GaAs-AlGaAs Core-shell Nanowires Research Articles

Erratum to: On the Luminescence of VLS-grown GaAs-AlGaAs Core-Shell Nanowires and its Dependence on MOVPE Growth Conditions - CORRIGENDUM

In regards to the list of authors on this article [1], the names should appear as follows: Paola Prete, Nico Lovergine, Ilio Miccoli, Fabio Marzo, Jonadan Ando Burger, Giancarlo Salviati and Laura Lazzarini

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs-AlGaAs Core-Shell Nanowires.

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core-shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core-shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65-85 µV K-1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S2 σ ≈ 60 µW m-1 K-2 and S2 G ≈ 0.06 pW K-2 within a single sub-band. Remarkably, these core-shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m-1 K-1 , about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.

Connecting Composition-Driven Faceting with Facet-Driven Composition Modulation in GaAs-AlGaAs Core-Shell Nanowires.

Ternary III-V alloys of tunable bandgap are a foundation for engineering advanced optoelectronic devices based on quantum-confined structures including quantum wells, nanowires, and dots. In this context, core-shell nanowires provide useful geometric degrees of freedom in heterostructure design, but alloy segregation is frequently observed in epitaxial shells even in the absence of interface strain. High-resolution scanning transmission electron microscopy and laser-assisted atom probe tomography were used to investigate the driving forces of segregation in nonplanar GaAs-AlGaAs core-shell nanowires. Growth-temperature-dependent studies of Al-rich regions growing on radial {112} nanofacets suggest that facet-dependent bonding preferences drive the enrichment, rather than kinetically limited diffusion. Observations of the distinct interface faceting when pure AlAs is grown on GaAs confirm the preferential bonding of Al on {112} facets over {110} facets, explaining the decomposition behavior. Furthermore, three-dimensional composition profiles generated by atom probe tomography reveal the presence of Al-rich nanorings perpendicular to the growth direction; correlated electron microscopy shows that short zincblende insertions in a nanowire segment with predominantly wurtzite structure are enriched in Al, demonstrating that crystal phase engineering can be used to modulate composition. The findings suggest strategies to limit alloy decomposition and promote new geometries of quantum confined structures.

Modeling of the growth of GaAs-AlGaAs core-shell nanowires.

Heterostructured GaAs–AlGaAs core–shell nanowires with have attracted much attention because of their significant advantages and great potential for creating high performance nanophotonics and nanoelectronics. The spontaneous formation of Al-rich stripes along certain crystallographic directions and quantum dots near the apexes of the shell are observed in AlGaAs shells. Controlling the formation of these core–shell heterostructures remains challenging. A two-dimensional model valid on the wire cross section, that accounts for capillarity in the faceted surface limit and deposition has been developed for the evolution of the shell morphology and concentration in AlxGa1−xAs alloys. The model includes a completely faceted shell–vapor interface. The objective is to understand the mechanisms of the formation of the radial heterostructures (Al-rich stripes and Al-poor quantum dots) in the nanowire shell. There are two issues that need to be understood. One is the mechanism responsible for the morphological evolution of the shells. Analysis and simulation results suggest that deposition introduces facets not present on the equilibrium Wulff shapes. A balance between diffusion and deposition yields the small facets with sizes varying slowly over time, which yield stripe structures, whereas deposition-dominated growth can lead to quantum-dot structures observed in experiments. There is no self-limiting facet size in this case. The other issue is the mechanism responsible for the segregation of Al atoms in the shells. It is found that the mobility difference of the atoms on the {112} and {110} facets together determine the non-uniform concentration of the atoms in the shell. In particular, even though the mobility of Al on {110} facets is smaller than that of Ga, Al-rich stripes are predicted to form along the {112} facets when the difference of the mobilities of Al and Ga atoms is sufficiently large on {112} facets. As the size of the shell increases, deposition becomes more important. The Al-poor dots are obtained at the apices of {112} facets, if the attachment rate of Al atoms is smaller there.

