Long-term relaxation of the surface conductivity of silicon

Abstract

In addition to the familiar fast and slow relaxations of the surface conductivity in germanium and silicon, silicon shows a relaxation with a time constant of 6.4 hr. This effect appears either after exposure to wet nitrogen or after heating to 150 °C for a short time in dry nitrogen. This long-term relaxation effect is attributed to the transformation of the electrically active chemisorbed water to the neutral silanol groups. The mechanism of chemisorption occurs only at discrete active centers which then become poisoned by the formation of the silanol groups. However upon heating for several days at 200 °C in dry nitrogen, the long-term relaxation effect disappears irreversibly and the surface conductivity changes from n-type to p-type. But if the sample is then exposed to wet nitrogen, the surface conductivity returns again to n-type.