Electronic structure and electron emission of lithium-containing amorphous hydrogenated carbon films

Abstract

Amorphous hydrogenated carbon films were prepared by ion beam deposition. The degree of sp3 coordination and the density of the films were determined from ultraviolet photoelectron valence band spectra and electron energy loss spectra. The films prepared can be described as highly sp3 coordinated (about 80%) and of relatively high density (about 2.3 g/cm3). An indication of a negative electron affinity is found in the valence band spectra. In an attempt to lower the work function, lithium-containing a-C:H films were deposited by lithium evaporation from dispensers during deposition of the a-C:H films. Photoelectron spectra of these films reveal parallel shifts of the core levels and valence band to higher binding energies with respect to the spectra of pure amorphous hydrogenated carbon. These shifts can be attributed to a shift of the Fermi level due to a charge transfer from the lithium to the carbon matrix. From the onset of the valence band spectra at low kinetic energy and from total photoyield spectra the work function of the samples was determined independently. A reduction of the work function from 3.4 eV in pure a-C:H to 2.45 eV in samples containing 35 at. % lithium is observed.