Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters.

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying a high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like domains with deformed QD layers, which originate at the first AlN buffer/superlattice interface and propagate vertically. The cones originate at a 30°-faceted shallow pit in the AlN, which appears to be associated with a threading dislocation that produces strong shear strain. The cone-like structures present Ga enrichment at the boundaring facets and larger QDs within the conic domain. The bimodality of the luminescence is attributed to the differing dot size and composition within the cones and at the faceted boundaries, which is confirmed by the correlation of microscopy results and Schrödinger-Poisson calculations.

Field-dependent abundances of hydride molecular ions in atom probe tomography of III-N semiconductors.

We investigate the microscopic behaviour of hydrogen-containing species formed on the surface of III-N semiconductor samples by the residual hydrogen in the analysis chamber in laser-assisted atom probe tomography (APT). We analysed AlGaN/GaN heterostructures containing alternate layers with a thickness of about 20 nm. The formation of H-containing species occurs at field strengths between 22 and 26 V/nm and is independent of the analysed samples. The 3D APT reconstruction makes it possible to map the evolution of the surface behaviour of these species issued by chemical reactions. The results highlight the strong dependence of the relative abundances of hydrides on the surface field during evaporation. The relative abundances of the hydrides decrease when the surface field increases due to the evolution of the tip shape or the different evaporation behaviour of the different layers.

The Photonic Atom Probe as a Tool for the Analysis of the Effect of Defects on the Luminescence of Nitride Quantum Structures.

By collecting simultaneously optical and chemical/morphological data from nanoscale volumes, the Photonic Atom Probe (PAP) can be applied not only to the study of the relationship between optical and structural properties of quantum emitter but also to evaluate the influence of other factors, such as the presence of point defects, on the photoluminescence. Through the analysis of multiple layers of InGaN/GaN quantum dots (QDs), grown so that the density of structural defects is higher with increasing distance from the substrate, we establish that the light emission is higher in the regions exhibiting a higher presence of structural defects. While the presence of intrinsic point defects with non-radiative recombination properties remains elusive, our result is consistent with the fact that QD layers closer to the substrate behave as traps for non-radiative point defects. This result demonstrates the potential of the PAP as a technique for the study of the optical properties of defects in semiconductors.

Behavior of the ε-Ga2O3:Sn Evaporation During Laser-Assisted Atom Probe Tomography.

The measurement of the composition of ε-Ga2O3 and the quantification of Sn doping in ε-Ga2O3:Sn by laser-assisted atom probe tomography (APT) may be inaccurate depending on the experimental conditions. Both the role of the laser energy and surface electric field were investigated, and the results clearly indicate that deviations from stoichiometry are observed changing the electric field conditions during APT. The measured atomic fraction of Ga can change from 0.45 at low field to 0.38 at high field, to be compared with the expected 0.4. This was interpreted in terms of preferential evaporation of Ga at high field and deficit of O at low field, which was caused by the formation of neutrals. The quantification of Sn-doping is accurate at low-field conditions, with an overestimation of the detected Sn-metallic fraction at high field. This suggests that Sn has a higher evaporation field compared to Ga. Finally, multiple detection events were in-depth studied, revealing that three dissociation reactions occur during APT: GaO2+ → Ga+ + O+; Ga2O22+ → Ga+ + GaO2+; Ga3O22+ → Ga+ + Ga2O2+. Nevertheless, only 2% of the detected events are related to such dissociation reactions, too small a fraction to fully explain the observed deviation from the stoichiometric composition in ε-Ga2O3.

Solubility Limit of Ge Dopants in AlGaN: A Chemical and Microstructural Investigation Down to the Nanoscale.

