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An acoustic focusing lens incorporated with periodically aligned subwavelength grooves corrugated on its spherical surface has been developed. It is demonstrated theoretically and experimentally that acoustic focusing achieved by using the lens can suppress the relative sidelobe amplitudes, enhance the focal gain, and minimize the shifting of the focus. Use of the lens coupled with a planar ultrasound transducer can generate an ultrasound beam with enhanced acoustic transmission and collimation effect, which offers the capability of improving the safety, efficiency, and accuracy of targeted surgery implemented by high intensity focused ultrasound. [ABSTRACT FROM AUTHOR]

Precipitates have recently been found to significantly enhance the mechanical quality factor in piezoelectric ceramics. Such a piezoelectric hardening effect was attributed to strong interactions between ferroelectric domains and precipitates. In the present work, the response of domains to applied electric fields is observed in situ via transmission electron microscopy in aged (Ba, Ca)TiO3 ceramics with precipitates to reveal the underlying mechanism of this phenomenon. Ferroelectric domains in the Ba-rich matrix grain are observed to be more concentrated near non-polar Ca-rich precipitates. With increasing applied voltage, domains separate from precipitates merge together first, while those near precipitates persist to higher voltages. During ramping down, domains nucleate from precipitates. These direct observations confirm the strong interactions between ferroelectric domains and precipitates in piezoelectric ceramics. [ABSTRACT FROM AUTHOR]

Fe-Au nanoclusters ranging in composition from ∼33 to 79 at. % Fe were prepared by inert gas condensation. Resulting phases were single crystalline solid solutions for all compositions with significant defects present. The as-deposited clusters formed in a bcc structure for Fe content >65 at. % and in a fcc structure for Fe < 65 at. %. Lattice parameters were expanded beyond rule-of-mixture estimates. The lattice expansion is explained by an analytical self-interstitial model. All clusters were ferromagnetic, although the fcc structures showed low magnetization. The low magnetizations are thought to arise from antiferromagnetic cores with uncompensated ferromagnetic surface spins. [ABSTRACT FROM AUTHOR]

GaN(Si)/AlInN multiple quantum wells were grown on GaN/Al2O3 (0001) templates by metalorganic vapor-phase epitaxy. Transmission electron microscopy observations showed well-defined GaN quantum wells and AlInN barrier layers. Electrostatic potential profiles across the heterostructure have been measured using off-axis electron holography. A polarization-induced electric field with magnitude of ∼2.2 ± 0.1 MV/cm was measured across the GaN quantum wells, in reasonable agreement with simulated values. However, the measured fields across the AlInN barriers were considerably less than predicted from simulations: possible reasons are briefly discussed. [ABSTRACT FROM AUTHOR]

Electrostatic potential profiles across wurtzite AlN/GaN quantum dot (QD) superlattices grown by molecular beam epitaxy have been measured using off-axis electron holography. The profiles for individual GaN QDs show large phase shifts which can be understood in terms of spontaneous polarization and piezoelectric fields and the accumulation of positive and negative charge at the GaN/AlN interfaces. An electric field with magnitude of ∼7.8 ± 2 MV/cm was measured across the center of a GaN QD, in reasonable agreement with reported simulations. [ABSTRACT FROM AUTHOR]

The microstructure of GaN(Si)/AlInN multiple quantum wells grown on GaN/Al2O3 (0001) templates by metalorganic vapor-phase epitaxy has been investigated using transmission electron microscopy and associated techniques. Dodecagon-shape V-defects with hexagonal apexes, which nucleate on screw-component threading dislocations, are observed at the film surface. The hexagonal apexes are bounded by {112_1} planes, whereas the dodecagons are bounded by {101_1} and {112_1} planes, where the {101_1} facets are generated from the edges between adjacent {112_1} planes. Indium segregation is observed along these edges. A possible reason for formation of these defects is briefly discussed. [ABSTRACT FROM AUTHOR]

