Dehydrogenation and the surface phase transition on diamond (111): Kinetics and electronic structure

The (1×1) to (2×1) surface phase transition of the hydrogen-covered diamond (111) surface is investigated by core level spectroscopy, low-energy electron diffraction, and measurements of the electron affinity. The latter method is shown to be a reliable measure of the hydrogen coverage. Prolonged annealing of the surface at 1000 K converts the hydrogen-terminated (1×1) structure with an electron affinity of -1.27 eV to a hydrogen-free (2×1) reconstruction, increases the separation of valence-band maximum from the Fermi level E_F from 0.68 to 0.88 eV, and results in a positive electron affinity of +0.38 eV. Annealing the surface at high temperature (up to 1400 K) yields the same (2×1) surface structure albeit with an increase in the separation of the valence-band maximum from E_F to 1.42 eV and a positive electron affinity of 0.8 eV which is associated with a partial surface graphitization. An analysis of the kinetics of the thermally induced hydrogen desorption yields an activation energy of 1.25±0.2eV. It was found that hydrogen desorption and reconstruction are surface phase transitions which are not directly linked. Instead, an intermediate phase with a high concentration of dangling bonds (up to 70%) is observed. The (1×1) to (2×1) phase transition is phenomenologically well described by a first-order transition provided a critical density of dangling bonds of about 70% is included in the analysis in such a way that the rate constant for reconstruction vanishes below that value.