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.

Growth and Modification of Planar and Self-Assembled Semiconductor Nanostructures

Jaakko Sormunen

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 Large Seminar Hall of Micronova at Helsinki University of Technology (Espoo, Finland) on the 10th of March, 2006, at 12 noon.

Overview in PDF format (ISBN 951-22-8038-8)   [2385 KB]
Dissertation is also available in print (ISBN 951-22-8037-X)


The epitaxial growth and modification of planar and self-assembled compound semiconductor nanostructures is studied. Quantum dot (QD), quantum ring (QR), and quantum well (QW) structures are grown by metalorganic vapor phase epitaxy. The surface morphology of the samples and nanostructure properties are studied by atomic force microscopy. Photoluminescence (PL) spectroscopy is used to characterize the optical properties of the structures.

A novel method for transforming self-assembled InAs islands on InP into QRs is developed. The fabrication of self-assembled semiconductor QRs relies typically on the partial capping of islands to induce the dot-to-ring transformation. In this work, the change in the morphology is achieved without capping, by annealing as-grown InAs/InP islands in a phosphorus-rich flow. With this method, 6–10-nm-high rings are achieved.

It is also demonstrated that InGaAs(P)/InP strain-induced quantum dots (SIQDs) can be realized using InAs stressor islands. To adjust the depth of the strain-induced lateral confinement potential, the height of the islands is modified by varying the growth conditions. Furthermore, by varying the composition of the nearly-lattice-matched InGaAsP/InP QW, the SIQD ground state emission wavelength is tuned from 1.3 to 1.7 µm. Luminescence from the excited states in the SIQDs is also observed. The redshift of the SIQD ground state transition from the QW PL peak increases up to 67 meV as the distance between the SIQD and the stressor is reduced to 4 nm. Simultaneously, the luminescence intensity of the SIQD peaks reduces notably. Time-resolved PL measurements reveal that the intensity reduction is accompanied by a faster decay in the carrier populations of the SIQD states. It was concluded that this is due to electron capture to the InAs stressor or surface states associated with it.

Finally, the growth of GaN layers for the surface passivation of GaAs is investigated. The passivation effect is probed by PL measurements of near-surface InGaAs/GaAs QWs. The luminescence intensity of samples passivated with GaN is clearly enhanced as compared to unpassivated samples. This shows that the growth of a thin epitaxial GaN layer is an effective means of in situ surface passivation of GaAs.

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

  1. 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).
  2. 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).
  3. 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).
  4. 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).
  5. 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).
  6. H. Koskenvaara, J. Riikonen, J. Sormunen, M. Sopanen, and H. Lipsanen, Carrier dynamics in strain induced InGaAsP/InP quantum dots, Physica E, accepted for publication.
  7. J. Sormunen, J. Toivonen, M. Sopanen, and H. Lipsanen, Morphology of ultra-thin cubic GaN layers on GaAs(1 0 0) grown by MOVPE with DMHy as nitrogen source, Applied Surface Science 222, 286-292 (2004).
  8. 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).

Keywords: nanotechnology, epitaxy, compound semiconductor, quantum well, quantum dot, quantum ring

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© 2006 Helsinki University of Technology

Last update 2011-05-26