Limitations upon photoinjection studies of charge distributions close to interfaces in MOS capacitors

Abstract

Carriers can be injected into the oxide of a metal‐oxide‐semiconductor (MOS)capacitor by illuminating either electrode with ultraviolet light. Once injected, a voltage across the capacitor will cause the injected carriers to flow, constituting a photocurrent. The voltage dependence of this photocurrent is affected by any charge distribution which may exist in the oxide near the injecting electrode. Such oxide charge affects the photocurrent in two ways. First, the oxide charge affects the barrier position, or the distance that an electron must travel before it becomes a candidate for injection. Since the probability of a scattering event en route increases with this distance, the effect of the charge on the barrier position will modify the photocurrent. Second, the oxide charge contributes to the electrostatic potential within the oxide and hence to the barrier height. Any such effect of the oxide charge on the barrier height will affect the number of electrons injected, and hence also the photocurrent. In this paper the two limiting cases when either one effect or the other dominates are examined. The minimum distance from the interface at which charge can be detected is determined for both limiting cases. It is found that this minimum distance is of the order of the scattering length of the injected carriers for the ``barrier‐position'' limit of measurement, and is limited only by the minimum absolute photocurrent detectable experimentally for the ``barrier‐height'' limit. Applying these results to the measurements of Powell and Berglund on gold‐oxide‐silicon structures we find that 90% of the so‐called ``fixed charge'' always found at the Si–SiO2interface cannot extend more than one scattering length (≈34 Å) into the SiO2. This result is obtained without assuming any special form for the charge distribution normal to the interface but assuming lateral uniformity parallel to the interface. Consequently, loss of sensitivity due to charge discreteness effects has not been considered. However, it is argued that such effects can be minimized by using the ``barrier‐height'' mode of measurement at low voltages. It is also suggested that lateral nonuniformities may be measurable via the dependence of the photocurrent on barrier position in the ``barrier‐height'' mode of measurement.