R. Gago

Production of ordered silicon nanocrystals by low-energy ion sputtering

We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si (100) substrate with 1.2 keV Ar+ ions at normal incidence. Atomic force and high-resolution transmission electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 h) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to the sputtering yield dependence on the local surface curvature.

Self-Organized Ordering of Nanostructures Produced by Ion-Beam Sputtering

We study the self-organized ordering of nanostructures produced by ion-beam sputtering of targets amorphizing under irradiation. By introducing a model akin to models of pattern formation in aeolian sand dunes, we extend consistently the current continuum theory of erosion by IBS. We obtain new nonlinear effects responsible for the in-plane ordering of the structures, whose strength correlates with the degree of ordering found in experiments. Our results highlight the importance of redeposition and surface viscous flow to this nanopattern formation process.

Bonding and hardness in nonhydrogenated carbon films with moderate sp3 content

Amorphous carbon films with an sp3 content up to 25% and a negligible amount of hydrogen have been grown by evaporation of graphite with concurrent Ar+ ion bombardment. The sp3 content is maximized for Ar+ energies between 200 and 300 eV following a subplantation mechanism. Higher ion energies deteriorate the film due to sputtering and heating processes. The hardness of the films increases in the optimal assisting range from 8 to 18 GPa, and is explained by crosslinking of graphitic planes through sp3 connecting sites.

Hydrogen quantification in hydrogenated amorphous carbon films by infrared, Raman, and x-ray absorption near edge spectroscopies

In this study, we have employed infrared (IR) absorption spectroscopy, visible Raman spectroscopy, and x-ray absorption near edge structure (XANES) to quantify the hydrogen (H) content in hydrogenated amorphous carbon (a-C:H) films. a-C:H films with a hydrogen content varying from 29 to 47 at. % have been synthesized by electron cyclotron resonance chemical vapor deposition at low substrate temperatures (<120 °C) applying a wide range of bias voltage, Vb, (−300 V<Vb<+100 V). With the application of high negative Vb, the a-C:H films undergo a dehydrogenation process accompanied by a sharp structural modification from polymer- to fullerenelike films. The trend in the H content derived from elastic recoil detection analysis (ERDA) is quantitatively reproduced from the intensity of the C–H bands and states in the IR and XANES spectra, respectively, as well as from the photoluminescence (PL) background drop in the Raman spectra. Using the H contents obtained by ERDA as reference data, semiquantitative expressi...

Self-Organized Surface Nanopatterning by Ion Beam Sputtering

The production of self-organized surface nanopatterns by ion beam sputtering (IBS) at low (<10 keV) and intermediate (10–100 keV) energies has emerged in the last decade as a promising bottom-up nanostructuring tool. The technique is remarkably universal, being applicable to metals, semiconductors or insulators, and it enables large degree of control over the main pattern features with high throughput (it requires low process time and can be used over extended areas). However, there is a wide scatter in the experimental results obtained as a function of system type and process parameters. In parallel, diverse theoretical models have been developed that differ in their capabilities to reproduce such a wide range of experimental features. We provide an overview of the most recent studies on the production of nanoripple, nanohole and nanodot periodic nanostructures by IBS, with special attention to the comparison between experiments and (continuum) models, and with a focus on those issues that remain open or, at least, ambiguous. The pattern properties to be considered are those of potential increased technological importance, such as the variation of size, shape, distance and ordering of the nanostructures as a function of parameters such as ion energy, target temperature and sputtering time (i.e., fluence). Finally, reported and proposed applications of IBS nanopatterns are briefly presented showing, in this way, the high-potential functionality of IBS nanostructured surfaces.

