Remotely doped In(0.35)Ga(0.65)As layers of different coverages 6, 9, 11, and 13 ML were grown by molecular beam epitaxy on (100) GaAs. Quantum dot (QD) nucleation was observed in situ by reflection high-energy electron diffraction at 8 ML growth of In(0.35)Ga(0.65)As, while for 6 ML, only two-dimensional (2D) growth was observed. Atomic force microscopy, low temperature photoluminescence, and Hall effect measurements confirmed this transition from 2D to three-dimensional growth. Low-frequency noise studies have been performed to probe defects in such heterostructures throughout the transition from a highly strained quantum well to QDs. Results were compared to a bulk n-type GaAs reference sample. We revealed three main defects in GaAs with activation energies of 0.8, 0.54, and 0.35 eV. These defects with the same activation energies were found in all samples. However, structures containing In(0.35)Ga(0.65)As QDs show an additional peak at low temperatures due to the presence of defects which are not observed for reference GaAs and quantum well samples. Detailed analysis shows that for 9 and 11 ML In(0.35)Ga(0.65)As QD samples this peak corresponds to the well known M1 defect in GaAs with an activation energy of 0.18 eV, while for a coverage of 13 ML the defect was found to have an activation energy of 0.12 eV. All defects were characterized quantitatively in terms of their activation energy, capture cross section, and density. These studies indicate that noise spectroscopy is a very sensitive tool for electronic material characterization on the nanoscale.

Long chains of quantum dots formed in InGaAs∕GaAs(100) multiple layers have been systematically investigated by scanning electron, transmission electron, and atomic force microscopies. In addition to the usual two-dimensional wetting layer involved in the Stranski-Krastanov growth, we have directly observed a one-dimensional postwetting layer along the [01−1] direction that strings together the quantum dots in each chain. In sharp contrast with the two-dimensional wetting layer, which exists before the quantum-dot chains form, this one-dimensional postwetting layer develops during the GaAs capping of the existing dot chains. This one-dimensional layer forms through the anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a one-dimensional postwetting layer as a result.

We report on Ga nano‐droplets on GaAs(100) that are not stable under arsenic flux. Spontaneous evolution in shape leads to many interesting GaAs nanostructures. GaAs nano‐crystals shaped like lighted candles and square‐holed round coins are observed under different growth conditions. The underlying physics of the formation of these interesting nano‐structures can be understood in terms of GaAs growth under a uniform arsenic flux and a non‐uniform Ga supply from the Ga nano‐droplets. These novel shaped GaAs nanostructures, in an AlGaAs matrix, offer promising applications in optoelectronics. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the use of (In,Ga)As/GaAs multilayer stacking at elevated growth temperatures to produce enhanced in-plane spatial ordering. Cross-sectional transmission electron microscopy images reveal that the (In,Ga)As islands are vertically correlated while atomic force microscopy images demonstrate lateral ordering of quantum dots that are closely aligned along the [0 (1) over bar1] direction as chains which are themselves positioned periodically along the [011] direction. The in-plane spatial ordering along the [0 (1) over bar1] and [011] directions is directly seen by asymmetric (311) glancing exit x-ray diffraction with the x-ray beam along the respective direction. Growth studies as a function of temperature indicated that the observed lateral ordering results from enhanced surface diffusion and the vertical transfer of corresponding anisotropic strain pattern due to the anisotropy of surface diffusion.

The behavior of InAs deposition on GaAs(111)B substrates and the corresponding routes toward strain relaxation have been investigated. InAs growth was for depositions ranging from 2 monolayers to 30 monolayers. Over this deposition range, different routes for strain relaxation caused by the lattice mismatch were observed. The strain relaxed through ragged step edge formation and Ga-In intermixing for low InAs deposition and through the formation of step bunching and dislocations for thicker depositions.

Molecular-beam-epitaxy growth of strained (In,Ga)As on GaAs vicinal (100) surfaces is investigated by scanning tunneling microscopy. Surface roughing as the consequence of step bunching driven by strain is explored. By tuning the In content over the range from 0.05 to 0.2, the step bunching is observed to exhibit considerable uniformity and order. These results experimentally demonstrate that strain-driven step bunching is a viable approach to provide templates for nanostructure growth.

The shape of InAs three-dimensional islands grown on GaAs(311)A substrates by molecular-beam epitaxy was investigated by in situ scanning tunneling microscopy. The island is found to be laterally surrounded by (111)A and {110} facets together with a convex curved region close to the (100) facet. The top ridge of the islands is atomically resolved to be the most recently discovered high-index surface {11,5,2}. This observation points to the importance of the study of nanostructure growth on high-index surfaces and their characterization.

Morphologies of GaAs(311) surfaces grown by molecular beam epitaxy were investigated by in situ reflection high-energy electron diffraction and scanning tunnelling microscope. In addition to the (8×1) reconstruction, two surface phases, GaAs(311)A-(4×1) and GaAs(311)B-(2×1) were observed. Both of these surfaces are characterized by wider, atomically smooth terraces with much lower structural anisotropy, when compared to the (8×1) reconstructed GaAs(311) surfaces. The observed surfaces have potential as templates for the growth of organized quantum dots, wires, and wells.

GaAs(311) surfaces grown by molecular beam epitaxy are investigated by in situ ultrahigh-vacuum scanning tunnelling microscopy. The observation of an atomically flat Ga(2x1)-reconstructed GaAs(311) surface and its transformation to a 8x1-reconstructed GaAs(311) surface leads to an improved understanding of the processes involved in the step formation. The high density of steps observed on the 8x1-reconstructed GaAs(311) surface along the [(2) over bar 33] direction originates from the change of surface atomic density required to accommodate the surface transition from the Ga(2x1) surface to the 8x1 surface. This understanding is further supported by the observation of independent step formation.

