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The effects of Si doping on the evolution of stress in AlxGa1-xN:Si thin films (x≈0.4–0.6) grown on 6H-SiC by metal organic chemical vapor deposition were investigated using in situ wafer curvature measurements. The results were correlated with changes in film microstructure as observed by transmission electron microscopy. The incorporation of Si into the films resulted in a compressive-to-tensile transition in the biaxial stress at the surface, and the magnitude of the tensile stress was found to increase in proportion to the Si concentration. The stress gradient was attributed to Si-induced dislocation inclination resulting from an effective climb mechanism. Si doping also resulted in a decrease in the threading dislocation density in the AlxGa1-xN layers, which was attributed to increased dislocation interaction and annihilation. The model describing tensile stress generated by dislocation effective climb was modified to account for the dislocation reduction and was found to yield an improved fit to the experimental stress-thickness data. [ABSTRACT FROM AUTHOR]

SrTiO3 films were grown by reactive molecular beam epitaxy to have varying degrees of both global and local cationic nonstoichiometries (with stoichiometry defined as a 1:1 ratio of Sr:Ti). Slight global excesses of Sr and Ti resulted in two-fold reconstructions in the reflection high-energy electron diffraction patterns along the [110] and [100] azimuths, respectively. Larger global nonstoichiometries (2:1 and 1:2 ratios) were also accommodated into the film’s crystalline structure and affected the long-range crystalline order as observed in the x-ray diffraction patterns, both of which were related to the parent perovskite pattern. Local nonstoichiometries were introduced by depositing multiple monolayers (MLs) (from 2 to 33) of SrO and TiO2 in an alternating fashion, while maintaining the global SrTiO3 stoichiometry. These layered structures of SrO and TiO2 blocks inter-reacted during growth to form highly crystalline epitaxial SrTiO3. Films grown in this manner with blocks thicker than 8 MLs were fully relaxed and, when the block thicknesses ranged between 8 and 10 MLs, the full widths at half maxima of 2θ peaks were narrower than the standard SrTiO3 films having blocks 1 ML thick. [ABSTRACT FROM AUTHOR]

We have investigated the mechanisms of GaAs1-xNx film growth by plasma-assisted molecular-beam epitaxy. A comparison of in situ reflection high-energy electron diffraction and scanning tunneling microscopy (STM), with ex situ atomic force microscopy, reveals a temperature-dependent interplay between surface and step-edge diffusion. At low temperatures, layer-by-layer growth is observed, presumably due to limited adatom surface mobility. As the temperature increases, the interplay between surface and step-edge diffusion leads to multilayer growth. For sufficiently high temperatures, adatoms overcome the step-edge diffusion barrier, resulting in layer-by-layer growth once again. The temperature range for multilayer growth is influenced by the Ga flux and may be narrowed by using As2. Using in situ STM, we quantified the activation energies for Ga surface diffusion, Ed, and step-edge diffusion, Ee, during layer-by-layer GaAsN growth. We estimate Ed=0.75 and 0.96 eV for growth using As4 and As2, respectively. Thus, the narrowing of the multilayer growth temperature range is likely due to the decrease in Ga surface diffusion length through the use of As2 in lieu of As4. Furthermore, we estimate Ee=80 meV, larger than what has been reported for GaAs growth. [ABSTRACT FROM AUTHOR]

We have investigated stress evolution in dilute nitride GaAs1-xNx alloy films grown by plasma-assisted molecular-beam epitaxy. For coherently strained films (x<2.5%), a comparison of stresses measured via in situ wafer curvature measurements, with those determined from x-ray rocking curves using a linear interpolation of lattice parameter and elastic constants, suggests significant bowing of the elastic properties of GaAsN. The observed stress differences are used to quantify the composition-dependent elastic constant bowing parameters. For films with x>2.5%, in situ wafer curvature measurements reveal a signature for stress relaxation. Atomic force microscopy and transmission electron microscopy measurements indicate that stress relaxation occurs by a combination of elastic relaxation via island formation and plastic relaxation associated with the formation of stacking faults. [ABSTRACT FROM AUTHOR]

