Effect of surface hydrogen on metal-diamond interface properties

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Abstract
Schottky barrier formation of metal contacts deposited on diamond (111) and (100) surfaces was investigated. Three different metals (Au, Al, and Ti) were studied because of their different chemical reactivity with C, i.e., Au being a nonreactive metal, Al a weak carbide‐forming metal, and Ti a strong carbide former. Both fully H‐terminated, unreconstruted (1×1) surfaces and H‐desorbed, reconstructed (2×2)/(2×1) surfaces with higher density of surface states were examined. Surface structures were determined via low‐energy electron diffraction, and the change of surface band bending (SBB) and the interface chemistry during the contact formation were monitored using x‐ray photoelectron spectroscopy. On the reconstructed surfaces, the SBB was independent of metal thicknesses. This was attributed primarily to Fermi‐level pinning by the high density of surface states on the reconstructed surfaces. On the other hand, the surface‐state densities were much lower on the unreconstructed surfaces and thus the Fermi level was not as strongly pinned as that on the reconstructed surfaces. When the metal coverage became large (≳0.5 monolayer) on these unreconstructed surfaces, the charge transferred from the metal contacts to the diamond, the Fermi level moved upward in the band gap and the SBB increased. However, even on the unreconstructed surfaces where the density of pinning states were low, the SBB for thick metal contacts did not correlate to metal properties such as work function and/or electronegativity. Regardless of the state of the diamond surfaces, it was also important to consider the interface chemistry (chemical reactivity of the metal on diamond) to understand the change of the SBB after annealing the metal contacts.