Local-field effects in reflectance anisotropy spectra of the (001) surface of gallium arsenide

Characteristic reflectance anisotropy spectra of the naturally oxidized (001) surfaces of GaAs undoped crystals and Ga_0.7Al_0.3As epitaxial films are measured in the energy range 1.5–5.7 eV. The spectra are interpreted in the framework of the microscopic model proposed for a GaAs(001)/oxide interface and the reflectance anisotropy (difference) theory developed for a multilayer medium with a monolayer of atomic dipoles located near one of the interfaces. The anisotropy of dipole polarizability and the anisotropy of the plane lattice formed by dipoles are taken into account within the unified Green function approach of classical electrodynamics. A good agreement between the measured and calculated reflectance anisotropy spectra of the oxidized GaAs(001) surfaces shows that the local field effects at the semiconductor-oxide interface make the main contribution to these spectra.     

A PCA study to determine how features in meteorite reflectance spectra vary with the samples’ physical properties

Meteorites have advanced our knowledge of processes in the Solar System with the application of high precision instruments here on Earth. The study of asteroids, the source of most meteorites, has in turn given us knowledge regarding the large scale evolution of the Solar System. Using the complementary information that asteroids and meteorites give us the story of our cosmic backyard can be more easily read. One efficient way to link meteorites to asteroids is by matching their respective reflectance spectra. There have been few convincing matches because of observational and scale differences as well as an incomplete knowledge of the light scattering physics involved. To better interpret the reflectance data we need to know the dependencies of the reflectance on physical properties and develop techniques for better comparisons of data sets. For these purposes we utilise our own measurements of 26 different meteorites together with spectra available on the NASA PDS.We find that normalisation of reflectance at a wavelength between 1.1 and 1.3 μm gives the closest match of spectra from meteorites common to both data sets. The depth of the spectra bands deepens by similar amounts for different types of surface texture alterations i.e. rock to sawn surface, rock to polished surface and rock to powdered surface. Principal Component Analysis (PCA) is able to easily place carbonaceous chondrites, ordinary chondrites and achondrites into distinct groups using their reflectance spectra. We track the variation of spectral features in principal component space by using a set of meteorite spectra synthesised from mineral and elemental spectra. A spectral agent that reduces the reflectance at all wavelengths is required, in addition to olivine, pyroxene and carbon, to generate a set of synthesised spectra to match the distribution of measured spectra, in principal component space.     

Optical anisotropy of the SiC((001))- (3x2) surface: evidence for the two-adlayer asymmetric-dimer model

The structure of the ( 3x2) reconstruction of beta-SiC(001) surface has been identified by comparing reflectance anisotropy spectra calculated from first principles with recent measurements. Only the calculations for the two-adlayer asymmetric-dimer model agree with experiment. The two prominent peaks at 3.6 and 5.0 eV found experimentally are assigned to electronic transitions between surface and bulklike electronic states. A further pronounced anisotropy at 2.0 eV, due to transitions between surface states, is predicted.     

A combined scanning tunneling microscopy and reflectance anisotropy spectroscopy investigation of tetraphenylporphyrin deposited on graphite

The ordering properties of tetraphenylporphyrin molecules sublimated on a highly oriented pyrolitic graphite surface have been investigated. The morphological information obtained by scanning tunneling microscopy has been linked with the optical spectra by reflectance anisotropy spectroscopy. Measurements were performed in situ as a function of the nominal coverage.     

Precipitation and re-solution of impurities at the surface of indium on traversing the melting-point

In the course of preparing a clean indium surface for solid-versus-liquid studies, changes in the surface concentrations of sulphur and oxygen were observed by AES and XPS while the metal was heated and cooled through its melting-point. Both impurities disappeared on melting, and reappeared on solidification, over a very narrow temperature range; the disappearance and reappearance were to a certain extent reproducible. The effect was found to be similar in characteristics to that observed for the behaviour of carbon on a nickel surface by Shelton et al., and the same Bragg-Williams model is invoked to explain the sharpness of the impurity concentration changes with temperature. Although the maximum temperature reached by the indium was only 200°C, traces of platinum were also observed on the indium surface after melting, in both the AES and XPS spectra, probably as a result of solution from the platinum boat.     

