Luca Francaviglia

Three-dimensional nanoscale study of Al segregation and quantum dot formation in GaAs/AlGaAs core-shell nanowires

GaAs/Al-GaAs core-shell nanowires fabricated by molecular beam epitaxy contain quantum confining structures susceptible of producing narrow photoluminescence (PL) and single photons. The nanoscale chemical mapping of these structures is analyzed in 3D by atom probe tomography (APT). The study allows us to confirm that Al atoms tend to segregate within the AlGaAs shells towards the vertices of the hexagons defining the nanowire cross section. We also find strong alloy fluctuations remaining AlGaAs shell, leading occasionally to the formation of quantum dots (QDs). The PL emission energies predicted in the framework of a 3D effective mass model for a QD analyzed by APT and the PL spectra measured on other nanowires from the same growth batch are consistent within the experimental uncertainties.

Quantum dots in the GaAs/AlxGa1−xAs core-shell nanowires: Statistical occurrence as a function of the shell thickness

Quantum dots (QDs) embedded in nanowires represent one of the most promising technologies for applications in quantum photonics. Self-assembled bottom-up fabrication is attractive to overcome the technological challenges involved in a top-down approach, but it needs post-growth investigations in order to understand the self-organization process. We investigate the QD formation by self-segregation in AlxGa1−xAs shells as a function of thickness and cross-section morphology. By analysing light emission from several hundreds of emitters, we find that there is a certain thickness threshold for the observation of the QDs. The threshold becomes smaller if a thin AlAs layer is pre-deposited between the GaAs nanowire core and the AlxGa1−xAs shell. Our results evidence the development of the quantum emitters during the shell growth and provide more guidance for their use in quantum photonics.

Bistability of Contact Angle and Its Role in Achieving Quantum-Thin Self-Assisted GaAs nanowires

Achieving quantum confinement by bottom-up growth of nanowires has so far been limited to the ability of obtaining stable metal droplets of radii around 10 nm or less. This is within reach for gold-assisted growth. Because of the necessity to maintain the group III droplets during growth, direct synthesis of quantum sized structures becomes much more challenging for self-assisted III-V nanowires. In this work, we elucidate and solve the challenges that involve the synthesis of gallium-assisted quantum-sized GaAs nanowires. We demonstrate the existence of two stable contact angles for the gallium droplet on top of GaAs nanowires. Contact angle around 130° fosters a continuous increase in the nanowire radius, while 90° allows for the stable growth of ultrathin tops. The experimental results are fully consistent with our model that explains the observed morphological evolution under the two different scenarios. We provide a generalized theory of self-assisted III-V nanowires that describes simultaneously the droplet shape relaxation and the NW radius evolution. Bistability of the contact angle described here should be the general phenomenon that pertains for any vapor-liquid-solid nanowires and significantly refines our picture of how nanowires grow. Overall, our results suggest a new path for obtaining ultrathin one-dimensional III-V nanostructures for studying lateral confinement of carriers.

Photophysics behind highly luminescent two-dimensional hybrid perovskite (CH3(CH2)2NH3)2(CH3NH3)2Pb3Br10 thin films

Two-dimensional (2D) Ruddlesden–Popper perovskites have emerged as a new class of hybrid materials with high photoluminescence and improved stability compared to their three-dimensional (3D) counterparts. Studies of the photophysics of these new 2D perovskites are essential for the fast development of optoelectronic devices. Here, we study the power and temperature dependences of the photoluminescence properties of the (PA)2(MA)2Pb3Br10 hybrid perovskite. High electron–phonon coupling near room temperature was found to be dominated by longitudinal optical (LO) phonons via the Frohlich interaction. However, we show that the presence of free carriers is also possible, with lower trapping states and higher and more stable emission compared to the 3D MAPbBr3. These characteristics make the studied 2D material very attractive for optoelectronic applications, including solar cells and light emitting diodes (LEDs). Our investigation provides new fundamental insights into the emission characteristics of 2D lead halide perovskites.

Anisotropic-Strain-Induced Band Gap Engineering in Nanowire-Based Quantum Dots

Tuning light emission in bulk and quantum structures by strain constitutes a complementary method to engineer functional properties of semiconductors. Here, we demonstrate the tuning of light emission of GaAs nanowires and their quantum dots up to 115 meV by applying strain through an oxide envelope. We prove that the strain is highly anisotropic and clearly results in a component along the NW longitudinal axis, showing good agreement with the equations of uniaxial stress. We further demonstrate that the strain strongly depends on the oxide thickness, the oxide intrinsic strain, and the oxide microstructure. We also show that ensemble measurements are fully consistent with characterizations at the single-NW level, further elucidating the general character of the findings. This work provides the basic elements for strain-induced band gap engineering and opens new avenues in applications where a band-edge shift is necessary.

Dopant-Induced Modifications of GaxIn(1–x)P Nanowire-Based p–n Junctions Monolithically Integrated on Si(111)

Today, silicon is the most used material in photovoltaics, with the maximum conversion efficiency getting very close to the Shockley-Queisser limit for single-junction devices. Integrating silicon with higher band-gap ternary III-V absorbers is the path to increase the conversion efficiency. Here, we report on the first monolithic integration of Ga xIn(1- x)P vertical nanowires, and the associated p-n junctions, on silicon by the Au-free template-assisted selective epitaxy (TASE) method. We demonstrate that TASE allows for a high chemical homogeneity of ternary alloys through the nanowires. We then show the influence of doping on the chemical composition and crystal phase, the latter previously attributed to the role of the contact angle in the liquid phase in the vapor-liquid-solid technique. Finally, the emission of the p-n junction is investigated, revealing a shift in the energy of the intraband levels due to the incorporation of dopants. These results clarify some open questions on the effects of doping on ternary III-V nanowire growth and provide the path toward their integration on the silicon platform in order to apply them in next-generation photovoltaic and optoelectronic devices.