We have examined the matrix-seeded growth of narrow-gap nitride nanostructures in nitrogen ion implanted GaAs and InAs. Low-energy implantation followed by rapid thermal annealing (RTA) results in the formation of 2–3 nm sized amorphous precipitates in a crystalline matrix. On the other hand, high-energy implantation results in an amorphous layer, with or without crystalline remnants. When the ion-beam-synthesized amorphous matrix is a continuous amorphous layer, subsequent RTA leads to the formation of 4–5 nm zinc blende (ZB)-GaN-rich crystallites in an amorphous matrix. When this matrix contains crystalline remnants, subsequent RTA leads to the formation of 2–4 nm ZB-GaN-rich crystallites within the amorphous regions. These results suggest that the matrix plays an important role in the nucleation and growth of narrow-gap nitride nanostructures, and that matrix-seeded growth may provide an opportunity to control the structure and properties of the nanostructures. [ABSTRACT FROM AUTHOR]

We have investigated the effects of various buffer layers on the structural and electronic properties of n-doped InSb films. We find a significant decrease in room-temperature electron mobility of InSb films grown on low-misfit GaSb buffers, and a significant increase in room-temperature electron mobility of InSb films grown on high-misfit InAlSb or step-graded GaSb+InAlSb buffers, in comparison with those grown directly on GaAs. Plan-view transmission electron microscopy (TEM) indicates a significant increase in threading dislocation density for InSb films grown on the low-misfit buffers, and a significant decrease in threading dislocation density for InSb films grown on high-misfit or step-graded buffers, in comparison with those grown directly on GaAs. Cross-sectional TEM reveals the role of the film/buffer interfaces in the nucleation (filtering) of threading dislocations for the low-misfit (high-misfit and step-graded) buffers. A quantitative analysis of electron mobility and carrier-concentration dependence on threading dislocation density suggests that electron scattering from the lattice dilation associated with threading dislocations has a stronger effect on electron mobility than electron scattering from the depletion potential surrounding the dislocations. Furthermore, while lattice dilation is the predominant mobility-limiting factor in these n-doped InSb films, ionized impurity scattering associated with dopants also plays a role in limiting the electron mobility. [ABSTRACT FROM AUTHOR]

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN. [ABSTRACT FROM AUTHOR]

We have investigated the evolution of structural and electronic properties of highly mismatched InSb films, with thicknesses ranging from 0.1 to 1.5 μm. Atomic force microscopy, cross-sectional transmission electron microscopy, and high-resolution x-ray diffraction show that the 0.1 μm films are nearly fully relaxed and consist of partially coalesced islands, which apparently contain threading dislocations at their boundaries. As the film thickness increases beyond 0.2 μm, the island coalescence is complete and the residual strain is reduced. Although the epilayers have relaxed equally in the <110> in-plane directions, the epilayer rotation about an in-plane axis (epilayer tilt) is not equal in both <110> in-plane directions. Interestingly, the island-like surface features tend to be preferentially elongated along the axis of epilayer tilt. Furthermore, epilayer tilt which increases the substrate offcut (reverse tilt) is evident in the [110] direction. High-resolution transmission electron microscopy indicates that both pure-edge and 60° misfit dislocations contribute to the relaxation of strain. In addition, as the film thickness increases, the threading dislocation density decreases, while the corresponding room-temperature electron mobility increases. The other structural features, including the residual strain, and the surface and interface roughness, do not appear to impact the electron mobility in these InSb films. Together, these results suggest that free-carrier scattering from the threading dislocations is the primary room-temperature mobility-limiting mechanism in highly mismatched InSb films. Finally, we show quantitatively that free-carrier scattering from the lattice dilation associated with threading dislocations, rather than scattering from a depletion potential surrounding the dislocations, is the dominant factor limiting the electron mobility. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which focused on a simplified method used for calculating the scattering rates in a hydrodynamic model based on the nonparabolic balance equations. Formulation; Results and discussion; Conclusions.

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. [ABSTRACT FROM AUTHOR]

Sea ice is modeled as a transversely isotropic brine-saturated porous medium in accordance with the model proposed by Schwarz and Weeks [J. Glaciol. 19, 499-531 (1977)]. The reflection and refraction of waves at the water-ice interface are studied. Numerical results indicate that the porosity of the ice has a noticeable effect on the wave reflectivity. Its effect, however, becomes small when the grazing angle of the incident wave becomes small. The existence of a thin and porous ice skeleton layer on the ice medium further reduces the reflectivity. The reduction of reflectivity becomes more distinct as the frequency of the incident wave increases. Numerical results point out that, in addition to the porosity, the ice layer thickness is the most dominant property influencing the under-ice wave reflectivity. The calculated results show a qualitative agreement with the in situ experimental results by Yang and Votaw [J. Acoust. Soc. Am. 70, 841-851 (1981)]. [ABSTRACT FROM AUTHOR]

