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A hybrid metal?semiconductor nanosystem for the generation of THz radiation, based on the fabrication of GaAs quantum molecules-Ga metal nanoparticles complexes through a self assembly approach, is proposed. The role of the growth parameters, the substrate temperature, the Ga and As flux during the quantum dot molecule (QDM) fabrication and the metal nanoparticle alignment are discussed. The tuning of the relative positioning of QDMs and metal nanoparticles is obtained through the careful control of Ga droplet nucleation sites via Ga surface diffusion. The electronic structure of a typical QDM was evaluated on the base of the morphological characterizations performed by atomic force microscopy and cross sectional scanning electron microscopy, and the predicted results confirmed by micro-photoluminescence experiments, showing that the Ga metal nanoparticle-GaAs quantum molecule complexes are suitable for terahertz generation from intraband transition.

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations., 14 pages, 3 figures

We investigate the effect of surface Ga accumulation on the growth of In0.5Ga0.5N by PAMBE at low temperatures (T= 440 °C). We find that the control of the surface metal condition is very important because the crystallization process strongly depends on the metal flux impinging on the surface. The growth rate rapidly decreases when the supply of metal flux exceeds certain amount. Such phenomenon can be explained by the model taking into account droplet effects on the incorporation of metals adatom into the crystal.

Entanglement plays a crucial role in many protocols for quantum cryptography and in various approaches for quantum computation. Semiconductor quantum dots (QDs) have been proposed as a source of polarization-entangled photons which can be integrated in an electrically driven solid state device [1]. The generation process relies on the biexciton-exciton radiative cascade. However, e-h exchange interaction often induces a fine structure splitting (FSS) between the bright exciton states and destroys quantum correlation. In the case of QDs grown on commonly used (100) substrates, this degeneracy lifting is caused by the C2v symmetry stemming from asymmetric interfaces, strain anisotropy, piezoelectric fields and shape elongation [2,3]. A viable alternative relies on growing QDs on the higher symmetry (111) substrate. Theoretical investigations have shown that QDs with C3v symmetry should exhibit zero FSS [4,5]. The most studied III-V materials do not grow in the Stranski-Krastanov mode on a (111) surface, however different techniques such as droplet epitaxy [6] or the use of patterned substrates [7] have demonstrated to be able to overcome this limitation. In our work we focus on GaAs/AlGaAs (111) QDs grown by droplet epitaxy. Polarization resolved single dot photoluminescence measurements on hexagonal QDs are presented. Charged and bi-excitonic complexes are consistently identified by means of power and polarization dependence analyses. A broad FSS energy distribution is observed, with an average value smaller than the one reported for QDs grown on (100) substrates using the same technique and emitting at similar wavelengths. The phase distribution of the polarization axis evidences the absence of systematic anisotropies. These results are in agreement with previous studies on similar samples [8]. Recent advances in fabrication have proven the ability to obtain atomically flat substrates and to gradually tune the shape from hexagonal to triangular by changing the growth parameters. This lays the groundwork for a systematic investigation of the impact of geometry on the excitonic fine structure, with the goal of finding the best conditions for vanishing FSS. [1] O. Benson, C. Santori, M. Pelton, Y. Yamamoto, in: Physical Review Letters 84, 2513 (2000). [2] G. Bester, S. Nair, A. Zunger, in: Physical Review B 67, 161306 (2003). [3] R. Seguin, A. Schliwa, S. Rodt, K. Poetschke, U. W. Pohl, D. Bimberg, in: Physical Review Letters 95, 257402 (2005). [4] R. Singh, G. Bester, in: Physical Review Letters 103, 063601 (2009). [5] A. Schliwa, M. Winkelnkemper, A. Lochmann, E. Stock, D. Bimberg, in: Physical Review B 80, 161307 (2009). [6] E. Stock, T. Warming, I. Ostapenko, S. Rodt, A. Schliwa, J. A. Toefflinger, A. Lochmann, A. Toropov, S. Moshchenko, D. Dmitriev, V. Haisler, D. Bimberg, in: Applied Physics Letters 96, 093112 (2010). [7] Y. Sugiyama, Y. Sakuma, S. Muto, N. Yokoyama, in: Applied Physics Letters 67, 256 (1995). [8] T. Mano, M. Abbarchi, T. Kuroda, B. McSkimming,, A. Ohtake, K. Mitsuishi, K. Sakoda, in: Applied Physics Express 3, 065203 (2010).

