Space-Charge Model for Surface Potential Shifts in Silicon Passivated with Thin Insulating Layers

Semipermanent changes in the semiconductor surface potential occur in insulator-covered semiconductors when external fields are applied for long times, particularly at elevated temperatures. An attempt to explain these changes in terms of the charging and discharging of interface states leads to conclusions that disagree with many of the experimental facts. Specifically, the semipermanent effects of interface-state charges can always be overcome by the application of a field smaller than that which is used to induce the effect, and of the same sign, while the experiments described in the accompanying papers generally show that a field much larger than the inducing field, and of the opposite sign, is required to return the insulator covered surface to its initial status. The accumulation of space charge in the insulating layer can give rise to very large fields at the semiconductor surface that persist after the removal of an external inducing field. The size and sign of such space-charge fields agree with the experimental observations. Measurements made after treatment at temperatures above 125°C show that the surface of silicon passivated with silicon dioxide can become strongly n-type as a result of such a positive space-charge layer formed at the interface. A model is presented based on the concept that this space charge arises from oxygen vacancies in the silicon dioxide. It is suggested that the improvement resulting from the use of phosphorus pentoxide on the outside surface is due to the elimination of vacancies by the oxidizing action of the phosphorus pentoxide.