MOVPE Self-Assembly and Physical Properties of Free-Standing III-V Nanowires

none2noThe different approaches being currently explored for the fabrication of free-standing quasi-1D nanostructures of III-V compound semiconductors utilizing the MOVPE technology are reviewed: major limitations and advantages are discussed. In particular, we focus on the self-assembly of semiconductor nanowires by the so-called metal-catalyst assisted ― or VLS ― mechanism. The latter is currently considered a most promising technology for the realization of high quality quasi-1D nanostructures. Examples of this approach are given based on results obtained in the author’s laboratory using low pressure MOVPE to growth nanowire structures of III-As compounds.Il Volume è OPEN ACCESS e può essere consultato e scaricato in formato elettronico all'indirizzo: http://sciyo.com/books/show/title/nanowires#openP. Prete; N. LOVERGINEPrete, Paola; Lovergine, Nicol

Enhanced Optical Absorption of GaAs Near-Band-Edge Transitions in GaAs/AlGaAs Core–Shell Nanowires: Implications for Nanowire Solar Cells

Dense arrays of core–shell nanowires possess great potential as superabsorptive media for the fabrication of efficient solar cells. We report on GaAs near-band-edge absorption properties of free-standing GaAs–AlGaAs core–shell nanowires having different shell thicknesses, by detailed line-shape analyses of room-temperature photoreflectance (PR) spectra, employing first-derivative Gaussian and Lorentzian models of the GaAs complex dielectric function. Line-shape analyses of the nanowire PR spectra returned a doublet of resonance lines at energies between 1.410 and 1.422 eV, ascribed to strain-split heavy- and light-hole exciton absorption transitions in the GaAs nanowire cores. The optical oscillator strengths of exciton resonances evaluated by Lorentzian analyses of PR features showed a significant enhancement (up to 30×) of GaAs band-edge optical absorption in nanowires with respect to the reference planar structure. Additionally, values of integrated Lorentzian moduli were normalized to the total GaAs core volume fill fraction (estimated in the range 0.5–7.0% with respect to a planar layer of the same height) within each nanowire ensemble, achieving a first ever experimental estimate of the GaAs near band-edge absorption enhancement factor for GaAs–AlGaAs core–shell nanowires in the range 22–190, depending on the nanowire inner core–shell structure. Such strong absorption enhancement is ascribed to improved wave-guiding of incident light into the GaAs cores by the surrounding AlGaAs shell (its average thickness being estimated between ∼14 and 100 nm in the present nanostructures)

Synthesis of vertically-aligned GaAs nanowires on GaAs/(111)Si heterosubstrates by metalorganic vapour phase epitaxy

We report on the Au-catalysed synthesis of GaAs nanowires on hetero-structured GaAs/(111)Si substrates by metalorganic vapour phase epitaxy. It is demonstrated that the deposition of a 40-50 nm thin GaAs epilayer onto Si guarantees a high percentage of straight and vertically-aligned GaAs nanowires. GaAs epilayers were grown at 400 °C and subsequently annealed at 700 °C. Growth experiments performed on 4°-miscut and exactly-oriented (111)Si substrates show that a higher yield (close to 90%) of vertical nanowires is obtained using miscut substrates, an effect ascribed to the smoother surface morphology of GaAs epilayers on these substrates. Comparison between the cross-sectional shape of nanowires grown on GaAs/(111)Si heterosubstrates and those on (111)A-GaAs and (111)B-GaAs substrates demonstrates that both GaAs epilayers and over-grown nanowires are (111)B-oriented

On the use of temporal series of L-and X-band SAR data for soil moisture retrieval. Capitanata plain case study

This paper investigates the use of time series of ALOS/PALSAR-1 and COSMO-SkyMed data for the soil moisture retrieval (mv) by means of the SMOSAR algorithm. The application context is the exploitation of mv maps at a moderate spatial and temporal resolution for improving flood/drought monitoring at regional scale. The SAR data were acquired over the Capitanata plain in Southern Italy, over which ground campaigns were carried out in 2007, 2010 and 2011. The analysis shows that the mv retrieval accuracy is 5%-7% m^3/m^3 at L- and X band, although the latter is restricted to a use over nearly bare soil only

