Effects of random alloy disorder, shape deformation, and substrate misorientation on the exciton lifetime and fine structure splitting of GaAs/AlxGa1-xAs(111) quantum dots

Using atomistic, million-atom screened pseudopotential theory together with configuration interaction, as well as atomically resolved structures based on experimental characterization, we perform numerical calculations on self-assembled GaAs/AlxGa1-xAs(111) quantum dots that we compare with our experimental data. We show that random alloy disorder in the barrier can cause a symmetry breaking at the single-particle level (distortions of wave functions and lifting of degeneracies) which translates into the appearance of a nonzero exciton fine structure splitting (FSS) at the many-body level. Nevertheless, our results indicate that varying the concentration of aluminum in the random alloyed barrier allows simultaneous tuning of the exciton fine structure splitting and emission wavelength without altering its radiative lifetime tau approximate to 200 ps. Additionally, the optical properties of these quantum dots are predicted to be very robust against both symmetric and asymmetric shape elongation (with FSS 2.2 mu eV), rendering postselection less essential under well-controlled growth conditions. On the other hand, the growth on miscut substrates introduces a structural anisotropy along the quantization axis to which the system is very sensitive: the FSS ranges between 5 and 50 mu eV while the radiative lifetime of the transition is increased up to tau = 400 ps. The numerical results for the FSS are in perfect agreement with our experimental measurements which give FSS = 10 +/- 9 mu eV for 2 degrees miscut angle at x = 0.15

GaAs Sub-Micron and Nano Islands by Droplet Epitaxy on Si

Merging the high effi?ciency light emitting III-V semiconductors with the state-of-the-art Silicon based electronics is of great interest for the realization of new optoelectronic devices. Unfortunately the heteroepitaxial growth of GaAs thin ?films on Si is a diffi?cult task because of the di?fference in the lattice constant, the polar/non-polar surface interaction and the di?fference in the thermal expansion coeffi?cients. We present for the ?first time the MBE growth of GaAs nanostructures on Si substrates by Droplet Epitaxy (DE) [1,2]. We believe this growth method to be promising for the growth of high quality GaAs nanoislands directly on Silicon. In the DE, the substrate is irradiated by a Ga molecular beam fl?ux ?first, leading to the formation of numerous fi?ne Ga droplets with uniform size, which are subsequently crystallized into GaAs nanostructures by an As molecular beam supply. By changing the Ga droplets deposition temperature is possible to change independently the size and the density of the droplets, while by varying the As ?flux for the crystallization we can change the fi?nal shape of the GaAs nanocrystals. We present the results for the growth of GaAs on Si by DE where the density of the GaAs nanoislands was changed by two orders of magnitude, while the size is varied from around 200 nm to around 20 nm. Measurements by X-ray microanalysis in the TEM con- fi?rmed the reaction between Ga and As with formation of GaAs. This has also seen by the presence of Moir? fringes in the TEM images taken in the two beam di?raction mode. The discontinuities of some Moir? fringes would suggest the presence of dislocations

Flat metamorphic InAlAs buffer layer on GaAs(111)A misoriented substrates by growth kinetics control

We have successfully grown, through the detailed control of the growth kinetics, flat InAlAs metamorphic buffer layers on 2 degrees -off GaAs(111)A substrates using molecular beam epitaxy. Almost full plastic relaxation is obtained for a layer thickness > 40 nm. The control of an adatom diffusion length and a step ejection probability from the bunches permits a reduction of the InAlAs epilayer root-mean-square surface roughness to 0.55 nm

Self-Assembled Local Artificial Substrates of GaAs on Si Substrate

We propose a self-assembling procedure for the fabrication of GaAs islands by Droplet Epitaxy on silicon substrate. Controlling substrate temperature and amount of supplied gallium is possible to tune the base size of the islands from 70 up to 250 nm and the density from 107 to 109 cm−2. The islands show a standard deviation of base size distribution below 10% and their shape evolves changing the aspect ratio from 0.3 to 0.5 as size increases. Due to their characteristics, these islands are suitable to be used as local artificial substrates for the integration of III–V quantum nanostructures directly on silicon substrate

Photoluminescence Study of Low Thermal Budget III–V Nanostructures on Silicon by Droplet Epitaxy

We present of a detailed photoluminescence characterization of high efficiency GaAs/AlGaAs quantum nanostructures grown on silicon substrates. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature ≤350°C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits

Control of the lateral growth morphology in GaAs Droplet Epitaxy

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Wurtzite nanowires strain control by DC electrical stimulation

Nanomechanics is a highly developed area of research, given the significant reported changes in material properties at the nanometer scale, requiring the development of new theories to explain the underlying mechanisms. Such theories must be based on measurements that are as accurate as possible, but unfortunately, conventional experimental techniques do not apply to such small components. Here we present a unique new method to control electro-mechanical forces on quasi −1D nanostructures through static electric fields with multiple ways of control of GaAs nanowires’ strain directly on the growth substrate

