This PDF file contains the front matter associated with SPIE Proceedings Volume 7610, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We present a general method that improves the emission efficiency of InAs quantum dots (QDs) fabricated by antimony surfactant-mediated growth. Unlike conventional InAs/GaAs QDs, we show that the control of the interface properties of the InAs/Sb:GaAs QDs is crucial. Our method consists in growing InAs QDs on an antimony-irradiated GaAs surface, in order to exploit the surfactant properties of antimony, and then removing the excess segregated antimony by applying a high arsenic pressure before capping. In such a way, one benefits from the advantages of the antimony-surfactant mediated growth (high density QDs, no coalescence, no emission blueshift after annealing), without the detrimental formation of antimony-induced non-radiative defects. We show that the lasing characteristics of InAs/Sb:GaAs QD lasers grown by metal organic chemical vapor deposition in the 1.3 μm band are drastically improved, with a reduced threshold current density and higher internal quantum efficiency. These studies advance the understanding of key processes in antimony-mediated growth of InAs QDs and will allow full utilization of its advantages for integration in opto-electronic devices.

Efficient generation of polarized single or entangled photons is a crucial requirement for the implementation of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection. We realized highly efficient single photon sources (SPS) based on well established semiconductor technology: In a pin structure a single electron and a single hole are funneled into a single InAs quantum dot using a submicron AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral filtering is demonstrated. Out-coupling efficiency and emission rate are increased by embedding the SPS into a micro-cavity of Q = 140. The design of the micro-cavity is based on detailed modeling to optimize its performance. The resulting resonant single-QD diode generates single polarized photons at a repetition rate of 1 GHz exhibiting a second order correlation function of g⁽²⁾(0) = 0. Eventually, QDs grown on (111) oriented substrate are proposed as source of entangled photon pairs. Intrinsic symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this substrate orientation. As a result the XX → X → 0 recombination cascade of a QD can be used for the generation of entangled photons without further tuning of the finestructure splitting via QD size and/or shape. We present first micro-photoluminescence studies on QDs grown on (111) GaAs, demonstrating a fine structure splitting less than the spectral resolution of our set-up.

We report on recent progress towards single photon sources based on quantum dot and quantum post nanostructures which are manipulated using surface acoustic waves. For this concept acoustic charge conveyance in a quantum well is used to spatially separate electron and hole pairs and transport these in the plane of the quantum well. When conveyed to the location of a quantum dot or quantum post these carriers are sequentially captured into the confined levels. Their radiative decays gives rise to the emission of a train of single photons. Three different approaches using (i) straininduced and (ii) self-assembled quantum dots, and (iii) self-assembled quantum posts are discussed and their application potential is discussed. First devices and initial experiments towards the realization of such an acoustically driven single photon source are presented and remote acoustically triggered injection into few individual emitters is demonstrated.

Self-assembled InAs quantum dots (QDs) have been the subject of intense research in part due to their potential for quantum information systems. However, many quantum information schemes require placing quantum dots at predetermined positions. Local anodic oxidation (LAO) on the base of atomic force microscope (AFM) is considered to be an effective tool for ex-situ patterning of GaAs substrate for further site-controlled growth of InAs quantum dots. We have experimentally shown that ex-situ AFM scanning without LAO (both in tapping and contact mode) of epitaxial GaAs surface modifies locally its properties while the surface topology remains unchanged. It has been revealed that AFM-treated area shows nucleating processes in MOCVD growth completely different from that of untreated area. The processes are found to be critical for growing of self-organized InAs quantum dots. Local surface density of grown quantum dots is significantly reduced in the AFM-treated area and its value depends on the number of the scan cycles. In the same epitaxial process the local surface density of quantum dots may be varied from 10¹¹ cm^-2 to 10⁷cm^-2. We discuss the nature of the observed phenomena in particular AFM-induced changes in surface potential. The observed effect in combination with LAO may be considered as a new tool for engineering surface density and position of epitaxially grown quantum dots.

