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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Approaches for Optimizing Light Emitting Diode Structures Based on III-N Materials

Pekka Törmä

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the School of Electrical Engineering for public examination and debate in the great hall in the Micronova building at the Aalto University School of Electrical Engineering (Espoo, Finland) on the 13th of May 2011 at 12 o'clock.

Overview in PDF format (ISBN 978-952-60-4082-0)   [3844 KB]
Dissertation is also available in print (ISBN 978-952-60-4081-3)

Abstract

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.

This thesis consists of an overview and of the following 8 publications:

  1. P. T. Törmä, O. Svensk, M. Ali, S. Suihkonen, M. Sopanen, M. A. Odnoblyudov, and V. E. Bougrov. 2009. Maskless roughening of sapphire substrates for enhanced light extraction of nitride based blue LEDs. Solid-State Electronics, volume 53, number 2, pages 166-169.
  2. P. T. Törmä, M. Ali, O. Svensk, S. Sintonen, P. Kostamo, S. Suihkonen, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov, and V. E. Bougrov. 2009. An investigation of structural properties of GaN films grown on patterned sapphire substrates by MOVPE. Physica B: Condensed Matter, volume 404, numbers 23-24, pages 4911-4915.
  3. Pekka T. Törmä, Muhammad Ali, Olli Svensk, Sami Suihkonen, Markku Sopanen, Harri Lipsanen, Mikael Mulot, Maxim A. Odnoblyudov, and Vladislav E. Bougrov. 2010. InGaN-based 405 nm near-ultraviolet light emitting diodes on pillar patterned sapphire substrates. CrystEngComm, volume 12, number 10, pages 3152-3156.
  4. S. Suihkonen, T. Lang, O. Svensk, J. Sormunen, P. T. Törmä, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov, and V. E. Bougrov. 2007. Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition. Journal of Crystal Growth, volume 300, number 2, pages 324-329.
  5. P. T. Törmä, O. Svensk, M. Ali, S. Suihkonen, M. Sopanen, M. A. Odnoblyudov, and V. E. Bougrov. 2008. Effect of InGaN underneath layer on MOVPE-grown InGaN/GaN blue LEDs. Journal of Crystal Growth, volume 310, number 23, pages 5162-5165.
  6. S. Suihkonen, O. Svensk, P. T. Törmä, M. Ali, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov, and V. E. Bougrov. 2008. MOVPE growth and characterization of InAlGaN films and InGaN/InAlGaN MQW structures. Journal of Crystal Growth, volume 310, numbers 7-9, pages 1777-1780.
  7. M. Ali, S. Suihkonen, O. Svensk, P. T. Törmä, M. Sopanen, H. Lipsanen, M. A. Odnoblyudov, and V. E. Bougrov. 2008. Study of composition control and capping of MOVPE grown InGaN/InxAlyGa1−x−yN MQW structures. physica status solidi (c), volume 5, number 9, pages 3020-3022.
  8. O. Svensk, P. T. Törmä, S. Suihkonen, M. Ali, H. Lipsanen, M. Sopanen, M. A. Odnoblyudov, and V. E. Bougrov. 2008. Enhanced electroluminescence in 405 nm InGaN/GaN LEDs by optimized electron blocking layer. Journal of Crystal Growth, volume 310, number 23, pages 5154-5157.

Keywords: LED, metal-organic vapor phase epitaxy, III-N materials, modified sapphire substrates

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Last update 2011-05-26