J.M. Albella

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.

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

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.

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.

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. %.

Optical emission characterization of CH4+H2 discharges for diamond deposition

Methane and hydrogen discharges has been studied at different discharge frequencies (35 kHz, 13.56 MHz, and 2.45 GHz) and feeding gas ratios (up to 100% of methane) during diamond and diamond‐like deposition by plasma chemical vapor deposition techniques. Optical emission spectroscopy shows that the intensity of atomic hydrogen line (Hα) is the highest at the microwave frequency (2.45 GHz). In addition, at this frequency and low methane concentrations (<7.5%) the emission of CH+ species is also detected, which has been associated to the presence of the diamond phase in the films. On the contrary, at the lower frequencies (35 kHz and 13.56 MHz), the emission spectra are dominated by neutral CH species that are supposed to be the precursor species in the diamond‐like films deposited at these frequencies.

X-ray absorption spectroscopy and atomic force microscopy study of bias-enhanced nucleation of diamond films

The bias-enhanced nucleation of diamond on Si(100) has been studied by x-ray absorption near-edge spectroscopy (XANES) and atomic force microscopy, two techniques well suited to characterize nanometric crystallites. Diamond nuclei of ∼15 nm are formed after 5 min of bias-enhanced treatment. The number of nuclei and its size increases with the time of application of the bias voltage. A nanocrystalline diamond film is attained after 20 min of bias-enhanced nucleation. At the initial nucleation stages, the Si substrate appears covered with diamond crystallites and graphite, without SiC being detected by XANES.

BCN films with controlled composition obtained by the interaction between molecular beams of B and C with nitrogen ion beams

Abstract Ternary BCN compounds with controlled composition have been synthesised by using independent molecular fluxes of B and C supplied from two different electron beam evaporators, and concurrent N2+ ions from a Kauffman ion gun fed with nitrogen gas. We have established experimentally the correspondence between the ratio of B/C/N atoms arriving at the substrate and the actual film composition measured by energy dispersive X-ray spectroscopy. The composition of the BCN films does not follow simply the ratio of the impinging B/C/N fluxes, but it is determined by the complex interplay of several factors, such as the reactivity between the different species, its sticking coefficient and the possibility of formation of volatile moieties.