Attaining low-resistivity AlxGa1-xN layers is one keystone to improve the efficiency of light-emitting devices in the ultraviolet spectral range. Here, we present a microstructural analysis of AlxGa1-xN/Ge samples with 0 ≤ x ≤ 1, and a nominal doping level in the range of 1020 cm-3, together with the measurement of Ge concentration and its spatial distribution down to the nanometer scale. AlxGa1-xN/Ge samples with x ≤ 0.2 do not present any sign of inhomogeneity. However, samples with x > 0.4 display µm-size Ge crystallites at the surface. Ge segregation is not restricted to the surface: Ge-rich regions with a size of tens of nanometers are observed inside the AlxGa1-xN/Ge layers, generally associated with Ga-rich regions around structural defects. With these local exceptions, the AlxGa1-xN/Ge matrix presents a homogeneous Ge composition which can be significantly lower than the nominal doping level. Precise measurements of Ge in the matrix provide a view of the solubility diagram of Ge in AlxGa1-xN as a function of the Al mole fraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN, the solubility increases linearly with the Ga mole fraction in the ternary alloy, which suggests that the Ge incorporation takes place by substitution of Ga atoms only. The maximum percentage of Ga sites occupied by Ge saturates around 1%. The solubility issues and Ge segregation phenomena at different length scales likely play a role in the efficiency of Ge as an n-type AlGaN dopant, even at Al concentrations where Ge DX centers are not expected to manifest. Therefore, this information can have direct impact on the performance of Ge-doped AlGaN light-emitting diodes, particularly in the spectral range for disinfection (≈260 nm), which requires heavily doped alloys with a high Al mole fraction.

Super-resolution Optical Spectroscopy of Nanoscale Emitters within a Photonic Atom Probe.

Atom Probe Tomography (APT) is a microscopy technique allowing for the 3D reconstruction of the chemical composition of a nanoscale needle-shaped sample with a precision close to the atomic scale. The photonic atom probe (PAP) is an evolution of APT featuring in situ and operando detection of the photoluminescence signal. The optical signatures of the light-emitting centers can be correlated with the structural and chemical information obtained by the analysis of the evaporated ions. It becomes thus possible to discriminate and interpret the spectral signatures of different light emitters as close as 20 nm, well beyond the resolution limit set by the exciting laser wavelength. This technique opens up new perspectives for the study of the physics of low dimensional systems, defects and optoelectronic devices.

Field-Dependent Measurement of GaAs Composition by Atom Probe Tomography.

The composition of GaAs measured by laser-assisted atom probe tomography may be inaccurate depending on the experimental conditions. In this work, we assess the role of the DC field and the impinging laser energy on such compositional bias. The DC field is found to have a major influence, while the laser energy has a weaker one within the range of parameters explored. The atomic fraction of Ga may vary from 0.55 at low-field conditions to 0.35 at high field. These results have been interpreted in terms of preferential evaporation of Ga at high field. The deficit of As is most likely explained by the formation of neutral As complexes either by direct ejection from the tip surface or upon the dissociation of large clusters. The study of multiple detection events supports this interpretation.

Carrier Localization in GaN/AlN Quantum Dots As Revealed by Three-Dimensional Multimicroscopy.

The localization of carrier states in GaN/AlN self-assembled quantum dots (QDs) is studied by correlative multimicroscopy relying on microphotoluminescence, electron tomography, and atom probe tomography (APT). Optically active field emission tip specimens were prepared by focused ion beam from an epitaxial film containing a stack of quantum dot layers and analyzed with different techniques applied subsequently on the same tip. The transition energies of single QDs were calculated in the framework of a 6-bands k.p model on the basis of APT and scanning transmission electron microscopy characterization showing that a good agreement between experimental and calculated energies can be obtained, overcoming the limitations of both techniques. The results indicate that holes effectively localize at interface fluctuations at the bottom of the QD, decreasing the extent of the wave function and the band-to-band transition energy. They also represent an important step toward the correlation of the three-dimensional atomic scale structural information with the optical properties of single light emitters based on quantum confinement.

Dissociation Dynamics of Molecular Ions in High dc Electric Field.

