Probing of impurity potential well at the Si/SiO2 interface by electric-field-enhanced emission

Abstract

A transient capacitance method is presented, which enables the study of electric-field-enhanced emission from a continuum of surface states at the Si/SiO2 interface. The enhancement of trap emission is measured in Al-SiO2-Si structures by the variation in the transient capacitance signal detected under a constant applied electric field, the trap emission having been previously activated by application of a higher electric field (excitation bias). The dipolar relaxation contribution to the transient signal observed in these structures is substracted during the experiment. The decrease in the resulting transient signal with the excitation bias clearly reveals the donor nature of the interface traps studied. Since the surface traps are highly localized at the Si-SiO2 interface, the electric field on the impurity is perfectly known. Assuming the sole presence of the Frenkel-Poole effect in our experimental conditions, the barrier lowering is obtained as a function of the applied electric field. From these data, the shape of a portion of the impurity potential well is deduced for different levels in the Si gap, in the direction normal to the Si-SiO2 interface. The results are in good agreement with a coulombic potential barrier at some distance from the impurities (∼10 nm).