A method using SH waves to estimate the quality of an adhesive layer in a bonded plate is presented in this paper. A mathematical analysis of SH-wave motions in a bonded plate consistent with the experimental arrangement was carried out in full. We have found that the cutoff frequency of the second mode waves in the plate is dependent upon the thickness and the mechanical properties of the adhesive layer, and the waves in the plate resonate at this frequency. This wave resonating phenomenon was used in the experimental determination of the property of the adhesive layer. An experimental verification of the method was carried out using adhesives of several different properties. We have demonstrated that the resonant frequency of the second mode wave in the plate is dependent upon the moduli and thickness ratio between the adhesive layer and plate as predicted by the analysis. [ABSTRACT FROM AUTHOR]

We have examined the formation of InN nanocrystals embedded in InAs. Low temperature (77K) N ion implantation into InAs leads to the formation of an amorphous layer with crystalline InAs remnants. Rapid thermal annealing up to 550 °C leads to the nucleation of zincblende InN nanocrystals with a maximum likelihood radius of 1.3 ± 0.2 nm. Rapid thermal annealing at 600 °C leads to nucleation of zincblende and wurtzite InN, with an increase in maximum likelihood radius to 2.6 ± 0.4 nm. These results are consistent with the predictions of a thermodynamic model for the nanoscale-size-dependence of zincblende and wurtzite InN nucleation. [ABSTRACT FROM AUTHOR]

The structure and chemistry of the (111)Sc2O3/(0001) GaN epitaxial interface grown by molecular-beam epitaxy have been investigated. High-resolution transmission electron microscopy reveals an abrupt Sc2O3/GaN interface with a hexagonal misfit dislocation network. These dislocations have Burgers vectors of (a/3)<1120>GaN and line directions parallel to <1100>GaN, with an average spacing of ∼3.8 nm. Scanning transmission electron microscopy and electron energy loss spectrometry reveal the intermixing of Sc, O, and N over a region with a width of ∼1.5 nm at the interface. [ABSTRACT FROM AUTHOR]

Sandwich-type MgB2/MgO/MgB2 Josephson junctions with Au or MgB2 interconnection were fabricated using hybrid physical-chemical vapor deposited MgB2 thin films and RF-magnetron-sputtered MgO barrier. The junctions show properties similar to those in high-Jc Nb junctions with Jc up to 275 kA/cm2 at 4 K, which remains nonzero up to 40 K. Critical current modulations by applied magnetic field and constant voltage steps under microwave radiation were observed. Combined with the larger energy gaps in MgB2 than in Nb, the junctions presented here allow simple MgB2 digital circuits to work over 20 K or with a clock speed above 1 THz. [ABSTRACT FROM AUTHOR]

We have fabricated planar superconductor-normal metal-superconductor MgB2 Josephson junctions using TiB2 as the barrier material. On a bilayer film of MgB2/TiB2 on SiC substrate, MgB2 was locally removed to create a gap of ≤=50 nm, separating the MgB2 film into two electrodes. A Josephson coupling is established through the TiB2 film across the gap by proximity effect. The junctions exhibit resistively shunted junction like current-voltage characteristics up to 31 K. An ac Josephson effect was observed and the behavior of the Shapiro steps are in good agreement with theoretical predictions. The magnetic field modulation of the critical current also agrees with the thin film planar junction behavior [ABSTRACT FROM AUTHOR]

We have examined the origins of luminescence in N-ion-implanted epitaxial GaAs, using a combination of cross-sectional transmission electron microscopy and low-energy electron-excited nanoscale-luminescence spectroscopy. A comparison of reference, as-implanted, and implanted-plus-annealed samples reveals a variety of emissions. In all samples, we observe the GaAs fundamental band-gap emission, as well as several emissions related to GaAs native defects. In the as-implanted and implanted-plus-annealed samples, an emission related to the implantation-induced defects, is also observed. Interestingly, in the implanted-plus-annealed samples, we identify a near-infrared emission associated with GaAsN nanocrystallites. [ABSTRACT FROM AUTHOR]