In an atom probe, molecular ions can be field evaporated from the analyzed material and, then, can dissociate under the very intense electric field close to the field emitter. In this work, field evaporation of ZnO reveals the emission of Zn2O2(2+) ions and their dissociation into ZnO(+) ions. It is shown that the repulsion between the produced ZnO(+) ions is large enough to have a measurable effect on both the ion trajectories and times of flight. Comparison with numerical simulations of the ion trajectories gives information on the lifetime of the parent ions, the energy released by the dissociation and repulsion, and also the dissociation direction. This study not only opens the way to a new method to obtain information on the behavior of molecular ions in high electric fields by using an atom probe, but also opens up the interesting perspective to apply this technique to a wide class of materials and molecules.

Correlation of microphotoluminescence spectroscopy, scanning transmission electron microscopy, and atom probe tomography on a single nano-object containing an InGaN/GaN multiquantum well system.

A single nanoscale object containing a set of InGaN/GaN nonpolar multiple-quantum wells has been analyzed by microphotoluminescence spectroscopy (µPL), high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT). The correlated measurements constitute a rich and coherent set of data supporting the interpretation that the observed µPL narrow emission lines, polarized perpendicularly to the crystal c-axis and with energies in the interval 2.9-3.3 eV, are related to exciton states localized in potential minima induced by the irregular 3D In distribution within the quantum well (QW) planes. This novel method opens up interesting perspectives, as it will be possible to apply it on a wide class of quantum confining emitters and nano-objects.

Assessing individual radial junction solar cells over millions on VLS-grown silicon nanowires.

Silicon nanowires (SiNWs) grown on low-cost substrates provide an ideal framework for the monolithic fabrication of radial junction photovoltaics. However, the quality of junction formation over a random matrix of SiNWs, fabricated via a vapor-liquid-solid (VLS) mechanism, has never been assessed in a realistic context. To address this, we probe the current response of individual radial junction solar cells under electron-beam and optical-beam excitations. Excellent current generation from the radial junction units, compared to their planar counterparts, has been recorded, indicating a high junction quality and effective doping in the ultra-thin SiNWs with diameters thinner than 20 nm. Interestingly, we found that the formation of radial junctions by plasma deposition can be quite robust against geometrical disorder and even the crossings of neighboring cell units. These results provide a strong support to the feasibility of building high-quality radial junction solar cells over high-throughput VLS-grown SiNWs on low-cost substrates.

Coupling atom probe tomography and photoluminescence spectroscopy: exploratory results and perspectives.

The development of laser-assisted atom probes makes it possible, in principle, to exploit the femtosecond laser pulse not only for triggering ion evaporation from a nanometric field emission tip, but also for generating photons via the radiative recombination of electron-hole pairs in tips made of dielectric materials. In this article we demonstrate a first step towards a correlation of micro-photoluminescence (µ-PL) and laser-assisted tomographic atom probe (LA-TAP) analysis applied separately on the same objects, namely on ZnO microwires. In particular, we assess that the use of the focused ion beam (FIB) tip preparation method significantly degrades the radiative recombination yield of the analyzed microwires. We discuss the strategies to avoid the FIB-induced damage on the optical properties of the sample and how to get beyond the correlated µ-PL and LA-TAP analysis with a coupled approach allowing to perform the two analyses within the same instrument.

Self-assembled GaN quantum wires on GaN/AlN nanowire templates.

