Laboratory for Epitaxial Nanostructures on Silicon and Spintronics

Affiliated Institutions

Droplet epitaxy for Advanced NanoTEchnology


29 Apr 2010Low Thermal Budget Fabrication of III-V Quantum Nanostructures on Si Substrates: poster presented at QD2010, Nottingham.


Former group members


Our method: Droplet Epitaxy

Droplet Epitaxy (DE) is a method for the fabrication of self-assembled high quality nanostructures, proposed and developed by Koguchi and co-workers since 1991. As an alternative to the more conventional Stranski-Krastanow growth mode, DE has been successfully applied to the fabrication of III-V semiconductor nanostructures. It exploits the self-organization of group III elements such as Ga, In and Al which, deposited on the substrate at temperatures higher than their melting points, automatically form nanometre-sized droplets. At this point, a group V element flux is supplied for the crystallization of every droplet into a III-V nanostructure. The main advantages of this approach are: the fabrication of 3D quantum nanostructures on both lattice-mismatched (InAs/GaAs) and lattice-matched systems (GaAs/AlGaAs); the control of the presence or absence of the wetting layer at the interface between the two materials, and the fine shape engineering of the nanocrystal. Indeed, by changing the growth conditions different shapes have been obtained during the last 20 years: quantum dots, quantum rings, concentric multiple quantum rings, quantum dot molecules, and so on.

Fig. 1. The Droplet Epitaxy method in the case of GaAs

Fabrication of unconventional GaAs nanostructures by Droplet Epitaxy

What makes Quantum Nanostructures (QNs) so attractive is the ability to tune their optoelectronic properties by careful design of their size, composition, strain and shape. These parameters set the confinement potential of electrons and holes thus determining the electronic and optical properties of the QNs. In fact, for the realization of QNs-based devices, the optical properties of the QNs such as the emission wavelength, the intersublevel spacing energy, and even the interactions between nearby QNs, should be freely accessible for engineering. Even though DE is a successful method for the fabrication of such semiconductor complex QNs, its full potential as source of nanostructures with designable and intriguing new geometries still remains mostly unexplored. Our aim is to show how to obtain, single, designable structures, with localized states, different dimensionality and tunable coupling. The DE fabrication procedure allows the possibility to finely tune the QN shape thus allowing the design of the desired electronic density of states: form follows function!

Fig. 2. AFM images of Ga droplet (A), Concentric Triple Quantum Ring (B) and Coupled Quantum Ring/Disk (C).

GaAs integration on Si for CMOS technology

Integrating III-V-based semiconductor devices for applications in optoelectronics and photonics directly on Si substrates would allow the use of the highly refined silicon infrastructure, based on CMOS technology, and offer the option of integrating a few specialized III-V devices within a large number of Si devices. Of particular technological interest is the possibility of carrying out the III-V device fabrication as a back-end process, that is, after the CMOS circuitry has been already realized. In this case, strict constraints on thermal budget for growth and processing of the epilayer are imposed by the compatibility with the underlying CMOS circuit. Nanostructures realized in III-V semiconductor materials hold a great technological interest, since quantum confinement leads to devices where strongly correlated few-electron/exciton systems play a fundamental role, such as single photon emitters. The key ingredient we adopted in order to achieve this goal is the use of DE and Ge virtual substrates to match the GaAs-based III-Vs and the Si substrate.

Fig. 3. Scheme of GaAs nanostructures on Si (A), AFM image of uncapped sample (B) and PL spectra of buried nanostructures (black line) and blank reference sample (red line) (C).


III-V Molecular Beam Epitaxy

Growth experiments are carried out by an EPI GEN II MBE system equipped with an As valved cell, Ga, In, Al, Si and Be as source materials. Reflection High Energy Electron Diffraction (RHEED) is used for the in-situ characterization of the samples.

Fig. 3. Our MBE system.

Atomic Force Microscopy

A Veeco Innova AFM is used for the morphological characterization of our samples.

Photoluminescence Spectroscopy

A photoluminescence spectroscopy (PL) apparatus is used to check the optical quality of the grown materials. The PL set up consists of: dispersive and Fourier Transform spectrometers, photomultipliers (0.7-3.5 eV range), CCD, diode detectors in a total detection range between 0.4 and 4.0 eV with resolution down to 0.1 meV. Available exciting sources are:

Working temperatures: 2-450 K.

