POSITIVE SAMPLE BIAS EFFECT IN SCANNING TUNNELING MICROSCOPE IMAGING OF LOW-COVERAGE ALKALI-METAL ATOMS ON SI(111)7X7 SURFACE

Already at coverages as low as 0.005 monolayer of potassium on room te mperature Si(111)7x7 surfaces, atomic resolution is gradually lost whe n imaging the surface with scanning tunneling microscope at positive s ample bias, giving rise to bright triangles over the faulted halves of the 7x7 unit cells and dark triangles above the unfaulted halves. We suggest that this is due to potassium atoms that are picked up by the tip from K islands at the surface, as is evident by the observed lower ing of the tunneling energy barrier. This phenomenon vanishes upon the rmal desorption of K islands or upon exposure to 0.1 L oxygen. (C) 199 8 American Vacuum Society.