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Dr. Sami Suihkonen – InGaN quantum wells for LED applications

25.03.2010 14:24

Thesis

Link to the dissertation: http://lib.tkk.fi/Diss/2008/isbn9789512292875/

Classification

Nano in electronics, NEMS, MEMS and optics; Characterization; Fabrication

Abstract

In this thesis fabrication and properties of InGaN quantum wells (QWs) for light emitting diode (LED) applications is studied. Metal-organic vapor phase epitaxy (MOVPE) is used to grow InGaN/(InAl)GaN multiple quantum well (MQW) and LED structures on GaN/sapphire substrates. Also a multistep growth method for the growth of GaN on sapphire is investigated. The method enables a tenfold reduction of threading dislocation (TD) density in the GaN layer compared to conventional growth methods. The objective of this work is to study the physics of InGaN QWs and to improve the performance of InGaN MQW structures used in near-UV, blue and green LEDs. The quality of quantum wells is analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The LED structures are characterized also by electroluminescence (EL) measurements.


Various MOVPE growth parameters of InGaN/GaN QWs are evaluated for growth of MQW structures emitting blue light. Smooth surface morphology of the MQW stack is achieved by introducing a small amount of H2during the MOVPE growth of the GaN barrier layers. The effect of TD density on the performance of near-UV, blue, and green LEDs is studied by fabricating LED structures on GaN buffers grown by the multistep method. Improved EL output power at high operating current density is observed in the blue LEDs fabricated on the multistep GaN buffers. 
MOVPE growth of quaternary InAlGaN layers is investigated and InGaN/InAlGaN MQW structures for near-UV emission are presented. The internal quantum efficiency (IQE) of InGaN/InAlGaN MQW structures is found to be sensitive to the InAlGaN barrier layer composition and the strain state of the structure. A MQW structure emitting at 383 nm with an IQE of 45 % is presented. 
Finally the origin of the high efficiency of InGaN QWs is discussed. The high efficiency is due to self-screening mechanism of TDs in In containing QWs. The height of the potential barrier formed around the TD depends on the In content of the QWs, and thus the effect of TDs on the performance of blue and green LEDs is different.

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