No abstract

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.

The work functions of silicon single crystals have been obtained by measuring the contact potential differences between the crystals and a gold reference whose work function was measured by the Fowler method. The measurements were carried out in high vacuum (p<10−9 mm Hg). For nonfloating-zone silicon, the work function was higher for samples which were radiation quenched from 990°C than it was for samples which were annealed at 500°C. This effect was not observed for floating-zone material and was presumably a function of occluded oxygen. The work functions for the different faces after ion-bombardment cleaning were in the sequence (100)>(110)>(111). The values after quenching were: (100), 4.92 ev; (110), 4.89 ev; (111), 4.77 ev. The values after annealing were: (100), 4.82 ev; (110), 4.70 ev; (111), 4.67 ev. The work function of the (111) face of a 2000-ohmcm, p-type, floating-zone sample was 4.73 ev after quenching or annealing. Changes in both surface and volume properties were observed when silicon was heated above 1000°C in contact with quartz. Oxygen exposures of 5×10−5 mm Hg min resulted in work function increases which were larger on the annealed surfaces than on the quenched ones. This was observed for both floating-zone and oxygen-doped material and is therefore a property of the silicon itself. Photoelectric evidence indicated a diffusion of oxygen into the silicon (1) when silicon was heated in oxygen, (2) when silicon was heated at 990°C in high vacuum after O2 adsorption, and (3) when silicon with residual surface films from chemical etching was heated at 990°C prior to ion bombardment. Hydrogen exposures of 7.5×10−4 mm Hg min resulted in work-function decreases only in the presence of a heated filament. Nitrogen exposures of 4×10−6 mm Hg min resulted in no work-function changes. Heating silicon in high vacuum at 990°C for about 100 hours developed thermal-etch patterns which were rectangular on the (100) face, hexagonal on the (110) face, and triangular on the (111) face. Heating at 1080°C for 24 hours caused further etching which developed the same triangular patterns on the (111) face but left the (100) and (110) faces with a sandblasted appearance.