(a) Large area STM scan of an -covered film (, gray scale , roughness , , and ). The height difference between the large terraces is corresponding to the atomic step height on the films. (b) Enlarged view of a subregion of (a) (, gray scale , and roughness ). The roughness within a single terrace (about ) is larger than within an atomically flat terrace. (c) STM image of another sample showing small islands (, gray scale , and roughness ). These islands have an average height of , which is close to the height of steps. The average width of the island is .
spectra of an -covered film: (a) locally determined differential conductivity measured for different tip-sample separations. The arrows indicate the change of the conductivity due to decreasing tip-sample distances. The dotted curve gives an example of the often observed case that a broad shoulder develops from the lower conduction-band edge into the band-gap region. (b) Normalized conductivity calculated from the average of single curves which have been acquired equally distributed across an area of . The energy of the small peaks (signatures) matches the energy of the critical points in the bulk band structure of and has been labeled accordingly.
(a) spectra of an -covered film. Exemplary calculated from a curve (solid curve, left scale) and, for comparison, measured while scanning another ramp (dashed curve, right scale). The peaks of at around are interpreted as being due to the band edges of the or corresponding to a band gap of as expected. (b) Energy scheme of the system vacuum tip at zero bias. Conduction-band edge (CBE) and valence-band edge (VBE) indicate the conduction- and the valence-band edges of the oxide, respectively. denotes the observed defect states within the band gap of the oxide.
A sequence of small area scans of an -covered film ( and gray scale ) taken at dc tunneling voltages of 0, 0.5, 1.0, 1.5, 2.0, and . Additionally to the dc bias, an ac tunneling voltage of was applied. The growing density of hillocks for increasing dc tunneling voltages is clearly visible.
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