Showing papers by "Le Si Dang published in 2010"

GaN-based nanowires: From nanometric-scale characterization to light emitting diodes

Abstract: We studied and improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs). PIN nanodiodes with and without InGaN/GaN multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) under N-rich conditions on n-doped Si(111) substrates. Thanks to the coalescence of the p-type region of the NWs grown at low temperature, an autoplanarization process has been performed to obtain LEDs. Ni/Au top contacts have been deposited and patterned in order to bias the devices. A multiple-scale characterization approach has been carried out through the comparison of localized cathodoluminescence (CL) and macroscopic electroluminescence (EL) spectra. It shows that the EL emission of PIN-based LED at room temperature is related to defects in the p-type region of the NWs. In order to enhance the radiative recombinations of NW-based LEDs, we have first added InGaN/GaN MQWs, and secondly an electron blocking layer (EBL) has been inserted between the MQWs and the p-type zone of the NWs. The LED with EBL exhibited an emission band at 420 nm. The blue-shift of this emission band with increasing injected current is attributed to quantum confined Stark effect (QCSE) and evidences the radiative emission of InGaN/GaN MQWs. At 50 mA dc current, this improved NW-based LED emits about 500 times more light than the heterostructure without EBL. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A single-step electron beam lithography of buried nanostructures using cathodoluminescence imaging and low temperature.

Abstract: We report a new electron beam lithography process using the cathodoluminescence properties of semiconductors to visualize nanostructures buried underneath the resist and to subsequently write the pattern associated with these nanostructures. This single-step process could be used, for example, to make electrical contacts to nanowires (as illustrated in this work) or to design a photonic crystal resonator centered on a single quantum dot. Fabrication speed and positioning accuracy are significantly increased as compared to the standard process since no alignment marks and the mapping step of the nanostructures with respect to these marks are needed. We show also that low temperature (down to 5 K) could be used to improve the observation of the nanostructures through the resist while keeping very good spatial resolution.

Exciton-Polariton Bose-Einstein Condensation: Advances and Issues

Abstract: In this review, we present a comprehensive set of experimental results on microcavity-polariton Bose-Einstein Condensation (BEC), obtained within a close collaboration between Institut Neel, Grenoble, France and EPFL, Lausanne, Switzerland. First, we recall the main observations, i.e., massive occupation of the ground state and build-up of long range order, which led us to conclude that polariton BEC indeed occurs. Then, the highly disordered environment in which the condensation takes place is considered: we show how interactions are a necessary ingredient for polariton BEC. Finally we discuss quantised vortices observed for the first time in polariton condensates. Their unusual features are shown to be inherited from the disordered environment and the driven-dissipative character of the polariton BEC.

Platinum assisted vapor-liquid-solid growth of GaN nanowires and their properties

Abstract: Growth of GaN nanowires by using a vapor-liquid-solid (VLS) mechanism with Pt catalysts was investigated. Single-crystal GaN nanowires with diameters of ∼100 nm and lengths of ∼5 μm were grown with Pt and with the well-known VLS catalyst of Ni. The growth rate of nanowires with Pt was lower than that with Ni under identical processing conditions. Pt-Ga alloy globules were observed by Transmission electron microscopy at the ends of the nanowires which lead the growth by the VLS mechanism. Electrical data from individual nanowires indicated high conductivity associated with high electron concentration in the Pt-GaN nanowires.

Room temperature one-dimensional polariton condensate in a ZnO microwire

Abstract: A cavity-polariton, formed due to the strong coupling between exciton and cavity mode, is one of the most promising composite bosons for realizing macroscopic spontaneous coherence at high temperature. Up to date, most of polariton quantum degeneracy experiments were conducted in the complicated two-dimensional (2D) planar microcavities. The role of dimensionality in coherent quantum degeneracy of a composite bosonic system of exciton polaritons remains mysterious. Here we report the first experimental observation of a one-dimensional (1D) polariton condensate in a ZnO microwire at room temperature. The massive occupation of the polariton ground state above a distinct pump power threshold is clearly demonstrated by using the angular resolved spectroscopy under non-resonant excitation. The power threshold is one order of magnitude lower than that of Mott transition. Furthermore, a well-defined far field emission pattern from the polariton condensate mode is observed, manifesting the coherence build-up in the condensed polariton system.

Electroluminescence from a n‐ZnO/p‐GaN hybrid LED

Abstract: In this work we report on the fabrication and characterization of a n-ZnO/p-GaN heterojunction LED. The p-GaN layer was fabricated using MOCVD on Al2O3 with Mg as the acceptor whereas the ZnO nanostructures were grown in a very simple vapor transport system without any additionally doping. Room temperature electroluminescence (EL) measurements show green deep band emission centered at 2.3 eV which is clearly visible with the naked eye when the structure is forward biased. Cathodoluminescence mapping was performed to explain the absence of the band edge emission in the EL spectrum. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous coherence within a gas of exciton-polaritons in Telluride microcavities

Abstract: Microcavity exciton-polaritons are the eigenstates resulting from strong light-matter coupling in high quality monolithic semiconductor microcavities. Owing to their mixed photonic and excitonic nature, polaritons are Bose particles of very light mass and short lifetime that can interact with their environment, forming a new class of Bose gas. In spite of their short lifetime, a polariton gas can show Bose-Einstein condensation or polariton lasing depending on the experimental conditions. The properties of these coherent states of polaritons are unique in many respects. In this chapter we rely on a comprehensive set of experimental and theoretical works carried out this last decade to give a detailed description of polariton coherent states. The influences of finite lifetime, disorder and interaction with the environment are addressed, and the analogies and differences between polariton condensate, polariton lasing, equilibrium Bose gas and photon lasing are outlined and discussed.