Deposition of polysilicon at low temperatures is important for the low cost production of electronic devices, especially large area devices like solar cells and active matrix liquid crystal displays (AMLCD). In this paper, a model for crystallization of hydrogenated amorphous silicon (a-Si:H), based on in-situ x-ray diffraction studies, is reported. The a-Si:H films, intrinsic, as well as phosphorus-doped were deposited on oxidized silicon substrates. Aluminum was deposited onto the a-Si:H using evaporation. X-ray diffraction analysis was done in an evacuated temperature controlled camera la glancing angle using thin film optics. The growth in the < 111 > silicon peak was monitored while annealing the samples at 250 and 275 degrees C. Area of the < 111 > silicon peak was used as an indicator of the volume crystallized. A crystallization model was developed for the change in the crystallized volume fraction. A good fit was obtained between the model and the experimental data. Phosphorus doping in a-Si:H was shown to retard crystallization rate.

The objective of this study was to evaluate the effect of 14 N phosphoric acid and 12 N hydrochloric acid treatments on soy hull carbon structure and adsorption performance. Scanning electron micrographs showed that acid treatments disrupted the carbon structure whereas X-ray diffraction data indicated that hydrochloric acid had a greater effect than phosphoric acid in creating a more amorphous carbon structure. Both acids significantly reduced carbon pH from 9.8, but hydrochloric acid reduced the pH to a much greater degree (pH 2.77) than phosphoric acid (pH 5.95). Both acids increased the carbon titratable surface charge. The greatest increase was in surface lactone and phenol groups, but increases were also seen in noncarboxylic carbonyls and the generation of carboxyls. Acidified carbon was most effective as a phospholipid adsorbent, and its lower affinity for other oil components may be due to competitive adsorption. Nonacidified carbon and hydrochloric acid-treated carbon adsorbed most free fatty acids, whereas the nonacidified control and commercial carbon control were most effective at adsorbing peroxides. However, vacuum adsorption conditions alone increased the contents of saturated carbonyls that were probably formed by peroxide cleavage.

Deposition of polysilicon at low temperatures is important for the low cost production of electronic devices, especially solar cells and active matrix liquid crystal displays (AMLCD). In this paper in situ X-ray diffraction studies of aluminum enhanced crystallization of hydrogenated amorphous silicon (a-Si:H) at low temperatures is reported. Hydrogenated amorphous silicon films were deposited on oxidized silicon substrates. Aluminum was deposited on the a-Si:H using evaporation. X-ray diffraction analysis was done in an evacuated temperature controlled camera at a glancing angle of 5° using thin film optics. Growth in the 〈111〉 silicon peak was monitored while annealing the samples at 200, 225, 250 and 275°C. Results show that the area under the 〈111〉 silicon peak grows linearly with time. The rate of crystallization was found to be higher at higher temperatures. Al-induced crystallization was found to follow an Arrhenius type dependence. The activation energy for the thermally activated phenomenon controlling crystallization was calculated to be 1.1 eV. This is close to the reported activation energy of 1.2 eV for the diffusion of Al in a-Si:H.

Impurities and growth-related defect structures are mainly responsible for low thermal conductivity of chemical vapor deposited diamond films. Different quality arc-jet-deposited, free-standing diamond samples were obtained from industry. Fourier transform infrared (FTIR), Raman, and x-ray photoelectron spectroscopy (XPS) were used to determine the quality of these samples. The nondiamond carbon was estimated from the 1560 cm−1 broad peak intensity, the CHx integrated peak absorbance, and the C1s plasmon loss features for Raman, FTIR, and XPS studies, respectively. The diamond quality was also determined from the Raman diamond peak full width at half maximum (FWHM) and XPS valence band spectra. It was observed that the higher the hydrogen content (determined by FTIR), the darker the color of the film, the larger the nondiamond 1560 cm−1 peak intensity, and the larger the FWHM of the Raman diamond peak at 1332 cm−1. Negligible difference in the C1s diamond bulk plasmon loss peak was observed for films of wide ranging quality. The FTIR CHx band exhibited the highest sensitivity to film quality. Impurity-related peaks were observed in the one phonon region of the FTIR spectra and the photoluminescence spectra. The photoluminescence background peak centered at 2.0 eV was found to be strongly related to nondiamond carbon impurities. It is shown that a combination of different analytical tools is required to determine diamond quality.

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO3: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10−9 A/W using a nonstationary measurement technique at a wavelength of 0.632 μm.

The transient response for beam fanning in photorefractive tungsten bronze crystals with an applied electric field and a focused laser beam is studied. Response times on the order of 1 ms for an incident power of 1 mW observed for incident beams focused to a 30 μm diameter in the crystal. This improved response time is accomplished without the significant reduction in the magnitude of the photorefractive effect normally observed for focused beams.

Self-trapping of optical beams due to the photorefractive (PR) effect, which has been recently predicted (1,2) , is now observed for the first time.

We present both a theoretical and experimental analysis of dynamic holographic interferometry using a reflection grating geometry in Bi12TiO20.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A physical model for the photorefractive effect consists of considering two laser beams which are allowed to cross in a photorefractive crystal, producing an interference pattern. This interference pattern, or alternating light and dark regions in the crystal, results in the selective excitation of free-carriers from impurity trapped sites in the illuminated regions into the conduction band. In the conduction band, there is a subsequent diffusion of these electrons to neighboring dark regions where they are re-trapped at empty impurity sites. This transport of charge develops a space-charge field which, at equilibrium, produces a restoring force that exactly balances the diffusive force experienced by the excited free-carriers.