When n-type silicon is heated to 1200 "C in ultra-nign vacuum to produce atomically clean surfaces it is found that the outer layer becomes p-type to a depth of several microns. The conductivity type was determined by thermal probing and the carrier density by four-point-probe measurement of sheet resistivity combined with a measurement of junction depth by scanning electron microscopy. The hole concentration could be as high as 2 x 10'5 ~r n -~ in material which had an electron concentration of 2 x 1013 (3111-3.

Atomically clean (111) silicon surfaces have been produced in ultra-high vacuum both by cleavage and by heat cleaning. These surfaces were deliberately contaminated with oxygen in a controlled manner. During this contamination the work-function changes were studied using the Kelvin vibrating capacitor technique. Schottky diodes were then made by evaporation of copper or gold on to these surfaces without breaking the vacuum. The barrier heights of these devices have been measured by the I-V , C-V and photoelectric methods. In general, the barrier height obtained from C-V data was greater than that obtained from the other two techniques and this difference was found a decreasing function of oxygen exposure. With copper the devices produced after exposure to oxygen aged, over a period of months, in such a way that the barrier height differences increased. The experimental results have not revealed any simple correlation between the work-function changes caused by oxygen adsorption and the corresponding barrier height changes caused by the presence of oxygen at the interface.