Analysis of defect-related localized emission processes in InGaN/GaN-based LEDs

This paper reports an extensive analysis of the defect-related localized emission processes occurring in InGaN/GaN-based light-emitting diodes (LEDs) at low reverse- and forward-bias conditions. The analysis is based on combined electrical characterization and spectrally and spatially resolved electroluminescence (EL) measurements. Results of this analysis show that: (i) under reverse bias, LEDs can emit a weak luminescence signal, which is directly proportional to the injected reverse current. Reverse-bias emission is localized in submicrometer-size spots; the intensity of the signal is strongly correlated to the threading dislocation (TD) density, since TDs are preferential paths for leakage current conduction. (ii) Under low forward-bias conditions, the intensity of the EL signal is not uniform over the device area. Spectrally resolved EL analysis of green LEDs identifies the presence of localized spots emitting at 600 nm (i.e., in the yellow spectral region), whose origin is ascribed to localized tunneling occurring between the quantum wells and the barrier layers of the diodes, with subsequent defect-assisted radiative recombination. The role of defects in determining yellow luminescence is confirmed by the high activation energy of the thermal quenching of yellow emission (Ea =0.64&eV). © 2012 IEEE.

Mode-localized displacement sensing

We report the construction of a new class of micromachined displacement sensors that employ the phenomenon of vibration-mode localization for monitoring minute inertial displacements. It is demonstrated both theoretically and experimentally that the eigenstate-shifted output signal of such mode-localized displacement sensors may be as high as 1000 times greater than corresponding resonant-frequency variations that serve as the output in the more traditional vibratory resonant micromechanical displacement/motion sensors. The high parametric sensitivities attainable in such mode-localized displacement sensors, together with their inherent advantages of improved environmental robustness and electrical tunability, suggest an alternative approach in achieving improved sensitivity and stability in high-resolution displacement transduction. © 1992-2012 IEEE.

Vibration response of a wind-turbine planetary gear set in the presence of a localized planet bearing defect

In a wind-turbine gearbox, planet bearings exhibit a high failure rate and are considered as one of the most critical components. Development of efficient vibration based fault detection methods for these bearings requires a thorough understanding of their vibration signature. Much work has been done to study the vibration properties of healthy planetary gear sets and to identify fault frequencies in fixed-axis bearings. However, vibration characteristics of planetary gear sets containing localized planet bearing defects (spalls or pits) have not been studied so far. In this paper, we propose a novel analytical model of a planetary gear set with ring gear flexibility and localized bearing defects as two key features. The model is used to simulate the vibration response of a planetary system in the presence of a defective planet bearing with faults on inner or outer raceway. The characteristic fault signature of a planetary bearing defect is determined and sources of modulation sidebands are identified. The findings from this work will be useful to improve existing sensor placement strategies and to develop more sophisticated fault detection algorithms. Copyright © 2011 by ASME.

Localized tail state distribution in amorphous oxide transistors deduced from low temperature measurements

In this paper, we extract density of localized tail states from measurements of low temperature conductance in amorphous oxide transistors. At low temperatures, trap-limited conduction prevails, allowing extraction of the trapped carrier distribution with energy. Using a test device with a-InGaZnO channel layer, the extracted tail state energy and density at the conduction band minima are 20 meV and 2 × 10 19 cm -3 eV -1, respectively, which are consistent with values reported in the literature. Also, the field-effect mobility as a function of temperature from 77 K to 300 K is retrieved for different gate voltages, yielding the activation energy and the percolation threshold. © 2012 American Institute of Physics.

Mechanism on Effect of Surface Plasmons Coupling with InGaN/GaN Quantum Wells: Enhancement and Suppression of Photoluminescence Intensity

The photoluminescence (PL) intensity enhancement and suppression mechanism on surface plasmons (SPs) coupling with InGaN/GaN quantum wells (QWs) have been systematically studied. The SP-QW coupling behaviors in the areas of GaN cap layer coated with silver thin film were compared at different temperatures and excitation powers. It is found that the internal quantum efficiency (IQE) of the light emitting diodes (LEDs) varies with temperature and excitation power, which in turn results in anomalous emission enhancement and suppression tendency related to SP-QW coupling. The observation is explained by the balance between the extraction efficiency of SPs and the IQE of LEDs

Enhanced mid-infrared transmission in heavily doped n-type semiconductor film based on surface plasmons

Transmission of electromagnetic wave in a heavily doped n-type GaAs film is studied theoretically. From the calculations, an extraordinary transmission of p-polarized waves through the film with subwavelength grooves on both surfaces at mid-infrared frequencies is found. This extraordinary transmission is attributed to the coupling of the surface-plasmon polariton modes and waveguide modes. By selecting a set of groove parameters, the transmission is optimized to a maximum. Furthermore, the transmission can be tuned by dopant concentrations. As the dopant concentration increases, the peak position shifts to higher frequency but the peak value decreases.