Natural electron oscillations in granular indium films deposited on rough surfaces of NaCl and KCl single crystals

Two plasma resonance bands were simultaneously observed in granular indium films deposited on rough surfaces of NaCl and KCl single crystals; one of them is caused by a combined action of the light-wave field and of the field created by the dipole grains, and the other, by the light-wave field alone with the frequency of natural electron oscillations ω₀ in the grains. The indium plasma frequency, calculated from the measured frequencies ω₀ and known dielectric constants of NaCl and KCl, agrees well with the known plasma frequency of bulk indium. An anisotropy in the plasma resonance band of granular indium films deposited on a rough NaCl surface was found and interpreted.     

Optical anisotropy of GaNAs grown on GaAs(0 0 1) substrate

In this paper GaN_xAs_1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaN_xAs_1−x surface can be classified into three types of the surface.     

薄膜层状外延生长的光学原位实时监测方法

综述了几种用于监控薄膜外延生长的光学原位实时监测方法的进展。其中光反射差法／光反射各向异性谱（ＲＤＳ／ＲＡＳ）和ｐ偏振反射谱（ＰＲＳ），表面光吸收（ＳＰＡ），椭偏仪（ＳＥ）等。在外延过程中已观测到了薄膜层状外延周期振荡。     

Spectroscopy of stop bands in magnetic photonic crystals

The shape of the spectra of continuous visible-range radiation reflection from the surface (111) of a globular photonic crystal filled with magnetic materials was determined. The bands observed in the reflectance spectrum correspond to the appearance of stop bands of the photonic crystal. It was shown that position of the intensity maximum in the observed reflectance spectra depends on the type of a magnetic material introduced into photonic crystal pores.     

Peculiarities of the reflection spectra of PECVD nanocrystalline porous silicon films

Photoluminescent nanocrystalline porous silicon films have been formed on silicon substrates by plasma-enhanced chemical vapor deposition. In the visible spectral range, a nontrivial character of the reflection spectra (dependent on the film thickness) has been found. The sizes of crystallites and the root-mean-square roughness of the deposited film surface have been determined by X-ray diffraction and atomic force microscopy. It is shown that the significant decrease in the reflectance in the spectral range from 200 to 900 nm and the red shift of the characteristic peaks in the reflection spectra are related to the sizes of nanocrystallites in the films formed.     

Optical anisotropy of the In/Si(1 1 1)(4 × 1)/(8 × 2) nanowire array

Ab initio calculations of the reflectance anisotropy of Si(1 1 1)-In surfaces are presented. A very pronounced optical anisotropy around 2 eV is found that is related to In-chain states. The distortion of the indium chains characteristic for the (4 × 1) → (8 × 2) phase transition results in a splitting of the 2 eV peak, as observed experimentally. The splitting occurs irrespective wether the phase transition occurs according to the trimer or hexamer model.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

RAS在线监测AlGaAs的MOCVD生长

通过瞬态反射各向异性谱和瞬态反射谱在线监测和研究了AlxGa1-xAs的生长过程,利用金属有机化合物汽相淀积技术在GaAs (001)衬底上生长了多层AlxGa1-xAs结构.选择最适合在线监测生长过程的探测光能量,在此探测光能量处所得到的反射各向异性谱和反射谱的信号在生长过程中有很明显的振荡行为产生.研究发现,通过瞬...     

Surface electron accumulation in indium nitride layers grown by high pressure chemical vapor deposition

Surface termination and electronic properties of InN layers grown by high pressure chemical vapor deposition have been studied by high resolution electron energy loss spectroscopy (HREELS). HREEL spectra from InN after atomic hydrogen cleaning show N-H termination with no indium overlayer or droplets and indicate that the layer is N-polar. Broad conduction band plasmon excitations are observed centered at 3400 cm⁻¹ in HREEL spectra with 7 eV incident electron energy which shift to 3100 cm⁻¹ when the incident electron energies are 25 eV or greater. The shift of the plasmon excitations to lower energy when electrons with larger penetration depths are used is due to a higher charge density on the surface compared with the bulk, that is, a surface electron accumulation. These results indicate that surface electron accumulation on InN does not require excess indium or In-In bonds.     

Study of the turbulence and inner waves in the stratosphere based on the observations of stellar scintillations from space: A model of scintillation spectra

Abstract

Stellar scintillations observed from space through the atmosphere show that density inhomogeneities in the stratosphere are stretched along the Earth's surface. This is true for vertical scales above dozens of metres. The observations reveal the existence of locally isotropic small-scale structure with fluctuation sizes up to fractions of a metre. The subject of this paper is to find out how the rotation of inhomogeneities with respect to the passing ray affects scintillations. Another subject of this study is chromatic aberration in the atmosphere which distorts the scintillation spectra. Numerical modelling within the weak-fluctuation approximation showed that the characteristic value of the anisotropy parameter is equal to the square root of the Earth's radius divided by the atmospheric scale. After the anisotropy exceeds this value, the growth of scintillation variance quickly becomes saturated. Chromatic aberration suppresses the high-frequency branch of the scintillation spectrum. However, information on the structure of isotropic fluctuations with scales up to the Fresnel radius is retained in scintillation spectra for oblique occultations. The model of composing blocks is suggested to develop the approximation for a three-dimensional fluctuation spectrum in the stratosphere. Parameters of these blocks can be determined from the set of measured scintillation spectra.     

