Strain Relief Analysis of InN Quantum Dots Grown on GaN

We present a study by transmission electron microscopy (TEM) of the strain state of individual InN quantum dots (QDs) grown on GaN substrates. Moiré fringe and high resolution TEM analyses showed that the QDs are almost fully relaxed due to the generation of a 60° misfit dislocation network at the InN/GaN interface. By applying the Geometric Phase Algorithm to plan-view high-resolution micrographs, we show that this network consists of three essentially non-interacting sets of misfit dislocations lying along the directions. Close to the edge of the QD, the dislocations curve to meet the surface and form a network of threading dislocations surrounding the system

Structural changes during the natural aging process of InN quantum dots

The natural aging process of InN nanostructures by the formation of indium oxides is examined by transmission electron microscopy related techniques. Uncapped and GaN-capped InN quantum dots (QDs) on GaN/sapphire substrates were grown under the same conditions and kept at room temperature/pressure conditions. The GaN capping layer is found to preserve the InN QDs in the wurtzite phase, avoiding the formation of group-III oxides, while in the uncapped sample, a thin layer of cubic phases are formed that envelops the nucleus of wurtzite InN. These cubic phases are shown to be mainly bcc- In 2 O3 for long aged samples where the nitrogen atoms in the InN surface layers have been substituted by atmospheric oxygen. This process implies the gradual transformation of the In sublattice from hcp to a quasi-fcc structure. Metastable zinc-blende InN phases rich in oxygen atoms are proposed to act as intermediate phases and they are evinced in samples less aged. The large concurrence of interplanar spaces, the twin formation, and the existence of a free surface that facilitates the transformation support this mechanism and would explain the high instability of the InN nanostructures at ambient conditions.Comisión Interministerial de Ciencia y Tecnología (CICYT) MAT2007-60643Junta de Andalucía TEP383 EspañaUnión Europea NMP4-CT-2004- 50010

Temperature-dependent low-frequency vibrational spectra of sodium magnesium chlorophyllin

Terahertz time-domain spectroscopy has been used to investigate the vibrational spectra of polycrystalline sodium magnesium chlorophyllin - one of the natural derivatives of chlorophyll - over the temperature range 88 K–298 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–2.5 THz, which are interpreted as originating from mixed character of intramolecular and intermolecular vibration modes. As the temperature is increased, the observed absorption features resolve into broader peaks. The peak centered at 1.83 THz shifts towards higher frequencies, indicating that for this feature, significant intermolecular anharmonicity exist

Terahertz probing of temperature-driven topological phase transition in HgCdTe bulk crystal and HgTe Quantum Well

International audienceBulk films and heterostructures based on HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic (quantum well thickness, Cd content) and external (magnetic field, temperature, strain or hydrostatic pressure) physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based Quantum Wells and bulk films. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively

Terahertz probing of temperature-driven topological phase transition in HgCdTe bulk crystal and HgTe Quantum Well

International audienceBulk films and heterostructures based on HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic (quantum well thickness, Cd content) and external (magnetic field, temperature, strain or hydrostatic pressure) physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based Quantum Wells and bulk films. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively

The influence of thermal annealing on optical and electrical properties indium nitride films grown by MOVPE

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Recent advances in the MOVPE growth of Indium Nitride

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Growth Studies and Nanoscale Strain Profiles of InN Quantum Dots

International audienceNitride semiconductors have been at the center of semiconductor research in the last decade. However Indium nitride is still poorly known, due to its growth difficulty, and its properties are still the subject of passionate debates. Still, it is clear that it is a very promising material for telecommunication optoelectronics and hyperfrequency/terahertz applications. In particular, nanostructures based on InN are extremely interesting, by combining a small bandgap, the incredible efficiency of nitride semiconductors and the tunability of quantum dots. In this paper, we present the latest advances in the growth and optical properties of InN quantum dots grown onto GaN. The optical properties of InN dots will be compared to those of InN films, and we will show that the thermal behaviour of InN dots is far superior. Using TEM and synchrotron radiation diffraction, we will analyze the strain state of the dots at the nanometer scale, along the growth axis of the dots. These results, combined with the thermodynamical analysis of the growth data, will allow us to demonstrate that the growth mechanism is not the usual Stransky-Krastanow mode, but is more related to the BCF (Burton-Cabrera-Franck) model