Effects of VLS and VS mechanisms during shell growth in GaAs-AlGaAs core-shell nanowires investigated by transmission electron microscopy

We investigated by means of transmission electron microscopy (TEM) the final growth stage of GaAs/AlGaAs core-shell nanowires (NWs) self-assembled by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). TEM observations and energy dispersive x-ray spectroscopy revealed the presence of an AlGaAs tapered region of varying chemical composition nearby the NW extreme end (i.e. between the core-shell NW trunk and the Au nanoparticle catalyst). Our findings evidence that this region exhibits an unintentional AlyGa1−yAs/AlxGa1−xAs core-shell structure, a result of the combined axial (vapor-liquid-solid, VLS) self-assembly and conventional (vapor-solid, VS) overgrowth of the material. While the VS-grown AlxGa1−xAs alloy retains the Al composition (x=0.3) of the AlGaAs shell along the NW trunk, the central AlyGa1−yAs section is made of an Al-rich (y≈0.8–0.9) alloy segment formed during AlGaAs shell overgrowth, followed by a graded-alloy segment formed upon deposition of the terminating GaAs cap layer, the latter segment due to the effect of the Al reservoir left in the Au catalyst nanoparticle (NP).

Controlling the exciton emission of gold coated GaAs–AlGaAs core–shell nanowires with an organic spacer layer

Excitons are the most prominent optical excitations and controlling their emission is an important step towards new optical devices. We have investigated the exciton emission from uncoated and gold/aluminum quinoline (Alq3) coated GaAs–AlGaAs–GaAs core–shell nanowires (NWs) using temperature-, intensity- and polarization dependent photoluminescence (PL). Plasmonic GaAs–AlGaAs–GaAs NWs with a ∼10 nm thick Au coating but without an Alq3 spacer layer reveal a significant reduction of the PL intensity of the exciton emission compared with the uncoated NW sample. Plasmonic NW samples with the same nominal Au coverage and an additional Alq3 interlayer of 3 or 6 nm thickness show a clearly stronger PL intensity which increases with rising Alq3 spacer thickness. Time-resolved (TR) PL measurements reveal an increase of the exciton decay rate by a factor of up to two with decreasing Alq3 spacer thickness suggesting the presence of Förster energy transfer from NW excitons to plasmon oscillations in the gold film. The weak change of the decay time, however, indicates that Förster energy-transfer is only partially responsible for the PL quenching in the gold coated NWs. The main reason for the reduction of the PL emission is attributed to a gold induced band-bending in the GaAs NW core which causes exciton dissociation. With increasing Alq3 spacer thickness the band-bending decreases leading to a reduction of the exciton dissociation and PL quenching. Our interpretation is supported by electron energy loss spectroscopy measurements which show a signal reduction and blue shift of defect (possibly EL2) transitions when gold particles are deposited on NWs compared with bare or Alq3 coated NWs.

Suppression of alloy fluctuations in GaAs-AlGaAs core-shell nanowires

Probing localized alloy fluctuations and controlling them by growth kinetics have been relatively limited so far in nanoscale structures such as semiconductor nanowires (NWs). Here, we demonstrate the tuning of alloy fluctuations in molecular beam epitaxially grown GaAs-AlGaAs core-shell NWs by modifications of shell growth temperature, as investigated by correlated micro-photoluminescence, scanning transmission electron microscopy, and atom probe tomography. By reducing the shell growth temperature from T > 600 °C to below 400 °C, we find a strong reduction in alloy fluctuation mediated sharp-line luminescence, concurrent with a decrease in the non-randomness of the alloy distribution in the AlGaAs shell. This trend is further characterized by a change in the alloy compositional structure from unintentional quasi-superlattices of Ga- and Al-rich AlGaAs layers at high T to a nearly homogeneous random alloy distribution at low T.

A Nanowire-Based Plasmonic Quantum Dot Laser.

Quantum dots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and high-temperature stability when used as a gain medium. However, quantum dot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantum dot-based plasmonic laser to reduce the cross-sectional area of nanowire quantum dot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs-AlGaAs core-shell nanowires containing InGaAs quantum dot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantum dot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 μJ/cm(2) was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantum dot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantum dot lasers can be realized with plasmonics.

Continuous wave lasing from individual GaAs-AlGaAs core-shell nanowires

We demonstrate single-mode continuous wave (cw) lasing from individual GaAs-AlGaAs core-shell nanowires (NWs) subject to optical excitation. By comparing the s-shaped input-output characteristics of cw-excited NW lasers with those obtained under pulsed excitation at 4 K, we observe a ∼4× lower equivalent pump power for cw-excitation and a lower minimum lasing linewidth of ∼200 μeV, indicative of the steady-state excitation conditions that result in improved temporal coherence of the emission. Analysis of the NW cavity length dependence of the mode characteristics reveals a clear inverse scaling behavior, with the spacing of Fabry–Perot modes corresponding to a group refractive index of ∼8. Remarkably, when subject to cw excitation heating of the NW-lasers is found to be negligible, as verified by a constant lasing linewidth as well as the absence of red-shifted lasing peak emission at high excitation levels.

