Self-assembled GaAs/AlGaAs coupled quantum ring-disk structures by droplet epitaxy

Elliptical Polarization of Localized States in an Anisotropic Single GaAs Quantum Ring

Localized states in an anisotropic single GaAs quantum ring were investigated in terms of polarization dependence of micro-photoluminescence spectrum at 5K. Given four Stokes parameters measured with a pair of linear polarizers and waveplates, the elliptical polarization states of two different vertical confinement states (k=1 and k=2) were compared with phase, rotation, and ellipticity angles. While the polarized emission intensity of the k=2 states becomes enhanced along [1,1,0] compared to that along [1,1¯,0], the polarization asymmetry of the k=1 states shows the opposite result. We conclude the polarization state is determined by the shape of the lateral wavefunctions. In the k=2 state, crescent-like wavefunctions are strongly localized, but the k=1 state consists of two crescent-like wavefunctions, which are connected weakly through quantum tunneling.

Strain Relaxation of InAs Quantum Dots on Misoriented InAlAs(111) Metamorphic Substrates

We investigate in detail the role of strain relaxation and capping overgrowth in the self-assembly of InAs quantum dots by droplet epitaxy. InAs quantum dots were realized on an In0.6Al0.4As metamorphic buffer layer grown on a GaAs(111)A misoriented substrate. The comparison between the quantum electronic calculations of the optical transitions and the emission properties of the quantum dots highlights the presence of a strong quenching of the emission from larger quantum dots. Detailed analysis of the surface morphology during the capping procedure show the presence of a critical size over which the quantum dots are plastically relaxed.

Polarization Anisotropies in Strain-Free, Asymmetric, and Symmetric Quantum Dots Grown by Droplet Epitaxy

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement compared to (001), where a wetting layer is present. This leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to the strong anti-binding character of the positive-charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting (FSS) ∼100 μeV), whereas for the three-fold symmetric (111)A counterpart, this figure of merit is reduced by about one order of magnitude. In all these cases, we do not observe any size dependence of the fine structure splitting. Heavy-hole/light-hole mixing is present in all the studied cases, leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine structure splitting, and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots.

Exciton Dynamics in Droplet Epitaxial Quantum Dots Grown on (311)A-Oriented Substrates

Droplet epitaxy allows the efficient fabrication of a plethora of 3D, III–V-based nanostructures on different crystalline orientations. Quantum dots grown on a (311)A-oriented surface are obtained with record surface density, with or without a wetting layer. These are appealing features for quantum dot lasing, thanks to the large density of quantum emitters and a truly 3D lateral confinement. However, the intimate photophysics of this class of nanostructures has not yet been investigated. Here, we address the main optical and electronic properties of s-shell excitons in individual quantum dots grown on (311)A substrates with photoluminescence spectroscopy experiments. We show the presence of neutral exciton and biexciton as well as positive and negative charged excitons. We investigate the origins of spectral broadening, identifying them in spectral diffusion at low temperature and phonon interaction at higher temperature, the presence of fine interactions between electron and hole spin, and a relevant heavy-hole/light-hole mixing. We interpret the level filling with a simple Poissonian model reproducing the power excitation dependence of the s-shell excitons. These results are relevant for the further improvement of this class of quantum emitters and their exploitation as single-photon sources for low-density samples as well as for efficient lasers for high-density samples.

Mechanism of nucleation and critical layer formation during In/GaAs droplet epitaxy

Fabrication of A^IIIB^V nanostructures by droplet epitaxy has many advantages over other epitaxial techniques. Although various characteristics of the growth by droplet epitaxy have been thoroughly studied for both lattice-matched and mismatched systems, little is known about physical processes hindering the formation of small size InAs/GaAs nanostructure arrays with low density and thin wetting layer. In this paper, we experimentally demonstrate that the indium droplet diameter can be reduced by decreasing the deposition time, but this reduction is limited by a critical thickness of droplet formation dependent on the substrate temperature. Using the kinetic Monte Carlo model, we propose a mechanism considering that the droplet formation begins when the system overcomes a barrier determined by the substrate attraction. As a result of physical and chemical balancing between adatom aggregation and substrate wetting, this attraction becomes weaker with increasing either temperature or deposition amount, which leads to the critical layer formation and subsequent nucleation. Using this mechanism, it is possible to provide a wide control over the nanostructure growth which is especially important at high temperatures when the processes of the island ripening are particularly intensive.

The key to controlling the morphologies of quantum nanocrystals: spherical carborane ligands

Carboranyl ligands with different functional moieties have been used to obtain different morphologies in the quantum regime using a colloidal route.

Tuning of few-electron states and optical absorption anisotropy in GaAs quantum rings

The electronic and optical properties of a GaAs quantum ring (QR) with few electrons in the presence of the Rashba spin-orbit interaction (RSOI) and the Dresselhaus spin-orbit interaction (DSOI) have been investigated theoretically. The configuration interaction (CI) method is employed to calculate the eigenvalues and eigenstates of the multiple-electron QR accurately. Our numerical results demonstrate that the symmetry breaking induced by the RSOI and DSOI leads to an anisotropic distribution of multi-electron states. The Coulomb interaction offers additional modulation of the electron distribution and thus the optical absorption indices in the quantum rings. By tuning the magnetic/electric fields and/or electron numbers in a quantum ring, one can change its optical properties significantly. Our theory provides a new way to control the multi-electron states and optical properties of a QR by hybrid modulations or by electrical means only.

