A bottom-up methodology to fabricate a nanostructured material by Au nanoclusters on 6H-SiC surface is illustrated. Furthermore, a methodology to control its structural properties by thermal-induced self-organization of the Au nanoclusters is demonstrated. To this aim, the self-organization kinetic mechanisms of Au nanoclusters on SiC surface were experimentally studied by scanning electron microscopy, atomic force microscopy, Rutherford backscattering spectrometry and theoretically modelled by a ripening process. The fabricated nanostructured materials were used to probe, by local conductive atomic force microscopy analyses, the electrical properties of nano-Schottky contact Au nanocluster/SiC. Strong efforts were dedicated to correlate the structural and electrical characteristics: the main observation was the Schottky barrier height dependence of the nano-Schottky contact on the cluster size. Such behavior was interpreted considering the physics of few electron quantum dots merged with the concepts of ballistic transport and thermoionic emission finding a satisfying agreement between the theoretical prediction and the experimental data. The fabricated Au nanocluster/SiC nanocontact is suggested as a prototype of nano-Schottky diode integrable in complex nanoelectronic circuits.
Understanding the effects of downscaling the devices dimensions to the
nanometer size is one of the most important topics in the modern material
science applied to microelectronics. In fact, the confinement of electrons in
dimensions typical of atoms and molecules obliges to consider their quantum behavior.
Therefore, a new class of effects are characterizing ultrascaled devices. In
the last years, these ideas led to the birth of
the “nanotechnology and nanoelectronic revolution” [
In particular, the nanometric level knowledge of the structural characteristics of such innovative materials and the nanometric control and manipulation of these characteristics acquired a fundamental importance in the design and realization of innovative electrical nanodevices. In fact, it is well known that the local electrical characteristics of such devices are dramatically dependent on the local structural ones. Hence, a precise control and manipulation (at atomic level) of the structural characteristics allow the precise control and manipulation of the electrical ones that are always innovative properties with respect to the traditional devices.
A promising topic of nanotechnology research is, surely, the
study of the structural and electrical properties of nanometric metal clusters
deposited on or embedded in semiconductor/insulating substrates in view of the
realization of nanostructured materials with electrical properties dependent on
and tuned by the
structural ones (clusters size, density, etc.) [
We developed a methodology to control and manipulate the clusters structural properties based on the self-organization mechanism of the Au nanoclusters (NCs) on the SiC surface induced by thermal processes. The Au clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory enabled us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process so that we achieved a control on size, size distribution, clusters distance distribution, and surface fraction of area covered by the clusters by simply controlling the process parameters.
We suggest to apply the self-organization of Au NCs as a nanotechnology step to fabricate innovative nanostructured devices. For main example, we studied, by the conductive atomic force microscopy (C-AFM) technique, the local electrical properties of the nanometric systems Au NC/SiC substrate. As expected, the main result was the strongly dependence of the electrical properties on the clusters size, density, fraction of covered area. In particular, we observed the Schottky barrier height dependence of the Au NC/SiC nanocontact on the cluster size. Furthermore, we propose a model to interpret such a behavior.
6H-SiC substrates (previously etched in 10% aqueous HF solution
to remove the native oxide) were
used. A set of substrates was covered by a 2 nm (nominally) thick Au
layer sputtered using an Emitech K550x Sputter coater apparatus (Ar plasma,
The RBS analyses were performed using 2 MeV
The local transversal current-voltage (
RBS analyses allowed to determine, in
particular, the Au atomic concentration
The change in morphology has been followed by AFM. Despite a tip-cluster, deconvolution was considered; surface morphology can present some artifacts derived by the tip-NCs interaction (not allowing an accurate determination of NCs shape and dimension). So, for a supplementary accuracy, we compared the information acquired by AFM with the NCs images obtained by SEM. From the AFM and SEM images, the NCs size distributions and the distributions of center-to-center distances between nearest NCs were determined by using a software that defines each NC area by the surface image sectioning of a plane that was positioned at half NC height. However, the results obtained by AFM and SEM analyses are in good agreement (the respective results are identical within the statistical error). So, AFM and SEM analyses were crossed to derive NC size distributions and center-to-center NC distance distributions.
As
an example, Figures
(a) AFM image of the Au clusters as-deposited on 6H-SiC substrate, (b) SEM image for the same sample, (c) AFM image of the Au clusters as-deposited on 6H-SiC substrate and annealed 1073 K-60 minutes, (d) SEM image for the same sample.
In particular, the mean NC radius
(a) Experimental
In the following, we briefly recall the kinetic growth evolution of the NCs by the coarsening (or ripening) model to explain the observed self-organization mechanism of the Au NCs on the SiC surface.
At any stage during coarsening there is a
so-called critical particle radius
The aim of mathematical modeling of
ripening process of particles dispersed (Au in our case) in/on a matrix is to
calculate the growth rate of individual particle. Despite the particular differences
derived by the
boundary conditions relative to the particular case examined, the general
theory of ripening process based on the LSW ideas has the same result for the
asymptotic temporal evolution of the system, and is summarized as follows [
The Au on SiC has a strong nonwetting
nature [
According to this considerations, if the
Au clustering on SiC surface, in the examined temperature range, is guided by a
ripening process of 3D structures limited by diffusion, the temporal variation
of the mean NC radius
Therefore, in the assessed growth modes,
the mean radius of the Au NCs on SiC in the examined temperature and time
ranges increases with time as indicated by (
Furthermore, the exposed model allows simulating the
The knowledge of the details of the self-organization mechanisms of Au NCs on SiC surfaces is a fundamental step towards innovative nanodevices design. For example, once the knowledge of the details of the Au NCs self-organization mechanisms on the SiC surface was established, we were able to probe the electrical behavior of the Au NCs/SiC nano-Schottky contacts observing their dependence on NCs size and fraction of area covered by cluster. Hence, by opportune annealing process we are able to control the structural properties of the fabricated nanostructured materials and, as a consequence, the electrical properties of nanodevices based on such systems.
The samples considered in these analyses were the as-deposited sample and
the 873 K-5 minutes, the 973 K-20 minutes, the 973 K-60 minutes, and the 1073 K-60 minutes
annealed samples. They showed NCs with mean diameter of
According to Giannazzo et al. [
Schottky
barrier height distributions. (a) Reference sample (sample without Au cluster);
sample with Au cluster mean diameter of (b)
Comparison between the data concerning the fraction of covered area by the Au clusters (normalized) derived by the structural analyses (square dots) and by the electrical ones (circular dots).
The data of Figure
In Figure
Experimental
values (dots) of the SBH as a function of mean cluster size and theoretical
prediction for
We based our interpretation of the SBH
dependence on the NCs size considering the
thermoionic transport theory through the MS barrier coupled with the concept of
ballistic transport and the constant interaction (CI) model for the electron
transport in few electrons quantum dots [
Comparing the theoretical prediction by (
The possibility of controlling and modeling size and size distribution of Au NCs deposited on SiC surface by process parameters such as thermal treatments has been demonstrated.
The clustering kinetic process and surface
diffusion of Au on SiC substrates were experimentally characterized by Rutherford backscattering spectrometry, scanning electron
microscopy, and atomic force microscopy. The evolution kinetics has been
interpreted by classical models involving surface diffusion limited ripening of
spherical three-dimensional clusters on a substrate. From the mass transfer
surface diffusion coefficients of gold on SiC hexagonal and SiO2 surfaces, determined in the