Tunnel junctions composed of two ferromagnetic electrodes separated by a ferroelectric barrier were fabricated from epitaxial La0.7Sr0.3MnO3/Ba0.95Sr0.05TiO3/La0.7Sr0.3MnO3 trilayers. Typical R-H curves with sharp-switched resistance states (magnetic parallel and antiparallel) of magnetic tunnel junctions have been observed up to room temperature. After applying a poling voltage, which reverses the barrier polarization, both the parallel and antiparallel resistance states will switch to different values. Clear tunneling magnetoresistance and tunneling electroresistance, hence the four resistance states have been observed at room temperature. [ABSTRACT FROM AUTHOR]

Ni films on (0001) and ([formula]) InN exhibited different reaction kinetics upon annealing at 673K. Structural and chemical analysis using grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray energy dispersive spectrometry indicated that an interfacial reaction did not occur between the Ni film and the In-polar (0001) InN layer. However, the N-polar face reacted with Ni to form the Ni3InNx ternary phase with an anti-perovskite structure. The difference in reactivity for Ni on In-face and N-face InN indicates that polarity alters the reaction and may also affect interactions between other metals and group III-nitride semiconductors. [ABSTRACT FROM AUTHOR]

The sea ice is modeled as a transversely isotropic brine-saturated porous medium, in accordance with the model proposed by Schwarz and Weeks (1977). The reflection and refraction of waves at the water-ice interface are studied. The numerical results obtained in this study indicate that the porosity of the ice has a noticeable effect on the wave reflectivity. Its effect, however, becomes small when the grazing angle of the incident wave becomes small. The existence of a thin and porous ice skeleton layer on the ice medium further reduces the reflectivity. The reduction of reflectivity becomes more distinct as the frequency of the incident wave is high. Numerical results point out that, in addition to the porosity, the ice layer thickness is the most dominant property influencing the under ice wave reflectivity. The calculated results show a qualitative agreement with the in situ experimental results by Yang and Votaw (1981). [ABSTRACT FROM AUTHOR]

A magnet system is used in the SPERF to create the magnetic field configuration for simulating the space plasma environment. In this paper, the parameters of the system are designed to achieve the target fields needed by the scaling laws, and the electromagnetic analysis has been performed to validate the results. A procedure to obtain the parameters is proposed based on the investigation into the physical and technological constraints. The vacuum magnetic fields for studying the 3D magnetic reconnection at the magnetopause, Earth's magnetosphere, and 3D magnetic reconnection driven by a plasma gun are computed. In addition, the engineering complexity is reviewed in brief. This research is crucial to the construction of the SPERF, and it is valuable to designing the magnets applied in other fields.

A new class of pulsed X-ray detection methods by sensing carrier changes in a diode laser cavity has been presented and demonstrated. The proof-of-principle experiments on detecting pulsed X-ray temporal profile have been done through the diode laser with a multiple quantum well active layer. The result shows that our method can achieve the aim of detecting the temporal profile of a pulsed X-ray source. We predict that there is a minimum value for the pre-bias current of the diode laser by analyzing the carrier rate equation, which exists near the threshold current of the diode laser chip in experiments. This behaviour generally agrees with the characterizations of theoretical analysis. The relative sensitivity is estimated at about 3.3 × 10 -17 C ⋅ cm 2 . We have analyzed the time scale of about 10 ps response with both rate equation and Monte Carlo methods.

A new microfluidic method that allows hydrodynamic focusing in a microchannel with two sheath flows is demonstrated. The microchannel network consists of a T-shaped main channel and two T-shaped branch channels. The flows of the sample stream and the sheath streams in the microchannel are generated by electroosmotic flow-induced pressure gradients. In comparison with other flow focusing methods, this novel method does not expose the sample to electrical field, and does not need any external pumps, tubing, and valves.

Barium fluoride (BaF2) is an inorganic scintillation material used for the detection of X∕gamma radiation due to its relatively high density, equivalent atomic number, radiation hardness, and high luminescence. BaF2 has a potential capacity to be used in gamma ray timing experiments due to the prompt decay emission components. It is known that the light output from BaF2 has three decay components: two prompt of those at approximately 195 nm and 220 nm with a decay constant around 600-800 ps and a more intense, slow component at approximately 310 nm with a decay constant around 630 ns which hinders fast timing experiments. We report here the development of a fast radiation detector based on a BaF2 scintillation crystal employing a special optical filter device, a multiple reflection multi-path ultraviolet region short-wavelength pass light guides (MRMP-short pass filter) by using selective reflection technique, for which the intensity of the slow component is reduced to less than 1%. The methods used for this study provide a novel way to design radiation detector by utilizing scintillation crystal with several emission bands.