Formation of silicon nanodots via ion beam sputtering of ultrathin gold thin film coatings on Si
© El-Atwani et al; licensee Springer. 2011
Received: 5 February 2011
Accepted: 31 May 2011
Published: 31 May 2011
Ion beam sputtering of ultrathin film Au coatings used as a physical catalyst for self-organization of Si nanostructures has been achieved by tuning the incident particle energy. This approach holds promise as a scalable nanomanufacturing parallel processing alternative to candidate nanolithography techniques. Structures of 11- to 14-nm Si nanodots are formed with normal incidence low-energy Ar ions of 200 eV and fluences above 2 × 1017 cm-2. In situ surface characterization during ion irradiation elucidates early stage ion mixing migration mechanism for nanodot self-organization. In particular, the evolution from gold film islands to the formation of ion-induced metastable gold silicide followed by pure Si nanodots formed with no need for impurity seeding.
Nanostructuring of semiconductor surfaces via ion beam sputtering has been shown to yield a variety of ordered nanostructures [1–3]. While there is speculation about the mechanism of nanostructure evolution on compound semiconductors, the structuring of single-component semiconductor materials, and more specifically silicon, remains elusive. Although structuring of silicon surfaces using ion beam bombardment at normal incidence was first reported by R. Gago et al., studies, later on, have shown that structuring of silicon dots on silicon surfaces at zero incidence angle is possible only if a certain level of impurity is available on the surface during the sputtering process . Moreoever, other studies have shown that irradiating silicon surfaces with no impurity seeding results in surface smoothing at normal incidence [6, 7], in contradiction to the results of R. Gago et al. The role of impurities, which usually comes from the ion gun and the clips holding the samples, was discussed by Ozaydin et al.[8, 9] and Sanchez-Garcia et al. who suggested several mechanisms on how impurity seeding can induce nanostructure formation on silicon. The formation of silicides, modification of the collision cascade, and stress generation during ion bombardment were the suggested possible impurity effects on silicon nanostructuring. In this work, we report the formation of silicon nanodots on silicon substrates via low-energy ion irradiation of ultrathin film gold coatings on Si. No impurity seeding was necessary to form Si nanostructures. The gold acted as a physical catalyst to form the structures, which was later eliminated from Si nanostructures via preferential sputtering. This process is unlike the previous studies where the impurities are kept implanted in the samples due to the continuous seeding of impurity particles from ion source grids or sample grips throughout the irradiation process.
Silicon (100) samples were prepared by cleaning silicon wafers with Piranha solution (1:1, hydrogen peroxide, sulfuric acid) and subsequent acetone, water, and alcohol baths, followed by coating with gold using an SPI sputter coater. Irradiation and the in situ characterization of the samples were performed in the same chamber at a pressure of 2 × 10-8 Torr. Irradiation was performed with 200 eV of argon ions using a low-energy, broad beam ion source. The temperature of the silicon samples was kept at nearly room temperature with active cooling. During the irradiation process, the samples were characterized in situ with X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) at different fluences. XPS scans were performed with a source analyzer angle of 54.7°. A nonmonochromatic Mg Kα (1,245.3 eV) X-ray source was used with an anode voltage of 13.0 kV and an emission current of 15.0 mA. An ISS characterization was performed using a 1,500-eV He+ and a backscattering angle of 145°. The total probing beam current was 150 nA corresponding to a maximum flux of 1.4 × 1013 cm-2 s-1.
where A Au and A Si are the areas under the curves of Au and Si, respectively, and σ Au and σ Si are the laboratory elastic scattering cross sections of Au and Si, respectively.
The formation of gold silicides is a strong indication of the mixing between silicon and gold and has been previously discussed in the literature in the context of xenon and krypton irradiation [14, 15]. Their formation is marked by a 1.0-eV shift in the XPS spectra to higher binding energies of gold after mixing; this indicates a reaction between gold and silicon . Figure 3b shows the in situ XPS data. Gold 4f5/2 and 4f7/2 peaks were at 83.8 and 87.5 eV, respectively. After a fluence of 3 × 1016 cm-2, the peaks shifted by 1 to 84.8 eV and 88.5 eV, respectively. This shift is due to the formation of gold silicide. The presence of the oxygen peak in the ISS and XPS data is due to the native oxide layer on top of the silicon present before coating the silicon substrates. This layer can be eliminated at higher fluences.
After mixing, both gold and silicon were sputtered, and the gold relative concentration decreases much less rapidly as marked by the higher fluence tail of the exponential decay in the data (Figure 4 region B). Two regions are observed when combining the LEISS and XPS data in situ. Below 3 × 1016 cm-2, since the penetration depth of Ar on Au is 2-3 nm at 200 eV, only monoelemental sputtering is the dominant erosion mechanism. However, binary collision approximation calculations show that mixing occurs at about 4 × 1016 cm-2, very close to the experimental value (3 × 1016). This difference is within the relative margin of error in the ion current density measurement. After mixing ensues at 3 × 1016 cm-2, low-energy ion scattering spectroscopy (LEISS) results indicate higher gold concentration. This is because the mixing layer thickness is less than the XPS probing depth. XPS probes the mixing layer and the silicon layers underneath, thus, is more silicon-biased. At higher fluences, however, ISS and XPS results begin to converge due to the very small amounts of gold left in the mixing layer. No impurities were found on the surface during or after the formation of the structures as revealed from the XPS and ISS data. Although the sputter yield of Au is ten times that of Si, we speculate the dominant Au concentration at the top 1-2 monolayers (along the surface nanostructures) compared to the subsurface which is likely due to the ion-induced segregation mechanism since the gold surface tension is known to be lower than Si.
After the first stage of erosion of the gold film, the second stage follows with the formation of gold silicides as indicated by the XPS data. It is well-known that gold silicide formation dominates at the bottom of the island or the Au/Si interface . We conjecture that after the formation of differential silicide regions at the Au/Si interface, sputtering occurs at different rates (the silicide regions sputtering less), and thus nanostructures are effectively self-organized primarily dominated by Si. Silicides can sputter less mainly due to the enhanced binding that occurs in these cases. For example, Silicides are known to sputter about a factor of two to four times less than the pure metal component . In the third and last stage at large fluences, the Au is sputtered away, and only silicon nanostructures remain.
In conclusion, silicon nanodots can be formed via low-energy ion irradiation without permanent impurity implementation. This was achieved by irradiating gold-coated silicon surfaces with argon ions at 200 eV, where gold acted as a catalyst during the nanopatterning process and was eliminated from the silicon samples after the formation of the nanodots. Silicide formation and preferential sputtering of the silicon surfaces after the gold silicide formation are the two phenomena that govern the nanodot formation process.
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