Y. S. Raptis

Temperature rise induced by a cw laser beam revisited

The problem of spatial distribution of the temperature rise due to a cw laser beam focused on the surface of an absorbing material is reexamined. The effect of temperature dependence of the absorption and reflection coefficients is incorporated in the calculations in a self‐consistent way. A Green’s function has been developed for the heat diffusion equation in the axially symmetric case and a general steady‐state solution is obtained for an arbitrary source function. Compared with previous results, our calculations predict melting at lower laser power densities and changes in the spatial temperature distribution. In the limit of large absorption constants these differences are small. However, they become increasingly significant as the absorption constant is decreased.

Stabilized cubic zirconia: A Raman study under uniaxial stress

Yttria‐stabilized cubic zirconia has been studied at 300 K by use of Raman spectroscopy under uniaxial stress along the [001] and [111] directions. The main effects observed are the frequency shifts of the F2g‐type Raman band at ∼610 cm−1 which vary linearly with the applied stress and tend to increase with Y2O3 concentration. Effective deformation potentials are determined for the F2g band. These potentials are necessary for strain characterization of stabilized cubic zirconia, in buffer or epitaxial film configurations. Polycrystalline yttria‐stabilized tetragonal zirconia is likewise studied.

Surface-related phonon mode in porous GaP

Abstract Porous GaP layers prepared by electrochemical anodization of (1 0 0) and (1 1 1) A-oriented n -GaP crystalline substrates in HF solution have been studied by Raman spectroscopy. A surface vibrational mode at 397 cm −1 was observed in porous GaP. The process of anodization results in downward shifts of the TO and LO phonon frequencies, which are attributed to phonon confinement in crystalline GaP particles of nanometer size.

Nanostructured titania films sensitized by quantum dot chalcogenides

The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

Micro-Raman Study of Orientation Effects of CuxSe-Crystallites on Cu-rich CuGaSe2 Thin Films

The structural properties of copper selenide (CuxSe), formed as a secondary phase on the surface of CuGaSe2 films grown under Cu-rich conditions on GaAs (100) substrates, were studied by micro-Raman spectroscopy. Raman bands at 45 and 263cm−1, observed on crystallites dispersed on the CuGaSe2 film, are in agreement with the Raman modes of CuSe and Cu2Se. Polarization- and angular-dependent micro-Raman measurements reveal that the CuxSe-crystallites are grown with a preferential orientation on the CuGaSe2 surface.

Arsenic passivation of MBE grown GaAs(100): structural and electronic properties of the decapped surfaces

Abstract The passivation properties and thermal stability of As on GaAs(100) were studied under ultra-high vacuum (UHV) conditions for As caps of different thicknesses and storage times in air. The As desorption was monitored by mass spectrometry (QMS) and the substrate surface quality by means of Auger electron spectroscopy (AES), electron diffraction (LEED) and Raman spectroscopy (RS). Additional desorption experiments were performed in an oven in N 2 atmosphere. These samples were then investigated by spatially resolved Raman spectroscopy under atmospheric conditions. The Raman spectra of the air exposed samples show that the cap consists of an amorphous As layer with small As 2 O 3 crystallites on top. AES investigations reveal in addition strong carbon contaminations. Arsenic desorption in UHV takes place in two steps, a first one at around 160°C which can be attributed to the desorption of the As 2 O 3 crystallites and a second one at around 350°C due to the desorption of the amorphous As layer. From 360°C on with further increasing temperature (1 × 1) (2 × 4) and (4 × 1) reconstructions are observed by LEED. The band bendings at the surfaces as determined by Raman scattering were found to be not systematically related to the differently reconstructed surfaces. Residual carbon contaminations were found after Arsenic desorption in many cases, whereas the surfaces are always free of oxygen contaminations.

