Study on Surface Plasmon Coupling with InGaN/GaN Quantum Wells
Date Issued
2009
Date
2009
Author(s)
Lu, Yen-Cheng
Abstract
In this dissertation, we first demonstrate the carrier trapping effects on photoluminescence (PL) decay time in InGaN/GaN quantum wells (QWs) with nano-cluster structures. Carrier dynamics in InGaN/GaN QWs with compositional fluctuations is studied with time-resolved PL experiment and Monte Carlo simulations of exciton hopping and recombination. In particular, the effects of indium-rich nano-clusters in such a QW structure on the photon-energy dependent PL decay time are investigated. In our experiments, two InGaN/GaN QW samples of different silicon doping conditions are used for demonstrating the two cases of different nano-cluster densities. An increasing trend of PL decay time on the high-energy side of the PL spectrum is observed in the sample of high nano-cluster density. Such a trend is not observed in another sample with few clusters. This difference is consistent with the simulation results which can help us in identifying the origin of the increasing trend as the exciton trapping by the local potential minima in the spectral range of the free-carrier states. Then, we study the temperature-dependent behavior of the surface plasmon (SP) coupling with an InGaN/GaN QW. The SP coupling efficiency relies on the availability of carriers with sufficient momentum for transferring the energy and momentum into the SP modes. At low temperatures, the carriers are trapped by the potential minima in the QW and the SP coupling is weak. As temperature increases, more and more carriers escape from the potential minima leading to the stronger and stronger SP coupling. When the temperature is close to the room condition, the SP coupling strength saturates because most carriers have escaped from the potential minima. The three temperature ranges of different SP coupling behaviors can be clearly identified from the data of PL enhancement ratio and PL intensity decay rate, and are consistent with those of temperature dependent PL feature variation, which clearly indicates the carrier localization process. Next, we report the observation of the enhancement of photoluminescence excitation through the couplings of an InGaN/GaN QW with localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs), which are generated on an Ag nanostructure deposited on the SiN-coated QW epitaxial sample. At the wavelengths corresponding to the LSP modes, the excitation light is first absorbed by the LSPs. The LSP energy is then transferred into the QW through the absorption of the LSP evanescent fields by the carriers. After the carriers are relaxed down to the PL emission levels, the coupling of the carriers with the SPPs enhances light emission. The coupling of the SPPs with the relaxed carriers becomes stronger as temperature increases because of the increased carrier momentum. Finally, the improved emission enhancement in SPP coupling with an InGaN/GaN QW by inserting a SiO2 layer of lower refractive index between the deposited Ag and GaN layers is experimentally and numerically demonstrated. The inserted SiO2 layer leads to reduced SPP dissipation rate, increased evanescent field intensity beyond a certain depth in GaN, and decreased SPP density of state. The combination of these factors can result in further emission enhancement of QW through SPP coupling. For light-emitting diode application, the elongated evanescent field coverage can release the constraint of thin p-type GaN for effective SPP coupling. More importantly, the reduced SPP dissipation can result in more effective emission in such an SPP-QW coupling mechanism.
Subjects
InGaN
Surface plasmon
Type
thesis
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