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Ultra-thin layers (< 8 nm) of a Bi 2 Te 3 topological insulator have been grown on GaAs (100) substrates using molecular beam epitaxy. The growth was performed from a single Bi 2 Te 3 effusion cell and one source of extra tellurium. Optical and structural characterizations were carried out through Raman spectroscopy, x-ray diffraction, atomic force microscopy, and scanning electron microscopy. The topological insulator properties were also investigated by angle-resolved photoelectron spectroscopy. A layer of 5 nm showed Dirac cone-like linear electronic band dispersion, indicating the signature of a topological insulator with the Dirac point having large binding energy relative to the Fermi level as expected for ultra-thin films. Topological insulator properties were also investigated at the initial growth stage where deposition follows an islandlike growth mode. Our results can contribute to the development of practical chalcogenide-based thin-film spintronics devices. [ABSTRACT FROM AUTHOR]

Cd1−xMnxTe (CMT) is a wide bandgap semiconductor that stands out among the ternary compounds in several industrial applications, such as optoelectronic and solar cell devices. Therefore, it is worth understanding the mechanisms of light interaction with CMT produced by growth techniques. In this work, we investigate the room temperature Raman scattering by longitudinal optical phonon modes CdTe-like (LO1) and MnTe-like (LO2) in CMT thin films grown on Si(111) by molecular-beam epitaxy. The well-known linear dependence of LO1 and LO2 frequencies on x is observed in Raman spectra when the excitation photon energy is above the bandgap energy. As expected, for the excitation energy of the incident laser line near the fundamental gap of CMT, the resonance becomes evident in the Raman results. However, for a specific resonance condition due to tuning of the bandgap energy dependence on Mn concentration, the frequencies of the LO1 and LO2 phonon modes remain constant as the average manganese concentration increases to values x ≳ 0.30. From micro-photoluminescence spectroscopy investigations, we concluded that for Mn concentration above 0.3, a broad range of optical transitions provides the required conditions for a resonant Raman scattering selected by the incident photon energy. It introduces a resonant selectivity of regions where the incident excitation energy coincides with a bandgap energy associated with a specific value of x, even for samples with nominally different compositions. The results show the sensitivity of resonant Raman to express diagnostics of ternary compound growth. [ABSTRACT FROM AUTHOR]

Hematite (α-Fe2O3) thin films were prepared by sol-gel route and investigated for application in H2 generation by photo-assisted water splitting. The photoelectrochemical (PEC) performance was shown to increase significantly for films deposited on SnO2:F/glass subjected to high temperature (T) annealing (>750?°C). Strong correlation was found between photogenerated current, donor concentration, and Sn concentration as determined by Mott-Schottky analysis and X-ray photoelectron spectroscopy. The effects of thermal annealing and Sn addition in the resulting microstructure and optical properties of hematite films deposited on fused silica substrates were determined by a combination of structural characterization techniques and spectroscopic ellipsometry. Thermal annealing (>600?°C) induces a higher optical absorption that is associated directly to film densification and grain growth; however, it promotes no changes in the energy positions of the main Fe2O3 electronic transitions. The band gap energy was found to be 2.21?eV and independent of microstructure and of Sn concentration for all studied films. On the other hand, Sn can be incorporated in the Fe2O3 lattice for concentration up to Sn/Fe ~2%, leading to an increase in energy split of the main absorption peak, attributed to a distortion of the Fe2O3 lattice. For higher concentrations, Sn incorporation leads to a reduction in absorption, associated with higher porosity and the formation of a secondary Sn-rich phase. In summary, the variation in the optical properties induced by thermal annealing and Sn addition cannot account for the order of magnitude increase of the current density generated by photoanodes annealed at high T (>750?°C); thus, it is concluded that the major contribution for the enhanced PEC performance comes from improved electronic properties induced by the n-type doping caused by Sn diffusion from the SnO2:F substrate. [ABSTRACT FROM AUTHOR]

We have investigated the crystalline configuration of CdTe quantum dots (QDs) grown on hydrogen passivated Si(111) substrates by hot wall epitaxy. Coplanar and grazing incidence diffraction were used for determination of dot strain state and the vertical and lateral dimensions of the crystalline domain. A change in aspect ratio was observed as a function of dot size. X-ray diffraction (XRD) results show that despite a mismatch of almost 20% the islands grow with a fairly good epitaxial orientation with respect to the Si(111) substrate. The dot mosaicity was also determined and was found to decrease with island size from 7° to about 4° for the samples studied, indicating an improvement in epitaxial quality even before the island coalescence. Careful observation of [formula] reflections in an azimuthal scan showed that an additional ensemble of islands is responsible for low-intensity peaks with a 30° symmetry besides the expected 60° symmetry. Transmission electron microscopy results have shown good accordance with atomic force microscopy and XRD and revealed the presence of an amorphous Tellurium rich oxide layer at the CdTe/Si interface, which could explain the fully unstrained QD state observed. [ABSTRACT FROM AUTHOR]

