Characterization and Effect of Thermal Annealing on InAs Quantum Dots Grown by Droplet Epitaxy on GaAs(111)A Substrates
© Bietti et al. 2015
Received: 26 February 2015
Accepted: 11 May 2015
Published: 2 June 2015
We report the study on formation and thermal annealing of InAs quantum dots grown by droplet epitaxy on GaAs (111)A surface. By following the changes in RHEED pattern, we found that InAs quantum dots arsenized at low temperature are lattice matched with GaAs substrate, becoming almost fully relaxed when substrate temperature is increased. Morphological characterizations performed by atomic force microscopy show that annealing process is able to change density and aspect ratio of InAs quantum dots and also to narrow size distribution.
The self–assembly of quantum dots (QDs) attracts a great interest for the possibility to fabricate advanced photoelectronic devices such as single photon emitters and entangled photon sources [1, 2]. In particular, droplet epitaxy (DE) technique [3–5] has recently demonstrated the possibility to grow high-quality quantum nanostructures in lattice-matched and mismatched systems with a high degree of control over density, size, and shape of the nanostructures [6–11] suitable for the fabrication of single photon emitters at liquid nitrogen temperature and entangled photon sources [12–14]. The flexibility of DE is due to the fact that the growth of III-V QDs is performed in two distinct steps. In the first one, the group-III element is deposited on the substrate to form liquid droplets; in the second step, a flux of group-V element is irradiated in order to crystallize the droplets in quantum nanostructures.
In order to shift the QD-DE emission towards telecommunication wavelength range (1.3–1.5 μm) and to reduce the fine structure splitting (FSS), many efforts have been devoted to the fabrication of InAs QDs on GaAs(111) surfaces [14–17]. Compared to (100), (111) surface is of extreme interest due to the fact that the C 3v symmetry of (111) surface allows to realize highly symmetric QDs with a vanishing FSS . Unfortunately, fabrication of InAs QDs on GaAs(111) by Stranski–Krastanow growth mode is impossible because strain relaxation takes place by the introduction of dislocations instead of three-dimensional island formation [18–20]. Recently, GaAs QDs grown by DE on GaAs(111)A surface were fabricated [21, 22] and control of nuclear spin , charge tuning , magneto-optical properties , interplay between exchange and Zeeman effect , and emission of entangled photon pairs  were studied. Successful deposition of InAs QDs by DE on GaAs(100) surface have been reported in [28–30], while recently studies about the formation and morphology of InAs QDs were reported in [31, 32].
Despite this interest, only few works showed the possibility to grow In(Ga)As QDs on (111) surfaces. Single photon emission is reported in , and a reduced FSS for entangled photon emission near telecommunication wavelength ranges is reported in . Anyway, a study on formation and morphology of InAs QDs grown on GaAs(111)A is not available in scientific literature.
In this work, we report the analysis of the different steps for the growth of InAs QDs by DE technique on GaAs(111)A surface by reflection high-energy electron diffraction (RHEED) to better understand the growth mechanism of InAs QDs, and we analyze the effect of annealing on density and size distribution of QDs by changing the initial size of the QDs and the annealing temperature. The annealing process is a step necessary to remove As excess on the surface exposed to high As flux at low temperature and in particular for QDs formed at low temperature in order to improve the crystalline quality [33–35]. In our experiments, we found evidence that the annealing process changes the density and the aspect ratio (the ratio between the height and the diameter of a QD) of InAs islands, narrowing the size distribution. We also observed from RHEED pattern that it is possible to fully convert a solid In nano–crystal into an InAs nano–crystal pseudomorphic with the GaAs substrate, becoming almost fully relaxed when substrate temperature is increased up to 300 °C.