Localized and free exciton spin relaxation dynamics in GaInNAs/GaAs quantum well

We have investigated the exciton spin relaxation in a GaInNAs/GaAs quantum well. The recombination from free and localized excitons is resolved on the basis of an analysis of the photoluminescence characteristics. The free exciton spin relaxation time is measured to be 192 ps at 10 K, while the localized exciton spin relaxation time is one order of magnitude longer than that of the free exciton. The dependence of the free exciton spin relaxation time on the temperature above 50 K suggests that both the D'yakonov-Perel' and the Elliot-Yafet effects dominate the spin relaxation process. The temperature independence below 50 K is considered to be due to the spin exchange interaction. The ultralong spin relaxation time of the localized excitons is explained to be due to the influence of nonradiative deep centers. (c) 2008 American Institute of Physics.

Localized-Surface-Plasmon Enhanced the 357 nm Forward Emission from ZnMgO Films Capped by Pt Nanoparticles

The Pt nanoparticles (NPs), which posses the wider tunable localized-surface-plasmon (LSP) energy varying from deep ultraviolet to visible region depending on their morphology, were prepared by annealing Pt thin films with different initial mass-thicknesses. A sixfold enhancement of the 357 nm forward emission of ZnMgO was observed after capping with Pt NPs, which is due to the resonance coupling between the LSP of Pt NPs and the band-gap emission of ZnMgO. The other factors affecting the ultraviolet emission of ZnMgO, such as emission from Pt itself and light multi-scattering at the interface, were also discussed. These results indicate that Pt NPs can be used to enhance the ultraviolet emission through the LSP coupling for various wide band-gap semiconductors.

Resonant Enhanced Wave Filter and Waveguide via Surface Plasmons

The authors present an analysis of plasmonic wave filter and curved waveguide, simulated using a 2-D finite-difference time-domain technique. With different dielectric materials or surface structures located on the interface of the metal/dielectric, the resonant enhanced wave filter can divide light waves of different wavelengths and guide them with low losses. And the straight or curved waveguide can confine and guide light waves in a subwavelength scale. Within the 20 mu m simulation region, it is found that the intensity of the guided light at the interface is roughly four times the peak intensity of the incident light.

Spin-Polarized Localized Exciton Photoluminescence Dynamics in GaInNAs Quantum Wells

We have investigated spin polarization-related localized exciton photoluminescence (PL) dynamics in GaInNAs quantum wells by time-resolved PL spectroscopy. The emission energy dependence of PL polarization decay time as well as polarization-independent PL decay time suggests that the acoustic phonon scattering in the process of localized exciton transfer from the high-energy localized states to the low-energy ones dominates the PL polarization relaxation. By increasing the excitation power from 1 to 10 mW, the PL polarization decay time is changed from 0.17 to more than 1 ns, which indicates the significant effect of the trapping of localized electrons by nonradiative recombination centers. These experimental findings indicate that the spin-related PL polarization in diluted nitride semiconductors can be manipulated through carrier scattering and recombination process. (C) 2009 The Japan Society of Applied Physics

Plasmons in vertically coupled InAs/GaAs quantum dots

We investigate plasmon excitations in a quantum wire that consists of an infinite one-dimensional array of vertically coupled InAs/GaAs strained quantum dots (QDs). The research is carried out in the framework of random-phase approximation using effective-mass theory. Our formalism is capable of studying plasmons with strong tunneling among QDs, which frustrate the conventionally adopted tight-binding approximation. Based on this formalism, a systematic study on the intraminiband or intrasubband plasmon in vertically coupled InAs/GaAs strained QDs is presented. It is found that an increase of the dot spacing will inevitably reduce the plasmon energy. In contrast, the role of dot height is relatively complex and depends on the dot spacing. The results demonstrate the possibility to engineer collective excitations in low dimensional systems by simply changing their geometric configuration.

Non-exponential photoluminescence decay dynamics of localized carriers in disordered InGaN/GaN quantum wells: the role of localization length

In this article, we report a combined experimental and theoretical study on the luminescence dynamics of localized carriers in disordered InGaN/GaN quantum wells. The luminescence intensity of localized carriers is found to exhibit an unusual non-exponential decay. Adopting a new model taking the radiative recombination and phonon-assisted hopping transition between different localized states into account, which was recently developed by Rubel et al., the non-exponential decay behavior of the carriers can be quantitatively interpreted. Combining with precise structure characterization, the theoretical simulations show that the localization length of localized carriers is a key parameter governing their luminescence decay dynamics. (c) 2006 Optical Society of America.