Environmental sensitivity of n-i-n and undoped single GaN nanowire photodetectors

In this work, we compare the photodetector performance of single defect-free undoped and n-in GaN nanowires (NWs). In vacuum, undoped NWs present a responsivity increment, nonlinearities and persistent photoconductivity effects (~ 100 s). Their unpinned Fermi level at the m-plane NW sidewalls enhances the surface states role in the photodetection dynamics. Air adsorbed oxygen accelerates the carrier dynamics at the price of reducing the photoresponse. In contrast, in n-i-n NWs, the Fermi level pinning at the contact regions limits the photoinduced sweep of the surface band bending, and hence reduces the environment sensitivity and prevents persistent effects even in vacuum

Intrinsic limits governing MBE growth of Ga-assisted GaAs nanowires on Si(111)

Diffusion-enhanced and desorption-limited growth regimes of Ga-assisted GaAs nanowires were identified. In the latter regime, the number of vertical NWs with a narrow length distribution was increased by raising the growth temperature. The maximum axial growth rate; which can be quantified by the supplied rate of As atoms, is achieved when a dynamical equilibrium state is maintained in Ga droplets i.e. the number of impinging As atoms on the droplet surface is equivalent to that of direct deposited Ga atoms combining with the diffusing ones. The contribution of Ga diffusion to the wire growth was evidenced by the diameter-dependent NW axial growth rate

Nanometer Scale Spectral Imaging of Quantum Emitters in nanowires and Its Correlation to Their Atomically Resolved Structure

International audienceWe report the spectral imaging in the UV to visible range with nanometer scale resolu-tion of closely packed GaN/AlN quantum discs in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as one nanometer and separated from each others by only few nanometers, and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disc size and emission wavelength allows us to evidence the quantum confined Stark effect leading to an emission below the bulk GaN band gap for discs thicker than 2.6 nm. Helped with simula-tions, we show that the internal electric field in the studied quantum discs is smaller than what is expected in the quantum well case. We evidence a clear dispersion of the emission wave-lengths of different quantum discs of identical size but different position along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire, and is thus attributed to the related strain variations along the wire. The present work opens the way both for fundamental studies of quantum confinement in closely packed quan-tum emitters and for characterizations of optoelectronic devices presenting carrier localization on the nanometer scale

Nanometer-scale monitoring of the quantum confined stark effect and emission efficiency droop in multiple GaN/AlN quantum disks in nanowires

21 pages, 11 figures, published in PRBInternational audienceWe report on a detailed study of the intensity dependent optical properties of individual GaN/AlN Quantum Disks (QDisks) embedded into GaN nanowires (NW). The structural and optical properties of the QDisks were probed by high spatial resolution cathodoluminescence (CL) in a scanning transmission electron microscope (STEM). By exciting the QDisks with a nanometric electron beam at currents spanning over 3 orders of magnitude, strong non-linearities (energy shifts) in the light emission are observed. In particular, we find that the amount of energy shift depends on the emission rate and on the QDisk morphology (size, position along the NW and shell thickness). For thick QDisks (>4nm), the QDisk emission energy is observed to blue-shift with the increase of the emission intensity. This is interpreted as a consequence of the increase of carriers density excited by the incident electron beam inside the QDisks, which screens the internal electric field and thus reduces the quantum confined Stark effect (QCSE) present in these QDisks. For thinner QDisks (<3 nm), the blue-shift is almost absent in agreement with the negligible QCSE at such sizes. For QDisks of intermediate sizes there exists a current threshold above which the energy shifts, marking the transition from unscreened to partially screened QCSE. From the threshold value we estimate the lifetime in the unscreened regime. These observations suggest that, counterintuitively, electrons of high energy can behave ultimately as single electron-hole pair generators. In addition, when we increase the current from 1 pA to 10 pA the light emission efficiency drops by more than one order of magnitude. This reduction of the emission efficiency is a manifestation of the efficiency droop as observed in nitride-based 2D light emitting diodes, a phenomenon tentatively attributed to the Auger effect

SiOx/Si radial superlattices and microtube optical ring resonators

Scanning and transmission electron microscopy reveal that SiOx/Si layers can roll-up into microtubes and radial superlattices on a Si substrate. These hybrid objects are thermally stable up to 850 C and emit light in the visible spectral range at room temperature. For tubes disengaged from the substrate surface, optically resonant emission with mode spacings inversely proportional to the tube diameter are observed and agree excellently with those obtained from Finite-Different-Time-Domain simulations. The resonant modes we record are strictly polarized along the tube axis.Comment: 3 pages including 4 figure

Visualising highly localised luminescence in GaN/AlN heterostructures in nanowires

The optical properties of a stack of GaN/AlN quantum discs (QDiscs) in a GaN nanowire have been studied by spatially resolved cathodoluminescence (CL) at the nanoscale (nanoCL) using a Scanning Transmission Electron Microscope (STEM) operating in spectrum imaging mode. For the electron beam excitation in the QDisc region, the luminescence signal is highly localized with spatial extension as low as 5 nm due to the high band gap difference between GaN and AlN. This allows for the discrimination between the emission of neighbouring QDiscs and for evidencing the presence of lateral inclusions, about 3 nm thick and 20 nm long rods (quantum rods, QRods), grown unintentionally on the nanowire sidewalls. These structures, also observed by STEM dark-field imaging, are proven to be optically active in nanoCL, emitting at similar, but usually shorter, wavelengths with respect to most QDiscs. This record was migrated from the OpenDepot repository service in June, 2017 before shutting down