The optical properties of solution-grown ZnO nanorods were investigated using photoluminescence and cathodoluminescence. The as-grown nanorods displayed a broad yellow-orange sub-band-gap luminescence and a small near-band-gap emission peak. The sub-band-gap luminescence can only be observed when exciting above band gap. Scanning cathodoluminescence experiments showed that the width of the sub-band-gap luminescence is not due to an ensemble effect. Upon reduction, the sub-band-gap luminescence disappeared and the near-band-gap emission increased. Compared to ZnO powders that are stoichiometric and oxygen deficient, we conclude that the yellow-orange sub-band-gap luminescence most likely arises from bulk defects that are associated with excess oxygen.

(Received s The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates *m is typically greater than 109/cm2. Such high dislocation densities degrade both the electronic so and photonic properties of the material. The density of dislocations can be decreased b~~@ orders of magnitude using cantilever epitaxy (CE), which employs prepattemed sapphire substrates to provide reduced-dmension mesa regions for nucleation and etched trenches ($)42 between them for suspended lateral growth of Gall or AIGaN. The substrate k prepattemed d~w ith narrow lines and etched to a depth that permits coalescence of laterally growing III-N~@ nucleated on the mesa stiaces before vertical growth fills the etched trench. Low a dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.The great potential of wide-band-gap Group III nitrides (III-N) has been limited in many applications by the very high density of treading dislocations (TDs) that form when the III-N materials are grown on latticemismatched substrates [1]. Growth of GaN on a planar substrate of sapphire, SiC, orSi(111) produces TD areal densities on~e order of 108to 1010/cm2.Although such high TD densltles do not appear to seriously degrade light-emitting diode (LED) performance due to the vertical character of the TDs and the short minority carrier difision lengths found in III-nitrides, they cause unacceptably short lifetimes for laser diodes (LDs) and excessive leakage current under reverse bias for p-n junction devices such as field-effect transistors (FETs) and high-electron-mobility transistors (HEMTs). To solve these problems, a GaN substrate with <1OG TDs/cm2 will be required.Several approaches have achieved considerable success in reducing TD densities to the 106/cm2range in selected regions of a wafer, but these techniques are very time-consuming to implement. These include epitaxial lateral overgrowth (ELO or LEO) [2,3], pendeoepitaxy (PE) [4], and lateral overgrowth from trenches (LOFT) [5]. While each technique produces selective areas on a wafer that possess the low TD densities (

Columnar and highly oriented (100) BaTiO3 and SrTiO3 thin films were prepared by a chelate‐type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum‐coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine‐grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO3 and SrTiO3 thin films were fabricated on LaAlO3 by a seeded growth process that appeared to follow a standard “two‐step” growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO3 throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO3 and PbTiO3 seed layers were effective in promoting the growth of highly oriented (100) BaTiO3 films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO3 and TiO2 appeared to dictate thin film microstructural evolution.

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thickness. We present studies of Nb based micron-scale ellipticallyshaped Josephson junctions containing ferromagnetic barriers of Ni 81 Fe 19 or Ni 65 Fe 15 Co 20 . By applying an external magnetic field, the critical current of the junctions are found to follow characteristic Fraunhofer patterns, and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0-π transitions based on existing theories.

We demonstrate that when vertical threading dislocations in (0001) GaN are imaged in plan-view by transmission electron microscopy, a surface-relaxation contrast operates in addition to that due to the strain fields of dislocations passing through the specimen. We show that all three dislocation types (edge, screw, and mixed) can be detected in the same image using g=(112̄0) and 18° specimen tilt from [0001], allowing total densities to be assessed properly. The type of an individual dislocation can also be readily identified.

The electronic transport mechanism in tetrahedrally-coordinated amorphous carbon was investigated using measurements of stress relaxation, thermal evolution of electrical conductivity, and temperature-dependent conductivity measurements. Stress relaxation measurements were used to determine the change in 3-fold coordinated carbon concentration, and the electrical conductivity was correlated to this change. It was found that the conductivity was exponentially proportional to the change in 3-fold concentration, indicating a tunneling or hopping transport mechanism. It was also found that the activation energy for transport decreased with increasing anneal temperature. The decrease in activation energy w a o the transport in this material is described by thermally activated conduction along 3-fol ,,pq?laol--23 q ' m 0 responsible for the observed increase in electrical conductivity. A model is described wherei linkages or chains with variable range and variable orientation hopping. Thermal annealing to chain ripening and a reduction in the activation energy for transport.

No abstract

Three important processes dominate the wet thermal oxidation of AlxGa1−xAs on GaAs: (1) oxidation of Al and Ga in the AlxGa1−xAs alloy to form an amorphous oxide, (2) formation and elimination of crystalline and amorphous elemental As and of amorphous As2O3, and (3) crystallization of the amorphous oxide film. Residual As can lead to strong Fermi-level pinning at the oxidized AlGaAs/GaAs interface, up to a 100-fold increase in leakage current, and a 30% increase in the dielectric constant of the oxide layer. Thermodynamically favored interfacial As may impose a fundamental limitation on the use of AlGaAs wet oxidation in metal-insulatorsemiconductor devices in the GaAs material system.