In this study, photopyroelectric spectroscopy (PPES) was used to obtain thermal and optical properties of CdTe thin films in the 300–1200 nm wavelength range. The samples studied were grown on glass or BaF2 substrates using the hot wall epitaxy technique with varying thicknesses. The normalized photopyroelectric signal intensity and its phase were independently measured as a function of wavelength λ and chopping frequency f. Equations of both the intensity and the phase of the PPES signal, taking into account the thermal and the optical characteristics of the pyroelectric detector, were used to fit the experimental results. From the fittings, we have obtained the values of thermal diffusivity coefficient α, thermal conductivity k, optical absorption coefficient, and the optical gap of CdTe. [ABSTRACT FROM AUTHOR]

We report on the growth and characterization of CdTe quantum dots on Si(111) by direct island nucleation. The samples were grown by hot wall epitaxy on Si(111) substrates treated with diluted HF solution only and the resulting hydrogen-terminated surface permitted the growth of good quality CdTe layers using substrate temperatures below 300 °C. The samples, grown at very low growth rates, were investigated by atomic force microscopy. Our measurements show that this system follows the Volmer-Weber growth mode, with nucleation of isolated CdTe islands on the Si substrate surface even for just 0.6 monolayers of evaporated material. As the growth proceeds, the density and size of quantum dots increase until the point at which they start to coalesce to form a uniform layer. We describe the size and density distribution of these islands as a function of growth time and substrate temperature. The results show that the Volmer-Weber growth mode can be successfully used to obtain self-assembled quantum dots of CdTe on Si, with reasonable size dispersion, using an inexpensive growth technique. [ABSTRACT FROM AUTHOR]

We demonstrate thin-film GaSb solar cells which are isolated from a GaSb substrate and transferred to a Si substrate. We epitaxially grow ∼3.3 μm thick GaSb P on N diode structures on a GaSb substrate. Upon patterning in 2D arrays of pixels, the GaSb films are released via epitaxial lift-off and they are transferred to Si substrates. Encapsulation of each pixel preserves the structural integrity of the GaSb film during lift-off. Using this technique, we consistently transfer ∼4 × 4 mm2 array of pixelated GaSb membranes to a Si substrate with a ∼ 80%–100% yield. The area of individual pixels ranges from ∼90 × 90 μm2 to ∼340 × 340 μm2. Further processing to fabricate photovoltaic devices is performed after the transfer. GaSb solar cells with lateral sizes of ∼340 × 340 μm2 under illumination exhibit efficiencies of ∼3%, which compares favorably with extracted values for large-area (i.e., 5 × 5 mm2) homoepitaxial GaSb solar cells on GaSb substrates. [ABSTRACT FROM AUTHOR]

A new subgrid-scale (SGS) model developed for large-eddy simulations (LES) of dilute polymer solutions, described by the finitely extensible nonlinear elastic constitutive equation closed with the Peterlin approximation, is presented. In this distortion similarity model (DSIM) the filtered conformation tensor evolution equation is based on the self-similarity of the polymer stretching terms, and on a global equilibrium of the trace of the conformation tensor, which is proportional to the elastic energy stored in the polymer molecules, while the SGS stresses are modelled with the classical Smagorinsky model. The DSIM closure is assessed in direct numerical simulations (DNS) of forced isotropic turbulence using classical a priori tests, and in a posteriori (LES) showing very good agreement with all the exact (filtered DNS) results. The DSIM model is simple to implement and computationally inexpensive and represents a major step forward in the numerical simulation of turbulent flows of Newtonian fluids with polymer additives. [ABSTRACT FROM AUTHOR]

Presents information on a study that investigated the coherent dynamics of excitons in Zn[sub1-x]Cd[subx]Se/ZnSe multiple quantum wells. Experimental procedure; Results and discussion on the study; Conclusion.

We have studied ferromagnetic nickel thin films patterned with square lattices of elongated antidots that are negative analogues of square artificial spin ice. Micromagnetic simulations and direct current magnetic moment measurements reveal in-plane anisotropy of the magnetic hysteresis loops, and the formation of a dense array of magnetic vortices with random polarization and chirality. These multiply-connected antidot arrays could be superior to lattices of disconnected nanodisks for investigations of vortex switching by applied electric current. [ABSTRACT FROM AUTHOR]

We have studied the influence of substrate temperature on the Hurst and growth exponents of CdTe thin films grown on glass substrates covered by fluorine doped tin oxide. The sample roughness profile was measured with a stylus profiler at different growth times and substrate temperatures in order to determine the critical exponents. The Hurst exponent increases linearly from 0.72 to 0.8, whereas the growth exponent increases exponentially from 0.14 to 0.62, for temperatures between 150 and 300 °C. The global roughness also increases with growth temperature, which turns to be a very good parameter for roughness control. [ABSTRACT FROM AUTHOR]