When n-type silicon is heated to 1200 degrees C in ultra-high 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*10 15 cm -3 in material which had an electron concentration of 2*10 13 cm -3 .
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Mottram et al. (1975) studied this question.
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