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Excess emission has been observed from velvet cathodes with total emission times >150 ns. In this diode experiment, we produce a sub-relativistic electron beam with the ability to consistently change γ from 1.2 to 1.5 and β = 0.5–0.75. Electron emission in this particular diode geometry requires electric fields >40 kV/cm. The current increases at steady rates >0.05 A/ns after the head of the pulse, indicating an expansion of the emission surface and reduction in the effective AK gap. Small transients of excess emission (or arcs) are consistently observed for current pulses exceeding 150 ns. The excess emission results in beam loaded levels ≥ 10 kV on the diode voltage. The effects described here are compounded as diode voltage is increased. The principal objective of these experiments is to quantify electric field emission thresholds, current ramps, excess emission delays, effective beam loading, and impedance collapse. [ABSTRACT FROM AUTHOR]

A spatially resolved air-wedge shearing interferometer and shadowgraph diagnostic provides measurements of electron density with a resolution of ∼ 40 μ m. A ∼ 100-ns-long, monoenergetic electron bunch at 19.8 MeV and a current of 1.4 kA (8.5 × 10 14 e −) heats 100- μ m-thick aluminum (Al) foils in a 1-mm-spot to T e ∼ 1 eV. A 5-ns-long, ∼ 60 mJ, frequency doubled Nd:YAG laser probes the dense Al plasma. Electron densities up to 10 20 cm − 3 are resolved; the maximum resolvable density is limited by opacity, transmission, and spatial fringe density achievable with the detector. This diagnostic provides measurements of the total phase shift, transmission, and electron density. Several measurements at different time slices provide the ability to determine the velocity of the leading edge of the shadowgraph and compare it to the motion of different density shells. These measurements are also compared to radiation hydrodynamics simulations. A rough quantitative agreement is shown between the hydro simulations and the measurements; there are differences in the exact density distributions. [ABSTRACT FROM AUTHOR]

Effective step index and the distributed Bragg reflector (DBR) scattering loss are used to characterize the physical and optical characteristics of asymmetric cladding surface-etched DBR lasers at 850 nm. The analysis has helped in the fabrication of frequency-selective DBR gratings with a predetermined grating index and reflectivity.

We measure the time-resolved photoluminescence characteristics of a novel type of lithographically patterned quantum dot fabricated by electron beam lithography, wet chemical etching, and overgrowth of the barrier layers by metalorganic chemical vapor deposition.We find that the quantum dot (QD) photoluminescence exhibits a bi-exponential decay that we explain in terms of the fast capture of carriers by defect states followed by a slower radiative relaxation process. We also perform a systematic investigation of the rise time and decay time as a function of the QD density, size, and temperature. These measurements indicate that the carrier capture process in this type of QD is limited by carrier drift within the GaAs barrier material. [ABSTRACT FROM AUTHOR]

We present an analysis of the physics of intersubband transitions in a patterned inverse quantum dot (IQD) array. Numerical techniques for computing the oscillator strength for intersubband transitions are outlined along with the selection and sum rules. It is demonstrated that the polarization dependence of intersubband absorption can be controlled through appropriate design of the IQD array geometry. [ABSTRACT FROM AUTHOR]

We present a detailed analytical study of the effect of nanopore lattice geometry, material parameters, and temperature on the density of states and intraband energy gaps in an ordered nanopore array diode laser. An improved fabrication process is proposed to increase the peak gain achievable in the nanopore laser by a factor of 3 based on the simulation results. [ABSTRACT FROM AUTHOR]

The upper cladding and residual guide thickness have been varied to develop an optimal asymmetric cladding ridge waveguide geometry for use in photonic integrated circuits (PICs). Measured values of the intrinsic loss, threshold current, and lateral modal characteristics allowed the determination of the geometry that simultaneously achieved good active device performance and a strong lateral index step. The bend loss was found by fabricating curved-waveguide y-branch lasers. These characteristics are important for fabricating low-loss routing structures in PICS. This experimental work analyzed an asymmetric cladding ridge waveguide for use with photonic integrated circuits; however, the method presented here generally can be used for the experimental optimization of ridge waveguide lasers. [ABSTRACT FROM AUTHOR]

We critically test the capabilities of the effective-medium approximation (EMA) and alloy models to describe multilayer samples with gradual interfaces by analyzing spectroscopic ellipsometric (SE) data of two AlGaAs samples grown expressly for this purpose. The dielectric functions [variant_greek_epsilon] of the interfaces are calculated in the EMA and alloy models, and the interfaces themselves simulated either as a single layer of Al0.5Ga0.5As or a stack of layers of AlxGa1-xAs with x increasing or decreasing between 0.1 and 0.9 in increments of 0.1. The EMA essentially fails completely for either interface representation. For the alloy model the stepwise-graded representation is significantly better, not only simulating the data more accurately but also yielding thicknesses in essential agreement with those obtained by cross-sectional transmission electron microscopy. The results highlight the types of errors that are encountered with the different models, and show that the analysis of SE data can provide information about these interfaces. [ABSTRACT FROM AUTHOR]