Fig. 4. Photoluminescence spectroscopy apparatus


  1. C. Frigeri, D. Scarpellini, A. Fedorov, S. Bietti, C. Somaschini, V. Grillo, L. Esposito, M. Salvalaglio, A. Marzegalli, F. Montalenti, and S. Sanguinetti: Structure, interface abruptness and strain relaxation in self-assisted grown InAs/GaAs nanowires, Appl. Surf. Sci. 395, 29 (2015).
  2. D. Scarpellini, C. Somaschini, A. Fedorov, S. Bietti, C. Frigeri, V. Grillo, L. Esposito, M. Salvalaglio, A. Marzegalli, F. Montalenti, E. Bonera, P. G. Medaglia, and S. Sanguinetti: InAs/GaAs sharply defined axial heterostructures in self-assisted nanowires, Nano Lett. 15, 3677 (2015).
  3. E. M. Sala, M. Bollani, S. Bietti, A. Fedorov, L. Esposito, and S. Sanguinetti: Ordered array of Ga droplets on GaAs(001) by local anodic oxidation, J. Vac. Sci. Technol. B 32, 061206 (2014).
  4. M. Bollani, S. Bietti, C. Frigeri, D. Chrastina, K. Reyes, P. Smereka, J. M. Millunchick, G. M. Vanacore, M. Burghammer, A. Tagliaferri, and S. Sanguinetti: Ordered arrays of embedded Ga nanoparticles on patterned silicon substrates, Nanotechnology 25, 205301 (2014).
  5. S. Bietti, A. Scaccabarozzi, C. Frigeri, M. Bollani, E. Bonera, C. V. Falub, H. von Känel, L. Miglio, and S. Sanguinetti: Monolithic integration of optical grade GaAs on Si(001) substrates deeply patterned at a micron scale, Appl. Phys. Lett. 103, 262106 (2013).
  6. S. Sanguinetti, M. Guzzi, E. Gatti, and M. Gurioli: Chapter 12 -- photoluminescence characterization of structural and electronic properties of semiconductor quantum wells.In G. Agostini and C. Lamberti (eds.), Heterostructures and Nanostructures (Second Edition), 509--556. Elsevier B.V. (2013).
  7. N. Accanto, S. Minari, L. Cavigli, S. Bietti, G. Isella, A. Vinattieri, S. Sanguinetti, and M. Gurioli: Kinetics of multiexciton complex in GaAs quantum dots on Si, Appl. Phys. Lett. 102, 053109 (2013).
  8. K. Reyes, P. Smereka, D. Nothern, J. Mirecki Millunchick, S. Bietti, C. Somaschini, S. Sanguinetti, and C. Frigeri: Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory, Phys. Rev. B 87, 165406 (2013).
  9. S. Bietti, S. Cecchi, C. Frigeri, E. Grilli, A. Fedorov, A. Vinattieri, M. Gurioli, G. Isella, and S. Sanguinetti: Fabrication of Ge-on-Si substrates for the integration of high-quality GaAs nanostructures on Si, ECS Transactions 50, 783 (2013).
  10. C. Frigeri, S. Bietti, G. Isella, and S. Sanguinetti: Structural characterization of GaAs self-assembled quantum dots grown by Droplet Epitaxy on Ge virtual substrates on Si, Appl. Surf. Sci. 267, 86 (2013).
  11. S. Minari, L. Cavigli, F. Sarti, M. Abbarchi, N. Accanto, G. Muñoz Matutano, S. Bietti, S. Sanguinetti, A. Vinattieri, and M. Gurioli: Single photon emission from impurity centers in AlGaAs epilayers on Ge and Si substrates, Appl. Phys. Lett. 101, 172105 (2012).
  12. L. Cavigli, S. Bietti, N. Accanto, S. Minari, M. Abbarchi, G. Isella, C. Frigeri, A. Vinattieri, M. Gurioli, and S. Sanguinetti: High temperature single photon emitter monolithically integrated on silicon, Appl. Phys. Lett. 100, 231112 (2012).
  13. L. Cavigli, S. Bietti, M. Abbarchi, C. Somaschini, A. Vinattieri, M. Gurioli, A. Fedorov, G. Isella, E. Grilli, and S. Sanguinetti: Fast emission dynamics in droplet epitaxy GaAs ring-disk nanostructures integrated on Si, J. Phys. Condens. Matt. 24, 104017 (2012).
