Plasmon-Enhanced Emission Rates from III-Nitride Quantum Wells Using Tunable Surface Plasmons
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Plasmon-Enhanced Emission Rates from III-Nitride Quantum Wells Using Tunable Surface Plasmons J. Henson, J. DiMaria, E. Dimakis, R. Li, S. Minissale, L. Dal Negro, T. D. Moustakas, and R. Paiella Department of Electrical and Computer Engineering and Photonics Center, Boston University ABSTRACT Two-dimensional arrays of silver nanocylinders fabricated by electron-beam lithography are used to demonstrate plasmon-enhanced near-green light emission from nitride semiconductor quantum wells. Large enhancements in peak photoluminescence intensity (up to a factor of over 3) are obtained, accompanied by a substantial reduction in recombination lifetime indicative of increased internal quantum efficiency. The measured enhancement factors exhibit a strong dependence on the nanoparticle dimensions, underscoring the importance of geometrical tuning for this application. INTRODUCTION The development of high-efficiency InGaN light emitting diodes (LEDs) is of great interest for a wide range of applications given their ability to cover the entire visible spectrum by the tuning of their alloy composition. At the same time, however, their internal quantum efficiency (IQE) strongly decreases with increasing In content, leading to limited luminescence yields at wavelengths in the green spectral region and beyond. As a result, several techniques for lightemission efficiency enhancement are currently being investigated for InGaN materials and devices, including the use of surface plasmon polaritons (SPPs) supported by metallic nanostructures [1-4]. In a simple description, this approach can be treated as a two-step process involving plasmon-enhanced spontaneous emission followed by scattering of the emitted SPPs into radiation [5]. First, energy is transferred from the excited electron-hole pairs in the active layer to the available SPP modes via near-field electromagnetic coupling. Given the large field enhancements and high densities of modes associated with SPPs, this process can be extremely fast and thus compete favorably with nonradiative recombination mechanisms [6]. This interaction is strongest when there is close matching between the emission wavelength and the SPP resonance. Second, the emitted SPPs are extracted via scattering into radiation modes from a suitable geometry providing the required phase matching. As a result of this two-step process, an overall increase in light emission efficiency can be obtained if the SPP extraction efficiency is larger than the emission efficiency in the absence of plasmonic nanostructures. In the context of nitride semiconductors, plasmon-enhanced photoluminescence (PL) has initially been demonstrated using InGaN/GaN quantum wells (QWs) coated with Ag or Al films [1,2], whose natural roughness was large enough to sufficiently scatter the SPPs. However, this geometry offers limited control of the plasmonic resonance wavelength and extraction efficiency and the requirement of near-field proximity to the active layer makes it impractical for many device configurations. These limitations can be
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