Off-axis electron holography has been used to measure the built-in potential profile across an Al0.85In0.15N/AlN/GaN high electron mobility transistor heterostructure. Profile measurements indicated a polarization-induced electric field of 6.9 MV/cm within the AlN layer. A two-dimensional electron gas with a density of ∼2.1×1013 cm-2 was located in the GaN layer at ∼0.8 nm away from the AlN/GaN interface in reasonable agreement with the reported simulations. Electron microscopy confirmed that the Al0.85In0.15N layer was uniform and that Al0.85In0.15N/AlN and AlN/GaN interfaces were abrupt and well defined. [ABSTRACT FROM AUTHOR]

The microstructure of InxAl1-xN/GaN heterostructures (where x∼0.13–0.19), grown by molecular beam epitaxy, was investigated by transmission electron microscopy. Observations in the cross-section and plan-view geometries show evidence for lateral phase separation originating at the GaN surface that results in a vertical honeycomblike structure within the InAlN layers. The lateral dimensions of the honeycomb cells are ∼5–10 nm. The vertical walls are In rich with a width of ∼1–2 nm and align roughly perpendicular to <1120> and <1100> directions. The phase separation is attributed to random compositional fluctuations during the early stages of growth, possibly associated with misfit-strain relaxation. [ABSTRACT FROM AUTHOR]

Propagation of an electromagnetic (EM) wave through a compound plasmonic crystal composed of perpendicularly arranged gold nanorod pairs has been studied. Because of the strong coupling between the EM wave and vibrations of free electrons inside nanorod pairs, the polarization-tunable polariton stop band as well as pronounced polarization conversion can be achieved in the compound plasmonic crystal. A theory based on Huang-Kun-like equations is presented to reveal the mechanism, which agrees well with the simulations. Our results may shed light on the structure design and physical understandings on the anisotropic metamaterials. [ABSTRACT FROM AUTHOR]

Domain boundary (DB) structures are of great importance for understanding the structure-property relationship in many ferroic crystals. Here, we present atomically resolved DB configurations in PbZrO3-based antiferroelectric ceramics. The Pb-cation displacement relative to B-site cations is precisely determined using aberration-corrected scanning transmission electron microscopy. We find that 90° DBs in undoped PbZrO3 can be as thin as one primitive cell of the perovskite structure, often appearing curved or zigzagged due to the complex dipole arrangement. In a chemically modified composition, Pb0.99Nb0.02[(Zr0.57Sn0.43)0.95Ti0.05]0.98O3, in which incommensurate modulations are present, the DB has a typical thickness of at least two primitive cells, with more or less aligned dipole moments. Our findings provide insights into establishing the structure-property relationship in antiferroelectrics, shedding light on the design and fabrication of domain-boundary electronics. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical study on the composite nanostructures composed of the LT-shaped metallic nanohole arrays. Multiple resonance transmission peaks are observed in the optical transmission spectra. By analyzing their electric field distributions on the interface, we infer the electric dipole and the electric quadrupole plasmonic resonances contribute to these transmission peaks. The electric quadrupole is a subradiant mode, which cannot be directly excited by the incident light. Here, we demonstrate that the surface plasmon polaritons can excite the electric quadrupole plasmonic resonance mode through the near-field interaction. [ABSTRACT FROM AUTHOR]

Deep-UV optical gain has been demonstrated in Al0.7Ga0.3N/AlN multiple quantum wells under femtosecond optical pumping. Samples were grown by molecular beam epitaxy under a growth mode that introduces band structure potential fluctuations and high-density nanocluster-like features within the AlGaN wells. A maximum net modal gain value of 118 ± 9 cm-1 has been measured and the transparency threshold of 5 ± 1 µJ/cm2 was experimentally determined, corresponding to 1.4 × 1017 cm-3 excited carriers. These findings pave the way for the demonstration of solid-state lasers with sub-250 nm emission at room temperature. [ABSTRACT FROM AUTHOR]