Our work, focused on the heteroepitaxial Ge/Si (001) system shows that patterned Si substrates in appropriate pillar arrays, featuring micrometric dimensions within the common deep-etching capabilities and array size suitable for several applications (up to few hundreds microns), do provide exceptional compliance to the thermal strain of a continuous Ge film deposited on top. Micro-Raman and XRD measurements on the Ge suspended layer, as obtained by Low Energy Plasma Enhanced Chemical Vapor Deposition, confirm the theoretical predictions. Actually, the sharp onset with pillar aspect ratio of such an effect is demonstrated to be triggered by the free rotation of the pillars, as induced by the thermal contraction of the film. The strain relaxation is deeply analyzed both by FEM simulations and experimental XRD results, showing a very good quantitative agreement. Our findings have far reaching consequences beyond the crystalline heteroepitaxial systems, whenever a film of any microstructure, or composition, is deposited in suspended patches on suitable pillar arrays.

We present a fully self‐assembly technique, based on Droplet Epitaxy, to fabricate semiconductor surfaces functionalized with asymmetric planar quantum dot molecules –metal nanoparticles hybrid systems for efficient THz generation. The role of growth parameters on molecule fabrication and metal nanoparticle alignment for the matching of the grown system with the design requirements is discussed.

Two-dimensional (2D) materials represents nowadays the new frontiers of semiconductor technology [1]. At one side, great effort has been done in research applied to graphene producing new technology that will rebuild many industrial sectors. On the other side non- graphene 2D materials like transition metal dichalcogenides (TMDs) are highly attractive because they offer complementary properties to graphene [2] but still lack a large-scale production method for high quality and well controlled layers. MoS2, one of the most studied TMDs materials, has been produced by using many techniques, but the deposition through chemical methods, i.e. based on the use of materials such as Mo or MoO3 and then the reaction with sulfur, already demonstrates to fit better the stringent requirements of lateral uniformity on the centimeter scale, vertical scalability and structural optimization as function of the growth parameters [3]. However, many details about the chemistry of the reaction between Mo precursor and sulfur needs further clarification in view of a large-area production [4]: in particular, it is not clear the role of the materials used as precursors, of the substrate type and of the deposition temperature, which drives Mo-S reaction, but also other undesired ones. This work is focused on the study of some of these fundamental parameters in the growth of MoS2 on SiO2/Si substrates by vapor-solid chemical reaction between Mo pre-deposited thin film precursor (TFP) and sulfur (figure 1(a)). We demonstrate that the control of the Mo film thickness allows us to have accurate control of the MoS2 thickness. This methodology leads to MoS2 nanosheets with a tunable number of layers and uniformly extended throughout the whole area of the support substrates (figure 1(b)). In this type of process, the role of the growth temperature and of the substrate on the structural properties of the layers becomes dramatically important. Various techniques are employed to characterize the MoS2 layers. In particular, Raman spectroscopy is the key-probe used for single- and multi-layers identification and for the validation of the high structural quality of the grown material (see figure 1(b)), but fundamental information about the chemical details, the morphology, and the structural quality are also drawn from X-ray photoelectron microscopy (XPS), atomic force microscopy (AFM), and photoluminescence (PL). The temperatures used for the study are 500, 750, and 1000 °C. All the grown films show granular morphology by AFM. Better results in term of layer quality is obtained with higher sulfurization temperature and detailed analysis of Raman spectra collected from quadrilayers demonstrates that the layer features are comparable to exfoliated MoS2 samples. We further demonstrate that SiO2/Si wafer bring intrinsic limitation as a substrate for the MoS2 growth at high deposition temperature because of the activation of silicon diffusion from the substrate and of the chemical reaction between Si and S with SiS2 byproduct formation [5]. Our study supplies some fundamental information on the chemistry of vapor-solid reaction growth, clarifying the role of temperature in giving high quality materials and putting a warning on the use of silicon substrate for this type of process [6]. References [1] Yazyev, O. V. and Chen, Y. P., Nature Nanotechnology 9, 755 (2014). [2] See editorial’s Nature Materials 13, 1073 (2014) and references therein. [3] Lee, Y.-H. et al. Adv. Mater. 24, 2320–2325 (2012); Liu, K. et al. Nano Lett. 12, 1538 (2012); Yu, Y. et al., Sci. Rep. 3, 1866 (2013); Zhan, Y. et al. Small 8, 966–971 (2012). [4] Liu, H. et al. Nanotechnology 25, 405702 (2014). [5] Niu, J. et al. Physica E 24, 278–281 (2004). [6] This research was partially supported by US-Army Contract with CNR-IMEM and CNR-IMM.

Large-scale integration of MoS2 in electronic devices requires the development of reliable and cost-effective deposition processes, leading to uniform MoS2 layers on a wafer scale. Here we report on the detailed study of the heterogeneous vapor-solid reaction between a pre-deposited molybdenum solid film and sulfur vapor, thus resulting in a controlled growth of MoS2 films onto SiO2/Si substrates with a tunable thickness and cm(2)-scale uniformity. Based on Raman spectroscopy and photoluminescence, we show that the degree of crystallinity in the MoS2 layers is dictated by the deposition temperature and thickness. In particular, the MoS2 structural disorder observed at low temperature (