Optical anisotropy and surface morphology of InGaAs lattice-mismatched with GaAs(0 0 1)

We investigated surface morphology and optical anisotropy of strained InGaAs films grown on GaAs(0 0 1) substrate using atomic force microscopy (AFM) and reflectance difference/reflectance anisotropy spectroscopy (RDS/RAS). High temperature (HT)-grown samples were found to have a rippled surface structure, however for films grown using a low temperature (LT) growth technique, the surface morphology was significantly improved, without the ripple structure seen on the HT samples. Furthermore, ex situ RD spectra of LT-grown samples showed notable peaks near the critical energies of band structure originated from bulk electronic transitions.     

Dynamics of electron scattering between bulk states and the C<Subscript>1</Subscript> surface state of InP(100)

Hot electron dynamics was investigated, with a focus on scattering between bulk states and the C₁ surface state that is formed on the (2×4)-reconstructed In-rich surface of InP(100). The latter surface was prepared via metal organic chemical vapor deposition (MOCVD) and monitored by reflectance anisotropy spectroscopy (RAS/RDS). Two-color twophoton photoemission (2PPE) was employed with laser pulses of about 50 fs duration. Hot electrons were generated in bulk states about 0.5 eV above the C₁ surface state, thereby avoiding any significant direct optical population of the surface state. A time constant of 35 fs was determined from the experimental data for electron scattering from isoenergetic bulk states to the C₁ surface state by analyzing the rise of a C₁-specific peak in the 2PPE spectrum. The decay of this C₁ peak was ascribed to energy relaxation of the photo-generated electrons to bulk states below the surface state. Analogous measurements were carried out with a (2×1/2×2)-reconstructed P-rich surface of InP(100) that was also grown via MOCVD. No sign of a surface statewas detected in the 2PPE spectra for the latter surface in the corresponding energy range of the conduction band. PACS 73.20.-r; 78.47.+p; 79.60.-i; 79.60.Bm     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Optical recognition of atomic steps on surfaces

Visible and UV light spectra display striking differences in optical reflectivity for the two types of monatomic steps on copper (111) surfaces. Electronic structure calculations trace these differences to the specific contributions of p(axially) and p(radially) local densities of states, parallel and perpendicular to the steps, of the distinctly coordinated corner atoms. The local origin of the spectral reflectance anisotropy is further corroborated by experiments on Cu(111) surfaces with varying step densities. Site specificity of the electronic structure of atoms in low coordinated sites on Cu(111) vicinals is thus revealed by reflectance anisotropy spectroscopy which can thereby detect step atom densities down to 10(13) atoms/cm(2).     

Thermal annealing dependence of some optical properties of plasma-modified porous silicon

The photoluminescence and reflectance of porous silicon (PS) with and without hydrocarbon (CH_x) deposition fabricated by plasma enhanced chemical vapour deposition (PECVD) technique have been investigated. The PS samples were then, annealed at temperatures between 200 and 800 °C. The influence of thermal annealing on optical properties of the hydrocarbon layer/porous silicon/silicon structure (CH_x/PS/Si) was studied by means of photoluminescence (PL) measurements, reflectivity and ellipsometry spectroscopy. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy. Photoluminescence and reflectance measurements were carried out before and after annealing on the carbonized samples for wavelengths between 250 and 1200 nm. A reduction of the reflectance in the ultraviolet region of the spectrum was observed for the hydrocarbon deposited polished silicon samples but an opposite behaviour was found in the case of the CH_x/PS ones. From the comparison of the photoluminescence and reflectance spectra, it was found that most of the contribution of the PL in the porous silicon came from its upper interface. The PL and reflectance spectra were found to be opposite to one another. Increasing the annealing temperature reduced the PL intensity and an increase in the ultraviolet reflectance was observed. These observations, consistent with a surface dominated emission process, suggest that the surface state of the PS is the principal determinant of the PL spectrum and the PL efficiency.