Using finite difference time domain simulations and e-beam assisted lithography we designed and fabricated high
transmission transparent contacts for UV nitride devices which consist in perpendicular sets of parallel aluminum lines
with a period as low as 260 nm. Transmittance values as high as 100% were predicted for aluminum meshes with the
optimized periods, metal line widths and thicknesses. Simulations were compared with optical transmittance
measurements. The critical parameters -such as grain size, edge roughness and mesh coating- were determined. The
large aluminum grain was decreased by performing a cold aluminum deposition. The aluminum oxide layer over the
aluminum mesh was found to reduce the mesh transmittance. Several alternatives were studied to overcome this issue
such as coating the mesh with a thin gold or silicon dioxide layer. While the second option appeared promising the
addition of the gold layer required much more improvement.
Using e-beam lithography on a single layer of polymethylmethacrylate (PMMA) we designed a relatively thick
subwavelength aluminum mesh on top of sapphire. The 100 nm thick mesh consisted of two perpendicularly oriented
sets of 100 nm wide parallel metal lines with a center to center distance as low as 260 nm. Due to the large proximity
effect during e-beam exposure and the small spacing between metallic lines the use of an adhesion promoting layer
appeared necessary to avoid premature peeling of the photoresist. Using a monoatomic layer of hexamethyldisilazane
(HMDS) as an adhesion promoter between the sapphire and the PMMA, a 500 nm thick photoresist layer could be
exposed and developed with excellent control over the features sizes. Line spacing distances from 500 nm down to 160
nm were achieved. An oxide plasma etch was found to be necessary for metal adhesion during the lift-off process. Due to
the small spacing between the aluminum lines, use of a bi-layer photoresist technique to achieve undercut was not
possible. Thermal evaporation of aluminum was performed and e-beam evaporation didn't help smoothing the metal
surface. An additional ultrasonic bath in acetone was found necessary to ease the lift-off process.
Using 2D finite element modeling with the ability to solve the current continuity equations, carrier energy transport
equation, Schrödinger and Poisson equations self-consistently, as well as the scalar wave equation for waveguiding
devices, we have investigated the possible improvements of the device efficiencies by introducing transparent p-type
contacts and multiple quantum shells (MQSs) in GaN / In<sub>0.14</sub>Ga<sub>0.86</sub>N / GaN / p-AlGaN / p-GaN core/multishell nanowires
(CMS NWs). The addition of a transparent p-type current spreading contact was found to promote more uniform current
injection into the CMS NWs, thus increasing the current injection efficiency. Despite the inclusion of a transparent ptype
contact, the current density remained non-uniform and weighted towards the n-contact side of the NW. This
asymmetry in the current density was found to be more important for higher injection current whereas it becomes much
more uniform with decreasing injection current. Light generation with the transparent contact was found to become more
uniformly distributed along the CMS NW, leading to more even light generation within the device in comparison to
NWs without transparent p-type contacts. The replacement of single quantum shells (SQS) by MQSs in the active
region of the nitride CMS NW-as has been used for conventional InGaN high brightness LEDs (HB-LEDs)-was found
to be advantageous up to three quantum shells, increasing light generation from 80.47 to 94.04 W/m under a 4V bias.
A comprehensive theoretical model for InGaN core/multishell nanowire (CMS NW) light emitting diodes (LEDs) has been developed that accurately predicts both electrical and optical properties of CMS NW structures. Using the model, the electron and hole injection in a CMS NW device was studied, showing that the InGaN quantum well (QW) serves as a high electron mobility channel for electron injection while the poor hole conductivity in the p-GaN outer layer confines hole injection to an area directly below the p-contact. Light generation in the CMS NW was found to occur only directly below the p-contact in the InGaN QW as a result of the isolated hole injection. The model was found to accurately predict the optical emission of the CMS NWs for both changes in thickness and Indium composition of the InGaN QW when compared to experimental values from the literature. Piezoelectric effects were not found to play a significant role in the CMS NWs, suggesting that alloy broadening and excitonic emission are the dominant role in determining the FWHM of the CMS NW emission.
Recent work in plasmon nanophotonics has shown the successful fabrication of surface plasmon (SP) based optical elements such as waveguides, splitters, and multimode interference devices. These elements enable the development of plasmonic integrated circuits. An important challenge lies in the coupling of conventional far-field optics to such nanoscale optical circuits. To address this coupling issue, we have designed structures that employ local resonances for far-field excitation of SPs. The proposed coupler structure consists of an array of ellipsoidal silver nanoparticles embedded in SiO<sub>2</sub> and placed close to a silver surface. To study the performance of the coupler we have performed simulations using the Finite Integration Technique. Our simulations show that normal incidence illumination at a free-space wavelength of 676 nm leads to the resonant excitation of SP oscillations in the Ag nanoparticles, accompanied by coherent near-field excitation of propagating SPs on the Ag film. The excitation efficiency can by maximized by tuning the aspect ratio of the nanoparticles, showing optimum coupling at an aspect ratio of 3.0 with the long axis (75 nm) along the polarization of the excitation signal. We discuss the origin of these observations.