GaN and its alloy with Al and In recently became the basic materials for short-wavelength optoelectronics. This was due mainly to their direct energy gaps covering the whole visible spectrum and a large part of UV range (6.2, 3.4 and 1.9 eV for AlN, GaN and InN respectively).
These general properties lend themselves to exploitation in nearly any electronic and optical device with special applications in high power electronics, high frequency applications, and use in hostile conditions such as high temperature probes.
Surface plasmons (SPs) have attracted much attention because optical properties can be greatly enhanced by coupling between SPs and the multiple quantum wells (MQWs) in LEDs. SPs are the collective oscillations of free electrons in a metal at the interfaces between metals and dielectrics. Especially, surface plasmon polaritons (SPPs) at metal surfaces and localized surface plasmons (LSPs) of metallic nanostructures can both be used to enhance the spontaneous emission rate and internal quantum efficiency of LEDs.
Photonic crystals (PCs) are periodic dielectric structures that have a band gap that forbids propagation of a certain frequency range of light. LEDs with PCs could be designed to efficiently couple light from the dielectric-guided modes into air. They can be utilized to inhibit emission of guided modes or redirect trapped light into radiated modes.
Antireflective sub-wavelength structure (SWS) is an ordered or disordered tapered feature with gradient refractive index. When the grating period becomes smaller than the wavelength, all higher-order diffracted fields are evanescent and only the zero-order propagates. At this time, the SWS can be regarded as a homogeneous medium with effective refractive index.
As the efficiency of LEDs improves, LEDs find more applications such as back light unit, automotive headlights, and general illumination. However, as injection current increases in InGaN/GaN MQWs LEDs, we observe a unique phenomenon called "efficiency droop" that is the reduction in LED efficiency at high injection current density. Therefore, the efficiency droop should be solved to develop the high-power LEDs.
