V. Hortelano

Non-radiative recombination centres in catalyst-free ZnO nanorods grown by atmospheric-metal organic chemical vapour deposition

We have investigated the cathodoluminescence (CL) emission and the Raman spectra along individual ZnO nanorods grown by a catalyst-free method. The spatial correlation between the CL emission and the defect related Raman modes permits establishing a correspondence between the non-radiative recombination centres (NRRCs) and the defects responsible for the 275 cm−1 Raman band. According to this relation, the NRRCs in these nanorods are tentatively associated with complexes of zinc interstitials.

Raman Microspectroscopy of Genuine and Fake Euro Banknotes

ABSTRACT To evaluate the effectiveness of Raman microspectroscopy in euro banknotes’ recognition, several genuine and fake 10 and 20 euro banknotes were analyzed. Raman microspectroscopy revealed itself to be very useful in the detection of differences in the inks used to provide the color on the banknotes. However, the study revealed that the Raman analysis results are not decisive to guarantee the authenticity of a specific banknote since similar Raman spectra were obtained for genuine and fake banknotes. Even the Raman microspectroscopy analysis obtained for the same color on different areas of fake banknotes revealed similar spectra, and this can help law enforcement agencies to identify counterfeit tracking routes.

Modification of the optical and structural properties of ZnO nanowires by low-energy Ar + ion sputtering

The effects of low-energy (≤2 kV) Ar+ irradiation on the optical and structural properties of zinc oxide (ZnO) nanowires (NWs) grown by a simple and cost-effective low-temperature technique were investigated. Both photoluminescence spectra from ZnO NW-coated films and cathodoluminescence analysis of individual ZnO NWs demonstrated obvious evidences of ultraviolet/visible luminescent enhancement with respect to irradiation fluence. Annihilation of the thinner ZnO NWs after the ion bombardment was appreciated by means of high-resolution scanning electron microscopy and transmission electron microscopy (TEM), which results in an increasing NW mean diameter for increasing irradiation fluences. Corresponding structural analysis by TEM pointed out not only significant changes in the morphology but also in the microstructure of these NWs, revealing certain radiation-sensitive behavior. The possible mechanisms accounting for the decrease of the deep-level emissions in the NWs with the increasing irradiation fluences are discussed according to their structural modifications.

InGaP Layers Grown on Different GaAs Surfaces for High Efficiency Solar Cells

InGaP layers grown on (111)Ga and (111)As GaAs substrate faces are investigated by microRaman spectroscopy, microphotoluminescence and cathodoluminescence. The growth on these polar faces benefits disorder with respect to the layers grown on (001) faces. It is shown that both (111)Ga and (111)As faces result in disordered InGaP layers. While the layers grown on (111)As faces present inhomogeneous compositions, the layers grown on (111)Ga faces present homogeneous compositions close to lattice matching and are almost disordered.

Cathodoluminescence study of Mg activation in non-polar and semi-polar faces of undoped/Mg-doped GaN core-shell nanorods

Spectrally and spatially resolved cathodoluminescence (CL) measurements were carried out at 80 K on undoped/Mg-doped GaN core-shell nanorods grown by selective area growth metalorganic vapor phase epitaxy in order to investigate locally the optical activity of the Mg dopants. A study of the luminescence emission distribution over the different regions of the nanorods is presented. We have investigated the CL fingerprints of the Mg incorporation into the non-polar lateral prismatic facets and the semi-polar facets of the pyramidal tips. The amount of Mg incorporation/activation was varied by using several Mg/Ga flow ratios and post-growth annealing treatment. For lower Mg/Ga flow ratios, the annealed nanorods clearly display a donor-acceptor pair band emission peaking at 3.26-3.27 eV and up to 4 LO phonon replicas, which can be considered as a reliable indicator of effective p-type Mg doping in the nanorod shell. For higher Mg/Ga flow ratios, a substantial enhancement of the yellow luminescence emission as well as several emission subbands are observed, which suggests an increase of disorder and the presence of defects as a consequence of the excess Mg doping.

Cathodoluminescence Study of Ammonothermal GaN Crystals

GaN crystals grown in supercritical ammonia by the ammonothermal method were studied by cathodoluminescence (CL), both in image and spectrally resolved modes. The main extended defects and the incorporation of point defects and impurities in different growth sectors were revealed. The influence of the seeds, the role of the growth planes and the changes in the crystal quality during the growth run are discussed.

Composition profiles in InP/InAsP quantum well structures under the effect of reactives gases during dry etching processes — Luminescence and SIMS

We have investigated the effects of reactive gases used during the deep reactive ion etching process of InP-based photonic structures in an inductively-coupled plasma (ICP) reactor. Samples with a specific structure, including 9 InAsP/InP quantum wells (QW) with graded As/P composition, were designed. Different chlorine-based gas chemistries were tested. Characterization was performed using cathodo-Iuminescence (CL) and photo-luminescence (PL) at different temperatures, and secondary ion mass spectrometry (SIMS). The luminescence lines display a blue shift upon exposure to the reactive gases, and a strong spectral sharpening. We discuss the influence of Cl diffusion and thermal processes during etching on these modifications.

Internal degradation of 980nm emitting single-spatial-mode lasers during ultrahigh power operation

Internal degradation of 980 nm emitting single-spatial-mode diode lasers during ultrahigh power operation is investigated for pulsed operation (2 μJ, 20 W). Analysis of the evolution of the emission nearfield with picosecond time resolution enables the observation of the transition from single- to multi-spatial-mode operation at elevated emission powers. Moreover, internal degradation events and subsequent defect propagation processes are in situ monitored by thermal imaging. Subsequently, these devices are opened and defect pattern are inspected by cathodo- and photoluminescence spectroscopy. The results complete earlier findings obtained with broad-area lasers and help to establish models covering defect generation and propagation in edge-emitting devices in general.

Combined EL and LBIC Study of the Electrical Activity of Defects in Solar Cells Based on Innovative Wafers Grown by Casting Methods

In this paper we combine LBIC and EL measurements of commercially multi-crystalline silicon solar cells, in order to obtain detailed information about the electrical activity around defect areas. This integrated analysis is suitable for the study of different crystal defects at both micrometric and full wafer scale. In particular, the electrical activity of some defect areas is studied in detail by means of highly spatially-resolved LBIC maps, showing important differences in their behaviours. A discussion about the origin of these differences is presented.