Tetrahedral amorphous carbon-silicon heterojunction band energy offsets

Non-hydrogenated tetrahedral amorphous carbon (ta-C) has shown superior fie ld emission characteristics. The understanding of the emission mechanism ha s been hindered by the lack of any directly measured data on the band offse ts between ta-C and Si. In this paper results from direct in situ X-ray pho toemission spectroscopy (XPS) measurements of the band-offset between ta-C and Si are reported. The measurements were carried out using a filtered cat hodic vacuum arc (FCVA) deposition system attached directly to an ultra-hig h vacuum (UHV) XPS chamber via a load lock chamber. Repeated XPS measuremen ts were carried out after monolayer depositions on in situ cleaned Si subst rates. The total film thickness for each set of measurements was approximat ely 5 nm. Analysis of the data from undoped ta-C on n and p Si show the une xpected result that the conduction band barrier between Si and ta-C remains around 1.0 eV, but that the valence band barrier changes from 0.7 to 0.0 e V. The band line up derived from these barriers suggests that the Fermi lev el in the ta-C lies 0.3 eV above the valence band on both p and n + Si. The heterojunction barriers when ta-C is doped with nitrogen are also presente d. The implications of the heterojunction energy barrier heights for field emission from ta-C are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.