The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Fabrication of InGaN Quantum Wells for LED Applications

Sami Suihkonen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Electronics, Communications and Automation for public examination and debate in Auditorium AS1 at Helsinki University of Technology (Espoo, Finland) on the 11th of April, 2008, at 12 noon.

Overview in PDF format (ISBN 978-951-22-9287-5)   [4168 KB]
Dissertation is also available in print (ISBN 978-951-22-9286-8)

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 H2 during 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.

This thesis consists of an overview and of the following 7 publications:

  1. S. Suihkonen, J. Sormunen, V. T. Rangel-Kuoppa, H. Koskenvaara and M. Sopanen, Growth of InN by vertical flow MOVPE, Journal of Crystal Growth 291, 8 (2006). © 2006 Elsevier Science. By permission.
  2. S. Suihkonen, T. Lang, O. Svensk, J. Sormunen, P. T. Törmä, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov and V. E. Bougrov, Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition, Journal of Crystal Growth 300, 324 (2007). © 2007 Elsevier Science. By permission.
  3. S. Suihkonen, O. Svensk, T. Lang, H. Lipsanen, M. A. Odnoblyudov and V. E. Bougrov, The effect of InGaN/GaN MQW hydrogen treatment and threading dislocation optimization on GaN LED efficiency, Journal of Crystal Growth 298, 740 (2007). © 2007 Elsevier Science. By permission.
  4. S. Suihkonen, O. Svensk, P. T. Törmä, M. Ali, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov and V. E. Bougrov, MOVPE growth and characterization of InAlGaN films and InGaN/InAlGaN MQW structures, Journal of Crystal Growth, in press. © 2008 Elsevier Science. By permission.
  5. T. Lang, M. A. Odnoblyudov, V. E. Bougrov, A. E. Romanov, S. Suihkonen, M. Sopanen and H. Lipsanen, Multistep method for threading dislocation density reduction in MOCVD grown GaN epilayers, physica status solidi (a) 203, No. 10, R76 (2006).
  6. T. Lang, M. Odnoblyudov, V. Bougrov, S. Suihkonen, M. Sopanen and H. Lipsanen, Morphology optimization of MOCVD-grown GaN nucleation layers by the multistep technique, Journal of Crystal Growth 292, 26 (2006). © 2006 Elsevier Science. By permission.
  7. M. Ali, S. Suihkonen, O. Svensk, P. T. Törmä, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov and V. E. Bougrov, Study of composition control and capping of MOVPE grown InGaN/InxAlyGa1−x−yN MQW structures, physica status solidi (c), accepted for publication.

Keywords: light emitting diodes, MOVPE, III-N materials, quantum wells

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© 2008 Helsinki University of Technology

Last update 2011-05-26