Work Function and Sorption Properties of Silicon Crystals

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 (p10−9 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.