We present self-assembly of InAs/InAlAs quantum dots by droplet epitaxy technique on vicinal GaAs(111)A substrates. The small miscut angle, while maintaining the symmetries imposed to the quantum dot from the surface, allows fast growth rate thanks to the presence of preferential nucleation sites at the step edges. A 100 nm InAlAs metamorphic layer with In content ≥ 50% is already almost fully relaxed with a very flat surface. The quantum dots emit at the 1.3 μm telecom O-band with the fine structure splitting as low as 10 μeV, thus making them suitable as photon sources in quantum communication networks using entangled photons. I. INTRODUCTIONEntangled photon emitters are fundamental components of the future quantum communication network and the basis of the photonic implementation of quantum information protocols [1, 2].Among possible entangled photon sources, self-assembled quantum dots (QD) of compound semiconductors are considered as ideal, being able to generate polarization entangled photon pairs on demand via the biexciton (XX)exciton (X) cascade [1][2][3][4][5]. The presence of the fine structure splitting (FSS) [6,7] of the X state, due to the QD anisotropy (shape, composition etc.), generates a decoherence mechanism, which complicates the observation of the entanglement. Highly symmetric QDs with natural low FSS can be achieved by self-assembled growth on (111) surfaces with C3v symmetry [5,[8][9][10].The growth of QDs on (111) compound semiconductor surfaces is not straightforward. The common Stranski-Krastanov (SK) growth mode seen in the InAs/GaAs system [11] is not able to induce the self-assembly of QDs on (111) surfaces because of the rapid relaxation of compressive strain due to the low threshold energy for the insertion of misfit dislocations at the substrate epilayer interface [12,13]. However, by turning from compressive to tensile strain epilayers, selfassembly SK GaAs QDs on InAl(Ga)As(111)A were demonstrated [14][15][16]. A more efficient and reliable method of obtaining self-assemble QDs on (111) substrate is Droplet Epitaxy (DE)

The fabrication and characterization of an infrared photodetector based on GaAs droplet epitaxy quantum dots embedded in Al 0.3 Ga 0.7 As barrier is reported. The high control over dot electronic properties and the high achievable number density allowed by droplet epitaxy technique permitted us to realize a device using a single dot layer in the active region. Moreover, thanks to the independent control over dot height and width, we were able to obtain a very sharp absorption peak in the thermal infrared region (3-8 μm). Low temperature photocurrent spectrum was measured by Fourier spectroscopy, showing a narrow peak at 198 meV (∼6.3 μm) with a full width at half maximum of 25 meV. The observed absorption is in agreement with theoretical prediction based on effective mass approximation of the dot electronic transition.

We demonstrate the growth of low density anti-phase boundaries, crack-free GaAs epilayers, by Molecular Beam Epitaxy on silicon (001) substrates. The method relies on the deposition of thick GaAs on a suspended Ge buffer realized on top of deeply patterned Si substrates by means of a three-temperature procedure for the growth. This approach allows to suppress, at the same time, both threading dislocations and thermal strain in the epilayer and to remove anti-phase boundaries even in absence of substrate tilt. Photoluminescence measurements show the good uniformity and the high optical quality of AlGaAs/GaAs quantum well structures realized on top of such GaAs layer.

The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size distribution below 15%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling allow us to deconvolve the contribution of phonon broadening in the ensemble photoluminescence and study it in a wide temperature range.

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