Controlled growth and increased radiative recombination rates of InGaN quantum dots (QDs) are demonstrated. The InGaN QDs are grown by a self-assembly (SA) method using metal-organic chemical vapor deposition on planar GaN and photoelectrochemically (PEC) etched InGaN QD templates. The PEC QD templates are formed from InGaN layers with a coherent source, which results in controlled QD sizes, and are capped and planarized with AlGaN/GaN layers. The PEC QDs behave as seeds via localizing strain near the QDs, which provide improved control of the SA QD growth. The SA QDs grown on PEC QD templates are smaller and have controlled sizes. Multiple quantum dots, consisting of 4 periods of SA QDs and AlGaN/GaN barrier layers, grown on PEC QD templates have higher radiative recombination rates. The higher rates are a result of the controlled and smaller-sized SA QDs. Forming SA QDs on PEC QDs templates can enhance the performance of InGaN-based QD emitters.
The challenges and approaches for high-efficiency InGaN quantum wells (QWs) light-emitting diodes (LEDs) are
presented. The studies include designs, growths, and device characteristics of 1) InGaN-based QWs LEDs with
enhanced matrix element for realizing green-emitting LEDs with high internal quantum efficiency, and 2) InGaN QW
LEDs device structure with lattice-matched AlInN-barrier to suppress efficiency-droop in nitride LEDs. Other
approaches to improve the efficiency of the nitride LEDs will be discussed as follow: 1) surface plasmon LEDs, 2) new
growth approach for dislocation density reduction in GaN, and 3) novel approaches for light extraction efficiency
improvement of III-Nitride LEDs.
The enhancement of light extraction efficiency of InGaN quantum well (QW) light emitting diodes (LEDs) was achieved
by employing the refractive index matched TiO2 microsphere arrays. The optimization studies of the dipping method and
rapid convective deposition (RCD) method were carried out for the deposition of TiO2 microsphere arrays onto LEDs.
The 2-dimensional relatively close-packed and close-packed TiO2 microsphere arrays were deposited by the using
optimized conditions of the dipping method and RCD method, respectively. The light extraction efficiencies of LEDs
under electrical injection were enhanced by 1.83 times by utilizing 520-nm diameter TiO2 microspheres. This
enhancement is primarily attributed to increase in the effective photon escape cone due to the matched index and
spherical shape of TiO2 microstructures arrays.
Being motivated by the possibility of fingerprinting and detecting VX nerve agent, we have investigated its stimulant,
i.e. malathion vapor, which is less toxic and commercially available, in the far-infrared/THz transition region and THz
frequency range. Such a spectroscopic study was carried out by using Fourier transform infrared spectroscopy (FTIR).
Our intention is to obtain a specific spectroscopic signature of VX nerve agent as a chemical warfare agent. Following
our experimental result, we have successfully observed eleven new absorption peaks from malathion vapor in the
spectral ranges from 15 cm-1 to 68 cm-1 and from 75 cm-1 to 640 cm-1. Specifically, in the far-infrared/THz transition
region, we have observed eight peaks and whereas in the THz region we have identified three relatively weak transition
peaks. In addition, we have investigated the dependence of the absorption spectra on temperature in the range from room
temperature to 60°C. In both of the frequency ranges, we have found that absorption coefficients significantly increase
with increasing temperature. By comparing the transition peaks in the two frequency ranges, we have concluded that the
frequency range of 400-640cm-1 is an optimal range for fingerprinting this chemical specie. We have designated two
peaks for effectively and accurately identifying the VX nerve agents and one peak for differentiating between malathion
and VX nerve agent.
Thin film multi-layered chalcogenide glass waveguide structures have been fabricated for evanescent wave sensing of bio toxins and other applications. Thin films of Ge containing chalcogenides have been deposited onto Si substrates, with a-GeSe2 as the cladding layer and a-GeSbSe as the core layer to form the slab waveguide. Channel waveguides have been written in the slab waveguides by appropriate light the through a mask. The photo-induced structural changes in the core layer selectively enhance refractive index at the portions of interest and thus confining the light to the channels. The waveguides have been characterized and tested for the guiding of light.
We have systematically designed, fabricated, and tested chalcogenide-glass waveguides. Among all the characterization techniques, we have found that the prism-coupling method is the most effective and accurate for determining all the parameters describing the performance of the slab waveguides. Furthermore, we have also achieved the end-fire coupling in these waveguides to study the characteristics of the transmitted beams. These waveguides can be optimized eventually for the biosensor applications.
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