Number and size control of InAs quantum dots (QDs) on truncated InP pyramids grown by selective area Metal Organic Vapor Phase Epitaxy (MOVPE) is reported. The facet composition of the pyramid top surface and the relative facet sizes are determined by the shape of the pyramid base and the pyramid height for a certain base size. This allows the precise position and distribution control of the QDs due to preferential nucleation on the {103} and {115} facets. The size of the QDs is adjusted by the growth parameters, e.g., InAs amount and growth rate together with the pyramid top surface size. The QD number, related to the specific shape of the pyramid top surface, is reduced by the shrinking pyramid top surface size during growth. Well defined positioning of four, three, two, and single QDs is realized successfully. Regrowth of a passive InP structure around the pyramids establishes submicrometer-scale active-passive integration for efficient microcavity QD nanolasers and single photon sources operating in the 1.55-μm telecom wavelength region and their implementation in photonic integrated circuits.

Quantum dots have the potential to produce devices with enhanced properties. However, many quantum dot devices require the quantum dots to have a precise size and a precise location for optimum operation. So far approaches such as directed assembly and self assembly have failed due to the random effects resulting during nucleation of the quantum dots. InAs grown under metal rich conditions can remain planar as opposed to forming the self assembled quantum dot morphology. Recently we have demonstrated that planar InAs when patterned via tip-based scribing and then annealed under an As pressure typical for self-assembled quantum dot growth reorganizes and assumes a 3D morphology. We have been studying this process as a potential method to precisely locate quantum dots with definable sizes. In this work we report change in the morphology for different thickness of planar InAs for various pattern dimensions and annealing temperatures. We have analyzed the composition of the films after annealing to determine the effect induced in the films from patterning resulting from scribing. Using this approach, arrays of 3D InAs mounds have been formed with mounds having base dimensions of 800, 500, and 350Å. These results demonstrate that the smaller patterns are less stable and coarsening becomes more dominant.

An all-optical switching device has been proposed by using self-assembled InAs/GaAs quantum dots (QDs) within a vertical cavity structure for ultrafast optical communications. This device has several desirable properties, such as the ultra-low power consumption, the micrometre size, and the polarization insensitive operation. Due to the threedimensional confined carrier state and the broad size distribution of self-assembled InAs/GaAs QDs, it is crucial to enhance the interaction between QDs and the cavity with appropriately designed 1D periodic structure. Significant QD/cavity nonlinearity is theoretically observed by increasing the GaAs/AlAs pair number of the bottom mirror. By this consideration, we have fabricated vertical-reflection type QD switches with 12 periods of GaAs/Al_0.8Ga_0.2As for the top mirror and 25 periods for the bottom mirror to give an asymmetric vertical cavity. Optical switching via the QD excited state exhibits a fast switching process with a time constant down to 23 ps, confirming that the fast intersubband relaxation of carriers inside QDs is an effective means to speed up the switching process. A technique by changing the light incident angle realizes wavelength tunability over 30 nm for the QD/cavity switch.

Dynamic effects in a quantum dot (QD) laser are studied theoretically. The frequency and decay rate of relaxation oscillations, and the modulation response are calculated as functions of injection current density, cavity length, and parameters of the QD structure. The highest possible bandwidth is calculated and shown to increase with increasing overlap integral between the electron and hole wave functions in a QD, number of QD-layers and surface density of QDs in a layer, and with reducing QD-size dispersion.

Effect of the wetting layer (WL) on the output power of a double tunneling-injection (DTI) quantum dot (QD) laser is studied. Such a laser was proposed earlier to suppress bipolar population and hence electron-hole recombination outside QDs. In the Stranski-Krastanow growth mode, QDs are formed on an initially grown WL. The WL is directly connected to QDs by the processes of carrier capture and thermal escape. These processes are described in terms of the temporal cross-sections of electron and hole capture from the WL into QDs. The electron and hole densities and parasitic electron-hole recombination current density in the WL, and the output power of the device are calculated as functions of the temporal cross-sections. These calculations provide the basis for optimization of a DTI QD laser with the WL aimed at maximizing the output power. The larger the temporal cross-section of electron capture into QDs, the more efficient is the electron capture from the WL into QDs, and hence the higher is the output power. The smaller the temporal crosssection of hole capture into QDs, the less intensive is the hole thermal escape from QDs into the WL, the less intensive is the recombination in the WL, and hence the higher is the output power.

