Growth and characterization of III-nitride semiconductors for high-efficient light-emitting diodes by metalorganic chemical vapor deposition
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The engineering of carrier dynamics in the MQW active region by modifying the p-type layers in the III-nitride based visible LEDs is described in this dissertation. It was found that the holes are preferentially injected into the QW adjacent to the p-InxGa1-xN layer with lower Indium mole fraction. Enhanced hole transport with increasing Indium mole fraction in the p-InxGa1-xN:Mg layer has been shown by analyzing the EL spectra. The improved hole transport and corresponding uniform distribution was achieved presumably by the potential barrier near the p-type layer and the MQW active region resulting in a modified kinetic energy of holes which creates a hole-transport-favorable environment in the MQW active region. At the same time, the limited hole injection due to the potential barrier for holes can be overcome under high injection conditions. The InAlN layers are widely used as an alternative high quality electron blocking layer in InGaN/GaN based visible LED structures. However, the Ga auto-incorporation of the InAlN layers has been recently reported during the growth of epitaxial layers by both MOCVD and MBE. The possible origins and a mechanism of Ga auto-incorporation of InAlN epitaxial layers were systematically investigated in this dissertation. It was found that the Ga-containing deposition on a wafer susceptor/carrier is the most dominant precursor for Ga auto-incorporation and the deposition on surrounding surfaces of quartz parts in a growth chamber is the other dominant source, while the effect of stainless-steel parts and interdiffusion of Ga atom from GaN underlayer are not critical. In addition, Mg or Cp2Mg in the growth chamber during InAl(Ga)N layer growth facilitates the auto-incorporation of Ga by modifying deposition conditions of GaN on the surrounding surfaces and the wafer susceptor/carrier. Based on experimental data of various cases, the Ga-containing deposition on any hot surfaces, which are also exposed to Indium precursor to form a liquid phase, is believed to be major origins of Ga auto-incorporation. In an effort to enhance the light extraction efficiency (LEE) in the LEDs, the direct patterning on the top surface of a LED structure, using laser interference ablation technique, has been studied in this dissertation. The 2-dimensional hexagonal lattice array of surface patterns was generated by direct irradiation of the laser source which is the interference of three laser beams onto the top p-GaN surface, without deterioration of electrical property of p-type layer and optical properties of MQW active region. The experimental results showed approximately 20 % improved LEE of the laser-patterned LED structure compared to the conventional LED structure without surface textures. Furthermore, the theoretical calculation using Monte-Carlo ray-tracing simulation confirmed the enhancement of LEE of the laser-patterned LED structure.