F. Ruffino

Atomic force microscopy study of the growth mechanisms of nanostructured sputtered Au film on Si(111): Evolution with film thickness and annealing time

Nanostructured Au films were deposited on Si(111) by room-temperature sputtering. By the atomic force microscopy technique we studied the evolution of the Au film morphology as a function of the film thickness h and annealing time t at 873 K. By the study of the evolution of the mean vertical and horizontal sizes of the islands forming the film and of their fraction of covered area as a function of h from 1.7×1017 to 1.0×1018 Au/cm2 we identified four different growth stages such as: (1) 1.7×1017≤h≤3.0×1017 Au/cm2, nucleation of nanometric three-dimensional (3D) hemispherical Au clusters; (2) 3.0×1017<h≤5.2×1017 Au/cm2, lateral growth of the Au clusters; (3) 5.2×1017<h≤7.7×1017 Au/cm2, coalescence of the Au clusters; (4) 7.7×1017<h≤1.0×1018 Au/cm2, vertical growth of the coalesced Au clusters. The application of the dynamic scaling theory of growing interfaces allowed us to calculate the dynamic scaling exponent z=3.8±0.3, the dynamic growth exponent β=0.38±0.03, the roughness exponent α=1.4±0.1 and the A...

Self-organization of gold nanoclusters on hexagonal SiC and SiO2 surfaces

Very thin Au layers were deposited on SiC hexagonal and SiO2 substrates by sputtering. The Au surface diffusion, clustering, and self-organization of Au nanoclusters on these substrates, induced by thermal processes, were investigated by Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. On both types of substrates, clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory allowed us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process. The system Au nanoclusters/SiC and Au nanoclusters/SiO2 are proposed as nanostructured materials for nanoelectronic and nanotechnology applications.

Novel approach to the fabrication of Au/silica core?shell nanostructures based on nanosecond laser irradiation of thin Au films on Si

We demonstrate the possibility of producing Au/SiO(2) core-shell nanoparticles by nanosecond laser irradiation of thin (5 and 20 nm) Au films on Si. The Au/Si eutectic reaction and dewetting process caused by the fast melting and solidification dynamics induced by the nanosecond laser irradiations are investigated as the origin of the formation of core-shell nanoparticles. Using several microscopic techniques (Rutherford backscattering spectrometry, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and energy filtered transmission electron microscopy) the formation and evolution of the core-shell structures are investigated as a function of the laser fluence in the 500-1500 mJ cm(-2) range for both film thicknesses. In particular, the mean height and diameter and surface density evolution of the core-shell structures are quantified and correlated to the laser fluence and Au film thickness.

Island-to-percolation transition during the room-temperature growth of sputtered nanoscale Pd films on hexagonal SiC

We have studied the growth of room-temperature sputtered Pd films on 6H-SiC by using the atomic force microscopy technique. In particular, we analyzed the Pd film surface morphology as a function of the film thickness from 3 to 72 nm observing that the Pd grows initially (thickness 2–12 nm) as three-dimensional (3D) islands. Then (thickness 12–36 nm) the Pd film morphology evolves from compact 3D islands to partially coalesced wormlike structures, followed (36–60 nm) by a percolation morphology and finally to a continuous and rough film (at 72 nm). The application of the interrupted coalescence model allowed us to evaluate the critical mean islands diameter Rc≈6.6 nm for the partial coalescence process while the application of the kinetic freezing model allowed us to evaluate the room-temperature Pd surface diffusion coefficient Ds≈1.4×10−17 m2/s on 6H-SiC. Finally, the application of the Vincent’s model allowed us to evaluate the critical Pd coverage Pc=68% for the percolation transition.

Atomic Force Microscopy Study of the Kinetic Roughening in Nanostructured Gold Films on SiO2

Dynamic scaling behavior has been observed during the room-temperature growth of sputtered Au films on SiO2using the atomic force microscopy technique. By the analyses of the dependence of the roughness, σ, of the surface roughness power,P(f), and of the correlation length,ξ, on the film thickness,h, the roughness exponent,α = 0.9 ± 0.1, the growth exponent,β = 0.3 ± 0.1, and the dynamic scaling exponent,z = 3.0 ± 0.1 were independently obtained. These values suggest that the sputtering deposition of Au on SiO2at room temperature belongs to a conservative growth process in which the Au grain boundary diffusion plays a dominant role.

