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A simple mathematical argument explains a recently identified route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained. [ABSTRACT FROM AUTHOR]

Pulsed-laser melting (PLM) is commonly used to achieve a fast quench rate in both thin films and nanoparticles. A model for the size evolution during PLM of nanoparticles confined in a transparent matrix, such as those created by ion-beam synthesis, is presented. A self-consistent mean-field rate equations approach that has been used successfully to model ion beam synthesis of germanium nanoparticles in silica is extended to include the PLM process. The PLM model includes classical optical absorption, multiscale heat transport by both analytical and finite difference methods, and melting kinetics for confined nanoparticles. The treatment of nucleation and coarsening behavior developed for the ion beam synthesis model is modified to allow for a nonuniform temperature gradient and for interacting liquid and solid particles with different properties. The model allows prediction of the particle size distribution after PLM under various laser fluences, starting from any particle size distribution including as-implanted or annealed simulated samples. A route for narrowing the size distribution of embedded nanoparticles is suggested, with simulated distribution widths as low as 15% of the average size. [ABSTRACT FROM AUTHOR]

We report a band gap bowing parameter for In1-xAlxN of 4.7 eV from a study of high quality and homogenous samples with x=0.017–0.60. Optical absorption data were modeled to extract the band gaps in order to consider the complications of the band structure of In-rich InAlN, including the Burstein–Moss shift, nonparabolic conduction band, and broadening of the absorption edge. The alloy compositions were accurately determined using Rutherford backscattering spectrometry and the sample quality was evaluated using x-ray diffraction and channeling-RBS. [ABSTRACT FROM AUTHOR]

We report on ultrahigh resolution studies of the bound exciton states associated with the shallow acceptor B and the shallow donor P in highly enriched 28Si using a tuneable single frequency laser to perform photoluminescence excitation spectroscopy. The linewidths and fine structure of the transitions, which were too narrow to be resolved previously using an available photoluminescence apparatus, are now fully revealed. The P bound exciton transition shows a complicated additional structure, which the Zeeman spectroscopy demonstrates to be a result of the splitting of the donor ground state by the hyperfine interaction between the spin of the donor electron and that of the 31P nucleus. The 31P nuclear spin populations can thus be determined, and hopefully modified, by optical means. The predominant Auger recombination channel of these bound excitons is used to observe the same resolved hyperfine transitions in the photocurrent spectrum. This demonstrates that donors in specific electronic and nuclear spin configurations can be selectively photoionized. Possible applications of these results to quantum computing and quantum information systems are discussed. [ABSTRACT FROM AUTHOR]

Isotopically pure 74Ge nanocrystals were formed in a sapphire matrix by the ion beam synthesis method. In contrast to those embedded in amorphous silica, sapphire-embedded nanocrystals are clearly faceted and are preferentially oriented with respect to the crystalline matrix. In situ transmission electron microscopy of heated samples reveals that the nanocrystals melt at 955±15 °C, very near to the bulk Ge melting point. The Raman spectra indicate that the sapphire-embedded Ge nanocrystals are under compressive stress in the range of 3–4 GPa. The magnitude of the stress is consistent with that expected for hydrostatic pressure arising from solidification. Stress relaxation was not observed for sapphire-embedded Ge nanocrystals; this is attributed to the slow self-diffusion rate of the alumina matrix atoms at temperatures below the nanocrystal melting point. [ABSTRACT FROM AUTHOR]

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In[sub 1-x]Ga[sub x]N alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in In[sub 1-x]Ga[sub x]N alloys. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Structural and electronic properties of amorphous and single-phase polycrystalline films of γ- and κ-In[SUB2]Se[SUB3] have been measured. The effect of deposition conditions on the film phase was studied extensively. The stable γ phase nucleates homogeneously in the film bulk and has a high resistivity, while the metastable k phase nucleates at the film surface and has a moderate resistivity. The microstructures of polycrystalline hot-deposited and postannealed, cold-deposited γ films are quite different but their electronic properties are similar. The increase in the resistivity of amorphous In[SUB2]Se[SUB3] films upon annealing is interpreted in terms of the replacement of In-In bonds with In-Se bonds during crystallization. Great care must be taken in the preparation of In[SUB2]Se[SUB3] films for electrical measurements as the presence of excess chalcogen or surface oxidation may greatly affect the film properties. [ABSTRACT FROM AUTHOR]

