Space-Charge Layers And Surface States At The Silicon/Electrolyte Interface

Pulse measurements on the silicon/electrolyte interface have been used to study space-charge layers and surface states on the (100) and (111) faces of silicon. The techniques used enable both the creation and study of space-charge layers at the semiconductor surface, ranging from large-depletion to strong-accumulation conditions. What is more important, they permit a straightforward separation of the different components of the induced charge at the silicon/electrolyte interface, so as to yield the variation of both the free-electron density in the space-charge layer and the density of occupied surface states with barrier height. The measured space-charge characteristics are in very good agreement with theory. The data in strong-accumulation layers indicate the presence of an insulating buffer layer (such as an oxide), 1–2 monolayers thick. As to surface states, we find that for CP-4 etched silicon, a discrete distribution of shallow states exists, located 0.1 eV on the (100) face and 0.14 eV on the (111) face below the conduction-band edge, with a total density of~6×1012cm−2. Furthermore, except for a very low continuous background, deeper surface states are absent over most of the energy gap. Addition of a minute amount of hydrofluoric acid to the indifferent electrolyte used reduces the density of the surface states by nearly two orders of magnitude. The distribution of the shallow states remains discrete but their energy position becomes somewhat deeper. It appears that Si-H bonds are formed at the Si/electrolyte interface, just as has been reported for the HF-treated free Si surface. It is very likely that the formation of such bonds leaves, on both types of surface, considerably fewer dangling bonds and hence considerably fewer surface states.