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 Heteroepitaxial Templates for GaN-Based Optoelectronic Devices

Teemu Lang

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 23rd of February, 2007, at 12 noon.

Overview in PDF format (ISBN 978-951-22-8612-6)   [2068 KB]
Errata (in PDF format)
Dissertation is also available in print (ISBN 978-951-22-8611-9)

Abstract

In this work the growth of GaN and AlGaN thin-films by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates is studied. The objective of the study is to improve the performance of optoelectronic devices by reducing the density of threading dislocations (TDs) in nitride semiconductor films. The quality of the thin-films is analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM).

A new multistep method for the growth of a GaN nucleation layer (NL) is presented. The method consists of cyclically depositing a layer of GaN at a lower temperature (LT-GaN) and recrystallizing this layer into GaN islands in an in-situ annealing step. After the recrystallization a new LT-GaN layer can be deposited and recrystallized onto the existing islands to increase their size. The method enables a reduction in the density of nucleation islands (NIs) and their boundaries. AFM is utilized to demonstrate a reduction in the density NIs to a value of 1.7×107 cm−2.

Process parameter optimization is utilized to stimulate growth of NIs in the vertical direction. This causes TDs to incline inside the NIs, which is demonstrated by TEM. A smaller density for the NIs enables a prolonged vertical growth period enhancing the inclination of TDs. Theoretical studies indicate that inclined TDs are able to react with each other by annihilation or fusion. The multistep method is used to fabricate GaN thin-films with a TD density of only 5×107 cm−2. This result is demonstrated by AFM and TEM measurements. Qualitatively the improvement in crystalline quality is demonstrated by XRD.

Finally the multistep method is adapted to the MOCVD growth of Al0.12Ga0.88N layers on sapphire substrates. The method is used to grow crack-free Al0.12Ga0.88N layers with a thickness of 2 µm on a GaN NL. A TD density of 5×108 cm−2 in these films is demonstrated by AFM and XRD. TEM is used to study the relaxation of the films and the formation of TDs at the Al0.12Ga0.88N-substrate interface.

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

  1. V. E. Bougrov, M. A. Odnoblyudov, A. E. Romanov, T. Lang, O. V. Konstantinov, Threading dislocation density reduction in two-stage growth of GaN layers, physica status solidi (a) 203 (2006) R25.
  2. T. Lang, M. A. Odnoblyudov, V. E. Bougrov, M. Sopanen, MOCVD growth of GaN islands by multistep nucleation layer technique, Journal of Crystal Growth 277 (2005) 64. © 2005 Elsevier Science. By permission.
  3. T. Lang, M. A. Odnoblyudov, V. E. Bougrov, S. Suihkonen, M. Sopanen, H. Lipsanen, Morphology optimization of MOCVD-grown GaN nucleation layers by the multistep technique, Journal of Crystal Growth 292 (2006) 26. © 2006 Elsevier Science. By permission.
  4. T. Lang, M. A. Odnoblyudov, V. E. Bougrov, A. E. Romanov, S. Suihkonen, M. Sopanen, H. Lipsanen, Multistep method for threading dislocation density reduction in MOCVD grown GaN epilayers, physica status solidi (a) 203 (2006) R76.
  5. T. Lang, M. A. Odnoblyudov, V. E. Bougrov, S. Suihkonen, O. Svensk, P. T. Törmä, M. Sopanen, H. Lipsanen, Reduction of threading dislocation density in Al0.12Ga0.88N epilayers by a multistep technique, Journal of Crystal Growth (2007), in press. © 2007 Elsevier Science. By permission.
  6. S. Suihkonen, O. Svensk, T. Lang, H. Lipsanen, M. A. Odnoblyudov, V. E. Bougrov, The effect of InGaN/GaN MQW hydrogen treatment and threading dislocation optimization on GaN LED efficiency, Journal of Crystal Growth (2007), in press. © 2007 Elsevier Science. By permission.
  7. O. Svensk, S. Suihkonen, T. Lang, H. Lipsanen, M. Sopanen, M. A. Odnoblyudov, V. E. Bougrov, Effect of growth conditions on electrical properties of Mg-doped p-GaN, Journal of Crystal Growth (2007), in press. © 2007 Elsevier Science. By permission.

Keywords: MOCVD, GaN, AlGaN, threading dislocation

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© 2007 Helsinki University of Technology

Last update 2011-05-26