Correlations between the morphology and emission properties of trench defects in InGaN/GaN quantum wells

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) with cathodoluminescence (CL) were performed on exactly the same defects in a blue-emitting InGaN/GaN multiple quantum well (QW) sample enabling the direct correlation of the morphology of an individual defect with its emission properties. The defects in question are observed in AFM and SEM as a trench partially or fully enclosing a region of the QW having altered emission properties. Their sub-surface structure has previously been shown to consist of a basal plane stacking fault (BSF) in the plane of the QW stack, and a stacking mismatch boundary (SMB) which opens up into a trench at the sample surface. In CL, the material enclosed by the trench may emit more or less intensely than the surrounding material, but always exhibits a redshift relative to the surrounding material. A strong correlation exists between the width of the trench and both the redshift and the intensity ratio, with the widest trenches surrounding regions which exhibit the brightest and most redshifted emission. Based on studies of the evolution of the trench width with the number of QWs from four additional MQW samples, we conclude that in order for a trench defect to emit intense, strongly redshifted light, the BSF must be formed in the early stages of the growth of the QW stack. The data suggest that the SMB may act as a non-radiative recombination center. © 2013 American Institute of Physics.

Laser microstructuring of Si surfaces for low-threshold field-electron emission

Dense arrays of high aspect ratio Si micro-pyramids have been formed by cumulative high intensity laser irradiation of doped Si wafers in an SF6 environment. A comparative study using nanosecond (XeCl, 308 nm) and femtosecond (Ti: Sapphire, 800 nm and KrF, 248 nm) laser pulses has been performed in this work. The influence of pulse duration and ambient gas pressure (SF6) is also presented. Scanning electron microscopy has shown that upon laser irradiation conical features appear on the Si surface in a rather homogenous distribution and with a spontaneous self alignment into arrays. Their lowest tip diameter is 800 nm; while their height reaches up to 90 mum. Secondary tip decoration appears on the surface of the formed spikes. Areas of 2 X 2 mm(2) covered with Si cones have been tested as cold cathode field emitters. After several conditioning cycles, the field emission threshold for the studied Si tips is as low as 2 V/mum, with an emission current of 10(-3) A/cm(2) at 4 V/mum. Even though these structures have smaller aspect ratios than good quality carbon nanotubes, their field emission properties are similar. The simple and direct formation of field emission Si arrays over small pre-selected areas by laser irradiation could lead to a novel approach for the development of electron sources. (C) 2003 Elsevier B.V. All rights reserved.

Field emission vacuum power switch using vertically aligned carbon nanotubes

The best field emission properties from carbon nanotube cathodes were obtained when their heights, diameters and spacings were optimized. Field emission currents as high as 10 mA were obtained from 1 cm × 1 cm vertically aligned CNT cathode with optimized parameters grown using dc plasma CVD in situ. It was found that in order to obtain large emission current of >10 mA, space charge effects within the electron beam must be taken into account.

Field emission site densities of nanostructured carbon

The field emission properties of nanostructured carbon films deposited by cathodic vacuum arc in a He atmosphere have been studied by measuring the emission currents and the emission site density. The films have an onset field of ∼ 3 V/μm. The emission site density is viewed on a phosphor anode and it increases rapidly with applied field. It is assumed that the emission occurs from surface regions with a range of field enhancement factors but with a constant work function. The field enhancement factor is found to have an exponential distribution.

Properties of trench defects in InGaN/GaN quantum well structures

The structural and optical properties of trench defects, which are poorly understood yet commonly occurring defects observed on the surfaces of InGaN multiple quantum wells (MQW), are reported. These defects comprise near-circular trenches which enclose areas of MQW which give rise to a red shift in peak photoluminescence emission and a change in cathodoluminescence intensity with respect to the surrounding material. Atomic force microscopy shows that the height of trench-enclosed areas differs from that of the surrounding quantum well structure, and that trenches are unrelated to the commonly observed V-defects in InGaN films, despite being occasionally intersected by them. Cross-sectional electron microscopy analysis of trenches with raised centres suggests that the red shift in the observed cathodoluminescence peak emission may be due to the quantum wells being thicker in the trench-enclosed regions than in the surrounding quantum well area. The mechanism of trench formation and its implication for the control of the emission properties of light-emitting diodes is discussed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: An investigation into the role of the density of photon states near resonance

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data. © 2013 American Physical Society.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: an investigation into the role of the density of photon states near resonance.

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data.