Crystal Phase Quantum Dots in the Ultrathin Core of GaAs-AlGaAs Core-Shell Nanowires.

Semiconductor quantum dots embedded in nanowires (NW-QDs) can be used as efficient sources of nonclassical light with ultrahigh brightness and indistinguishability, needed for photonic quantum information technologies. Although most NW-QDs studied so far focus on heterostructure-type QDs that provide an effective electronic confinement potential using chemically distinct regions with dissimilar electronic structure, homostructure NWs can localize excitons at crystal phase defects in leading to NW-QDs. Here, we optically investigate QD emitters embedded in GaAs-AlGaAs core-shell NWs, where the excitons are confined in an ultrathin-diameter NW core and localized along the axis of the NW core at wurtzite (WZ)/zincblende (ZB) crystal phase defects. Photoluminescence (PL)-excitation measurements performed on the QD-emission reveal sharp resonances arising from excited electronic states of the axial confinement potential. The QD-like nature of the emissive centers are suggested by the observation of a narrow PL line width, as low as ~300 μeV, and confirmed by the observation of clear photon antibunching in autocorrelation measurements. Most interestingly, time-resolved PL measurements reveal a very short radiative lifetime <1 ns, indicative of a transition from a type-II to type-I band alignment of the WZ/ZB crystal interface in GaAs due to the strong quantum confinement in the ultrathin NW core.

Alloy Fluctuations Act as Quantum Dot-like Emitters in GaAs-AlGaAs Core-Shell Nanowires.

GaAs-AlxGa1-xAs (AlGaAs) core-shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core-shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters.

Low-Threshold near-Infrared GaAs–AlGaAs Core–Shell Nanowire Plasmon Laser

We demonstrate plasmonic lasing from metal–organic chemical vapor deposition (MOCVD)-grown GaAs–AlGaAs core–shell nanowires (NWs) with subdiffraction limit diameters of ∼150 nm placed directly on a silver thin film. The absence of a low-index dielectric spacer layer between the NW and the metal layer allows for surface plasmon polariton (SPP) lasing using a nonhybridized plasmonic mode. Unlike previously reported plasmonic NW lasers using the fundamental SPP mode, we demonstrate for the first time plasmonic NW lasing under pulsed optical excitation by using the higher order SPP mode. The higher order mode allows us to alleviate the high losses associated with the fundamental plasmonic mode. We observed lasing at temperatures up to 125 K. Our demonstration of a plasmonic laser based on GaAs emitting in the near-infrared region will be useful for the on-chip integration of nanophotonic and electronic devices and the development of GaAs-based plasmonic devices.

Nanoscale three-dimensional reconstruction of electric and magnetic stray fields around nanowires

Static electromagnetic stray fields around nanowires (NWs) are characteristic for a number of important physical effects such as field emission or magnetic force microscopy. Consequently, an accurate characterization of these fields is of high interest and electron holographic tomography (EHT) is unique in providing tomographic 3D reconstructions at nm spatial resolution. However, several limitations of the experimental setup and the specimen itself are influencing EHT. Here, we show how a deliberate restriction of the tomographic reconstruction to the exterior of the NWs can be used to mitigate these limitations facilitating a quantitative 3D tomographic reconstruction of static electromagnetic stray fields at the nanoscale. As an example, we reconstruct the electrostatic stray field around a GaAs-AlGaAs core shell NW and the magnetic stray field around a Co2FeGa Heusler compound NW.

Radio frequency occupancy state control of a single nanowire quantum dot

The excitonic occupancy state of a single, nanowire-based, heterostructure quantum dot is dynamically programmed by a surface acoustic wave. The quantum dot is formed by an interface or thickness fluctuation of a GaAs QW embedded in a AlGaAs shell of a GaAs–AlGaAs core–shell nanowire. As we tune the time at which carriers are photogenerated during the acoustic cycle, we find pronounced intensity oscillations of neutral and negatively charged excitons. At high acoustic power levels these oscillations become anticorrelated which enables direct acoustic programming of the dot's charge configuration, emission intensity and emission wavelength. Numerical simulations confirm that the observed modulations arise from acoustically controlled modulations of the electron and electron–hole pair concentrations at the position of the quantum dot.

