Field ion microscopy of germanium: Field ionization and surface states

Field evaporated Ge tips were imaged with Ar and Ne at 80° K. The ions of these gases display identical field ion images. A striking correlation between the regional brightness of FIM and FEM patterns was found. Electron shadow microscopy and the behavior of FIM images above BIV indicate no local field enhancements, thus this correlation must be explained by the electronic structure of the surface. Explanation is furnished by a model, where electrons of the image gas tunnel preferentially into surface states. This model, based on the known fact that surface states of Ge contribute in field emission, is supported by the observation that in regions of low index poles, oxygen adsorption decreases the surface density of centers above which field ionization occurs.