy, and x-ray diffraction. While the preparation of the Ga-rich surface is hardly influenced by the presence of the nitrogen precursor 1,1-dimethylhydrazine (UDMH), we find that stabilization with UDMH after growth hinders well-defined formation of the V-rich GaP0.98N0.02/Si(100) surface. Additional features in the reflection anisotropy spectra are suggested to be related to nitrogen incorporation in the GaP bulk.

Reflectance anisotropy spectroscopy of metal nanoclusters formed on semiconductor surface

Optical anisotropy of indium nanoclusters formed on the (001) surface of indium arsenide was found by differential reflectance anisotropy spectroscopy. The fact of such an observation for nanocluster arrays unambiguously evidences the presence of their macroscopic anisotropy which could not be disclosed by conventional diagnostics techniques. The scale of the observed plasmonic anisotropy signal exceeds by two orders of magnitude the scale of anisotropy signals from valence-bond structures formed on a semiconductor surface. A resonant feature observed in reflectance anisotropy spectra is interpreted in the model of coupled dipole plasmons belonging to ellipsoidal nanoparticles. Estimation based on the experimental spectra shows that within the sample surface the lengths of ellipsoid semiaxes differ from each other by a few percent.

The influence of the structure of the Au(110) surface on the ordering of a monolayer of cytochrome P450 reductase at the Au(110)/phosphate buffer interface.

The reflection anisotropy spectra (RAS) observed initially from Au(110)/phosphate buffer interfaces at applied potentials of −0.652 and 0.056 V are very similar to the spectra observed from ordered Au(110) (1 × 3) and anion induced (1 × 1) surface structures respectively. These RAS profiles transform to a common profile after cycling the potential between these two values over 72 h indicating the formation of a less ordered surface. The RAS of a monolayer of a P499C variant of the human flavoprotein cytochrome P450 reductase adsorbed at 0.056 V at an ordered Au(110)/phosphate buffer interface is shown to arise from an ordered layer in which the optical dipole transitions are in a plane that is orientated roughly normal to the surface and parallel to either the [11̄0] or [001] axes of the Au(110) surface. The same result was found previously for adsorption of P499C on an ordered interface at −0.652 V. The adsorption of P499C at the disordered surface does not result in the formation of an ordered monolayer confirming that the molecular ordering is strongly influenced by both the local structure and the long range macroscopic order of the Au(110) surface.

Reflectance anisotropy spectroscopy of Si(111)-(3×1)Li and Ag surfaces

The Si(111)-($3\ifmmode\times\else\texttimes\fi{}1$)Li and Ag surface structures consist of Si honeycomb chains separated by channels occupied by Li or Ag atoms. The dimerization of neighboring Ag atoms results in a larger $c(12\ifmmode\times\else\texttimes\fi{}2)$Ag superstructure. We report calculations of the electronic surface states, dielectric function, and reflectance anisotropy spectrum of these surfaces using hybrid density functional theory (DFT) methods. The positions of surface states and the reflectance anisotropy spectrum calculated using DFT are in very good agreement with experimental results, where they have been reported. The surface states which contribute to the reflectance anisotropy below the band gap are identified for each orientation of the incident electromagnetic field.