Angular dispersion of ‘‘backward’’ Raman scattering: Weakly absorbing cubic materials (Si)

We have shown that the angular distribution of Raman scattering power from the surface of diamond‐type opaque materials can be adequately described by use of classical optics. It turns out that within the commonly‐used nearly backward scattering geometries there are significant variations of the scattered power due to purely geometrical factors. Only crystals with small extinction coefficients are considered. Families of curves for the scattered power are derived in terms of the angles of incidence and detection and of the index of refraction for three of the most widely‐used crystal orientations. The analysis reveals the optimum scattering configuration for a given geometrical situation and provides a quantitative measure for the deviation from the selection rules when a ‘‘backward’’ geometry for forbidden scattering is less than exact. The importance of incidence and/or detection at the Brewster angle is examined. The experimental data obtained in Si are consistent with the theoretical predictions. The ...

Dynamic Stereochemical Activity of the Sn2+ Lone Pair in Perovskite CsSnBr3

Stable s(2) lone pair electrons on heavy main-group elements in their lower oxidation states drive a range of important phenomena, such as the emergence of polar ground states in some ferroic materials. Here we study the perovskite halide CsSnBr3 as an embodiment of the broader materials class. We show that lone pair stereochemical activity due to the Sn(2+) s(2) lone pair causes a crystallographically hidden, locally distorted state to appear upon warming, a phenomenon previously referred to as emphanisis. The synchrotron X-ray pair distribution function acquired between 300 and 420 K reveals emerging asymmetry in the nearest-neighbor Sn-Br correlations, consistent with dynamic Sn(2+) off-centering, despite there being no evidence of any deviation from the average cubic structure. Computation based on density functional theory supports the finding of a lattice instability associated with dynamic off-centering of Sn(2+) in its coordination environment. Photoluminescence measurements reveal an unusual blue-shift with increasing temperature, closely linked to the structural evolution. At low temperatures, the structures reflect the influence of octahedral rotation. A continuous transition from an orthorhombic structure (Pnma, no. 62) to a tetragonal structure (P4/mbm, no. 127) is found around 250 K, with a final, first-order transformation at 286 K to the cubic structure (Pm3̅m, no. 221).

Micro-Raman characterization of In x Ga 1 − x N ∕ Ga N ∕ Al 2 O 3 heterostructures

In{sub x}Ga{sub 1-x}N/GaN/Al{sub 2}O{sub 3} (0001) heterostructures with x=10.5%, 13.5%, 19.0%, 19.6%, and 26.5% are studied, by polarized micro-Raman spectroscopy, under plane and side backscattering geometries. The combination of both scattering geometries, together with variable excitation wavelengths, enabled the possibility to check independently strain-vs-depth distribution and selective-resonance effects from In-rich regions. Several Raman modes have been detected and were attributed to either the GaN or the InGaN films. Particular modes, which are not permitted in the bulk materials, are activated in the InGaN layers. Shifts of the frequencies relative to the ones expected in the bulk materials are explained as due to the elastic strains present in the hetero-structures. The results are evaluated in combination with compositional RBS analysis and the strain values obtained are compared with high resolution x-ray diffraction results including reciprocal space mapping, leading to very good consistency between Raman and XRD. Consequent relaxation values are obtained and the underlying mechanisms are discussed.

Angular dispersion of ‘‘backward’’ Raman scattering: Absorbing III‐V semiconductors (GaAs)

A previously developed model for the angular dispersion of Raman scattering power from the surface of absorbing centrosymmetric cubic materials is extended to absorbing materials of the zinc blende type. Independent expressions have been obtained which give the scattered power due to the longitudinal and transverse long‐wavelength optical phonons as a function of external scattering geometry parameters; from these calculations, it has been found that the ratio of the scattered powers shows significant dependence on the angles of incidence and detection. These results provide a quantitative method in assessing the effects of choosing an approximate rather than the exact backward scattering geometry. Experimental results obtained from GaAs are in very good agreement with the predictions of the model. Examples of physical situations are given where angular dispersion corrections become necessary.