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Laser-assisted atom probe tomography (APT) and high-resolution dark-field electron holography (HR-DFEH) were performed to investigate the composition of a polar [0001] GaN/AlxGa1 − xN/InyGa1 − yN light emitting diode. In particular, the III-site fraction of both AlxGa1 − xN and InyGa1 − yN alloys was studied adopting a comparative approach. HR-DFEH allows mapping the projected strain with a subnanometer spatial resolution which is used for the calculation of the two-dimensional alloy composition distribution. APT provides three-dimensional alloys composition distribution with a nanometer spatial resolution. However, here we reveal that important inaccuracies affect local composition measurements. A Ga-poor composition is obtained in high DC-electric field regions. Moreover, such inaccuracies may be locally enhanced where the [0001] pole intersects the surface of the analyzed specimen, leading to a lower fraction of Ga measured. III-site fractions closer to the nominal values were measured at low field conditions. Ga loss is thought to be due to preferential DC field induced evaporation of Ga ions between laser pulses. This is explained in terms of formation of a metallic layer on the tip surface during APT analysis, where weak Ga-Ga bonds are formed, promoting the loss of Ga at high field conditions. [ABSTRACT FROM AUTHOR]

The use of a tip-shaped sample for the atom probe tomography technique offers the unique opportunity to analyze the dynamics of molecular ions in strong DC fields. We investigate here the stability of AlN2+ and GaN2+ dications emitted from an Al0.25Ga0.75N sample in a joint theoretical and experimental study. Despite the strong chemical resemblance of these two molecules, we observe only stable AlN2+, while GaN2+ can only be observed as a transient species. We simulate the emission dynamics of these ions on field-perturbed potential energy surfaces obtained from quantum chemical calculations. We show that the dissociation is governed by two independent processes. For all bound states, a mechanical dissociation is induced by the distortion of the potential energy surface in the close vicinity of the emitting tip. In the specific case of GaN2+, the relatively small electric dipole of the dication in its ground 13Σ− and excited 11Δ states induces a weak coupling with the electric field so that the mechanical dissociation into Ga+ + N+ lasts for sufficient time to be observed. By contrast, the AlN2+ mechanical dissociation leads to Al2+ + N which cannot be observed as a correlated event. For some deeply bound singlet excited states, the spin-orbit coupling with lower energy triplet states gives another chance of dissociation by system inter-system crossing with specific patterns observed experimentally in a correlated time of flight map. [ABSTRACT FROM AUTHOR]

Laser enhanced field evaporation of surface atoms in laser-assisted Atom Probe Tomography (APT) can simultaneously excite photoluminescence in semiconductor or insulating specimens. An atom probe equipped with appropriate focalization and collection optics has been coupled with an in situ micro-photoluminescence (μPL) bench that can be operated during APT analysis. The photonic atom probe instrument we have developed operates at frequencies up to 500 kHz and is controlled by 150 fs laser pulses tunable in energy in a large spectral range (spanning from deep UV to near IR). Micro-PL spectroscopy is performed using a 320 mm focal length spectrometer equipped with a CCD camera for time-integrated and with a streak camera for time-resolved acquisitions. An example of application of this instrument on a multi-quantum well oxide heterostructure sample illustrates the potential of this new generation of tomographic atom probes. [ABSTRACT FROM AUTHOR]

The structural, compositional, and optical properties of ZnO/MgxZn1-xO m-plane heterostructures are investigated using scanning transmission electron microscopy, laser-assisted atom probe tomography, and micro-photoluminescence. Coupled with electron tomography, atom probe tomography is currently the only technique providing a 3D reconstruction of the position of the atoms of a nanoscale specimen with their chemical nature. The multi-quantum well system investigated exhibits a V-groove grating profile along the a-axis accompanied by the formation of Zn- and Mg-enriched regions corresponding to the edges pointing towards the substrate and towards the upper surface, respectively. The optical signature of these heterostructures has been investigated by performing micro-photoluminescence on atom probe tip specimens. Effective mass calculations based on the 3D microscopy data indicate that the quantum well geometry and barrier composition yield a localization of hole states at the bottom of the V-groove. [ABSTRACT FROM AUTHOR]

We have investigated the morphology of a nanotip under femtosecond laser pulse illumination and a high electric field. We show that both the symmetry and the local radius of the tip change with the direction of laser polarization as against the tip axis. The experiments were performed on the very same GaN nanotip by laser-assisted atom probe tomography and electron tomography. This allowed an accurate assessment of the tip features by following the order of evaporation of single atoms from the surface. A change of atom emission sites was observed when a change of the angle between the tip axis and the linearly polarized electric field of the laser was imposed. This enables an optical control of field-evaporation sites. A close optical control of the tip morphology on a scale below 10 nm is thus achievable. Calculations of the field at nanotip apex and absorption maps support the experimental observations. Based on the present study, methods can be developed for reshaping nanotips at the nanometer level. This finding opens perspectives for numerous applications, making use of nanotips as probes or field emitters, and for plasmonic devices. [ABSTRACT FROM AUTHOR]

The use of a tip-shaped sample for the atom probe tomography technique offers the unique opportunity to analyze the dynamics of molecular ions in strong DC fields. We investigate here the stability of AlN 2+ and GaN 2+ dications emitted from an Al 0.25 Ga 0.75 N sample in a joint theoretical and experimental study. Despite the strong chemical resemblance of these two molecules, we observe only stable AlN 2+ , while GaN 2+ can only be observed as a transient species. We simulate the emission dynamics of these ions on field-perturbed potential energy surfaces obtained from quantum chemical calculations. We show that the dissociation is governed by two independent processes. For all bound states, a mechanical dissociation is induced by the distortion of the potential energy surface in the close vicinity of the emitting tip. In the specific case of GaN 2+ , the relatively small electric dipole of the dication in its ground 1 3 Σ - and excited 1 1 Δ states induces a weak coupling with the electric field so that the mechanical dissociation into Ga + + N + lasts for sufficient time to be observed. By contrast, the AlN 2+ mechanical dissociation leads to Al 2+ + N which cannot be observed as a correlated event. For some deeply bound singlet excited states, the spin-orbit coupling with lower energy triplet states gives another chance of dissociation by system inter-system crossing with specific patterns observed experimentally in a correlated time of flight map.