The Si-SiO2 Interface - Electrical Properties as Determined by the Metal-Insulator-Silicon Conductance Technique

The electrical properties of semiconductor surfaces have been studied for a long time. Until recently most investigators were concerned with either etched surfaces covered by a thin natural oxide 1055 1056 TI-IE BELL SYSTEM TECHNICAL JOURNAL, J U L Y - A U G U S T 1967 layer or with ultraclean surfaces that exist only in high vacuum. Development of improved oxidation techniques for silicon have made it now possible to study the electrical phenomena occurring at the interface between silicon and silicon dioxide. These studies resulted in the recognition that a semiconductor-insulator interface behaves differently in many ways from a "bare" surface. Of practical importance is the fact that an oxide-covered surface is more stable and can be made electrically more perfect than an unprotected surface. The difference between oxidized and bare surfaces will be listed here briefly. (i) There is no charge exchange with states at the air oxide interface. This type of state, the "slow surface state," has a long time constant and is encountered on etched surfaces. For thick oxide layers, only states at the oxide-semiconductor interface have to be considered. They are equivalent to "fast states" and will be called interface states in this paper. In addition to interface states, there may be traps within the oxide. 1,2 Not much is known about these traps and they will not be a major topic of this paper. (w) All oxidized silicon surfaces contain a so-called surface charge. It consists of positive charges residing close to the interface.