Control of interchain separation enables the optical properties of polymer thin films to be altered. We present a thin film preparation technique that increases the free volume fraction in spin coated polymer thin films resulting in reduced interchain interaction. The polaron yield, measured using photoinduced absorption, was significantly reduced as a direct result of the increased interchain separation, leading to a higher value for the measured photoluminescence efficiency. Impedance spectroscopy showed an increase in permittivity, probably due to greater polarizability. Increases approaching one order of magnitude were observed for direct current hole conductivity and mobility values. Space charge limited conduction analysis suggests a narrowing of the highest occupied molecular orbital band tail on treatment resulting in reduced trapping. Single layer light emitting devices prepared using this technique were found to be significantly brighter and to have longer operating lifetimes. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Alternating current (ac) and direct current (dc) conductivities have been measured in polymer-nanotube composite thin films. This was carried out for a range of concentrations of multiwall nanotubes in two polymer hosts, poly(m-phenylenevinylene-co-2,5-dioctyloxyp-phenylenevinylene) (PmPV) and polyvinylalcohol (PVA). In all cases the dc conductivity σ[sub DC] was ohmic in the voltage range studied. In general the ac conductivity displayed two distinct regions, a frequency independent region of magnitude σ[sub 0] at low frequency and a frequency dependent region at higher frequency. Both σ[sub DC] and σ[sub 0] followed a percolation scaling law of the form σ∝(p-p[sub c])[sup t] with p[sub c] = 0.055% by mass and t = 1.36. This extrapolates to a conductivity of 1 × 10[sup -3] S/m for 100% nanotube content. Such a low value reflects the presence of a thick polymer coating, resulting in poor electrical connection between tubes. This leads to the suggestion that charge transport is controlled by fluctuation induced tunneling. In the high frequency regime the conductivity increases with frequency according to an approximate power law with exponent s ≈ 0.92, indicative of hopping transport. The onset of this frequency independent conductivity scales with mass fraction for the PmPV composite due to the variation of correlation length with nanotube content. This behavior is discussed in terms of a biased random walk in three dimensions. In addition ac universality is demonstrated by the construction of a mastercurve. [ABSTRACT FROM AUTHOR]

We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer, and poly(9-vinylcarbazole) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4phenylene-vinylene-2-methoxy- 5(2′-ethylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current–voltage–luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0%, 2%, 4%, 8%, 16%, and 32%) for all device structures studied. dc transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was observed using the composite. The best efficiency was obtained for those devices with an electron-transport layer of mass ratio 8%. In addition, on doping with nanotubes, electron conductivity in the composite increased by over 4 orders of magnitude with little quenching of photoluminescence. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm [sup 4]I[sub 13/2]-[sup 4]I[sub 15/2] emissions of Er[sup 3+] in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er[sup 3+] centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er[sup 3+]PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er[sup 3+]PL spectra. The investigations of selectively excited Er[sup 3+]PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called violet-pumped Er[sup 3+]PL band and its associated broad violet PLE band are significantly enhanced relative to the PL and PLE of the other selectively excited Er[sup 3+]PL centers. More importantly, the violet-pumped Er[sup 3+]PL spectrum dominates the above-gap excited Er[sup 3+]PL spectrum of Er-implanted Mg-doped GaN, whereas it was unobservable under above-gap excitation in Er-implanted undoped GaN. These results confirm the hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er[sup 3+] emission in Er-implanted GaN. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the interface characteristics of different heterostructures grown by metalorganic chemical vapor deposition (MOCVD) using secondary ion mass spectroscopy and high-resolution transmission electron microscopy (TEM). Description of MOCVD; Illustration of compositional transients; Analysis of high-resolution TEM.

Presents a study which investigated the uniformity of epitaxial growth of gallium arsenide by metalorganic chemical vapor deposition in a vertical-flow rotating-disk reactor. Observations on the thickness of the epitaxial layer versus radial distance on the susceptor surface; Model proposed in consideration of hydrodynamic and thermal boundary layer effects at the susceptor surface; Description of the effects of a parasitic process in the limit of smaller boundary layers and transition.

Presents a study that examined the gain characteristics for a series of five separate confinement heterostructure indium compound strained-layer quantum well lasers. Use of indium compound strained-layer quantum well heterostructure laser diodes; Dependence of emission wavelength on cavity length for five different quantum well compositions.

Provides information on a study which examined the effect of the buffer-layer structures on the generation and propagation of dislocations in epitaxial layers of high composition aluminum[subx]gallium[sub1--x]arsenic through chemical vapor deposition. Experimental procedure; Results and discussion; Conclusions.

