Browsing by Author "Suihkonen, Sami, Dr."

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Approaches for optimizing light emitting diode structures based on III-N materials
(2011) Törmä, Pekka
Sähkötekniikan korkeakoulu | Doctoral dissertation (article-based)
In this thesis fabrication and properties of light emitting diode (LED) structures based on III-N materials were studied. LED structures were grown by metal-organic vapor phase epitaxy (MOVPE). Especially increasing the light extraction efficiency (LEE) by using modified sapphire substrates was investigated. Optimization of InGaN/GaN and InGaN/InAlGaN multiple quantum well (MQW) structures was studied to improve internal quantum efficiency (IQE) of the LEDs. Electron blocking layer (EBL) was developed to increase carrier confinement into the active MQW region. The samples were characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and electroluminescence (EL) measurements. Roughening and periodic patterning of the sapphire substrates were investigated in order to improve LEE. Significant improvements were achieved in LED performance with these methods. Also GaN material quality on patterned sapphire substrates (PSSs) was studied. It was found that using of PSSs enables reduction of threading dislocation (TD) density in the GaN layer. However, the effect on the LED performance caused by improved material quality was insignificant compared to the effect caused by improved LEE. Various MOVPE processes were also evaluated for growth of InGaN/GaN MQW structures. Smooth surface morphology of the MQW stack was achieved by introducing a small amount of H2 during the MOVPE growth of the GaN barrier layers. The homogeneity between the quantum wells was improved by inserting an InGaN underneath layer. Quaternary InAlGaN layers were investigated and InGaN/InAlGaN MQW structures for near-UV emission were presented. IQE of InGaN/InAlGaN MQW structures was found to be sensitive to the InAlGaN barrier layer composition and the strain state of the structure. Finally an AlGaN EBL for better carrier confinement is presented. It was found that the thickness and optimal Mg doping significantly affect the functionality of the EBL. A significant improvement in the LED performance was achieved by inserting an optimized AlGaN EBL into the LED structure.