Interaction of Radiation with Surfaces and Electron Tunneling · Figs. 24 - 36

This chapter discusses scanning tunneling microscopy (STM) images of semiconductors includes germanium (Ge), gallium arsenide (GaAs) and graphite. Tunneling spectroscopy measurements on the Ge(001) surfaces with two occupied surface states at 0.9 eV and 2.7 eV below E_F, and an unoccupied surface state at 0.9 eV above E_F are observed. Tunneling spectroscopy measurements of the Ge(111 )c(2 x 8) reconstruction show occupied states near -1.6 and -0.8 eV and unoccupied states near +0.4 and +1.5 eV. In GaAs(100), the gallium-rich face shows a 4 x 2 or c(8 x 2) reconstruction, and the arsenic-rich face shows a 2 x 4 or c(2 x 8) reconstruction. The STM measurements show that the surface atoms pair together, forming arsenic dimers. STM images of GaAs(110) obtained at positive sample bias reveal the positions of the unoccupied states concentrated on the Ga atoms, while images at negative bias reveal the occupied states concentrated on the As atoms. The GaAs(111)-A surface exhibits a 2 x 2 reconstruction. STM images at positive sample bias are interpreted as revealing the locations of the Ga atoms. Graphite is a rather unusual material due to its 2-dimensional layer structure. STM image of graphite acquired under ultrahigh vacuum conditions with a clean tip, along with a superimposed unit cell for scale is shown. STM images of graphite routinely observe only half the surface-layer atoms.