Tailoring the morphology of carbon nanotube assemblies using microgradients in the catalyst thickness

In addition to the structural control of individual carbon nanotubes (CNTs), the morphological control of their assemblies is crucial to realize miniaturized CNT devices. Microgradients in the thickness of catalyst are used to enrich the variety of available self-organized morphologies of CNTs. Microtrenches were fabricated in gate/spacer/cathode trilayers using a conventional self-aligned top-down process and catalyst exhibiting a microgradient in its thickness was formed on the cathode by sputter deposition through gate slits. CNTs, including single-walled CNTs, of up to 1μm in length were grown within 5-15 s by chemical vapor deposition. The tendency of thin CNTs to aggregate caused interactions between CNTs with different growth rates, yielding various morphologies dependent on the thickness of the catalyst. The field emission properties of several types of CNT assemblies were evaluated. The ability to produce CNTs with tailored morphologies by engineering the spatial distribution of catalysts will enhance their performance in devices. © 2011 The Japan Society of Applied Physics.

Excellent oxidation endurance of boron nitride nanotube field electron emitters

Boron nitride nanotubes (BNNTs) are considered as a promising cold electron emission material owing to their negative electron affinity. BNNT field emitters show excellent oxidation endurance after high temperature thermal annealing of 600 °C in air ambient. There is no damage to the BNNTs after thermal annealing at a temperature of 600 °C and also no degradation of field emission properties. The thermally annealed BNNTs exhibit a high maximum emission current density of 8.39mA/cm2 and show very robust emission stability. The BNNTs can be a promising emitter material for field emission devices under harsh oxygen environments. © 2014 AIP Publishing LLC.

Comparison of the coherence properties of superradiance and laser emission in semiconductor structures

The coherence properties of a transient electron-hole state developing during superradiance emission in semiconductor laser structures have been studied experimentally using a Michelson interferometer and Young's classic double-slit configuration. The results demonstrate that, in the lasers studied, the first-order correlation function, which quantifies spatial coherence, approaches unity for superradiant emission and is 0.2-0.5 for laser emission. The supercoherence is due to long-range ordering upon the superradiant phase transition. © 2012 Kvantovaya Elektronika and Turpion Ltd.

Properties of electrons and holes during femtosecond cooperative emission

Spectral and energetic characteristics of the cooperative recombination of high density electrons and holes in bulk GaAs are experimentally studied at room temperature. It is shown that the properties and parameters of femtosecond superradiant pulses are conditioned by the collective properties of electrons and holes. Electron-hole pairing and the formation of a short-living coherent e-h BCS state distinguish strongly the regime of cooperative emission from all radiative e-h recombination regimes, which have been observed earlier. The dependences of the energy gap (the order parameter), the Fermi energy, and the band gap of the coherent e-h BCS state on the concentration of electron-hole pairs are obtained.

Optical and micro-structural properties of high photoluminescence efficiency InGaN/AlInGaN quantum well structures

We have studied the optical properties of a series of InGaN/AlInGaN 10-period multiple quantum wells (MQW) with differing well thickness grown by metal-organic vapor-phase epitaxy that emit at around 380 nm. The aim of this investigation was to optimise the room temperature internal quantum efficiency, thus the quantum well (QW) thicknesses were accordingly chosen so that the overlap of the electron/hole wave function was maximised. At low temperature, we observed a reduction of the photo luminescence decay time with decreasing well width in line with the theoretical predictions. For a structure with well thicknesses of 1.5 nm, we measured a photoluminescence internal quantum efficiency of 67% at room temperature with a peak emission wavelength of 382 nm. (c) 2006 Elsevier B.V. All rights reserved.

Electrical and field emission investigation of individual carbon nanotubes from plasma enhanced chemical vapour deposition

Plasma enhanced chemical vapour deposition (PECVD) is a controlled technique for the production of vertically aligned multiwall carbon nanotubes for field emission applications. In this paper, we investigate the electrical properties of individual carbon nanotubes which is important for designing field emission devices. PECVD nanotubes exhibit a room temperature resistance of 1-10 kΩ/μm length (resistivity 10-6 to 10-5 Ω m) and have a maximum current carrying capability of 0.2-2 mA (current density 107-108 A/cm2). The field emission characteristics show that the field enhancement of the structures is strongly related to the geometry (height/radius) of the structures and maximum emission currents of ∼ 10 μA were obtained. The failure of nanotubes under field emission is also discussed. © 2002 Elsevier Science B.V. All rights reserved.