  14. S. Bietti, L. Cavigli, M. Abbarchi, A. Vinattieri, M. Gurioli, A. Fedorov, S. Cecchi, F. Isa, G. Isella, and S. Sanguinetti: High quality GaAs quantum nanostructures grown by droplet epitaxy on Ge and Ge-on-Si substrates, phys. stat. sol. (c) 9, 202 (2012).
  15. L. Cavigli, M. Abbarchi, S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, A. Vinattieri, and M. Gurioli: Individual GaAs quantum emitters grown on Ge substrates, Appl. Phys. Lett. 98, 103104 (2011).
  16. S. Bietti, C. Somaschini, N. Koguchi, C. Frigeri, and S. Sanguinetti: Self-assembled GaAs local artificial substrates on Si by droplet epitaxy, J. Cryst. Growth 323, 267 (2011).
  17. C. Somaschini, S. Bietti, A. Fedorov, N. Koguchi, and S. Sanguinetti: Outer zone morphology in GaAs ring/disk nanostructures by droplet epitaxy, J. Cryst. Growth 323, 279 (2011).
  18. C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti: Shape control via surface reconstruction kinetics of droplet epitaxy nanostructures, Appl. Phys. Lett. 97, 203109 (2010).
  19. C. Somaschini, S. Bietti, A. Fedorov, N. Koguchi, and S. Sanguinetti: Growth interruption effect on the fabrication of GaAs concentric multiple rings by droplet epitaxy, Nanoscale Res. Lett. 5, 1897 (2010).
  20. C. Somaschini, S. Bietti, A. Fedorov, N. Koguchi, and S. Sanguinetti: Concentric multiple rings by droplet epitaxy: Fabrication and study of the morphological anisotropy, Nanoscale Res. Lett. 5, 1865 (2010).
  21. S. Bietti, C. Somaschini, N. Koguchi, C. Frigeri, and S. Sanguinetti: Self-assembled local artificial substrates of GaAs on Si substrate, Nanoscale Res. Lett. 5, 1905 (2010).
  22. S. Bietti, C. Somaschini, E. Sarti, N. Koguchi, S. Sanguinetti, G. Isella, D. Chrastina, and A. Fedorov: Photoluminescence study of low thermal budget III--V nanostructures on silicon by droplet epitaxy, Nanoscale Res. Lett. 5, 1650 (2010).
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  24. C. Somaschini, S. Bietti, N. Koguchi, F. Montalenti, C. Frigeri, and S. Sanguinetti: Self-assembled GaAs islands on Si by droplet epitaxy, Appl. Phys. Lett. 97, 053101 (2010).
  25. C. Somaschini, S. Bietti, S. Sanguinetti, N. Koguchi, and A. Fedorov: Self-assembled GaAs/AlGaAs coupled quantum ring-disk structures by droplet epitaxy, Nanotechnology 21, 125601 (2010).
  26. S. Bietti, S. Sanguinetti, C. Somaschini, N. Koguchi, G. Isella, D. Chrastina, and A. Fedorov: Fabrication of GaAs quantum dots by droplet epitaxy on Si/Ge virtual substrate, IOP Conf. Ser.: Mat. Sci. Eng. 6, 012009 (2009).
  27. C. Somaschini, S. Bietti, S. Sanguinetti, N. Koguchi, A. Fedorov, M. Abbarchi, and M. Gurioli: Fabrication of GaAs concentric multiple quantum rings by droplet epitaxy, IOP Conf. Ser.: Mat. Sci. Eng. 6, 012008 (2009).
  28. S. Bietti, C. Somaschini, M. Abbarchi, N. Koguchi, S. Sanguinetti, E. Poliani, M. Bonfanti, M. Gurioli, A. Vinattieri, T. Kuroda, T. Mano, and S. Sakoda: Quantum dots to double concentric quantum ring structures transition, phys. stat. sol. (c) 6, 928 (2009).
  29. S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, G. Isella, and D. Chrastina: Fabrication of high efficiency III-V quantum nanostructures at low thermal budget on Si, Appl. Phys. Lett. 95, 241102 (2009).
  30. C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti: Fabrication of multiple concentric nanoring structures, Nano Lett. 9, 3419 (2009).

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Last updated: 11th July 2016