New phases of amorphous carbon and silicon films obtained by low energy cluster beam deposition

Abstract

Since the last two decades, the physics of free clusters and assembled materials obtained from cluster deposition have opened a large field of potential applications [1]. Molecular dynamics simulations of the thin film growth by low energy cluster beam deposition (LECBD) show that impact energy (i.e. the initial free cluster energy) is the main characteristic parameter [2]. In the LECBD technique, the impact energy is low enough to have a “soft landing” process. Thus, the film grows by ballistic deposition of the free clusters if no rearrangement into clusters or coalescence between abutting supported clusters occurs. In the case of metallic clusters, small particles down to 2 nm diameter coalesce and the memory effect is destroyed [3] whereas covalent clusters present several features allowing the memory effect. Firstly, the covalent bond is highly oriented. Two abutting supported clusters can merge only if their own dangling bonds are in close direction. Secondly, the bond energy is high enough to avoid a large reconstruction because the break between two bonds cost appreciable energy. Thirdly, the equilibrium shape of the clusters present a large proportion of five-fold rings (see for example C 60 having an I h symmetry). Since the five-fold axis is not compatible with the translation symmetry, the film growth presents a large void component as in pure ballistic deposition. To study the specific properties of these new materials, we have deposited carbon and silicon clusters produced in a laser vaporization source. The properties of the films are studied using several techniques (Raman spectroscopy, XPS, EXAFS, etc.).