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


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  2. A. Tuktamyshev, S. Vichi, F. Cesura, A. Fedorov, S. Bietti, D. Chrastina, S. Tsukamoto, and S. Sanguinetti: Flat metamorphic InAlAs buffer layer on GaAs(111)A misoriented substrates by growth kinetics control, J. Cryst. Growth 600, 126906 (2022).
  3. L. Anzi, A. Tuktamyshev, A. Fedorov, A. Zurutuza, S. Sanguinetti, and R. Sordan: Controlling the threshold voltage of a semiconductor field-effect transistor by gating its graphene gate, npj 2D Mater. Appl. 6, 28 (2022).
  4. G. Tavani, A. Chiappini, A. Fedorov, F. Scotognella, S. Sanguinetti, D. Chrastina, and M. Bollani: Tailoring of embedded dielectric alumina film in AlGaAs epilayer by selective thermal oxidation, Opt. Mater. Express 12, 835 (2022).
  5. A. Tuktamyshev, A. Fedorov, S. Bietti, S. Vichi, K. D. Zeuner, K. D. Jöns, D. Chrastina, S. Tsukamoto, V. Zwiller, M. Gurioli, and S. Sanguinetti: Telecom-wavelength InAs QDs with low fine structure splitting grown by droplet epitaxy on GaAs(111)A vicinal substrates, Appl. Phys. Lett. 118, 133102 (2021).
  6. M. Azadmand, E. Bonera, D. Chrastina, S. Bietti, S. Tsukamoto, R. Nötzel, and S. Sanguinetti: Raman spectroscopy of epitaxial InGaN/Si in the central composition range, Jpn. J. Appl. Phys. 58, SC1020 (2019).
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  8. F. Biccari, L. Esposito, C. Mannucci, A. G. Taboada, S. Bietti, A. Ballabio, A. Fedorov, G. Isella, H. von Känel, L. Miglio, S. Sanguinetti, A. Vinattieri, and M. Gurioli: Site-controlled natural GaAs(111) quantum dots fabricated on vertical GaAs/Ge microcrystals on deeply patterned Si(001) substrates, Nanosci. Nanotechnol. Lett. 9, 1108 (2017).
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  10. R. Bergamaschini, S. Bietti, A. Castellano, C. Frigeri, C. V. Falub, A. Scaccabarozzi, M. Bollani, H. von Känel, L. Miglio, and S. Sanguinetti: Kinetic growth mode of epitaxial GaAs on Si(001) micro-pillars, J. Appl. Phys. 120, 245702 (2016).
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  12. 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).
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  21. 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).
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  24. 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).
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  37. 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).
  38. 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).
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