Photomodulation spectroscopy studies of the transitions at the F point of the Brillouin zone of thin films of B[sub x]Ga[sub 1-x]As alloys grown by metal-organic chemical-vapor deposition are presented. A very small increase of the fundamental band gap is found in samples with B content up to 3%. Under hydrostatic pressure, the band gap increases at a rate almost identical to the pressure dependence of the GaAs band gap. In contrast to the case of N incorporation at similar concentrations into GaAs, the experimental results show that B incorporation does not cause large modifications of the conduction-band structure in B[sub x]Ga[sub 1-x]As alloys. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the structural characteristics of zinc selenide thin films grown by organometallic vapor phase epitaxy and implanted heavily with chlorine ions using ion beam techniques, x-ray diffraction and Raman spectroscopy. Details of the experiment; Results and discussion.

Presents information on a study which investigated the effects of radiation damage and stoichiometry on the electrical activity of carbon implanted in gallium arsenide. Methods; Results; Discussion.

Focuses on a study which investigated diamond nucleation on unscratched silicon substrates using a conventional microwave plasma-enhanced chemical vapor deposition system. Effect of pretreatment process parameters on diamond nucleation; Methodology of the study; Results and discussion.

Presents a study that determined the combined electron and hole mobility of single-crystal type IIa natural diamond and a polycrystalline diamond film deposited by chemical vapor deposition using transient photoconductivity as a function of excitation decay and temperature. Background on the use of diamond for electronic applications; Experimental procedure; Results of signals taken at various temperatures at similar intensities and electric fields.

Deals with a study which investigated the effects of substrate temperature on chemical structure of amorphous carbon films. Background to the study; Experimental procedure; Structure of the microcrystallites.

Focuses on a study which discussed the isochronal and isothermal annealing characteristics of acceptor-doped GaAs:Be grown at low substrate temperatures by molecular-beam epitaxy (LTMBE). Result of the analysis of isothermal annealing kinetics for the removal of As[subGa]-related defects; Material that has generated great technological and scientific interest; Benefits from LTMBE buffer layers.

Presents a study which measured the tribology of a series of carbon thin films deposited on untextured magnetic disks by magnetron sputtering. Experimental details; Results and discussion; Conclusion.

We report measurements of the thermal conductivity of isotopically modulated silicon that consists of alternating layers of highly enriched silicon-28 and silicon-29. A reduced thermal conductivity of the isotopically modulated silicon compared to natural silicon was measured by means of time-resolved x-ray scattering. Comparison of the experimental results to numerical solutions of the corresponding heat diffusion equations reveals a factor of three lower thermal conductivity of the isotope structure compared to natural Si. Our results demonstrate that the thermal conductivity of silicon can be effectively reduced with isotopically modulated structures. This offers a promising approach to optimize silicon for thermoelectric applications. [ABSTRACT FROM AUTHOR]

We present a few electron double quantum dot device defined in an isotopically purified 28Si quantum well (QW). An electron mobility of 5.5·104cm2(Vs)-1 is observed in the QW, which is the highest mobility ever reported for a two-dimensional electron system in 28Si. The residual concentration of 29Si nuclei in the 28Si QW is lower than 103ppm, at the verge where the hyperfine interaction is theoretically no longer expected to dominantly limit the T2 spin dephasing time. We also demonstrate a complete suppression of hysteretic gate behavior and charge noise using a negatively biased global top gate. [ABSTRACT FROM AUTHOR]

Plasma-assisted molecular beam epitaxial growth of Mg-doped GaN and InGaN on a sapphire substrate is investigated in this study. Electrical characteristics of p-type GaN strongly depend on the flux of Mg acceptors and the growth temperature. Only the intermediate range of Mg fluxes (beam equivalent pressures near 1×10-9T) produce p-type GaN with good electrical properties, and a maximum hole concentration of 3.5 × 1018 cm-3 is obtained with a Hall mobility of 2.1 cm2/V·s. Due to the strong surface accumulation of electrons, Hall measurements do not indicate p-type polarity for In fraction beyond 11%. In contrast, hot probe measurements show that p-polarity can be measured for the entire range of Mg-doped In mole fractions. Electroluminescence also indicates p-polarity for Ga-rich mole fractions. InxGa1-xNp-n homojunctions are fabricated and tested. All GaN devices show low series resistance (0.03 ohm-cm2) and insignificant parasitic leakage. IV curves of all three InGaN homojunctions show rectifying characteristics under dark conditions and photo-response under outdoor sunlight, indicating the existence of holes in InGaN with up to 40% In content. [ABSTRACT FROM AUTHOR]

