Quantum nanostructures by droplet epitaxy for advanced devices

S. Bietti
LNESS and Dip. di Scienza dei Materiali, Univ. di Milano Bicocca Milano, Italy

Semiconductor quantum dots are often denoted as artificial atoms because the energy spectra of the electronic carriers become discrete, and for each energy level the electronic state is described by an atom-like wavefunction. The absorption and emission wavelengths can be tuned by simply varying nanocrystal size and shape, which change the carrier confinement. These properties make quantum dots very interesting for the fabrication of devices based on the interaction between light and charge carriers.
In the last years, Droplet Epitaxy (DE) [1], a growth technique based on molecular beam epitaxy, was exploited to fabricate quantum nanostructures with a precise and independent control on the shape, size and density. This control allows to tailor the optical and electronic property of the nanostructures to fit the needs of different applications.
In this talk the main aspects of the growth of DE nanostructures will be discussed, together with different applications recently developed with DE. In particular, I will show the fabrication of hybrid systems involving GaAs/AlGaAs quantum dot molecules (QDMs) and Ga metal nanoparticles (MNs) suitable for terahertz emission, discussing  the role of growth parameters during for the relative positioning of QDMs and MNPs.
I will also show the fabrication of GaAs single photon emitters integrated on Si substrates demonstrating how DE makes also possible the growth of bright III-V quantum emitters on silicon substrates thus paving the route to the integration of optically efficient III-V nanostructures on CMOS technology.
The last application I will describe is the growth of ultra high density QDs for the fabrication of quantum dot infrared photodetectors (QDIP) active in the range between 2 and 8 μm, as a first step toward the the fabrication of a III-V based QDIP integrated on Si.

Figure 1. Panel (a): DE procedure for the fabrication of quantum nanostructures with the separate supply of group III and group V atoms. Panel (b): nano-positioning of Ga MNs (red) next and on top of QDM (blue). Panel (c): ultra high density QDs for the fabrication of a QDIP device active in the MWIR region.

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