The Research On The Improvement Of Lighting Efficiency Of LED By Surface Plasmon Technique

The 21st century has witnessed a boom of optoelectronic technologies. LED is one of the key elements in a variety of contemporary optoelectronic systems. The light emission in active region of LED is generated by spontaneous electron-hole pair radiative recombination. As semiconductor solid light source, LED's incomparable advantages over other light sources include-ruggedness, reliability, and scalability, making LED the ideal choice in a broad range of applications. LEDs not only appear in high power lighting system as single chip, but also can be integrated in pixel point of flat panel display. LEDs can reliably work in extreme weather conditions, and seldom fail in treacherous environment. In their early childhood, LEDs were developed on GaAsP and GaP based materials, emitting red and blue light. GaN based blue light LED is the newer member of this big family, but with the greatest potential for development. GaN LED can be used as white light source. With the rapid improvement of related technologies, GaN LED is expected to be the right replacement of conventional light sources and the ideal choice for green lighting. But a sad fact of LEDs is:most of the photons generated inside the device cannot escape from the semiconductor to produce available optical output power. The problem is due to the unique structure of semiconductor solid light source:the refraction index of solid light-emitting body is usually higher than that of its surrounding dielectrics (eg. air), thus total internal reflection occurs on the semiconductor/air interface, preventing light from escaping. Apart from that, even a small fraction of light with incident angle smaller than critical angle is not subjected to total internal reflection, it is still subjected to Fresnel reflection when making the path through the semiconductor/air interface. In a word, the high refraction index of semiconductor light-emitting body is the trouble maker. Current technologies for the improvement of light extraction efficiency mainly rely on surface texturing, including surface roughening and photonic crystals, and so on. The recombination events of electrons and holes in semiconductor are basically divided into two categories - radicative recombination and non-radiative recombination. Photons are generated only in radiative recombination, whereas non-radiative recombination usually ends up in heat. The ratio of radiative versus non-radiative recombination defines the internal quantum efficiency of LED. Therefore improvement of radiative recombination rate plays a key role in promotion of LED efficiency and energy conservation. And for applications such as white lighting, the internal quantum efficiency droop of GaN LED under heavy current injection is another annoying problem.The improvement of light extraction efficiency, spontaneous radiative recombination rate and internal quantum efficiency should definitely accelerate the process of massive commercial applications of GaN LED, and make the device comply with energy-saving rules. Currently available technologies can respectively solve the above problems to some extent; however, a once-for-all solution for all of the problems has never emerged.Sub-wavelength metal structure optics is a recent hot topic. A great number of optical phenomena can occur in sub-wavelength metal structures, typically, extraordinary optical transmission. Surface plamson excited by sub-wavelength metal structure usually plays a significant role behind scene. Surface plasmons are quantization of collective oscillations of free electron gas density at a metal/dielectric interface under electromagnetic illumination, just as photons are quantization of light and phonons of sound waves. These electron oscillations can interact with the incoming light resulting in evanescently confined electromagnetic modes propagating along the metal interface. This unique property opens up a tunnel for direct coupling of light and electronic components, and has been exploited in a wide range of contemporary optoelectronic devices. Sub-wavelength structures in GaN LED can excite extraordinary optical transmission, release the trapped light, and enhance the light transmission through semiconductor/air interface; moreover, surface plasmon on sub-wavelength structure can couple with the light-emitting body of GaN LED, improving spontaneous emission rate. Thus sub-wavelength metal structure reveals a possible way to realize simultaneous promotion of light extraction efficiency and internal quantum efficiency, enlighten us about a shortcut toward high efficiency GaN LED.The normal procedure for development of novel optoelectronic devices usually starts with computer simulation. To shorten development cycle and cut down expenses, the design is usually verified in computer as a substitute for complicated, expensive, and time-consuming trial manufacture process. Theoretically, the rigorous description of optoelectronic devices comprises optical, electrical, thermal, and structural mechanic parameters; based on our needs, we may be interested in all of them or just one or several of the four kinds. The research of sub-wavelength metal GaN LED follows the above normal procedure, beginning with computer simulation. And for LED, we are usually concerned about the optical, electrical and thermal characteristics.To study the optical phenomena on sub-wavelength metal structure in GaN LED, it is advisable to use electromagnetic simulation technology to solve Maxwell's equations under specific boundary conditions. Due to the complexity of the structure, there are hardly any analytical methods to solve Maxwell's equations in LED; instead, any reasonable results must be obtained with numeric computational methods. Finite-difference time-domain (FDTD) method is a most popular numeric method for computational electromagnetics, and very suitable for analysis of electromagnetic field in GaN LED with sub-wavelength metal structure. Theoretically, by setting up dielectric parameters, boundary conditions and source excitation, the time evolution of electromagnetic field in GaN LED can be obtained by FDTD iteration. For the same LED structure, the electrical, thermal characteristics can be analyzed by semiconductor simulation. Similar to electromagnetic simulation, semiconductor simulation calculate LED's energy band model, carrier diffusion and drift model, and heat generation and dissipation model with numeric methods.The purpose of this dissertation is to investigate sub-wavelength metal structure and its role in improvement of GaN LED efficiency. It focuses on the following aspects: In this dissertation, the physical mechanisms for extraordinary optical transmission in sub-wavelength metal structure under illumination of electric dipole sources of GaN LED are discussed. The role of surface plasmon and Fabry-Perot resonance in extraordinary optical transmission, and their contributions to GaN LED light extraction improvement are compared. The influence of the geometrical parameters of sub-wavelength periodic array, the position and polarization of electric dipole source upon light extraction improvement is discussed. The results indicate extraordinary optical transmission on nano metal periodic array structures provides a channel for the photons inside GaN LED to escape from the semiconductor structure and therefore improves the light extraction efficiency of GaN LED.The principle of coupling of surface plasmon with quantum wells inside GaN LED is discussed, and so the influence on the spontaneous emission rate of quantum wells. With numeric simulation, the influence of geometrical parameters of surface plasmon-quantum well coupling structure, such as the thickness of metal thin film and GaN layer, the distance between source and metal thin film, upon the spontaneous emission rate of quantum well is discussed. The results indicate that nano metal film of appropriate thickness can couple with quantum wells in GaN LED and this process can serve to increase the spontaneous emission rate of quantum wells.An approach for energy transmission across metal thin film in GaN LED is proposed. This approach can realize energy transmission by mutual coupling of surface plasmon on the two surfaces of metal thin film. A FDTD simulation is launched to verify the idea, and the results confirm the successful transmission of optical energy by this approach.An approach to extract visible light from surface plasmon is proposed. This approach uses dielectric grating on metal thin film to extract light from surface plasmon as well as control the light propagation direction. A FDTD simulation is launched to optimize the parameters of the grating and verify the idea; and the calculated optical field profiles confirm the effectiveness of this approach.With semiconductor simulation technology, the electrical and thermal characteristics of GaN LED with sub-wavelength metal structure are investigated. The carrier diffusion and drift, heat generation and dissipation, and influencing factors on internal quantum efficiency inside GaN LED are discussed. The work in this dissertation produces simulation results in regard to optical, electrical, thermal characteristics of GaN LED with sub-wavelength metal structures.