Internal quantum efficiency of AlGaN/AlN quantum dot superlattices for electron-pumped ultraviolet sources.

In this paper, we describe the growth and characterization of ≈530 nm thick superlattices (100 periods) of AlxGa1-xN/AlN (0 ⩽ x ⩽ 0.1) Stranski-Krastanov quantum dots (QDs) for application as the active region of electron-beam pumped ultraviolet lamps. Highly dense (>1011 cm-2) QD layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance. The study considers structures emitting in the 244-335 nm range at room temperature, with a relative linewidth in the 6%-11% range, mainly due to the QD diameter dispersion inherent in self-assembled growth. Under electron pumping, the emission efficiency remains constant for acceleration voltages below ≈9 kV. The correlation of this threshold with the total thickness of the SL and the penetration depth of the electron beam confirms the homogeneity of the nanostructures along the growth axis. Below the threshold, the emission intensity scales linearly with the injected current. The internal quantum efficiency (IQE) is characterized at low injection, which reveals the material properties in terms of non-radiative processes, and high injection, which emulates carrier injection in operation conditions. In QDs synthesized with III/V ratio <0.75, the IQE remains around 50% from low injection to pumping power densities as high as 200 kW cm-2, being the first kind of nanostructure that present such stable behaviour.

Assessment of AlGaN/AlN superlattices on GaN nanowires as active region of electron-pumped ultraviolet sources.

In this paper, we describe the design and characterization of 400 nm long (88 periods) Al x Ga1-x N/AlN (0 ≤ x ≤ 0.1) quantum dot superlattices deposited on self-assembled GaN nanowires for application in electron-pumped ultraviolet sources. The optical performance of GaN/AlN superlattices on nanowires is compared with the emission of planar GaN/AlN superlattices with the same periodicity and thickness grown on bulk GaN substrates along the N-polar and metal-polar crystallographic axes. The nanowire samples are less sensitive to nonradiative recombination than planar layers, attaining internal quantum efficiencies (IQE) in excess of 60% at room temperature even under low injection conditions. The IQE remains stable for higher excitation power densities, up to 50 kW cm-2. We demonstrate that the nanowire superlattice is long enough to collect the electron-hole pairs generated by an electron beam with an acceleration voltage V A = 5 kV. At such V A, the light emitted from the nanowire ensemble does not show any sign of quenching under constant electron beam excitation (tested for an excitation power density around 8 kW cm-2 over the scale of minutes). Varying the dot/barrier thickness ratio and the Al content in the dots, the nanowire peak emission can be tuned in the range from 340 to 258 nm.

Effect of Bias on the Response of GaN Axial p-n Junction Single-Nanowire Photodetectors.

We present a comprehensive study of the performance of GaN single-nanowire photodetectors containing an axial p-n junction. The electrical contact to the p region of the diode is made by including a p+/n+ tunnel junction as cap structure, which allows the use of the same metal scheme to contact both ends of the nanowire. Single-nanowire devices present the rectifying current-voltage characteristic of a p-n diode but their photovoltaic response to ultraviolet radiation scales sublinearly with the incident optical power. This behavior is attributed to the dominant role of surface states. Nevertheless, when the junction is reverse biased, the role of the surface becomes negligible in comparison to the drift of photogenerated carriers in the depletion region. Therefore, the responsivity increases by about 3 orders of magnitude and the photocurrent scales linearly with the excitation. These reverse-biased nanowires display decay times in the range of ∼10 µs, limited by the resistor-capacitor time constant of the setup. Their ultraviolet/visible contrast of several orders of magnitude is suitable for applications requiring high spectral selectivity. When the junction is forward biased, the device behaves as a GaN photoconductor with an increase of the responsivity at the price of a degradation of the time response. The presence of leakage current in some of the wires can be modeled as a shunt resistance which reacts to the radiation as a photoconductor and can dominate the response of the wire even under reverse bias.

Shot-Noise-Limited Nanomechanical Detection and Radiation Pressure Backaction from an Electron Beam.

Detecting nanomechanical motion has become an important challenge in science and technology. Recently, electromechanical coupling to focused electron beams has emerged as a promising method adapted to ultralow scale systems. However the fundamental measurement processes associated with such complex interaction remain to be explored. Here we report a highly sensitive detection of the Brownian motion of µm-long semiconductor nanowires (InAs). The measurement imprecision is found to be set by the shot noise of the secondary electrons generated along the electromechanical interaction. By carefully analyzing the nanoelectromechanical dynamics, we demonstrate the existence of a radial backaction process that we identify as originating from the momentum exchange between the electron beam and the nanomechanical device, which is also known as radiation pressure.

Probing the light hole/heavy hole switching with correlated magneto-optical spectroscopy and chemical analysis on a single quantum dot.

