Hydrogen-related structural changes on CVD diamond (1 0 0) surfaces by ultra-high-vacuum annealing

Using an ultra-high-vacuum (UHV) scanning tunneling microscope (STM), we have investigated surface atomic structures of single-crystalline (1 0 0) diamond homoepitaxially grown by means of a microwave-plasma (MWP) chemical-vapor-deposition (CVD) method. STM images taken from B-doped (p-type) as-grown (H-terminated) diamond samples showed partially amorphous-like structures and partially unclear features related to (2x1)/(1x2) structures characteristic of H-terminated diamond (1 0 0). While the latter structures became clearer with increasing periods and temperatures of UHV annealing treatments well below 500 deg. C, substantially clear (2x1)/(1x2) structure images were obtained only after a sufficient annealing above 500 deg. C. A moderate UHV annealing at 150-200 deg. C sometimes resulted in the presence of disordered short-range (3x1) structures featured by row separations of 0.35 and 0.40 nm in very limited areas beside the (2x1)/(1x2) structures. These observations suggest the existence of both C-H and C-H2 bonds at the topmost surfaces, substantially small but possible changes in surface fractions of the C-H and C-H2 species during the UHV annealing and the final surface occupation of the C-H bonds. In addition, STM images differently featured for both occupied and unoccupied states of the (2x1) structure are discussed in relation to bonding and anti-bonding states of the surface C-H bonds