Imaging atomic orbitals in STM experiments on a Si(1 1 1)-(7 × 7) surface

We report on scanning tunnelling microscopy (STM) studies demonstrating the subatomic features in the images of a Si(1 1 1)-(7 × 7) surface. The STM images with double atomic features corresponding to individual surface adatoms were measured with a Si terminated tip at different bias voltages and tip-sample separations. The observed features related to the contribution of two dangling bonds of the silicon apex atom are qualitatively similar to the ones observed earlier in high resolution atomic force microscopy experiments [F.J. Giessibl, S. Hembacher, H. Bielefeldt, J. Mannhart, Science 289 (2000) 422]. The gap resistance dependence of the images demonstrates that the observed effect is more pronounced at small tip-surface separations and low bias voltages.