Growth parameters and morphological data for the two sets of samples: In amount deposited (here is reported the equivalent amount on GaAs(100) surface), substrate temperature during the annealing procedure, density of InAs QDs, percentage of deposited In incorporated in InAs QDs, mean value of radius, mean value of aspect ratio
% of In deposited
(×108 cm −2)
incorporated in QDs
25.2 ± 4.9
0.123 ± 0.015
18.4 ± 4.7
0.055 ± 0.008
24.0 ± 2.6
0.084 ± 0.028
22.9 ± 3.1
0.062 ± 0.015
Results and Discussion
After the annealing step at 300 °C, (2×2) reconstruction on GaAs(111)A surface is again clear, due to removal of As excess accumulated during the As irradiation, while the spots related to InAs QDs are slightly shifted towards specular beam (see Fig. 1d). Calculating the ratio between spacing of GaAs streaks and the one of InAs spots, we can estimate a different lattice parameter of about 7 %, corresponding to almost fully relaxed InAs. This change can be related to the nucleation of dislocation at the interface between InAs and GaAs driven by the thermal energy added to the system by annealing procedure.
In scientific literature, it is reported that InAs layers on GaAs(111)A grow in planar mode [19, 20] instead of Stranski-Krastanow mode as reported for the case of InAs on GaAs(001). This behavior is due to strain relaxation induced by the introduction of dislocations at the interface between InAs and GaAs. The presence of these defects is expected to affect quality and optoelectronic properties of InAs QDs. The formation of strained (and consequently non–dislocated) InAs islands on GaAs(111)A was observed for low InAs coverage in [18, 20] and in our experiments is observed until substrate temperature is maintained low.
where D 0 is the diffusivity prefactor, E A the activation energy for diffusion, T the substrate temperature, N s the number of surface sites, and J As the arsenic flux.
where AR and AR’ are aspect ratios before and after the annealing, r is the initial radius of the QD (equal to base edge of the hexagonal pyramid) and ℓ the diffusion length of In ad–atoms as defined in Eq. 1. The effect of the annealing process is then to reduce the height and increase the radius of the QDs and is more evident on smaller dots. With r dot≲ℓ, we expect a reduction of aspect ratio of eight times or more. In these conditions, it is then quite easy to understand that smaller dots can be flattened to a single monolayer height on the surface with the formation of a InAs 2–D layer on the surface, as observed in . The decrease observed in mean aspect ratio for InAs QDs on samples L1 and H2 confirms this model, as reported in Table 1. Also an increase of mean radius is expected, but we have to consider that the formation of a layer originated by flattening of InAs QDs explains the reason why the centre of radius distribution is not apparently increased from sample L1 to sample H2, as reported in Fig. 3c. We have to consider that on sample H2, InAs QDs are partially buried by the InAs layer formed on the surface. The flattening of the smaller dots is also confirmed by the reduction in density of InAs QDs observed in Table 1 for higher annealing temperature (wider ℓ) and decreasing amount of In deposited (smaller mean radius of the dots). As shown by the data presented in Fig. 3d, the increased diffusion length leads to the formation of InAs QDs with lower aspect ratio  and to completely flatten smaller dots present on surface.
It is also interesting to note that size distribution of QDs is reduced by thermal annealing. As reported in Table 1, the radius of QDs on samples annealed at higher temperatures has a standard deviation on radius reduced by a factor ∼1.6.
We reported the study on formation of InAs QDs grown by droplet epitaxy on GaAs (111)A surface, performed by a mean of RHEED pattern and of AFM analysis. We demonstrated that InAs QDs arsenized at low temperature are lattice matched with GaAs substrate and become almost fully relaxed when substrate temperature is increased with the insertion of dislocations. We also studied the effect of annealing on density and aspect ratio of InAs QDs, showing that increasing annealing temperature, size dispersion is reduced, while density and aspect ratio decrease up to complete flattening of smaller dots.
The research was supported by the CARIPLO Foundation and Regione Lombardia (prj. COSMOS 2013-0382). The authors like to thank Prof. Richard Nötzel for useful discussions.
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