Tunable hot-electron transfer within a single core-shell nanowire

We report the hot photoexcited electron transfer across the coaxial interface of a cylindrical core-shell nanowire. Modulation of the transfer rates, manifested as a large tunability of the voltage onset of negative differential resistance and of voltage-current phase, is achieved using three different modes. The coupling of electrostatic gating, incident photon energy, and the incident photon intensity to transfer rates is facilitated by the combined influences of geometric confinement and heterojunction shape on hot-electron transfer, and by electron-electron scattering rates that can be altered by varying the incident photon flux, with evidence of weak electron-phonon scattering. Dynamic manipulation of this transfer rate permits the introduction and control of a continuously adjustable phase delay of up to about 130° within a single nanometer-scale device element

PRIN 2004: "Crescita e proprietà di nanocristalli quasi-unidimensionali di ossidi semiconduttori"

Il Progetto mira a: 1) sviluppare metodi di crescita auto-organizzata/nanofabbricazione di nanocristalli quasi 1-dimensionali (1D) di ossidi metallici semiconduttori a larga gap (ZnO, MgO, SnO2, TiO2, ecc.); 2) comprendere i meccanismi che ne sovrintendono la crescita; 3) studiare gli effetti di ridotta dimensionalità sulle proprietà elettroniche, vibrazionali ed ottiche; 4) chiarire il ruolo di specie gassose reattive sulla ricombinazione radiativa di nanocristalli quasi-1D. 1) crescita auto-organizzata/nanofabbricazione di nanocristalli quasi 1D ed arrays: Studio dell’auto-organizzazione di nanocristalli e strutture quasi-1D: i) nanorod epitassiali di ZnO, MgO, e ZnMgO; ii) insiemi (bundles) di nanowire/nanobelt casualmente orientati di SnO2 e ossidi metallici similari; iii) array periodici di nanorod epitassiali. Sarà dapprima studiata la crescita di nanocristalli quasi-1D mediante metodi di trasporto chimico da fase vapore; saranno quindi identificati dei precursori per il passaggio ai metodi CVD. L’auto-organizzazione di nanocristalli quasi-1D di ossidi semiconduttori sarà studiata impiegando sia la crescita assistita da catalizzatore metallico (meccanismo vapore-liquido-solido, VLS), sia la crescita in assenza di catalizzatore (meccanismo vapore-solido). ‘Array’ periodici di nanorod saranno realizzati via CVD e nano-fabbricazione da fascio ionico focalizzato (FIB), quest’ultimo per ottenere il controllo della posizione e delle dimensioni del catalizzatore nella crescita VLS 2) modelli termo/fluodinamici e atomistici di auto-organizzazione di nanocristalli quasi-1D: Saranno sviluppati modelli termo-/fluido-dinamici per predire la composizione e la supersaturazione del vapore e la velocità del trasporto di massa durante i processi CVD. Lo sviluppo di tali modelli sarà basato su dati ottenuti dallo studio delle fasi vapore con spettrometria di massa. Saranno considerati modelli atomistici per investigare i meccanismi di crescita e nucleazione quasi-1D. Questi modelli saranno applicati sia a nanowire/nanobelt non orientati che ai nanorod epitassiali. In quest’ultimo caso, sarà considerata la presenza del catalizzatore durante crescita VLS. Le previsioni dei modelli atomistici saranno confrontate con i risultati delle analisi morfologiche 3) proprietà elettroniche, vibrazionali e ottiche di nanocristalli quasi-1D ed array: La risposta dielettrica anisotropa dei nanorod sarà determinata utilizzando tecniche ottiche sensibili alla polarizzazione. Gli stati elettronici saranno studiati con tecniche spettroscopiche convenzionali, concentrandosi sull’emissione di singoli (o pochi) nanorod/nanobelt. Questo permetterà di localizzare i livelli di energia elettronici e paragonarli con le previsioni teoriche. Le proprietà vibrazionali, studiate mediante µ-Raman e scattering Raman risonante, consentiranno di chiarire gli effetti della bassa dimensionalità sulle costanti di forza interatomica. Saranno determinate le proprietà ottiche non lineari e le dinamiche di rilassamento dei portatori fotoeccitati, effettuando esperimenti di spettroscopia ultra-veloce. Ciò consentirà di determinare i tempi di rilassamento, misurare il guadagno ottico e la banda spettrale dei nanorod. Infine, misure di dispersione angolare di luce in array periodici di nanorod consentiranno di studiare le loro proprietà di confinamento fotonico 4) proprietà radiative di nanocristalli quasi-1D in ambienti gassosi controllati: Si studieranno i canali di ricombinazione radiativa e non radiativa per determinare le energie di attivazione di difetti superficiali. Sarà investigata la presenza di bande di foto-assorbimento, insieme alle modifiche della luminescenza dei nanorod/nanobelt causate da interazioni gas-superficie in condizioni ambientali controllate. Tali effetti saranno studiati in funzione delle dimensioni e dei trattamenti