A whole series of complementary studies have been performed on the same single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive x-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si3N4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn, Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

Strained GaAs/InGaAs Core-Shell Nanowires for Photovoltaic Applications.

We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 10(8) cm(-2), length between 3 and 3.5 µm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900-1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.

Comparison of Three E-Beam Techniques for Electric Field Imaging and Carrier Diffusion Length Measurement on the Same Nanowires.

Whereas nanowire (NW)-based devices offer numerous advantages compared to bulk ones, their performances are frequently limited by an incomplete understanding of their properties where surface effect should be carefully considered. Here, we demonstrate the ability to spatially map the electric field and determine the exciton diffusion length in NW by using an electron beam as the single excitation source. This approach is performed on numerous single ZnO NW Schottky diodes whose NW radius vary from 42.5 to 175 nm. The dominant impact of the surface on the NW properties is revealed through the comparison of three different physical quantities recorded on the same NW: electron-beam induced current, cathodoluminescence, and secondary electron signal. Indeed, the space charge region near the Schottky contact exhibits an unusual linear variation with reverse bias whatever the NW radius. On the contrary, the exciton diffusion length is shown to be controlled by the NW radius through surface recombination. This systematic comparison performed on a single ZnO NW demonstrates the power of these complementary techniques in understanding NW properties.

Effect of doping on the far-infrared intersubband transitions in nonpolar m-plane GaN/AlGaN heterostructures.

This paper assesses the effects of Si doping on the properties of nonpolar m-plane GaN/AlGaN quantum wells (QWs) designed for intersubband (ISB) absorption in the far-infrared spectral range. For doping levels up to 3 × 10(12) cm(-2), structural analysis reveals uniform QWs with abrupt interfaces and no epitaxially induced defects. Cathodoluminescence spectroscopy confirms the homogeneity of the multiple QWs along the growth direction. Increasing the doping density in the QWs from 1 × 10(11) cm(-2) to 3 × 10(12) cm(-2) induces a broadening of the photoluminescence as well as a reduction of the exciton localization energy in the alloy. Also, enhancement of the ISB absorption is observed, along with a blue shift and widening of the absorption peak. The magnitude of the ISB absorption saturates for doping levels around 1 × 10(12) cm(-2), and the blue shift and broadening increase less than theoretically predicted for the samples with higher doping levels. This is explained by the presence of free carriers in the excited electron level due to the increase of the Fermi level energy.

Direct imaging of p-n junction in core-shell GaN wires.

While core-shell wire-based devices offer a promising path toward improved optoelectronic applications, their development is hampered by the present uncertainty about essential semiconductor properties along the three-dimensional (3D) buried p-n junction. Thanks to a cross-sectional approach, scanning electron beam probing techniques were employed here to obtain a nanoscale spatially resolved analysis of GaN core-shell wire p-n junctions grown by catalyst-free metal-organic vapor phase epitaxy on GaN and Si substrates. Both electron beam induced current (EBIC) and secondary electron voltage constrast (VC) were demonstrated to delineate the radial and axial junction existing in the 3D structure. The Mg dopant activation process in p-GaN shell was dynamically controlled by the ebeam exposure conditions and visualized thanks to EBIC mapping. EBIC measurements were shown to yield local minority carrier/exciton diffusion lengths on the p-side (∼57 nm) and the n-side (∼15 nm) as well as depletion width in the range 40-50 nm. Under reverse bias conditions, VC imaging provided electrostatic potential maps in the vicinity of the 3D junction from which acceptor Na and donor Nd doping levels were locally determined to be Na = 3 × 10(18) cm(-3) and Nd = 3.5 × 10(18) cm(-3) in both the axial and the radial junction. Results from EBIC and VC are in good agreement. This nanoscale approach provides essential guidance to the further development of core-shell wire devices.

Photonic hall effect in ferrofluids: theory and experiments

An experimental and theoretical study on the photonic Hall effect (PHE) in liquid and gelled samples of ferrofluids is presented. The ferrofluids are aqueous colloidal suspensions of Fe2CoO4 particles, which can be considered as anisotropic and absorbing Rayleigh scatterers. The PHE is found to be produced by the orientation of the magnetic moments of the particles, as is also the case for the Faraday effect. The dependence of the PHE with respect to the concentration of the scatterers, the magnetic field, and the polarization of the incident light is measured in liquid and in gelled samples, and is compared to a simple model based on the use of a scattering matrix and the single scattering approximation.

Pure Faraday rotator: a ferrofluid mixing method.

We present a method for obtaining a pure Faraday rotator by using a ferrofluid material. We demonstrate indeed that compatible magnetic liquids can be mixed in suitable proportions so as to cancel the circular dichroism at selected wavelengths in the visible-near-IR spectrum.