Presents a study which focuses on the amorphization mechanism operative in Al0.6Ga0.4As/GaAs structures. Examination of the damage produced by the ion implantations; Consistency of the recovered structure; Basis of structures; Indication of results.

Presents a study which examined the ion implantation damage in AlxGa1 - xAs. Information on the X-ray diffraction and channeling-Rutherford backscattering spectrometry employed in this study; Preparation of the samples used in this study; Results of this study.

Presents a study aimed at monitoring the thickness of GaAs growth by surface photoabsorption in metalorganic chemical vapor deposition. Information on metalorganic chemical vapor deposition; Explanation of the experiment used in the study; Results of and discussion on the experiment.

Provides information on a study that determined the influence of interface type on the accumulation of damage and ion mixing in Al[subx]Ga[sub1-x]As/GaAs heterostructures by using a combination of Rutherford backscattering spectrometry and transmission electron microscopy. Experimental procedure; Experimental results and discussion; Conclusions.

Provides information on a study that examined luminescence properties of GaN layers grown by metalorganic chemical vapor deposition on sapphire substrates with GaN/AIN double buffer layers or GaN single buffer layers. Methodology of the study; Results and discussion on the study.

Presents a study that investigated the effect of electron beam exposure on the room-temperature electroluminescence spectra on a p-n junction GaN light emitting diode. Background on GaN based III-V nitride semiconductor system; Analysis of the change in electroluminescence spectral features; Discussion on the origin of each electroluminescence band.

Deals with a study which examined the effect of deposition temperature of a-Si:H films deposited by the de glow discharge technique on the structural and electrical properties of the laser-crystallized films. Experimental procedures; Results and discussion; Conclusion.

Deals with a study that investigated the ion implantation damage in Al[sub0.6]Ga[sub0.4]As/gallium arsenide heterostructures. Factor to which the difference in the damage states at the bottom and top interfaces is attributed; Method used to determine the depth dependence and nature of the damage produced by the ion implantation; Reason for the greater degree of damage produced at the bottom interface.

Presents a study which analyzed data from a series of separate confinement heterostructure quantum well Al[subx]Ga[sub1-x]As lasers of various structural design. Analysis of experimental data; Characterization of various SCH structures; Conclusion.

Investigates the optical transmission loss in dielectric waveguide bends using the finite-difference time-domain approach. Value of an absorbing boundary condition for the waveguide structure; Structures of multiple-bend waveguides; Investigation of bending loss in nonplanar laser arrays.

Provides data on strong carrier-induced antiguiding present in strained-layer indium gallium arsenide-gallium arsenide-aluminum gallium arsenide lasers. Magnitude of the carrier-induced index depression; Emission from short cavity devices; Discussion of results.

Presents a study which observed the characteristics of step-graded separate confinement quantum well lasers with direct and indirect barriers. Details on the experiment; Results of the study; Discussion of findings.

Focuses on a study which investigated the growth of gallium arsenide irradiated with ultraviolet laser light in a metalorganic chemical vapor deposition reactor. Information on laser photochemical vapor deposition; Methodology of the study; Results and discussion.

Presents information on a study which examined the switching speed of a heterostructure hot-electron diode (HHED). Illustration of an experimental current-voltage characteristic; Mechanisms underlying the switching of HHED and the effect of electron-electron interactions; Transient analysis.

Describes the surface acoustic wave (SAW) properties of aluminum gallium arsenide. Parameters used to characterize SAW properties; Measurement of the piezoelectric coupling coefficient of the substrate; Information on SAW velocities as compared to those measured on gallium arsenide.

Presents a theoretical and experimental analysis of the switching mechanism in heterostructure hot-electron diodes. Prerequisite for the development of a theory for the direct current properties of hot electron diodes; Discussion of the switching mechanism in hot-electron diodes; Details of the experimental techniques used.

Presents the preliminary results of a study of defects in gallium arsenide (GaAs)-aluminum GaAs superlattices using deep-level transient spectroscopy. Defect levels observed in two superlattices; Ionization of defect levels in the presence and absence of miniband conduction; Reinterpretation of observed activation energies.

Focuses on a study which characterized gold-nickel-germanium and gold-silver-germanium ohmic contracts to modulation-doped gallium arsenide/aluminum gallium arsenide heterostructures, using both secondary ion mass spectroscopy and transmission electron microscopy. Methodology of the study; Results and discussion.

Presents a study that examined the photoluminescence and absorption data on silicon (Si)- and germanium (Ge)-disordered Al[subx]Ga[sub1-x]As-gallium arsenide superlattices. Examination of the disordering of the superlattice; Evaluation of the Si- and Ge-disordered superlattice; Analysis of the transmission electron micrograph of the superlattice.