Droplet epitaxy for Advanced NanoTEchnology
|29 Apr 2010||Low Thermal Budget Fabrication of III-V Quantum Nanostructures on Si Substrates: poster presented at QD2010, Nottingham.|
- Stefano Sanguinetti, group leader
- Alexey Fedorov, researcher
- Sergio Bietti, researcher
- Luca Esposito, PhD student
- Andrea Scaccabarozzi, PhD student
- Andrea Castellano, undergraduate student
Former group members
- Silvia Adorno, undergraduate student
- Nobuyuki Koguchi, visiting professor
- Elisa Maddalena Sala, undergraduate student
- Claudio Somaschini, PhD student
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
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).
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).
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.
A Veeco Innova AFM is used for the morphological characterization of our samples.
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:
- CW lasers in the range 454-1064 nm (Ar+, Nd:YAG duplicated and not, semiconductor diodes, He:Neon)
- CW tunable laser in the 700-1000 nm range (Ti:Sapphire)
- Pulsed N2 UV laser (337 nm), 40 Hz, < 10 ns pulse duration Pulsed Dye laser, tunable in the 370-700 nm range
- Incandescent and high pressure lamps.
Working temperatures: 2-450 K.
Fig. 4. Photoluminescence spectroscopy apparatus
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Last updated: 14th May 2013