Dual-layer Structures Research Articles

Monolitically grown dual wavelength InGaN LEDs for improved CRI

AbstractIn this study we have grown dual wavelength epitaxial LED layer structures by MOCVD. A secondary InGaN MQW is added to the layer structure to optically convert a certain fraction of photons which have been electrically generated in the primary InGaN‐MQW. Photoluminescence spectra are compared to electrical ThinGaN® chip operation data. The emission spectra clearly display two peaks. The height of the peak generated by the converted photons was adjusted by the number of QWs used in the secondary MQW. Additionally, the influence of the optical ThinGaN® light extraction design on the final emission spectrum has been investigated. These emission spectra are discussed in dependence of current density and temperature. Finally, dual MQW ThinGaN® devices have been assembled into phosphor converted white high‐power LED packages. The measured spectra confirm improved CRI values and are compared to simulated data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization-dependent plasmonic coupling in dual-layer metallic structures at terahertz frequencies

Dual-layer metallic wire-hole structures were fabricated and their terahertz transmission properties were measured. They exhibit polarization-dependent transmittance with large extinction ratios. Simulation and experimental results on structures with different wire-to-hole orientations provide strong evidence that the resonance peaks are caused by plasmonic coupling between the two metallic layers. A simplified LC-circuit model is proposed to explain the coupling mechanism and to estimate the peak frequencies. Our results suggest that specific electromagnetic response can be achieved by appropriate design of the geometrical patterns on the two metallic layers and a suitable polarization of the incident wave.

Surface-enhanced Raman scattering on dual-layer metallic grating structures

Dual-layer Metallic grating (DMG) structures as surface-enhanced Raman scattering (SERS) substrates are studied using benzenethiol as the probe analyte. The DMG structure consists of a SiO2 grating and 100-nm-thick gold coating layers. An enhancement factor of 105 is achieved by optimizing the SiO2 grating height within the range from 165 to 550 nm. The enhancement factor dependence on the SiO2 grating height is due to the surface plasmon excitation, which is dependent on the polarization of the incident light, and confirmed by finite difference time domain simulations. This study demonstrates the advantages of high uniformity, reproducibility and sensitivity in the DMG structures for SERS applications.

Plastic masters—rigid templates for soft lithography

We demonstrate a simple process for the fabrication of rigid plastic master molds for soft lithography directly from (poly)dimethysiloxane devices. Plastics masters (PMs) provide a cost-effective alternative to silicon-based masters and can be easily replicated without the need for cleanroom facilities. We have successfully demonstrated the use of plastics micromolding to generate both single and dual-layer plastic structures, and have characterized the fidelity of the molding process. Using the PM fabrication technique, world-to-chip connections can be integrated directly into the master enabling devices with robust, well-aligned fluidic ports directly after molding. PMs provide an easy technique for the fabrication of microfluidic devices and a simple route for the scaling-up of fabrication of robust masters for soft lithography.

The effect of insulating spacer layers on the electrical properties of polymeric Langmuir-Blodgett film light emitting devices

We report on the use of insulating Langmuir-Blodgett (LB) films to improve the quantum efficiency of light emitting devices based on polymeric LB films. The insertion of arachidic acid layers between a poly-(2-methoxy,5-(2'ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) film and an aluminium cathode was found to decrease the turn-on voltage for electroluminescence and to double the quantum efficiency. In contrast, depositing the fatty acid layer between the polymer and the indium tin oxide anode produced little change in the device efficiency. Dual-layer structures using MEH-PPV as the light emissive layer and poly-(p-(3-hexyl pyridylene)) as an electron transport layer were also tested. Again, an increase in device efficiency could be obtained by placing arachidic acid LB layers immediately beneath the metallic cathode.

Formation effects and optical absorption of Ag nanocrystals embedded in single crystal SiO2 by implantation

Ag + ions of 200 keV were implanted into single crystal SiO2 at room temperature to five different doses: 5×1015, 2.3×1016, 4.5×1016, 5.6×1016, and 6.7×1016/cm2. With increasing dose, the implanted Ag distributions change from usual Gaussian-type profiles to abnormal bimodal profiles with narrow full width at half maximum, which are associated with Ag nanoparticles forming during high dose implantation. The implanted Ag depth profile evolution with dose can be clearly observed during Rutherford backscattering spectroscopy analysis. The nanoparticles form dual-layer structures at high doses: as far as the dose of 6.7×1016/cm2 is concerned, transmission electron microscopy proves that the shallower implanted layer contains noninteracting small Ag nanoparticles with the diameters of about 7 nm; the deeper layer contains a high density of interacting large nanoparticles with the diameters of about 25 nm. High resolution electron microscopy identifies that the nanoparticles are perfect single crystals. Although plasmon resonance frequency of the Ag nanoparticles formed at relatively low dose agrees well with the Mie’s theoretical prediction, great redshift due to multipole interactions between high density nanoparticles occurs for high doses, moreover, the magnitude of redshift increases with implanted dose.

Generalized impedance boundary conditions for isotropic multilayers

Second-order approximate boundary conditions for multilayered absorbing covering are developed. For dual-layer structures, the coefficients are given in explicit form. The results are applicable to all practically interesting cases, as is demonstrated by numerical examples. These results can be used in analytical and numerical studies of scattering from complex-shaped bodies covered with microwave absorbers. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 17: 262–265, 1998.

Inter-diffusion phenomena and electrical conduction in thick-film segmented-resistor structures

Inter-facial processes and charge-transport mechanisms in a segmented resistor have been investigated using single- and double-layer resistive test structures. Microstructure studies of the cross section of the specimens supported by elemental mapping using energy- and wavelength-dispersive X-ray analysis show that, in dual-layer structures, RuO2 agglomerates in a thin region near the interface and the current flow takes place principally through this region. From the Arrhenius plots of the specimens combined with Mott's T1/4 law, it is found that the conduction mechanism at the junction of two cofired thick resistive films is governed by the mechanism of the lower-sheet-resistivity film.