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Present and past Quantum Dot technology has been largely dominated by Stransky Krastanov (SK) growth leading to important successes both in standard optoelectronic (LED, lasers) and in quantum nanophotonics (single and entangled photon emitters). However SK has intrinsic limitations due to its thermodynamic 3D nucleation driven by minimization of the elastic energy during growth. Indeed SK growth applies only to lattice mismatched compounds such as the archetypal InGaAs/GaAs. On the contrary, QDs growth by DE is based on the kinetic control of the 3D growth and it is intrinsically a strain free self aggregation technique. This introduces a high control on QD shape which permits to engineer QD electronic wave function for the targeted application. DE is nowadays a mature methods capable not only to reproduce most of the SK achievements but also to overcome its intrinsic limitations introducing novel degrees of freedom in the design and the control of the QDs.
Droplet Epitaxy QD
Following the list of QD requirements for optoelectronics and quantum applications reported in the previous box, we highlighted the DE achievements, stressing the additional degree of freedom that DE guarantees with respect to SK growth.
(i) DE-QDs formation does not depend on the lattice mismatched between the deposited material and the substrate (i.e.the surrounding barrier material). This largely expands the possible compounds to be used, including for instance the well optimized GaAs/AlGaAs system. DE allows for easy control of the nanostructure shape and even topology, allowing for intriguing spin physics opening new and still unexplored routes in spintronics
(ii) The kinetic control of the QD nucleation can be also exploited for using a large variety of substrates, some of them not accessible for SK. So far DE-QDs has included single photon emission up to 80 K on Si, and Ge. Another example is the growth of GaAs/AlGaAs QD on (111) GaAs producing high symmetric QDS with vanishing neutral exciton fine-structure splitting and then to high fidelity source of entangled photons.
(iii) The size and the density of QDs can be tuned independently in a very simple way by DE, thus allowing to achieve extremely low and high density QDs without losing the possibility of fine tuning the QD electronic properties.
(iv) Site control of high quality QDs is still an open challenge for both SK and DE. Nevertheless site controlled formation of DE QD have been demonstrated. Lateral and vertical aligned QDs enable complicated quantum molecules. DE epitaxy allows self-assembly of coupled QDs in-plane and out-of-plane.
DROPLET EPITAXY PROCESS
Group III reservoir deposition (droplets)
III-V crystallization under group V flux
(Nanostructure shape control)
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