Optical and structural characterisation of epitaxial nanoporous GaN grown by CVD

In this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 μm. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain

Rectifiers, MOS diodes and LEDs made of fully porous GaN produced by chemical vapor deposition

Here we present the fabrication of LEDs based on porous GaN produced by chemical vapor deposition (CVD) and reviewed the work done that allowed demonstrating p-n junction rectifiers and MOS diodes in a simple manner and without involving post-growth steps to induce porosity. p-n junction rectifiers exhibited stable rectification in the range ±1–±5 V, with very stable values of current with time. MOS diodes were fabricated in a single growth step formed by a MgO dielectric interlayer in between Mg-doped porous GaN and a Mg-Ga metallic alloy. Despite the high resistivity observed in the LEDs fabricated, that induced a turn on voltage of ∼13 V, the emission consisted only in one peak centered at 542 nm. Our porous GaN films exhibit random porosity when compared to arrays of nanostructures, however, their easy deposition over large areas without dominating leakage currents is promising for wideband gap applications

Plasmonic enhancement of second harmonic generation from nonlinear RbTiOPO4 crystals by aggregates of silver nanostructures

We demonstrate a 60–fold enhancement of the second harmonic generation (SHG) response at the nanoscale in a hybrid metal-dielectric system. By using complex silver nanostructures photochemically deposited on the polar surface of a ferroelectric crystal, we tune the plasmonic resonances from the visible to the near-infrared (NIR) spectral region, matching either the SH or the fundamental frequency. In both cases the SHG signal at the metal-dielectric interface is enhanced, although with substantially different enhancement values: around 5 times when the plasmonic resonance is at the SH frequency or up to 60 times when it matches the fundamental NIR radiation. The results are consistent with the more spatially-extended near-field response of complex metallic nanostructures and can be well explained by taking into account the quadratic character of the SHG process. The work points out the potential of aggregates of silver nanostructures for enhancing optical nonlinearities at the nanoscale and provides an alternative approach for the development of nanometric nonlinear photonic devices in a scalable way.This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under projects MAT2013-43301-R, FIS2013-41184-P, MAT2013-47395-C4-4-R and TEC2014-55948-R. The authors also acknowledge Comunidad de Madrid under grant S2013/MIT-2740, Dept. of Industry of the Basque Country under project nanoGUNE’14, and Catalan Authority under project 2014SGF1358. F.D. acknowledges additional support through the ICREA Academia awards 2010ICREA-02 for excellence in research. LSG acknowledges FPU13/02476 grant from the Spanish Ministry of Education.Peer reviewe

Er-doped KY_(1-x-y)Gd_xLu_y(WO₄)₂ surface channel waveguides

Channel waveguides on KY1-x-yGdxLuy(WO4)2 epitaxial films doped with Er3+ were obtained by ion beam milling and guided modes at wavelengths near 1.5µm confirmed single mode behaviour. Absorption and emission spectra of these waveguides agree with those of bulk crystals of the same family, showing potential for a planar waveguide laser

Reduced workfunction intermetallic seed layers allow growth of porous n-GaN and low resistivity, ohmic electron transport

Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt- and Au-coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH3 gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN–metal interface, allowing vapor–solid–solid seeding and subsequent growth of porous GaN. Current–voltage and capacitance–voltage measurements confirm that the intermetallic seed layers prevent interface oxidation and give a high-quality reduced workfunction contact that allows exceptionally low contact resistivities. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission

Blue SHG enhancement by silver nanocubes photochemically prepared on a RbTiOPO4 ferroelectric crystal

This is the peer reviewed version of the following article: Blue SHG enhancement by silver nanocubes photochemically prepared on a RbTiOPO4 ferroelectric crystal, which has been published in final form at http://doi.org/10.1002/adma.201401603. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionsSilver nanocubes with low size dispersion have been selectively photo-deposited on the positive surface of a periodically poled RbTiOPO 4 ferroelectric crystal. The obtained nanocubes show preferential orientations with respect to the substrate suggesting ep itaxial growth. The plasmonic resonances supported by the nanocubes are exploited to enhance blue SHG at the domain wallsThis work has been supported by the Spanish Government under projects MAT2010–17443, MAT2011–29255-C02–02 and MAT2013– 43301-R, Comunidad de Madrid under grant S2009/1756 and Generalitat de Catalunya under project 2009SGR23

Porous GaN and high-k MgO-GaN MOS diode layers grown in a single step on silicon

Porous GaN polycrystalline layers with n-type conduction characteristics were catalytically grown from Mg films formed by decomposition of a Mg2N3 precursor typically employed for activating p-type conduction in GaN. After being exposed to oxygen, the Mg film oxidized to a polycrystalline high-κ oxide between the ohmic alloy interlayer contact and the porous GaN, while maintaining a clean interface. Electrical measurements on devices coupled to composition analysis and electron microscopy of the component layers confirm that a MOS-type porous GaN diode on silicon can be formed by chemical vapor deposition in a single growth regime

Surface ablation of RbTiOPO4 by femtosecond laser

We report here the results obtained in surface ablation of RbTiOPO4 single crystals by femtosecond laser. We fabricated and characterized one-dimensional (1D) diffraction gratings with different lattice spacings of 15 and 20 μm, and with a sub-modulation of the period introduced in the later. The optical and electronic microscopy characterization and filling factor analysis of these diffraction gratings are reported. We also show that the roughness generated on the grooves by the ablation process can be improved by chemical etching.This work was partially funded by the European Commission under the Seventh Framework Program under Project Cleanspace FP7-SPACE-2010-1-GA-263044, supported by the Spanish Government under Projects PI09/90527, TEC2009-09551, AECID A/024560/09, FIS2009-09522, HOPE CSD2007-00007 and SAUUL CSD2007-00013 (Consolider-Ingenio 2010), by Catalan Government under Projects 2009SGR235 and 2009SGR549, by Junta de Castilla y León under Project GR27, and by the Research Center on Engineering of Materials and Systems (EMaS) of the URV. J.J.C. is supported by the Education and Science Ministry of Spain and European Social Fund under the Ramón y Cajal program, RYC2006-858. We also acknowledge support from the Centro de Laseres Pulsados, CLPU, Salamanca, Spain