Abstract
Buckminsterfullerene (C60) is a molecule fully formed of carbon that can be used, owing to its electronic and mechanical properties, as “clean” precursor for the growth of carbon-based materials, ranging from -conjugated systems (graphenes) to synthesized species, e.g. carbides such as silicon carbide (SiC). To this goal, C60 cage rupture is the main physical process that triggers material growth. Cage breaking can be obtained either thermally by heating up the substrate to high temperatures (630°C), after C60 physisorption, or kinetically by using Supersonic Molecular Beam Epitaxy (SuMBE) techniques. In this work, aiming at demonstrating the growth of SiC thin films by C60 supersonic beams, we present the experimental investigation of C60 impacts on Si(111) 7x7 kept at 500°C for translational kinetic energies ranging from 18 to 30 eV. The attained kinetically activated synthesis of SiC submonolayer films is probed by in-situ surface electron spectroscopies (XPS and UPS). Furthermore, in these experimental conditions the C60-Si(111) 7×7 collision has been studied by computer simulations based on a tight-binding approximation to Density Functional Theory, DFT. Our theoretical and experimental findings point towards a kinetically driven growth of SiC on Si, where C60 precursor kinetic energy plays a crucial role, while temperature is relevant only after cage rupture to enhance Si and carbon reactivity. In particular, we observe a counterintuitive effect in which for low kinetic energy (below 22 eV), C60 bounces back without breaking more effectively at high temperature due to energy transfer from excited phonons. At higher kinetic energy (22 < K < 30 eV), for which cage rupture occurs, temperature enhances reactivity without playing a major role in the cage break. These results are in good agreement with ab-initio molecular dynamics simulations. SuMBE is thus a technique able to drive materials growth at low temperature regime.
Highlights
The synthesis of carbon-based thin films and nanostructured compounds, such as carbides and graphene, on top of semiconductor or metal surfaces represents a serious challenge to the production of electronic devices and materials coating
Using photoelectron spectroscopy and low-energy electron diffraction (LEED), we explore the chemical, structural, and electronic changes induced by varying the supersonic C60 beam kinetic energies (KEs) at fixed substrate temperature
The ultra-high vacuum (UHV) chamber is equipped with a CLAM2 Electron Hemisperical Analyzer, an MgKα X-ray source, and a helium discharge lamp, the excitation photon for X-ray photoelectron spectroscopy (XPS) is at 1253.6 eV and for ultraviolet photoelectron spectroscopy (UPS) the HeI at 21.22 eV
Summary
The synthesis of carbon-based thin films and nanostructured compounds, such as carbides and graphene, on top of semiconductor or metal surfaces represents a serious challenge to the production of electronic devices and materials coating. The low energy deposition of fullerene on silicon has been deeply studied with experimental approaches, mainly using standard techniques (Sakamoto et al, 1999; De Seta et al, 2000; Sanvitto et al, 2000; Balooch and Hamza, 1993). MBE, in particular, has shown to be a viable approach to silicon carbide (3C–SiC) synthesis at about 800°C, using fullerene (C60) as carbon precursor and silicon as a growth substrate (Sanvitto et al, 2000; De Seta et al, 2000). The required high temperature, leads to structural defects at the nano- and micro-scale, mainly due to high lattice mismatch between SiC and Si and due to increasing thermal diffusion through the SiC film. Only amorphous Si films can be achieved at 800°C, and postdeposition thermal treatments at higher temperatures are necessary to improve crystallinity
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