LEDs based on III-nitride quantum dots and quantum wells grown by molecular beam epitaxy

The research described in this dissertation addresses the study of materials issues related to the growth and characterization of nitride semiconductor quantum wells (QWs) and quantum dots (QDs) and their application to blue-green and ultra-violet light emitting diodes (LEDs).; The materials component involves the study of the growth of self-assembled gallium nitride (GaN), indium nitride (InN) and indium gallium nitride (InGaN) QDs by plasma-assisted molecular beam epitaxy. The growth of the QDs was monitored by reflection high energy electron diffraction (RHEED). The QD morphology and microstructure were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM), while the QD emission properties were studied by cathodoluminescence spectroscopy. GaN QDs grown on an aluminum nitride (AlN) buffer follow the classical pattern of the Stranski-Krastanov mode, where the formation of a two dimensional layer is followed by the formation of GaN QDs. AFM studies indicate that the GaN QDs on AlN have a density of the order of 1011/cm2 and have a bimodal size distribution, with the larger size QDs dominating as the number of deposited monolayers increases. Contrary to GaN QDs grown on AlN, TEM studies show InN QDs grown on a GaN buffer to be fully relaxed, a result attributed to the larger lattice mismatch between GaN and InN (Volmer-Weber Mode). The formation of InGaN QDs on a GaN buffer is more difficult and the mode of growth should depend on InN mole fraction. AFM studies indicate that InGaN QDs with up to 40% InN mole fraction are formed on a GaN buffer, and their density and size depend strongly on deposition temperature. It was found that the cathodoluminescence emission spectra are significantly red shifted in multiple layers of InGaN/GaN quantum dots (MQDs), a result attributed to Quantum Confined Stark effect due to the additional stress on QDs by the GaN barriers.; LED structures based on InGaN/GaN multiple quantum wells (MQWs) and emitting at 440 nm were grown with or without GaN QDs in the nucleation layer. TEM studies found that the inclusion of QDs in the nucleation layer acts as a dislocation filtering mechanism leading to superior devices. Green and red LEDs based on InGaN MQDs emitting at 560 nm and 640 nm, respectively, were grown. Such structures were fabricated into 800 mum x 800 mum LED devices using standard photolithography and metallization schemes. The electroluminescence spectra of these devices were investigated as a function of injection current, and bare-die blue LEDs with total power output of 4.5 mW have been obtained.