B. Lacroix

A mechanism for damage formation in GaN during rare earth ion implantation at medium range energy and room temperature

A detailed investigation of the crystallographic damage has been carried out in GaN following 300 keV rare earth ion implantation at room temperature by varying the fluence from 7×1013 to 5×1016 at/cm2. It is shown that above a threshold fluence around 2×1015 at/cm2, nanocrystallization takes place from the surface, subsequent to the formation of a planar defects network consisting of basal and prismatic stacking faults. This network starts to form at the lowest analyzed fluence mostly around the mean projected range. When the fluence increases, it propagates toward the surface, reaching it just before the on-set of the nanocrystallization. A model based on the mechanical breakdown of the GaN wurtzite structure mediated by prismatic stacking faults is proposed.

Mechanisms of damage formation in Eu-implanted GaN probed by X-ray diffraction

At low fluence, 300 keV Eu implantation in GaN leads to a strain increase followed by a saturation as observed by X-ray diffraction, while Rutherford backscattering/channeling remains insensitive to the radiation damage. Based on transmission electron microscopy, this saturation regime is attributed to a damaged region in the crystal bulk in which interaction between point defects and stacking faults (SFs) occurs, leading to the densification of the network of planar defects by the trapping of point defects. At higher fluences, above 2×1014 Eu/cm2, the evolutions of strain state in another region and of the microstructure as observed by TEM indicate a modification of the degradation mechanisms which now involve a migration of point defects out of the region of SFs. This results in the formation of a highly strained area below the region of SFs made up of large point defect clusters, and in the extension of the SFs network towards the surface that eventually leads to its nanocrystallization.

Polarity determination of polar and semipolar (112¯2) InN and GaN layers by valence band photoemission spectroscopy

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band ph...

Nonlinear absorption of InN/InGaN multiple-quantum-well structures at optical telecommunication wavelengths

We report on the nonlinear optical absorption of InN / InxGa1−xN (x=0.8, 0.9) multiple-quantum-well structures characterized at 1.55 µm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~−9x103 cm /GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.

Microstructural and conductivity changes induced by annealing of ZnO:B thin films deposited by chemical vapour deposition

Zinc oxide (ZnO) thin films have attracted much attention in recent years due to progress in crystal growth for a large variety of technological applications including optoelectronics and transparent electrodes in solar cells. Boron (B)-doped ZnO thin films are deposited by low pressure chemical vapour deposition (LPCVD) on Si(100). These films exhibit a strong (002) texture with a pyramidal grain structure. The ZnO films were annealed after growth; the annealing temperature and the atmosphere appear to strongly impact the layer conductivity. This work will first present the modification of the physical properties (carrier concentration, mobility) extracted from the simulation of layer reflection in the infrared range. At low annealing temperatures the mobility increases slightly before decreasing drastically above a temperature close to 250 °C. The chemical and structural evolution (XPS, x-ray diffraction) of the films was also studied to identify the relationship between microstructural modifications and the variations observed in the film conductivity. An in situ XRD study during annealing has been performed under air and low pressure conditions. As observed for electrical properties, the microstructural modifications shift to higher temperatures for vacuum annealing.

Efficient blocking of planar defects by prismatic stacking faults in semipolar (112¯2)-GaN layers on m-sapphire by epitaxial lateral overgrowth

For the next-generation solid state lighting, the production of high quality semipolar (112¯2) GaN layers on sapphire obtained using asymmetric epitaxial lateral overgrowth (ELO) method has been investigated. This type of ELO leads to efficient blocking of the basal stacking faults (BSFs) in the bulk, and enables the formation of nondefective layers at the surface. The BSFs terminate due to generation of prismatic stacking faults along a well defined boundary. The corresponding intensity of GaN band edge photoluminescence emission is increased by more than four orders of magnitude in comparison to that from semipolar templates.

Mechanisms of damage formation in Eu-implanted AlN

X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the evolution of damage during implantation of 300 keV Eu ions at room temperature in AlN. At low fluence, a strain increase is observed in a buried layer where clusters of point defects and stacking faults (SFs) coexist. At higher fluence, a saturation of the strain is observed in this layer, and the XRD curves exhibit characteristic features which coupled with TEM results enable the identification of additional, spatially separated, dilated and contracted regions. From these observations, the following damage mechanisms are proposed. As the SFs grow by trapping point defects, a dense network of basal and prismatic SFs forms, which leads to the ejection of point defects from the buried damaged layer and consequently to the saturation of the strain. In this process, interstitials in excess migrate towards the undamaged bulk where they form clusters inducing large strain values. In contrast, defects ejected towards ...

STEM?EELS analysis reveals stable high-density He in nanopores of amorphous silicon coatings deposited by magnetron sputtering

A broad interest has been showed recently on the study of nanostructuring of thin films and surfaces obtained by low-energy He plasma treatments and He incorporation via magnetron sputtering. In this paper spatially resolved electron energy-loss spectroscopy in a scanning transmission electron microscope is used to locate and characterize the He state in nanoporous amorphous silicon coatings deposited by magnetron sputtering. A dedicated MATLAB program was developed to quantify the helium density inside individual pores based on the energy position shift or peak intensity of the He K-edge. A good agreement was observed between the high density (∼35-60 at nm(-3)) and pressure (0.3-1.0 GPa) values obtained in nanoscale analysis and the values derived from macroscopic measurements (the composition obtained by proton backscattering spectroscopy coupled to the macroscopic porosity estimated from ellipsometry). This work provides new insights into these novel porous coatings, providing evidence of high-density He located inside the pores and validating the methodology applied here to characterize the formation of pores filled with the helium process gas during deposition. A similar stabilization of condensed He bubbles has been previously demonstrated by high-energy He ion implantation in metals and is newly demonstrated here using a widely employed methodology, magnetron sputtering, for achieving coatings with a high density of homogeneously distributed pores and He storage capacities as high as 21 at%.

The high sensitivity of InN under rare earth ion implantation at medium range energy

In this work, the damage formation in InN layers has been investigated subsequent to europium implantation at 300 keV and room temperature. The layers of several micrometres were produced by hydride vapour phase epitaxy and used as matrices for ion implantation experiments due to their good crystalline quality. From this investigation, it is shown that InN exhibits a low stability under rare earth ion implantation. Starting at a low fluence of around 5 × 1012 Eu cm−2, an extensive modification of the surface layer takes place. The dissociation of InN and the presence of misoriented nanograins are observed in the damaged area. Analysis by electron diffraction indicates that the nanograins correspond to indium oxide In2O3.

Fabrication of Optical Multilayer Devices from Porous Silicon Coatings with Closed Porosity by Magnetron Sputtering

The fabrication of single-material photonic-multilayer devices is explored using a new methodology to produce porous silicon layers by magnetron sputtering. Our bottom-up methodology produces highly stable amorphous porous silicon films with a controlled refractive index using magnetron sputtering and incorporating a large amount of deposition gas inside the closed pores. The influence of the substrate bias on the formation of the closed porosity was explored here for the first time when He was used as the deposition gas. We successfully simulated, designed, and characterized Bragg reflectors and an optical microcavity that integrates these porous layers. The sharp interfaces between the dense and porous layers combined with the adequate control of the refractive index and thickness allowed for excellent agreement between the simulation and the experiments. The versatility of the magnetron sputtering technique allowed for the preparation of these structures for a wide range of substrates such as polymers while also taking advantage of the oblique angle deposition to prepare Bragg reflectors with a controlled lateral gradient in the stop band wavelengths.