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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Self-Assembled Nanorings and Stressor Quantum Dots

Juha Riikonen

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 AS1 at Helsinki University of Technology (Espoo, Finland) on the 2nd of June, 2006, at 12 noon.

Overview in PDF format (ISBN 951-22-8182-1)   [1104 KB]
Dissertation is also available in print (ISBN 951-22-8181-3)

Abstract

In this thesis, the main focus is in the fabrication and characterization of self-assembled III-V compound semiconductor nanostructures. The samples were fabricated by metalorganic vapor phase epitaxy (MOVPE). Atomic force microscopy (AFM) and photoluminescence (PL) measurements were used to examine surface morphology and optical characteristics of the samples, respectively.

Quantum dot (QD) heterostructures are typically realized by embedding self-assembled islands in a matrix of another semiconductor material with a higher band gap. In this work, however, islands were used as stressors on top of a near-surface quantum well (QW) to induce three-dimensional potential enclosures into the QW. These QD structures are known as strain-induced QDs (SIQDs) or stressor QDs. Firstly, a new material system utilizing InAs stressor islands to create strain-induced InGaAs(P)/InP QDs was demonstrated. Up to four PL peaks from the QD ground and excited states were clearly resolved in the spectra. The island ensemble was optimized using a growth temperature ramp-down during the InAs island deposition. The luminescence wavelength was tuned by varying the QW composition. On the other hand, the QD confinement was modified by altering the height of the stressor islands or by varying the distance of the QW from the surface.

Secondly, transformation of self-assembled InAs islands into volcano-like nanorings was investigated. In the method introduced in this thesis, contrary to previous techniques utilizing partial capping of islands, only phosphorus annealing was applied. It is assumed that the material redistribution, i.e., the island-to-ring transformation, is caused by the exchange of As atoms to P atoms along with the strain-driven migration of In atoms outwards from the island.

Thirdly, surface passivation of GaAs by an in situ grown epitaxial ultrathin GaN layer was studied. Near-surface InGaAs/GaAs QWs were used as low-dimensional test structures, on which the passivation was applied. PL measurements were used to assess the effect of the passivation. Significant enhancement of the PL intensity shows that the growth of an ultrathin GaN layers is an efficient method in the passivation of GaAs surfaces.

This thesis consists of an overview and of the following 9 publications:

  1. J. Riikonen, J. Sormunen, M. Mattila, M. Sopanen, and H. Lipsanen, InGaAs/InP quantum dots induced by self-organized InAs stressor-islands, Japanese Journal of Applied Physics 44, L518-L520 (2005).
  2. J. Sormunen, J. Riikonen, M. Mattila, M. Sopanen, and H. Lipsanen, Modified self-assembly of InAs islands acting as stressors for strain-induced InGaAs(P)/InP quantum dots, Nanotechnology 16, 1630-1635 (2005). © 2005 Institute of Physics Publishing. By permission.
  3. J. Riikonen, J. Sormunen, H. Koskenvaara, M. Mattila, M. Sopanen, and H. Lipsanen, Highly tunable emission from strain-induced InGaAsP/InP quantum dots, Japanese Journal of Applied Physics 44, L976-L978 (2005).
  4. H. Koskenvaara, J. Riikonen, J. Sormunen, M. Sopanen, and H. Lipsanen, Carrier dynamics in strain-induced InGaAsP/InP quantum dots, Physica E 32, 179-182 (2006). © 2006 Elsevier Science. By permission.
  5. J. Riikonen, J. Sormunen, H. Koskenvaara, M. Mattila, A. Aierken, T. Hakkarainen, M. Sopanen, and H. Lipsanen, Effect of surface states on carrier dynamics in InGaAsP/InP stressor quantum dots, Nanotechnology 17, 2181-2186 (2006). © 2006 Institute of Physics Publishing. By permission.
  6. J. Sormunen, J. Riikonen, M. Mattila, J. Tiilikainen, M. Sopanen, and H. Lipsanen, Transformation of self-assembled InAs/InP quantum dots into quantum rings without capping, Nano Letters 5, 1541-1543 (2005). © 2005 American Chemical Society. By permission.
  7. J. Sormunen, J. Riikonen, T. Hakkarainen, M. Sopanen, and H. Lipsanen, Evolution of self-assembled InAs/InP islands into quantum rings, Japanese Journal of Applied Physics 44, L1323-L1325 (2005).
  8. J. Sormunen, J. Riikonen, M. Sopanen, and H. Lipsanen, GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor, Journal of Crystal Growth 270, 346-350 (2004). © 2004 Elsevier Science. By permission.
  9. J. Riikonen, J. Sormunen, H. Koskenvaara, M. Mattila, M. Sopanen, and H. Lipsanen, Passivation of GaAs surface by ultrathin epitaxial GaN layer, Journal of Crystal Growth 272, 621-626 (2004). © 2004 Elsevier Science. By permission.

Keywords: MOVPE, epitaxy, nanotechnology, self-assembled, compound semiconductor, quantum dot, nanoring, quantum ring, passivation

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© 2006 Helsinki University of Technology

Last update 2011-05-26