Designing nanoparticles during the drawing step

International audienceNanoparticles in the core of optical fibres are widely studied due to the opportunity they give to tailor spectroscopic properties. Such fibres are usually obtained by drawing at high temperature a preform containing nanoparticles. This study focuses on the effect of the fibre drawing on nanoparticles. We fabricated an MCVD optical preform by doping the porous layer with nanoparticles. The optical fibre was studied by a FIB/SEM tomography.Figure 1 is the volume reconstruction of the core of the optical fibre. The yellow phase represents nanoparticles inside the core of the optical fibre. This reconstruction shows evidences of break-up, elongation and coalescence of particles. These features will be discussed according to phenomena well known from the rheology of emulsions and polymers. It comes from a competition between viscous stresses of the flow and surface tension.Observation of these size-controlling phenomena occuring during fibre drawing offer new perspectives to tailor the size of nanoparticles and are therefore of great interest for light scattering issues

Topical Review: Development of overgrown semi-polar GaN for high efficiency green/yellow emission

The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters. However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters. One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics

Control of the polarity of GaN films using an Mg adsorption layer

The polarity of GaN epilayers grown by molecular beam epitaxy is controlled using Mg. This is achieved by simultaneously exposing the surface to Mg and NH3 fluxes during growth interruption. Reflection high-energy electron diffraction (RHEED) indicates the formation of a Mg3N2, layer. Overgrowing GaN on this surface leads to a polarity inversion either from Ga to N or N to Ga. The change of the polarity is followed in situ by RHEED via surface reconstructions of the GaN surface. The polarity inversion is further confirmed by convergent beam electron diffraction experiments. Finally, high-resolution transmission electron microscopy images show different interface morphologies between Ga/N and N/Ga polarity boundaries. The control of the GaN polarity opens the way for novel periodic polarity structures dedicated to non-linear optics. (C) 2002 Elsevier Science B.V. All rights reserved

In situ imaging of threading dislocation terminations at the surface of GaN(0001) epitaxially grown on Si(111)

Scanning tunneling microscopy (STM) is used in situ to study threading dislocation terminations at the surface of GaN grown by molecular beam epitaxy on a Si(111) surface. The associated surface depressions are imaged at the nanometer scale, giving a new insight into the relationship between growth kinetics and dislocation-related surface morphology. Pure edge and dislocations with a screw component are easily differentiated and their density is determined. Pure edge dislocations are located at the boundaries of subgrains slightly misoriented in the surface plane. The direct measurement of the spacing of these dislocations provides the subgrain misorientation determination. Transmission electron microscopy well confirms the findings obtained from the STM study

Effect of the nucleation layer deposition temperature on the nature of defects in GSMBE GaN films

The microstructure of gas source molecular beam epitaxy GaN films deposited on (0001) sapphire is studied by transmission electron microscopy. For a nucleation layer deposited at 500 degrees C, high-quality materials, with only dislocations (density = 5 x 10(9) cm(-2)) in the volume of the film, are obtained. For a nucleation layer deposited at 550 degrees C, the resulting structural quality is poor. Inversion Domains and {11 (2) over bar 0} prismatic defects are observed. (C) 1999 Elsevier Science B.V. All rights reserved

Influence of Stacking Sequences and Lattice Parameter Differences on the Microstructure of Nonpolar AlN Films Grown on (11(2)over-bar0) 6H-SiC by Plasma-Assisted Molecular Beam Epitaxy

International audienceThanks to close crystalline structures and low lattice mismatches, nonpolar (11 (2) over bar0) 6H-SiC is expected to be a well- adapted substrate for the growth of nonpolar (11 (2) over bar0) III-nitride films. We demonstrate that the local reproduction of the basal planes stacking induces the presence of numerous planar defects (1.3 x 10(6) cm(-1)) in AlN films deposited on (11 (2) over bar0) 6H-SiC. Moreover, the tensile strain of AlN along the [0001] direction results in the cracking of the film for a thickness as low as 100 nm. This fragile plastic relaxation is favored because the only slip systems available for ductile plastic relaxation are pyramidal systems involving a + c dislocations which have a high activation energy

Polarity inversion of GaN(0001) by a high Mg doping

The control of the GaN-polarity is promising in view of achieving periodic polarity structures for non-linear optics. It is shown here that the polarity of GaN(0 0 0 1) epilayers can be reversed from Ga to N using a high Mg doping during molecular beam epitaxy with NH3 as nitrogen precursor. Transmission electron microscopy (TEM) studies indicate that the N-polar crystal quality is not degraded compared to the initial Ga-polar GaN. The structure, thickness and distance from the doping start of the inversion domain boundaries are studied by TEM. The polarity inversion dependence upon the Mg doping level is explored. The results show that a critical Mg surface coverage is necessary for polarity inversion and that Mg surface segregation plays a key role. Finally, a transition from hexagonal N-polar GaN to cubic GaN is demonstrated at high doping level. (C) 2004 Elsevier B.V. All rights reserved

Effect of the nitridation of the sapphire (0001) substrate on the GaN growth

The analysis of the sapphire surface nitridation by in situ reflection high-energy electron diffraction evidences the formation of a relaxed AIN layer. Its role on the early stage of the GaN growth is investigated by transmission electron microscopy (TEM). GaN crystallites of high structural quality, with the c axis perpendicular to the sapphire basal plane, are observed when the starting surface is nitridated. On the other hand, the growth of GaN on a bare substrate involves the formation of larger islands with numerous defects, TEM study reveals that the c axis of these latter crystallites is systematically tilted by about 19 degrees with respect to the sapphire basal plane. Actually, this orientation corresponds to a particular epitaxial relationship between GaN and sapphire (0001) substrates. Finally, the optical properties of GaN thin layers are shown to be strongly dependent on the nitridation state of the sapphire surface

Epitaxial relationships between GaN and Al2O3(0001) substrates

GaN thin layers (200 Angstrom) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates, Transmission electron microscopy reveals that two different epitaxial relationships may occur, The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and [](GaN)parallel to[](Al2O3) is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: [](GaN)parallel to[](Al2O3) and [](GaN)parallel to[](Al2O3). This corresponds to a tilt of about 19 degrees of the c axis with respect to the substrate surface. (C) 1997 American Institute of Physics