•InGaN/GaN superlattice was used as a buffer layer on InGaN pseudo-substrate.•The V pit density was reduced by one order of magnitude.•(0 0 2) XRD ω scan linewidth reduces from 3000 arcsec to ...780 arcsec.•InGaN based quantum wells have an internal quantum efficiency of 6.5% at 624 nm.
The InGaN pseudo-substrate, namely InGaNOS (InGaN On Sapphire), is used to enhance the In incorporation rate in InyGa1-yN/InxGa1-xN multiple quantum wells (MQWs) to get red emission for micro-display applications. However, the starting material for the InGaNOS fabrication is a non-optimized In0.08Ga0.92N layer grown on GaN on sapphire substrate which exhibits V shaped defects (V pits). Such V pits remain afterwards in the final InGaNOS substrate. We demonstrate here that InxGa1-xN/GaN superlattice has the potential to cover or fill the native V pits while maintaining a pseudomorphic growth. A combination of a thin GaN interlayer and an InGaN layer in a slight tensile strain state for each pair of the superlattice is necessary to achieve this goal. In addition, it is shown that the presence of GaN interlayers improves the material quality and the surface roughness. (0 0 2) X-ray diffraction rocking curve linewidth reduces to 780 arcsec compared to 3000 arcsec for the substrate. Finally, InyGa1-yN/InxGa1-xN multiple quantum wells grown on InxGa1-xN/GaN superlattice buffer layer on InGaNOS 3.205Ȧ substrate shows a central emission wavelength, measured by photoluminescence, of 624 nm at 290 K with an optical internal quantum efficiency value of 6.5%.
The fast-growing market of GaN-based power electronics requires the development of performing ohmic contacts. The Ti/Al stack, being one of the most popular, has been thoughtfully investigated over ...the years in terms of electrical performances but remains to be better understood in terms of contact formation and related solid-state reactions, especially when targeting small Ti to Al ratios (typically, down to lower than 5 at. %). In this paper, the influence of the deposited Ti layer thickness on the resulting contact is investigated using in-situ and ex-situ XRD methods as well as TEM imaging. It is found that a threshold exists, below which the diffusion phenomena ruling alloy formation during annealing are controlled, leading to a frozen Al3Ti/Al bi-layer morphology. Past this threshold, the erratic formation of non-textured Al3Ti grains is observed, which is detrimental to devices performances. The result interpretation is supported by an extended bibliography survey focused on material science, the first of the sort.
Single-mode waveguides for GRAVITY Perraut, K.; Jocou, L.; Berger, J. P. ...
Astronomy and astrophysics (Berlin),
06/2018, Volume:
614
Journal Article
Peer reviewed
Open access
Context.
Within the framework of the second-generation instrumentation of the Very Large Telescope Interferometer of the European Southern Observatory we have developed the four-telescope beam ...combiner in integrated optics.
Aims.
We optimized the performance of such beam combiners, for the first time in the near-infrared
K
band, for the GRAVITY instrument dedicated to the study of the close environment of the galactic centre black hole by precision narrow-angle astrometry and interferometric imaging.
Methods.
We optimized the design of the integrated optics chip and the manufacturing technology as well, to fulfil the very demanding throughput specification. We also designed an integrated optics assembly able to operate at 200 K in the GRAVITY cryostat to reduce thermal emission.
Results.
We manufactured about 50 beam combiners by silica-on-silicon etching technology. We glued the best combiners to single-mode fluoride fibre arrays that inject the VLTI light into the integrated optics beam combiners. The final integrated optics assemblies have been fully characterized in the laboratory and through on-site calibrations: their global throughput over the
K
band is higher than 55% and the instrumental contrast reaches more than 95% in polarized light, which is well within the GRAVITY specifications.
Conclusions.
While integrated optics technology is known to be mature enough to provide efficient and reliable beam combiners for astronomical interferometry in the
H
band, we managed to successfully extend it to the longest wavelengths of the
K
band and to manufacture the most complex integrated optics beam combiner in this specific spectral band.
In this work, 200 nm-thick LiNbO 3 layers have been grown on Ru/Si and Pt/Si substrates using an industrialized Pulsed Laser Deposition (PLD) system. Film's properties have been investigated by means ...of X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Secondary Ions Mass Spectrometry (SIMS). In both cases, XRD analysis highlights the coexistence of LiNbO 3 phase and a Li-deficient secondary phase (LiNb 3 O 8 ). A c-axis preferential growth orientation have been obtained for LiNbO 3 film deposited on Pt/Si substrate. The formation of the Li-deficient secondary phase is related to Li diffusion, as highlighted by SIMS measurements. Finally, the fabricated Film Bulk Acoustic Resonator (FBAR) testifies to the piezoelectric activity of the as-deposited LiNbO 3 film. The FBAR based on pulsed laser deposited LiNbO 3 film displays an electromechanical coupling coefficient close to 6.7% at 6.9 GHz. Process optimizations (electrodes, piezoelectric films) are in progress in order to improve the microstructural properties of the as-deposited LiNbO 3 layer and thus the quality factor.
Room-temperature CW laser operation at 1.55 mum of Yb:Er:Ca(2)Al(2)SiO(7) (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, ...Ce(3 ), decreases the Er (3 4)I(11/2) excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of ~5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented
PDF