A lithography-free method for producing freestanding one-dimensional gold nanoparticle arrays encapsulated within silicon dioxide nanowires is reported. Silicon nanowires grown by the vapor-liquid-solid technique with diameters ranging from 20 nm to 50 nm were used as the synthesis template. The gold nanoparticle arrays were obtained by coating the surface of the silicon nanowires with a 10 nm gold film, followed by thermal oxidation in an oxygen ambient. It was found that the thermal oxidation rate of the silicon nanowires was significantly enhanced by the presence of the gold thin film, which fully converted the silicon into silicon dioxide. The gold-enhanced oxidation process forced the gold into the core of the wire, forming a solid gold nanowire core surrounded by a silicon dioxide shell. Subsequent thermal treatment resulted in the fragmentation of the gold nanowire into a uniformly spaced array of gold nanoparticles encapsulated by a silicon dioxide shell, which was observed by in situ annealing in transmission electron microscopy. Analysis of many different silicon nanowire diameters shows that the diameter and spacing of the gold nanopaticles follows the Rayleigh instability, which confirms this is the mechanism responsible for formation of the nanoparticle array.

We report some results on the analysis of thermo-electromechanical effects in low dimensional semiconductor nanostructures (LDSNs). A coupled model of thermoelectroelasticity has been applied to the analysis of quantum dots and quantum wires. Finite element solutions have been obtained for different thermal loadings and their effects on the electromechanical properties in quantum dots and quantum wires are presented. Our model accounts for a practically important range of internal and external thermoelectromechanical loadings. Results are obtained for typical quantum dot and quantum wire systems with cylindrical geometry. The comparative analysis of thermoelectromechanical effects in quantum dots and quantum wires is also presented. It is observed that the electromechanical effects in LDSNs are noticeably influenced by thermal loadings. The influence is more significant in quantum dots as compared to that of quantum wires.

Within the framework of effective mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende GaN/Al_xGa_1-xN spherical quantum dot (QD) is investigated using the plane wave basis. The dependencies of the binding energy on electric field, magnetic field, QD radius and impurity position are obtained. The maximum of impurity binding energy is shifted from the centre of QD and the degenerating energy located for symmetrical positions with respect to the centre of QD are split in presence of the external electric field. The binding energy increases with the increases of magnetic field when the impurity located at the centre of QD. In the presence of electric and magnetic field simultaneously, an increase in the electric field strength leads to a decrease of the maxima of binding energy with an increase in magnetic field.

Zinc Oxide nanoparticles and nanorods have been synthesized at an optimum temperature of 60°C using aqueous solution of zinc acetate and potassium hydroxide in methanol. Particle and rod like structures were obtained by merely varying the relative concentration of the reagents. A variety of techniques like UV-Vis absorption spectroscopy, X-ray diffraction (XRD), photoluminescence, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were used to carry out structural and spectroscopic characterizations. FTIR confirms the preparation of zinc oxide. XRD shows the formation of well crystalline nature and wurtzite structure of prepared zinc oxide samples. Grain sizes were also calculated using XRD data and found to be in 11-15nm range for all preparations. Presence of one-dimensional structures in the rod samples were confirmed by SEM images. Blue shift of the absorption peaks were found due to quantum confinement of excitons. Capping action of polyvinyl pyrrolidone (PVP) was also studied. Use of PVP leads to the decrement in aspect ratio of rods but provides spherical shaped nanostructures. Enhancement of UV-emission intensity with suppression of green emission intensity was observed by the use of PVP during preparation.

We studied the energy states in In_0.8Ga_0.2As SAQDs (self-assembled quantum dots) which depended on W(001) and the misorientation angle of the substrate. Starting materials used in this study were SiO₂-patterend exact and 5 degree - off (001) GaAs substrates. In_0.8Ga_0.2As SAQDs had only ground state emissions for SiO₂-patterned exact (001) GaAs substrate, whereas those had ground and excited state emissions for SiO₂-patterned 5 degree-off (001) GaAs substrate. These results suggest that discrete nature of the density of states in SAQDs was improved by using SiO₂-patterned vicinal (001) GaAs substrate with higher misorientation angle of substrate.