Lasing from individual GaAs-AlGaAs core-shell nanowires up to room temperature

Semiconductor nanowires are widely considered to be the next frontier in the drive towards ultra-small, highly efficient coherent light sources. While NW lasers in the visible and ultraviolet have been widely demonstrated, the major role of surface and Auger recombination has hindered their development in the near infrared. Here we report infrared lasing up to room temperature from individual core-shell GaAs-AlGaAs nanowires. When subject to pulsed optical excitation, NWs exhibit lasing, characterized by single-mode emission at 10 K with a linewidth <60 GHz. The major role of non-radiative surface recombination is obviated by the presence of an AlGaAs shell around the GaAs-active region. Remarkably low threshold pump power densities down to ~760 W cm(-2) are observed at 10 K, with a characteristic temperature of T(0)=109±12 K and lasing operation up to room temperature. Our results show that, by carefully designing the materials composition profile, high-performance infrared NW lasers can be realised using III/V semiconductors.

Probing the trapping and thermal activation dynamics of excitons at single twin defects in GaAs–AlGaAs core–shell nanowires

Time resolved and time-integrated photoluminescence (PL) spectroscopy is used to investigate the trapping and thermal activation dynamics of excitons bound to single twin defects in individual GaAs–AlGaAs core–shell nanowires. The GaAs core exhibits two distinct spectral emission features that are attributed to free and bound excitons. Time resolved measurements reveal lifetimes of and for the free and bound excitons, respectively. For temperatures above 30 K, the global PL intensity is quenched due to non-radiative carrier recombination. In contrast, for temperatures below 20 K we observe clear evidence for thermal detrapping of bound excitons into the continuum. By comparing the time-resolved PL spectra with a two-level rate equation model, quantitative values are obtained for both the exciton trapping and detrapping rates. Our data is consistent with a temperature independent exciton trapping rate >20 GHz that dominates the population dynamics of bound excitons at T = 6 K. At elevated temperature, the detrapping rate is found to adhere to a thermally activated behavior characterized by a thermal activation energy of , very close to the energy spacing of bound and free exciton emission features. This observation implies that bound excitons are thermally activated into the excitonic continuum without the need to overcome additional energetic barriers.

Built‐in elastic strain and localization effects on GaAs luminescence of MOVPE‐grown GaAs–AlGaAs core–shell nanowires

Abstractmagnified imageThe core photoluminescence emission of MOVPE‐grown GaAs–Al0.33Ga0.67As core–shell nanowires is studied as function of the relevant geometrical parameter of these nanostructures, namely the shell‐thickness to core‐radius ratio hs/Rc. The energy of the dominant emission peak was compared with values of the GaAs heavy‐ and light‐hole excitons redshifted by a uniaxial tensile strain, the latter calculated assuming perfect coherence at the core/shell interface and elastic energy equilibrium within the nanowires. Good agreement is obtained for hs/Rc &lt; 1, the intrinsic strain‐free excitonic emission being identified at 1.510 eV, and further ascribed to bound heavy‐hole excitons. For hs/Rc &gt; 1 increasingly larger redshifts (up to ∼9 meV in excess of values calculated based on our elastic strain model) are observed, and tentatively ascribed to shell‐dependent exciton localization effects.magnified imageExperimental and calculated bound exciton peak energies for GaAs–Al0.33Ga0.67As core–shell nanowires as function of their shell‐thickness to core‐radius ratio hs/Rc. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spontaneous Alloy Composition Ordering in GaAs-AlGaAs Core–Shell Nanowires

By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 μm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.

Terahertz emission from GaAs-AlGaAs core-shell nanowires on Si (100) substrate: Effects of applied magnetic field and excitation wavelength

Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) substrates was investigated using THz time-domain spectroscopy. The applied magnetic field polarity dependence strongly suggests that THz emission originated from photo-carriers in the CSNWs. Optical excitation of the GaAs-AlGaAs core-shell yielded a wider THz emission bandwidth compared with that of just the GaAs core material. This result is currently attributed to faster carrier lifetimes in the AlGaAs shell. The THz emission spectral data are supported by time-resolved photoluminescence studies.

Electron holographic tomography for mapping the three-dimensional distribution of electrostatic potential in III-V semiconductor nanowires

Electron holographic tomography (EHT), the combination of off-axis electron holography with electron tomography, is a technique, which can be applied to the quantitative 3-dimensional (3D) mapping of electrostatic potential at the nanoscale. Here, we show the results obtained in the EHT investigation of GaAs and GaAs-AlGaAs core-shell nanowires grown by Au-catalysed metalorganic vapor phase epitaxy. The unique ability of EHT of disentangling the materials mean inner potential (MIP) from the specimen projected thickness allows reconstruction of the nanowire 3D morphology and inner compositional structure as well as the measurement of the MIP.