We present a novel approach for self-assembled growth of GaN quantum wires (QWRs) exhibiting strong confinement in two spatial dimensions. The GaN QWRs are formed by selective nucleation on {112[combining macron]0} (a-plane) facets formed at the six intersections of {11[combining macron]00} (m-plane) sidewalls of AlN/GaN nanowires used as a template. Based on microscopy observations we have developed a 3D model explaining the growth mechanism of QWRs. We show that the QWR formation is governed by self-limited pseudomorphic growth on the side facets of the nanowires (NWs). Quantum confinement in the QWRs is confirmed by the observation of narrow photoluminescence lines originating from individual QWRs with emission energies up to 4.4 eV. Time-resolved photoluminescence studies reveal a short decay time (~120 ps) of the QWR emission. Capping of the QWRs with AlN allows enhancement of the photoluminescence, which is blue-shifted due to compressive strain. The emission energies from single QWRs are modelled assuming a triangular cross-section resulting from self-limited growth on a-plane facets. Comparison with the experimental results yields an average QWR diameter of about 2.7 nm in agreement with structural characterization. The presented results open a new route towards controlled realization of one-dimensional semiconductor quantum structures with a high potential both for fundamental studies and for applications in electronics and in UV light generation.

Cathodoluminescence spectra of gallium nitride nanorods.

Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.

M-plane core-shell InGaN/GaN multiple-quantum-wells on GaN wires for electroluminescent devices.

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (∼16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.

High degree of polarization of the near-band-edge photoluminescence in ZnO nanowires.

We investigated the polarization dependence of the near-band-edge photoluminescence in ZnO strain-free nanowires grown by vapor phase technique. The emission is polarized perpendicular to the nanowire axis with a large polarization ratio (as high as 0.84 at 4.2 K and 0.63 at 300 K). The observed polarization ratio is explained in terms of selection rules for excitonic transitions derived from the k·p theory for ZnO. The temperature dependence of the polarization ratio evidences a gradual activation of the XC excitonic transition.PACS: 78.55.Cr, 77.22.Ej, 81.07.Gf.

Nanometer scale spectral imaging of quantum emitters in nanowires and its correlation to their atomically resolved structure.

We report the spectral imaging in the UV to visible range with nanometer scale resolution of closely packed GaN/AlN quantum disks 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 1 nm and separated from each other by only a few nanometers and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disk size and emission wavelength provides evidence of the quantum confined Stark effect leading to an emission below the bulk GaN band gap for disks thicker than 2.6 nm. With the help of simulations, we show that the internal electric field in the studied quantum disks is smaller than what is expected in the quantum well case. We show evidence of a clear dispersion of the emission wavelengths of different quantum disks of identical size but different positions 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 to fundamental studies of quantum confinement in closely packed quantum emitters and to characterizations of optoelectronic devices presenting carrier localization on the nanometer scale.

Investigation of the electronic transport in GaN nanowires containing GaN/AlN quantum discs.

We report the investigation of electronic transport in GaN nanowires containing GaN/AlN quantum discs (QDiscs). The nanowires were grown by plasma-assisted molecular beam epitaxy and contacted by electron-beam lithography. Three nanowire samples containing QDiscs are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The current-voltage measurements on single nanowires show that if the QDiscs are covered with a lateral GaN shell, the current mainly flows through the shell close to the lateral surface and the wire conductivity is extremely sensitive to the environmental conditions. On the contrary, if no GaN shell is present, the current flows through the QDisc region and a reproducible negative differential resistance related to electron tunneling through the QDiscs can be observed for temperatures up to 250 K. The demonstration of the resonant tunneling in GaN/AlN superlattices is of major importance for the development of nitride-based far-infrared quantum cascade lasers operating at high temperature.

Visible-blind photodetector based on p-i-n junction GaN nanowire ensembles.

We report the synthesis, fabrication and extensive characterization of a visible-blind photodetector based on p-i-n junction GaN nanowire ensembles. The nanowires were grown by plasma-assisted molecular beam epitaxy on an n-doped Si(111) substrate, encapsulated into a spin-on-glass and processed using dry etching and metallization techniques. The detector presents a high peak responsivity of 0.47 A W(-1) at - 1 V. The spectral response of the detector is restricted to the UV range with a UV-to-visible rejection ratio of 2 x 10(2). The dependence on the incident power and the operation speed of the photodetector are discussed.