Nanocharacterization of nanowires, nanomagnets and laser diodes for sensors, optoelectronics and data storage

Progetto Bilaterale di Grande Rilevanza Italia-Giappone (finanziato dal Ministero degli Esteri)

DEVELOPMENT OF NOVEL OPTICAL TECHNIQUES AND DEVICES FOR NON-DESTRUCTIVE CHARACTERIZATION OF SEMICONDUCTING COMPOUNDS AND STRUCTURES (SfP 974476)

NATO 'Science for Peace' Project (Years 2000-2005). Scope: A principal goal of the project was the development of novel optical (holographic) measurement techniques and their implementation into a novel diagnostic devices for nondestructive characterization of semiconducting crystals and structures. It was foreseen to develop the technique of dynamic holography, exploring nonresonant mechanisms of optical nonlinearities in a photoexcited semiconductor. The developed techniques and the prototypes of device were to be adapted for diagnostics of semiconductor materials, important for industry and market of semiconductor electronics and optoelectronics

Shape, size evolution and nucleation mechanisms of GaAs nanoislands grown on (111)Si by low temperature metalorganic vapor phase epitaxy

The shape, size evolution, and nucleation mechanisms of GaAs nanoislands grown at 400 degrees C on As-stabilized (111)Si by metal-organic vapor phase epitaxy are reported for the first time. GaAs crystallizes in the zincblende phase in the very early nucleation stages until a continuous epilayer is formed. GaAs nanoislands grow (111)-oriented on Si as truncated hexagonal pyramids, bound by six equivalent {120} side facets and a (111) facet at the top. Their diameter and height appear to increase linearly with the deposition time, yielding a constant aspect ratio of similar to 1/4. The nanoisland density (before coalescence) stays constant with time at similar to 2 x 10^(10) cm(-2), suggesting that the nucleation occurs at specific Si surface sites (defects) during the very early growth stages, rather than being due to the continuous formation of new nuclei. To understand the molecular-level mechanisms driving the low-temperature MOVPE growth of GaAs on Si, we applied a deposition-diffusion-aggregation (DDA) nucleation model, which predicts a linear evolution of the overall GaAs growth rate with surface coverage, in good agreement with experimental observations, under the assumption that direct impingement of trimethylgallium (Me3Ga) molecules onto the nanoisland surface dominates the material nucleation and growth rate; the contribution of Me3Ga adsorbed onto the As-stabilized (111)Si is negligible, pointing out the reduced reactivity of the Si surface (As passivation). Our DDA model allows estimation of the effective reactive sticking coefficient of Me3Ga onto GaAs, which is equal to 2.82 X 10^(-5) : the small value is compatible with the Me3Ga large steric hindrance and the competitive role of methyl radicals in surface adsorption at low temperature

Correlating spectroscopic and nanoscale structural properties in quantum well tubes III-V nanowires

We report on a combined spectroscopic/structural study of MOVPE-grown GaAs-AlGaAs core-multishell nanowires, containing thin GaAs quantum well tubes (QWTs) wrapped around the central GaAs core. Low temperature (7K) cathodoluminescence (CL) spectroscopic imaging combined with Z-contrast scanning transmission electron microscopy (STEM) tomography performed on single core-multishell nanowires allowed robust correlation between QWT emission and the nanowire inner structure down to the nano-scale. Besides the core luminescence and minor defects-related contributions, each nanowire showed one or more QWT peaks in the 1.53-1.65 eV spectral region, which correlated with sections of the nanowire trunk having different diameters. Average values of QWT thickness (in the 3-7 nm range) were thus extracted from measured nanowire diameter through the application of a multishell growth model, the latter validated against experimental data (core/nanowire diameter, shell thicknesses) obtained from 3-dimensional (3D) reconstructed STEM tomograms of single QWT nanowires. Our data evidenced that the QWT emissions appear redshifted (by about 40-120 meV) with respect to values expected for uniform QWTs of the same thickness. CL mapping evidenced nanoscale localization of QWT exciton emissions along the nanowire, demonstrating that their emission is affected by carrier localization at confinement-potential inhomogeneity. The latter have been ascribed to azimuthal asymmetries as well as (azimuthal and axial) random fluctuations of the GaAs QWT thickness within each nanowire, as evidenced by detailed statistical analysis of the 3D tomograms