Attitudes toward abortion, as obtained in a 1971 nationwide survey, are presented for students and faculty in nursing, medicine, and social work, and are compared to attitudes of the general population. Attitudinal differences are examined among the professions, between students and faculty, and in terms of religion. The results reveal implications for the abortion-related services provided by health professionals. [ABSTRACT FROM AUTHOR]

PN junction rectification is demonstrated in indium nitride. The junction is formed between the n-type surface accumulation layer and the Mg-doped, p-type bulk in a top-contacted thin film structure. The parasitic shorting path between the top contacts is controlled by gating with an electrolyte. For positive gate voltages, electrons accumulate at the surface and the current flows preferentially through n type regions, and linear current-voltage (I-V) behavior is observed. However, for negative gate voltages, surface electrons are depleted and current flows through the p-type bulk. This creates an n-p-n structure and a characteristic non-linear I-V curve is observed. [ABSTRACT FROM AUTHOR]

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge-Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge-Au. [ABSTRACT FROM AUTHOR]

We report a systematic investigation on Mg doped InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy. Electrolyte capacitance voltage (ECV) combined with thermopower measurements find p-type conduction over an Mg concentration range. For InN:Mg in this p-type 'window' the Seebeck coefficients dramatically change their signs from negative to positive when the thickness of undoped InN interlayer decreases to zero. This notable sign change of Seebeck coefficient explains the previous inconsistency between ECV and thermopower results and confirms the existence of mobile holes in the InN:Mg. Taking into account the undoped InN interlayer, the hole density and mobility are extracted. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) of Er-doped silica films containing Ge nanoclusters synthesized by ion implantation was investigated. The area of the 1540 nm Er3+ PL peak was enhanced by up to a factor of 200 by the addition of Ge nanoclusters. The PL enhancement was found to be proportional to the concentration of Ge atoms. Control experiments with argon ion implantation were used to show that the enhancement is due to the presence of Ge and not radiation damage. Furthermore, the Er3+ PL was found to be strongly influenced by the postgrowth annealing and the crystallinity of the Ge nanoclusters. [ABSTRACT FROM AUTHOR]

The optical transitions of the shallow donor bound exciton associated with phosphorus in silicon are a subject of renewed interest due to the recent discovery that these transitions can be used to both read out and initialize the donor electron and nuclear spin in highly enriched 28Si. The ultimate limit of these processes will be determined by the natural or homogeneous linewidth which we determine here using spectral hole burning. The observed 10 neV linewidth is only four times the limit set by the bound exciton lifetime. [ABSTRACT FROM AUTHOR]

Ion beam synthesis of nanocrystals is explored using a recently developed kinetic Monte Carlo model for the process. The model suggests that temperature can be used to engineer nanocrystal size distributions. Specifically, by initiating implants at low temperature and then ramping the temperature upward, one can both tune the average size of the nanocrystals and restrict size distribution widths to less than 20% of the average size. [ABSTRACT FROM AUTHOR]

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface. [ABSTRACT FROM AUTHOR]

The surface contribution to the electrical transport properties of InN was directly measured and modulated by the electrolyte gated Hall effect. Undoped and Mg-doped films show different behaviors that can be effectively described by a multilayer model, taking into account the conduction contribution from both the surface and interface with the buffer layer. Gated photoluminescence experiments further show the surface accumulation layer enhances radiative electron-hole recombination in undoped InN. [ABSTRACT FROM AUTHOR]

The equilibrium structure of embedded nanocrystals formed from strongly segregating binary alloys is considered within a simple thermodynamic model. The model identifies two dimensionless interface energies that dictate the structure and allows prediction of the stable structure for any choice of these parameters. The resulting structure map includes three distinct nanocrystal morphologies: core/shell, lobe/lobe, and completely separated spheres. [ABSTRACT FROM AUTHOR]