Pd/Au/SiC Nanostructured Diodes for Nanoelectronics: Room Temperature Electrical Properties

Pd/Au/SiC nanostructured Schottky diodes were fabricated embedding Au nanoparticles (NPs) at the metal-semiconductor interface of macroscopic Pd/SiC contacts. The Au NPs mean size was varied controlling the temperature and time of opportune annealing processes. The electrical characteristics of the nanostructured diodes were studied as a function of the NPs mean size. In particular, using the standard theory of thermoionic emission, we obtained the effective Schottky barrier height (SBH) and the effective ideality factor observing their dependence on the annealing time and temperature being the signature of their dependence on the mean NP size. Furthermore, plotting the effective SBH as a function of the effective ideality factor we observe a linear correlation, indicating that the Au NPs act as lateral inhomogeneities in the Schottky diodes according to the Tungs model. Therefore, we can control the size, fraction of covered area, and surface density of such intentionally introduced inhomogeneities. The application of the Tungs model for the electronic transport in inhomogeneous Schottky contacts allow us to obtain, in particular, the homogeneous SBH. These nanostructured diodes are proposed as possible components of integrated complex nanoelectronic devices.

Kinetic mechanism of the thermal-induced self-organization of Au/Si nanodroplets on Si(100): Size and roughness evolution

Very thin Au layer was deposited on Si(100) using the sputtering technique. By annealing at 873 K Au/Si nanodroplets were formed and their self-organization was induced changing the annealing time. The evolution of droplet size distribution, center-to-center distance distribution, and droplet density as a function of the annealing time at 873 K was investigated by Rutherford backscattering spectrometry, atomic force microscopy (AFM), and scanning electron microscopy. As a consequence of such study, the droplet clustering is shown to be a ripening process of hemispherical three-dimensional structures limited by the Au surface diffusion. The application of the ripening theory allowed us to calculate the surface diffusion coefficient and all other parameters needed to describe the entire process. Furthermore, the AFM measurements allowed us to study the roughness evolution of the sputtered Au thin film and compare the experimental data with the dynamic scaling theories of growing interfaces.

Atomic force microscopy investigation of the kinetic growth mechanisms of sputtered nanostructured Au film on mica: towards a nanoscale morphology control

The study of surface morphology of Au deposited on mica is crucial for the fabrication of flat Au films for applications in biological, electronic, and optical devices. The understanding of the growth mechanisms of Au on mica allows to tune the process parameters to obtain ultra-flat film as suitable platform for anchoring self-assembling monolayers, molecules, nanotubes, and nanoparticles. Furthermore, atomically flat Au substrates are ideal for imaging adsorbate layers using scanning probe microscopy techniques. The control of these mechanisms is a prerequisite for control of the film nano- and micro-structure to obtain materials with desired morphological properties. We report on an atomic force microscopy (AFM) study of the morphology evolution of Au film deposited on mica by room-temperature sputtering as a function of subsequent annealing processes. Starting from an Au continuous film on the mica substrate, the AFM technique allowed us to observe nucleation and growth of Au clusters when annealing process is performed in the 573-773 K temperature range and 900-3600 s time range. The evolution of the clusters size was quantified allowing us to evaluate the growth exponent 〈z〉 = 1.88 ± 0.06. Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters. From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T)=[(7.42×10−13)±(5.94×10−14)m2/s]exp(−(0.33±0.04)eVkT). These quantitative data and their correlation with existing theoretical models elucidate the kinetic growth mechanisms of the sputtered Au on mica. As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.

Normal and abnormal grain growth in nanostructured gold film

Thin nanostructured gold films were deposited on SiO2 by the sputtering technique at room temperature. Films of different thicknesses were deposited ranging from 2 to 16 nm. The film morphology as a function of the thickness was analyzed by microscopic techniques such as atomic force microscopy and transmission electron microscopy. These analyses allowed us to clarify the growth mechanism of the gold nanograins forming the film: in a first stage of growth (2–6 nm) normal grain growth proceeds; then (8–16 nm) the grain surface energy anisotropy drives the growth of abnormal large gold grains by annihilation of the normal ones. During the abnormal growth other normal grain continue to growth. The normal grain size distribution is showed to be a monomodal log-normal distribution that evolves toward larger mean grain radius continuously following a scaling law. By determination of the grain growth exponent, the kinetic mechanism responsible of the grain growth is demonstrated to be the gold atomic diffusion o...

Size-dependent Schottky Barrier Height in self-assembled gold nanoparticles

(Au nanocluster)/6H-SiC Schottky contacts were electrically characterized by conductive atomic force microscopy, collecting a high number of current-voltage (I-V) curves. The main observed result is the Schottky barrier height (SBH) dependence on the cluster size. The SBH increases from 1.35±0.01to1.77±0.01eV when the cluster size increases from 1.5to6.8nm and it tends, asymptotically, to the theoretical SBH of the macroscopic contact Au∕SiC (∼1.9eV). This behavior is interpreted considering the thermoionic transport theory through the Au cluster/SiC barrier coupled with the concept of ballistic transport within few electron quantum dots.