Chemical Composition of Nanoporous Layer Formed by Electrochemical Etching of p-Type GaAs

We have performed a detailed characterization study of electrochemically etched p-type GaAs in a hydrofluoric acid-based electrolyte. The samples were investigated and characterized through cathodoluminescence (CL), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that after electrochemical etching, the porous layer showed a major decrease in the CL intensity and a change in chemical composition and in the crystalline phase. Contrary to previous reports on p-GaAs porosification, which stated that the formed layer is composed of porous GaAs, we report evidence that the porous layer is in fact mainly constituted of porous As₂O₃. Finally, a qualitative model is proposed to explain the porous As₂O₃ layer formation on p-GaAs substrate.

Dot-ring nanostructure: Rigorous analysis of many-electron effects

We discuss the quantum dot-ring nanostructure (DRN) as canonical example of a nanosystem, for which the interelectronic interactions can be evaluated exactly. The system has been selected due to its tunability, i.e., its electron wave functions can be modified much easier than in, e.g., quantum dots. We determine many-particle states for N_e = 2 and 3 electrons and calculate the 3- and 4-state interaction parameters, and discuss their importance. For that purpose, we combine the first- and second-quantization schemes and hence are able to single out the component single-particle contributions to the resultant many-particle state. The method provides both the ground- and the first-excited-state energies, as the exact diagonalization of the many-particle Hamiltonian is carried out. DRN provides one of the few examples for which one can determine theoretically all interaction microscopic parameters to a high accuracy. Thus the evolution of the single-particle vs. many-particle contributions to each state and its energy can be determined and tested with the increasing system size. In this manner, we contribute to the wave-function engineering with the interactions included for those few-electron systems.

The entangled role of strain and diffusion in driving the spontaneous formation of atolls and holes in Ge/Si(111) heteroepitaxy

We investigate the interdependent processes of strain and diffusion in the formation of holes and atolls obtained by rapid annealing of Ge/Si(111) islands at T ≈ 970 °C. We show that the shape evolution from islands to atolls and holes is closely captured by an analytical model including strain-driven diffusion. In the model, strain profiles obtained by finite element solutions of continuum elasticity equations are introduced in the diffusion equation as the source of a diffusion flux driven by the strain gradient. When the shape of the elastic field in Ge/Si(111) islands is coupled to diffusion, the morphology of the SiGe nanostructures observed after annealing is reproduced.

Crystallization kinetics of Ga metallic nano-droplets under As flux

We present an experimental investigation of the crystallization dynamics of Ga nano-droplets under As flux. The transformation of the metallic Ga contained in the droplets into a GaAs nano-island proceeds by increasing the size of a tiny ring of GaAs which is formed just after the Ga deposition at the rim of a droplet. The GaAs crystallization rate depends linearly on the liquid-solid interface area. The maximum growth rate is set by the As flux impinging on the droplet, thus showing an efficient As incorporation and transport despite the predicted low solubility of the As in metallic Ga at the crystallization temperatures.

Fast emission dynamics in droplet epitaxy GaAs ring-disk nanostructures integrated on Si

The emission dynamics in GaAs/AlGaAs coupled ring-disk (CRD) quantum structures fabricated on silicon substrates is presented. The CRD structures are self-assembled via droplet epitaxy, a growth technique which, due to its low thermal budget, is compatible with the monolithic integration of III-V devices on Si based electronic circuits. Continuous wave, time resolved photoluminescence and theoretical calculations in the effective mass approximations are presented for the assessment of the electronic and carrier properties of the CRDs. The CRDs show a fast carrier dynamics which is expected to be suitable for ultrafast optical switching applications integrated on silicon.

Micro-photoluminescence of GaAs/AlGaAs triple concentric quantum rings

A systematic optical study, including micro, ensemble and time resolved photoluminescence of GaAs/AlGaAs triple concentric quantum rings, self-assembled via droplet epitaxy, is presented. Clear emission from localized states belonging to the ring structures is reported. The triple rings show a fast decay dynamics, around 40 ps, which is expected to be useful for ultrafast optical switching applications.

Coupled quantum dot-ring structures by droplet epitaxy

The fabrication, by pure self-assembly, of GaAs/AlGaAs dot-ring quantum nanostructures is presented. The growth is performed via droplet epitaxy, which allows for the fine control, through As flux and substrate temperature, of the crystallization kinetics of nanometer scale metallic Ga reservoirs deposited on the surface. Such a procedure permits the combination of quantum dots and quantum rings into a single, multi-functional, complex quantum nanostructure.

Growth Interruption Effect on the Fabrication of GaAs Concentric Multiple Rings by Droplet Epitaxy

WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

Concentric Multiple Rings by Droplet Epitaxy: Fabrication and Study of the Morphological Anisotropy

We present the Molecular Beam Epitaxy fabrication of complex GaAs/AlGaAs nanostructures by Droplet Epitaxy, characterized by the presence of concentric multiple rings. We propose an innovative experimental procedure that allows the fabrication of individual portions of the structure, controlling their diameter by only changing the substrate temperature. The obtained nanocrystals show a significant anisotropy between [110] and [1-10] crystallographic directions, which can be ascribed to different activation energies for the Ga atoms migration processes.