Several kinds of (100) surfaces of homoepitaxial diamond films doped with boron are investigated. The valence band maximum at the surface and electronic affinity are measured using ultraviolet and X-ray photoelectron spectroscopies. In p-type doped films, resistivity measurements indicate that the bulk Fermi level is pinned close to 0.4 eV above the valence band edge, in contrast to usual natural or synthetic crystals. This fact permits to derive the band curvature at these surfaces. All of them show a downward band bending (hole depletion) of 0.3 eV for the H-surface and 0.9 eV for the free surface, respectively, but only the hydrogenated surface exhibits a negative electronic affinity. Refractory and fairly adhesive metallic contacts can be achieved with the help of carbide formation on diamond and are able to increase band bending. New peaks in the C1s core level spectrum, characteristics of the carbide formation, appear at lower binding energies only on the hydrogenated and free surfaces but the carbide growth is favoured on the free surface. When the carbide layer exceeds some thickness, the contact is Ohmic whereas if the carbide is limited to a few monolayers, it can show rectifying properties with a potential barrier height up to 3.4 eV.