Surface states at the silicon-electrolyte interface

Pulse measurements on the silicon-electrolyte interface are used to study surface states at the Si surface. The authors find that for CP-4-etched Si, surface states exist in the upper half of the energy gap, with a total density of 10¹² cm^-2. Addition of a minute amount of hydrofluoric acid to the electrolyte reduces their density by an order of magnitude. It has been reported that the surface recombination velocity on an oxidized surface is lowered by HF treatment, suggesting a corresponding reduction in the density of surface recombination centres. The results are the first direct evidence for such a reduction. It appears that Si-H bonds are formed at the interface, just as has been reported for an HF-treated free Si surface. Very likely, the formation of such bonds leaves, on both types of surface, considerably fewer dangling bonds and hence fewer surface states.