Identification of ternary boron–carbon–nitrogen hexagonal phases by x-ray absorption spectroscopy

Boron carbon nitride (BCN) films have been grown by B4C evaporation with concurrent N2+ ion assistance, and have been characterized by x-ray absorption near edge (XANES) spectroscopy. Upon the nitrogen insertion, the film structure evolves from BxC-like to h-BN-like. The hexagonal structure corresponds to a true ternary BCN compound that can be understood as h-BN with carbon incorporated in substitutional sites. The C(1s)XANES presents π* states characteristic of the BCN arrangement. The basal planes of the h-BCN phase are oriented perpendicular to the substrate, as derived from the angle dependence of the XANES signal.

Nanopatterning of silicon surfaces by low-energy ion-beam sputtering: dependence on the angle of ion incidence

We report on the production of nanoscale patterning on Si substrates by low-energy ion-beam sputtering. The surface morphology and structure of the irradiated surface were studied by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). Under ion irradiation at off-normal incidence angle (∼50 ◦ ), AFM images show the formation of both nanoripple and sawtooth-like structures for sputtering times longer than 20 min. The latter feature coarsens appreciably after 60 min of sputtering, inducing a large increase in the surface roughness. This behaviour is attributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading to shadowing effects among surface features in the sputtering process. Under irradiation at normal incidence, the formation of an hexagonal array of nanodots is induced for irradiation times longer than 2 min. The shape and crystallinity of the nanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged even after 16 h of sputtering. For the two angle conditions studied, the formation of the corresponding surface structures can be understood as the interplay between an instability due to the sputtering yield dependence on the local surface curvature and surface smoothing processes such as surface diffusion.

Stress-induced solid flow drives surface nanopatterning of silicon by ion-beam irradiation

Ion Beam Sputtering (IBS) is known to produce surface nanopatterns over macroscopic areas on a wide range of materials. However, in spite of the technological potential of this route to nanostructuring, the physical process by which these surfaces self-organize remains poorly understood. We have performed detailed experiments of IBS on Si substrates that validate dynamical and morphological predictions from a hydrodynamic description of the phenomenon. Our results elucidate flow of a nanoscopically thin and highly viscous surface layer, driven by the stress created by the ion-beam, as a description of the system. This type of slow relaxation is akin to flow of macroscopic solids like glaciers or lead pipes, that is driven by defect dynamics.

X-Ray Absorption Studies of Cubic Boron carbon nitrogen Films Grown by Ion Beam Assisted Evaporation

Abstract We have synthesised boron carbon nitride (BCN) thin films with tetrahedral structure by evaporation of B 4 C and concurrent ion bombardment. The film structure and composition are characterised by infrared and X-ray absorption near edge (XANES) spectroscopies. The addition of Ar to the N 2 assisting gas is necessary to reach a momentum transfer above the threshold to promote tetrahedral bonding. Under these conditions, c-BCN compounds are achieved, but the carbon content is limited to ∼5 at.%. This phase grows on the top of a ∼50-nm h-BCN layer oriented perpendicular to the substrate. The hexagonal BCN phase permits the accommodation of a carbon content between 10 and 15 at.%. The percentage of cubic and hexagonal phases is controlled by the temperature and ion assisting parameters. There is a narrow window for promoting the cubic structure with values similar to those reported for c-BN.

Transition from amorphous boron carbide to hexagonal boron carbon nitride thin films induced by nitrogen ion assistance

Boron carbon nitride films (BCN) were grown by B4C evaporation under concurrent N2 ion beam assistance. The films were characterized by x-ray absorption near-edge spectroscopy, infrared and Raman spectroscopies, and high-resolution transmission electron microscopy. The bonding structure and film composition correlate with the momentum transfer per incoming atom during deposition. As the momentum transfer is increased, the film structure evolves from an amorphous boron carbide network towards a hexagonal ternary compound (h–BCN) with standing basal planes. The growth of h–BCN takes place for momentum transfer in the window between 80 and 250 (eV×amu)1/2. The characteristic vibrational features of the h–BCN compounds have also been studied. Finally, the solubility limit of carbon in the hexagonal BN structure, under the working conditions of this article, is found to be ∼15 at. %.