Excitonic Diffusion in InGaN/GaN Core-Shell Nanowires.

We report on the direct observation of the diffusion of carriers in graded InGaN/GaN quantum wells in a nanowire. By probing the local dynamics at the nanoscale, along the wire for different temperatures between 4 and 250 K, we conclude that this diffusion process is thermally activated. In addition, the analysis of the cathodoluminescence lifetime for different temperatures shows that the carrier motion is isotropic and does not follow the indium gradient. Our observations are interpreted in terms of a hopping process between localized states. We find that the random alloy fluctuations prevent any directional drift of excitons along the In gradient and therefore any carrier accumulation. Our results therefore confirm the potential of core-shell nanowires for lighting devices. Indeed, the short lifetime of m-plane quantum wells together with their large active area and the homogeneous distribution of carrier along the quantum well will decrease influence of any high carrier density effect on the efficiency of these light-emitting diodes.

InGaN/GaN core-shell single nanowire light emitting diodes with graphene-based p-contact.

We report on the demonstration of MOVPE-grown single nanowire InGaN/GaN core-shell light emitting diodes (LEDs) with a transparent graphene contact for hole injection. The electrical homogeneity of the graphene-contacted LED has been assessed by electron beam induced current microscopy. By comparing graphene-contacted and metal-contacted nanowire LEDs, we show that the contact layout determines the electroluminescence spectrum. The electroluminescence changes color from green to blue with increasing injection current. High-resolution cathodoluminescence on cleaved nanowires allows the location with high precision of the origin of different emitted wavelengths and demonstrates that the blue peak originates from the emission of the radial quantum well on the m-planes, whereas the green peak arises from the In-rich region at the junction between the m-planes and the semipolar planes. The spectral behavior of the electroluminescence is understood by modeling the current distribution within the nanowire.

Integrated photonic platform based on InGaN/GaN nanowire emitters and detectors.

We report the fabrication of a photonic platform consisting of single wire light-emitting diodes (LED) and photodetectors optically coupled by waveguides. MOVPE-grown (metal-organic vapor-phase epitaxy) InGaN/GaN p-n junction core-shell nanowires have been used for device fabrication. To achieve a good spectral matching between the emission wavelength and the detection range, different active regions containing either five narrow InGaN/GaN quantum wells or one wide InGaN segment were employed for the LED and the detector, respectively. The communication wavelength is ∼400 nm. The devices are realized by means of electron beam lithography on Si/SiO2 templates and connected by ∼100 µm long nonrectilinear SiN waveguides. The photodetector current trace shows signal variation correlated with the LED on/off switching with a fast transition time below 0.5 s.

Optical properties of GaN-based nanowires containing a single Al(0.14)Ga(0.86)N/GaN quantum disc.

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.

Visualizing highly localized 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 extent as low as 5 nm, due to the high band gap difference between GaN and AlN. This allows the discrimination between the emission of neighbouring QDiscs and 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 proved to be optically active in nanoCL, emitting at similar, but usually shorter, wavelengths with respect to most QDiscs.

Photovoltaic properties of GaAsP core-shell nanowires on Si(001) substrate.

We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs0.88P0.12 in the nanowire core and GaAs0.76P0.24 in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires.

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (µ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.

Ultraviolet photodetector based on GaN/AlN quantum disks in a single nanowire.

We report the demonstration of single-nanowire photodetectors relying on carrier generation in GaN/AlN QDiscs. Two nanowire samples containing QDiscs of different thicknesses are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The responsivity of a single wire QDisc detector is as high as 2 x 10(3) A/W at lambda = 300 nm at room temperature. We show that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanowires and enhances the photosensitivity factor (i.e., the ratio between the photocurrent and the dark current) up to 5 x 10(2) for an incoming light intensity of 5 mW/cm(2). Photocurrent spectroscopy allows identification of the spectral contribution related to carriers generated within large QDiscs, which lies below the GaN band gap due to the quantum confined Stark effect.

Core-shell InGaN/GaN nanowire light emitting diodes analyzed by electron beam induced current microscopy and cathodoluminescence mapping.

We report on the electron beam induced current (EBIC) microscopy and cathodoluminescence (CL) characterization correlated with compositional analysis of light emitting diodes based on core/shell InGaN/GaN nanowire arrays. The EBIC mapping of cleaved fully operational devices allows to probe the electrical properties of the active region with a nanoscale resolution. In particular, the electrical activity of the p-n junction on the m-planes and on the semi-polar planes of individual nanowires is assessed in top view and cross-sectional geometries. The EBIC maps combined with CL characterization demonstrate the impact of the compositional gradients along the wire axis on the electrical and optical signals: the reduction of the EBIC signal toward the nanowire top is accompanied by an increase of the CL intensity. This effect is interpreted as a consequence of the In and Al gradients in the quantum well and in the electron blocking layer, which influence the carrier extraction efficiency. The interface between the nanowire core and the radially grown layer is shown to produce in some cases a transitory EBIC signal. This observation is explained by the presence of charged traps at this interface, which can be saturated by electron irradiation.