Electronic properties of diamond/nondiamond carbon heterostructures

In this paper the properties of diamond/amorphous carbon heterostructures are studied using photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. The nondiamond carbon films are deposited on a p-doped polycrystalline diamond substrate, and produced by first, electron beam evaporation of graphite forming hydrogen-free amorphous carbon (a-C:H) films. The overlayer formation is monitored step-by-step, and the changes in band bending in the diamond substrate and the valence band discontinuities are deduced from the UPS and XPS spectra. In the diamond/a-C structure a downward band bending in diamond evolves continuously with overlayer thickness and a final value of about 1.1 eV is obtained, resulting in a band offset of 1.5±0.1eV. In the diamond/a-C:H structure a downward band bending of 1.4 eV is observed after only a brief deposition time, when the estimated overlayer thickness is still less than a monolayer. The ion energies employed for the film deposition were 200 and 600 eV and although the resulting overlayers exhibit characteristic structural differences, the band bending and band offset (1.4±0.15eV) are not noticeably influenced. It appears likely that in this case the defects formed in the diamond lattice through the energetic ions used for the overlayer deposition, are responsible for a pinning of the Fermi level at the surface. In an additional experiment the diamond substrate was irradiated with Ar⁺ ions (3 keV) and the resulting band bending, induced through the creation of a defect-rich surface layer, amounts to about 1.3 eV. The introduction of dangling bonds and/or {π}-bonded regions, which are energetically located in the gap of diamond, leads to the observed increase in downward band bending in the p-doped diamond.