Sduty On Designand Manufacturing Technology For High Power Gallium Nitride-Based Light Emitting Diodes

It has been widely known that solid state lighting, which is based on high power light emitting diodes (LEDs), has potential applications in general lighting due to its capability of energy saving. It is very important to improve luminous efficiency of LED, which is the key issue for the massive application of solid state lighting. There are two factors affecting the luminous efficiency of LED chip, including internal quantum efficiency and light extraction efficiency. Consequently, more and more researchers are exploring to improve internal quantum efficiency and light extraction efficiency of LED chip. Howerver, most of the studies focused on on the low power LED chip with small size. Additionally, high power LED performance is related to fabrication processes. The fabrication processes have effect on the LED's luminous efficiency, reliability, and durability. Some key technical problems relating to high power LED chip manufacturing have not been fully solved, which lead to the lack of experimental data support for optimized design of LED chip, material selection and fabrication process control. To solve these problems, a new method was established to enhance the light extraction efficiency of LED, and effects of material properties, chip structure parameters, and fabrication process parameters on the LED performance are investigated in three aspects: numerical simulation, theory and experiment. The detailed contents of this thesis contain:1) Effects of LED structure parameters and material properties on the light extraction efficiency of LED were investigated based on Monte-Carlo ray tracing method. A three-dimensional electrothermal-coupled simulation model based on finite element method was established to study the electrical and thermal properties of high power LED chip. The current spreading, heat generation and transfer in the LED chip were simultaneously analyzed in this model. Effects of the electrode pattern, ITO transparent conductive layer, and etched mesa depth on the uniform current spreading performance and thermal performance of LED chip were investigated in detail based on the model. A novel method by integrating patterned sapphire substrate with patterned ITO was employed to enhance light extraction efficiency of high power LED chip. The periodic semisphere shaped pattern formed on sapphire substrate can eliminate the total internal reflection existing along the interface between GaN buffer layer and sapphire substrate. The periodic and nonperiodic pattern formed on ITO transparent conductive layer can eliminate the total internal reflection existing along the interface between n-GaN layer and air.2) In order to realize high rate, low damage etching for GaN material, processing technique using Cl2/BCl3 inductively coupled plasma (ICP) was systemically investigated. ICP power/RF power, operating pressure, and Cl2/BCl3 gas mixing ratio were altered to investigate the effect of input process parameters on the GaN etch rate, microstructure profile, surface roughness, and photoluminescence spectra intensity by scanning electron microscope (SEM), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. A two-step process including high-rate etching followed by low-damage etching is suggested for deep etching of GaN. Based on the investigation, the optimized etching process, used for shallow and deep etching of GaN during the LED fabrication, was presented.3) Micro machining techniques, including lithography, etching, film deposition, metallization, and grinding & polishing, were employed to fabricate LED chip. Periodic semisphere shaped patterns with 3.5um bottom size and 1.2um height were formed on sapphire substrate by ICP etching. The periodic and nonperiodic circular pattern and hexagonal pattern were form on ITO transparent conductive layer by wet etching. A method integrating mechanical grinding, chemical mechanical polishing (CMP) and dry etching process to remove sapphire substrate for fabricating vertical structure LED was presented.4) A measurement system has been developed for LED characteristic parameters testing including photometric parameters, colorimetric parameters, thermal parameter, and electrical parameters. Two novel algorithms including look-up table method and curve fitting method were employed to determine the dominant wavelength of LED chip. In combination with linear interpolation, the look-up table method can locate dominant wavelength of LED with a resolution of 0.1nm. Effect of patterned sapphire substrate on optical power of LED was analyzed based on the developed measurement system. Effects of pattern geometry, size, and spacing on the optical power, I-V characteristic of LED were also studied in detail. Additionally, effects of sapphire substrate thickness on the junction temperature and I-V characteristic of LED chip were also analyzed. The experimental results agreed well with simulation results of this thesis.