Telecom-wavelength InAs QDs with low fine structure splitting grown by droplet epitaxy on GaAs(111)A vicinal substrates

Abstract

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)

Fine structure splitting analysis of cavity-enhanced telecom-wavelength InAs quantum dots grown on a GaAs(111)A vicinal substrate

“…Fitting 30 different transitions reveals no correlation between the emission wavelength and the linewidth, which varies between 100 µeV and 550 µeV (Figure 1d). Those rather broad linewidths are consistent with previously reported values [24] and originate from the presence of point defects and threading dislocations in the InAlAs barrier layers. We also notice an improvement in the signal/background ratio and an important reduction in the density of spectral lines, which makes it possible to isolate the emission pattern of single QDs.…”

“…In Figure 1b we report a typical O-band photoluminescence (PL) spectrum from the QDs. The inset shows an example of PL measurement taken in the same setup on InAs/InAlAs QDs grown without DBR mirrors [24], confirming that the presence of the optical cavity leads to an enhancement of the signal intensity by a factor > 5. The FWHM of the spectral lines is extracted with a Gaussian fit as shown in Figure 1c.…”

“…Further improvements in the extraction efficiency can be achieved by etching nanostructures such as mesas or micropillars. Also, linewidths and FSS can be reduced by changing the composition of the barrier layer from In-AlAs to InGaAs [24] and employing a smaller miscut angle, respectively. Finally, we reported that the QDs tend to have dipoles aligned along a specific direction, despite the symmetry of the (111)-oriented substrate.…”

“…DE offers better control over the QD self-assembly dynamics and guarantees fine tuning of the shape, size and density of the nanostructures [22,23]. The formation of telecom-wavelength InAs QDs with FSS as low as 16 µeV on GaAs(111)A has been recently demonstrated [24]. Nevertheless, previous works on (100) substrates suggest that incorporating those emitters in a one-dimensional microcavity could improve the photon extraction efficiency and generate sufficient signal intensity for applications [25][26][27].…”

Recent Developments of Quantum Dot Materials for High Speed and Ultrafast Lasers

“…In addition, it is worth mentioning that the strain-free GaAs/AlGaAs QD can be fabricated by the droplet epitaxy technique, which has a definite advantage in enabling a large number of high-quality QD layers as there are no strain induced dislocations [ 60 , 61 ]. The dot density and the dot size can also be precisely controlled by the droplet epitaxy technique on vicinal GaAs(111)A substrates, by which the fine structure splitting as low as 16 μeV was realized, thus making them suitable as photon sources in quantum communication networks [ 62 ]. Today, many QD devices have been commercialized, such as lasers, broadband light emitters, and passive devices, and among them, almost all are based on self-assembled QD structures.…”