TEM Characterization of GaAs Nanoislands on Si

A TEM study of GaAs nanoislands grown on (001) Si substrate by the droplet epitaxy technique is presented. The nanoislands turn out to be monocrystalline in perfect epitaxial relationship with Si. By X-ray microanalysis in the TEM it is also seen that the islands are stoichiometric. TEM images of the moir? fringes revealed the presence of dislocations at the nanoislands suggesting strain relaxation

Optical characterization of individual GaAs quantum dots grown with height control technique

We show that the epitaxial growth of height-controlled GaAs quantum dots, leading to the reduction of the inhomogeneous emission bandwidth, produces individual nanostructures of peculiar morphology. Besides the height controlled quantum dots, we observe nanodisks formation. Exploiting time resolved and spatially resolved photoluminescence we establish the decoupling between quantum dots and nanodisks and demonstrate the high optical properties of the individual quantum dots, despite the processing steps needed for height control.Sarti, F.; Muñoz Matutano, G.; Bietti, S.; Vinattieri, A.; Sanguinetti, S.; Gurioli, M. (2013). Optical characterization of individual GaAs quantum dots grown with height control technique. Journal of Applied Physics. 114(2):1243011-1243014. doi:10.1063/1.4821901S124301112430141142Shields, A. J. (2007). Semiconductor quantum light sources. Nature Photonics, 1(4), 215-223. doi:10.1038/nphoton.2007.46Koguchi, N. (1993). New selective molecular-beam epitaxial growth method for direct formation of GaAs quantum dots. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 11(3), 787. doi:10.1116/1.586789Watanabe, K., Koguchi, N., & Gotoh, Y. (2000). Fabrication of GaAs Quantum Dots by Modified Droplet Epitaxy. Japanese Journal of Applied Physics, 39(Part 2, No. 2A), L79-L81. doi:10.1143/jjap.39.l79Sanguinetti, S., & Koguchi, N. (2013). Droplet epitaxy of nanostructures. Molecular Beam Epitaxy, 95-111. doi:10.1016/b978-0-12-387839-7.00004-xKeizer, J. G., Bocquel, J., Koenraad, P. M., Mano, T., Noda, T., & Sakoda, K. (2010). Atomic scale analysis of self assembled GaAs/AlGaAs quantum dots grown by droplet epitaxy. Applied Physics Letters, 96(6), 062101. doi:10.1063/1.3303979(s. f.). doi:10.1021/nl048192Lee, J. H., Wang, Z. M., Abuwaar, Z. Y., Strom, N. W., & Salamo, G. J. (2006). Evolution between self-assembled single and double ring-like nanostructures. Nanotechnology, 17(15), 3973-3976. doi:10.1088/0957-4484/17/15/061Somaschini, C., Bietti, S., Koguchi, N., & Sanguinetti, S. (2011). Coupled quantum dot–ring structures by droplet epitaxy. Nanotechnology, 22(18), 185602. doi:10.1088/0957-4484/22/18/185602Somaschini, C., Bietti, S., Koguchi, N., & Sanguinetti, S. (2009). Fabrication of Multiple Concentric Nanoring Structures. Nano Letters, 9(10), 3419-3424. doi:10.1021/nl901493fReyes, K., Smereka, P., Nothern, D., Millunchick, J. M., Bietti, S., Somaschini, C., … Frigeri, C. (2013). Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory. Physical Review B, 87(16). doi:10.1103/physrevb.87.165406Bietti, S., Somaschini, C., & Sanguinetti, S. (2013). Crystallization kinetics of Ga metallic nano-droplets under As flux. Nanotechnology, 24(20), 205603. doi:10.1088/0957-4484/24/20/205603Jo, M., Mano, T., & Sakoda, K. (2010). Morphological control of GaAs quantum dots grown by droplet epitaxy using a thin AlGaAs capping layer. Journal of Applied Physics, 108(8), 083505. doi:10.1063/1.3493262Ohtake, A. (2008). Surface reconstructions on GaAs(001). Surface Science Reports, 63(7), 295-327. doi:10.1016/j.surfrep.2008.03.001Mano, T., Abbarchi, M., Kuroda, T., Mastrandrea, C. A., Vinattieri, A., Sanguinetti, S., … Gurioli, M. (2009). Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy. Nanotechnology, 20(39), 395601. doi:10.1088/0957-4484/20/39/395601Adorno, S., Bietti, S., & Sanguinetti, S. (2013). Annealing induced anisotropy in GaAs/AlGaAs quantum dots grown by droplet epitaxy. Journal of Crystal Growth, 378, 515-518. doi:10.1016/j.jcrysgro.2012.11.006Horikoshi, Y., Kawashima, M., & Yamaguchi, H. (1988). Migration-Enhanced Epitaxy of GaAs and AlGaAs. Japanese Journal of Applied Physics, 27(Part 1, No. 2), 169-179. doi:10.1143/jjap.27.16