InAs QDs grown on high-index GaAs(h11) surfaces seem to exhibit superior optical properties compared to the usual QD growth on GaAs(001), due to their prominent piezoelectric field, which can be functional in nano-photonics and quantum computing. However, the morphology of the QDs, as well as their strain state and chemical composition, influence significantly the light emission and absorption, the lasing efficiency, and other optoelectronic properties of QD-based devices. In order to shed light to these effects, we have explored the nanostructure, the strain properties, and the related chemical composition of buried InAs QDs grown on (211)B GaAs surface employing quantitative HRTEM techniques.

The InAs QD layer was grown by MBE under the Stranski-Krastanow (S-K) regime at 480^oC with a growth rate of 0.9 ML/s for 2s, over a 20 nm GaAs layer grown at 620^oC. The corresponding BEP was 8.5×10^-7 mbar. The QD layer was then overgrown by a 30 nm-thick GaAs cap layer, without growth interruption, by ramping up the temperature back to 620^oC.

The QDs adopted an anisotropic pyramidal shape, elongated along the <111> direction and were delimited by the {110}, {100}, and {213} or {214} crystal facets [Fig. 1(a)], as clearly evidenced by the equivalent larger uncapped InAs QDs grown on (211)B GaAs surface under similar growth conditions [1]. Local strain measurements by the geometric phase analysis (GPA) method [2] showed that buried dots were pseudomorphically grown on GaAs without the presence of any interfacial or extended defects [Figs. 1(a) and 1(b)]. Assuming a plane stress state of the QDs, following the transformation of the elastic stiffness tensor in order to comply with growth along the [211] direction [3], we found a systematic increase of the local strain from the base area to their apex region [Figs. 1(c) and 1(d)]. Then, applying Vegard’s law, we calculated the chemical composition of the QDs that was found to exhibit an indium composition gradient along the growth direction, obviously suggesting gallium segregation inside the dots (Fig. 2). Even though the gradual increase of indium concentration is a common trend for all QDs, various In-content maxima (0.50 to 0.92) were measured at the apex area of different QDs. This variation can be attributed to the corrugated form of the (211) surface [4], resulting in local compositional fluctuations of the wetting layer at the nucleation sites of the QDs. Therefore, gallium segregation is already involved at the onset of the S-K growth. Furthermore, photoluminescence (PL) and μ-PL experiments, as well as simulations of the QDs’ transition energies, showed variations in the emission energy of the QDs, which is in line with a graded In-content along the growth direction instead of pure InAs, thus verifying the chemical composition profile of the QDs revealed by quantitative strain measurements.

[1] N. Florini, G. P. Dimitrakopulos, J. Kioseoglou, S. Germanis, C. Katsidis, Z. Hatzopoulos, N. T. Pelekanos, and Th. Kehagias, J. Appl. Phys. 119, 034304 (2016).

[2] M. J. Hÿtch, E. Snoeck, R. Kilaas, Ultramicroscopy 74, 131 (1998).

[3] T. Hammerschmidt, P. Kratzer, and M. Scheffler, Phys. Rev. B 75, 235328 (2007).

[4] R. Nötzel, L. Däweritz, and K. Ploog, Phys. Rev. B 46, 4736 (1992).

Acknowledgements

Work supported by the European Union (ESF) and Greek national funds – Research Funding Program: THALES, project “NANOPHOS”.

Figures:

Fig. 1. (a) HRTEM image of a buried InAs QD, along the [0-11] zone axis, elongated along the [-111] direction. (b) GPA surface plot of the εxx in-plane strain, which is roughly zero suggesting a fully registered InAs/GaAs heterostructure. (c) GPA strain map along the growth direction, illustrating the gradual increase of the εzz strain inside the QD with the GaAs lattice as reference. (d) Corresponding GPA surface plot of the out-of-plane strain.

Fig. 2. Indium chemical profiles of two In(Ga)As QDs, showing the composition gradient along the growth direction. Maximum In-content values of 0.92 and 0.79 were measured at the apex region of the left and the right QD, respectively. The variation of indium concentration is pigmented according to the color bar.

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/quantitative-evaluation-of-the-211b-gaasinas-quantum-dot-heterostructure/