III-Nitride Photonic Crystals for Blue and UV Emitters
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III-Nitride Photonic Crystals for Blue and UV Emitters J. Shakya, K. H. Kim, J. Li, J. Y. Lin, and H. X. Jianga) Department of Physics, Kansas State University, Manhattan, Kansas 66506-2601, USA T. N. Oder Department of Physics and Astronomy, Youngstown State University, Youngstown, Ohio 44555, USA ABSTRACT We report the nanofabrication and characterization of triangular lattice array of photonic crystals (PCs) with diameter/periodicity as small as 100/180 nm on III-nitride Light Emitting Diodes (LEDs) using electron beam lithography and inductively-coupled-plasma dry etching. Under optical pumping, a maximum enhancement factor of 20 was obtained from the PCs for emission light intensity at the wavelength of 475 nm at room temperature. Under current injection, the total power at 20 mA of 300 x 300 µm2 unpackaged LED chips revealed an increase by 63% and 95% for 460 nm blue and 340 nm UV LEDs, respectively, as a result of the PC formation. Our results show that the fabrication of PCs enhances the power output significantly on the III-nitride LEDs, which currently have very low external quantum efficiency especially in the UV range. INTRODUCTION Light emitting diodes (LEDs) with high external efficiency are currently in high demand for a variety of applications including flat panel displays, printers, optical interconnects in computers and general lighting. High efficient ultraviolet (UV) emitters are particularly sought for applications including chemical and biological agent detection and medical uses. However, while the internal quantum efficiency (QE) of visible LEDs is close to 100%; most of the light is lost to guided modes in the semiconductor materials and only about 1/(4n2) of the light emitted radiates through the top and bottom [1]. For nitride materials of refractive index n ~ 2.4, this amounts to only about 5% of the blue and green light extracted from the top and bottom surfaces. The need for improvement of extraction efficiency in LEDs is exceptionally great, especially for deep UV LEDs (λ < 340 nm) based on III-nitride wide bandgap semiconductors, which presently have very low QE. Much effort has been exerted in improving LED QE including the use of photon recycling schemes and novel geometrical designs aimed at enlarging escape cones of emitted light [2-4]. Our group has also previously developed interconnected microdisk LED architecture as a method of enhancing extraction efficiency [5]. Multiple scattering of photons by lattices of periodically varying refractive indices in PCs acts to form photonic bandgaps (PBGs) in which propagation of certain wavelengths of the electromagnetic waves are prohibited. This can be exploited to confine light propagation only in the vertical direction and thus enhance light extraction efficiency in LEDs. Ideal PBG is achieved by periodicity in 3 dimensions but for extraction of light in LEDs, it is sufficient to eliminate light propagation only in the horizontal plane with the use of 2-D PCs. For this purpose, triangular arrays of holes are typically etched in
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