INTERRELATION OF CORRELATION AND DISORDER. EXPERIMENT AND THEORY.POSSIBLE CONSEQUENCES OF NEGATIVE U CENTERS IN AMORPHOUS MATERIALS

Abstract

I have proposed, and Mott and others elaborated, a model of amorphous semiconductors in which there is a fairly high density of localized centers near EF which have effectively negative U and can hence accommodate zero or two electrons, in conjunction with a mobility gap which is of order |U|. Two new aspects of these centers will be mentioned here : (1) Varma and Pandey (private communication) have proposed that such centers form at metal-insulator contacts and there may be direct experimental evidence for them. They could be responsible for the wellknown Fermi level pinning effect ascribed by Bardeen to surface states (2). Several arguments suggest that the one-electron gap itself will be a function of the pair state occupation. If so, and the relaxation rates of these states are very slow and cover a broad range as expected, they may be seen to lead to : (1) l/f resistance noise ; (2) Long-period photo-electric phenomena ; (3) Switching