Work-Function Studies of Germanium Crystals Cleaned by Ion Bombardment

The work functions of germanium single crystals have been obtained by measuring the contact potential differences between the germanium crystals and a gold reference whose work function was measured by the Fowler method. Work function values for the bombardment-cleaned surfaces of three samples having comparable resistivities were the same within experimental error (4.78±0.015 ev). Variation in the doping concentration resulted in work-function changes smaller than those predicted by a simple Fermi level shift. Adsorption of oxygen at pressures of about 1×10−7 mm Hg resulted in work-function increases of about 0.20 ev. The clean surface work-function values could be restored after O2 adsorption by heating at 500°C for 15 min. For those cases in which changes were noted, adsorption of hydrogen and nitrogen at higher pressures (10−3 mm Hg) resulted in decreases of work function, while CO adsorption increased the work function. Measurements of the effects of strong electric fields and intense illumination on the work function suggested that these effects were associated with rather thick surface layers which could be removed by vacuum heating or ion-bombardment cleaning. The photoelectric-threshold level was near or slightly above the Fermi level for the ion-bombardment cleaned surfaces. After oxygen adsorption the threshold level was below the Fermi level.