Single-crystal, single-phase Pb(ZrxTi1-x)O3 films (x=0–0.4) were grown on (001) SrTiO3 and SrTiO3:Nb substrates by molecular beam epitaxy. Layer-by-layer growth of the Pb(Zr,Ti)O3 films was achieved by using PbTiO3 buffer layers between the SrTiO3 substrates and the Pb(Zr,Ti)O3 films. The layers with low Zr content showed high crystallinity with full width at half maximum of ω-rocking curves as low as 4 arc min, whereas increase in Zr concentration led to pronounced angular broadening. The PbZr0.07Ti0.93O3 films exhibited remanent polarization as high as 83 μC/cm2, but local areas suffered from nonuniform leakage current. [ABSTRACT FROM AUTHOR]

AlN/GaN/AlGaN coupled quantum wells grown by molecular beam epitaxy have been developed and characterized via intersubband absorption spectroscopy. In these structures, an AlGaN layer of sufficiently low Al content is used to achieve strong interwell coupling without the need for ultrathin inner barriers. At the same time, AlN is used in the outer barriers to provide the large quantum confinement required for near-infrared intersubband transitions. The composition of the inner barriers also provides a continuously tunable parameter to control the coupling strength. Double intersubband absorption peaks are measured in each sample, at photon energies in good agreement with theoretical expectations. [ABSTRACT FROM AUTHOR]

The authors report the growth of semipolar [formula] GaN films on nominally on-axis [formula] m-plane sapphire substrates using metal organic chemical vapor deposition. High-resolution x-ray diffraction (XRD) results indicate a preferred (1122) GaN orientation. Moreover, epitaxial lateral overgrowth (ELO) of GaN was carried out on the (1122) oriented GaN templates. When the ELO stripes were aligned along [1120]sapphire, the Ga-polar wings were inclined by 32° with respect to the substrate plane with smooth extended nonpolar a-plane GaN surfaces and polar c-plane GaN growth fronts. When compared with the template, the on-axis and off-axis XRD rocking curves indicated significant improvement in the crystalline quality by ELO for this mask orientation (on-axis 1700 arc sec for the template, 380 arc sec for the ELO sample, when rocked toward the GaN m axis), as verified by transmission electron microscopy (TEM). For growth mask stripes aligned along [0001]sapphire with GaN m-plane as growth fronts, the surface was composed of two {1011} planes making a 26° angle with the substrate plane. For this mask orientation XRD and TEM showed no improvement in the crystalline quality by ELO when compared to the non-ELO template. [ABSTRACT FROM AUTHOR]

Structures with incommensurate ordering along the [0001] direction are observed in wurtzite Al0.72Ga0.28N alloys grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. Films grown in environments with group-III/N ratios greater than 1 exhibit ordered superlattice structures that are incommensurate with the wurtzite crystal lattice. In contrast, films grown under nitrogen-rich conditions exhibit ordered structures with a periodicity of four cation-N monolayers. The increasing complexity of the ordering with increasing Ga-rich growth environment suggests that the ordering is related to the presence of a Ga overlayer believed to exist on the surface of the growing film. [ABSTRACT FROM AUTHOR]

We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistor structures have been grown by plasma-assisted molecular beam epitaxy on semi-insulating 4H-SiC utilizing an AlN nucleation layer. The electron Hall mobility of these structures increases from 1050 cm2/V s to greater than 1450 cm2/V s when the Al/N flux ratio during the growth of the nucleation layer is increased from 0.90 to 1.07. Buffer leakage currents increase abruptly by nearly three orders of magnitude when the Al/N ratio increases from below to above unity. Transmission electron microscopy indicates that high buffer leakage is correlated with the presence of stacking faults in the nucleation layer and cubic phase GaN in the buffer, while low mobilities are correlated with high dislocation densities. [ABSTRACT FROM AUTHOR]