Photoinjection Studies of Charge Distributions in Oxides of MOS Structures

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Abstract
Determination of the nature and distribution of oxide charge in MOS structures is important to an understanding of oxide charging phenomena. In addition to their relevance to device failure mechanisms, it appears that charging phenomena may be used to advantage in memory device applications. The physical mechanism dominating the voltage dependence of photoinjected currents in SiO2 is the scattering of electrons in the region of the oxide between the injecting electrode and the image force potential maximum. Since the spatial position of the potential maximum depends on the electric field resulting from space charge in the oxide as well as the field due to the applied voltage, analysis of the V‐I characteristics of photoinjected currents can provide information about the spatial location of oxide charge. Presented in this paper are the results of an analysis of the effects of oxide space charge on the voltage dependence of photoinjected currents. It is shown that effective charge distributions in SiO2 can be nondestructively profiled over a range from about ten to several hundred angstroms from the injecting electrode. Since photoinjection V‐I characteristics depend on electric field at the potential maximum in the oxide, whereas C‐V characteristics depend on the field just inside the semiconductor, analysis of both characteristics permits distinction between charge in the oxide a distance greater than about 10 Å and charge located at the interface. The results of experiments on Au–SiO2–Si structures in which charge has been purposely introduced into the oxide are presented, and it is shown that the interface fixed charge and the charge associated with fast surface states extend no further than about 20 Å into the oxide from the Si–SiO2 interface.