The kinetic behavior of mobile ions in the Al-SiO2-Si system

Abstract

Thermally stimulated ionic current (TSIC) measurements have been used to study the kinetic behavior of mobile ions in Al-SiO2-Si structures formed by evaporation of Al electrodes onto thermally oxidized Si slices. It is shown that the ionic current under bias-temperature stress is limited primarily by release of ions from traps at the interfaces, and that traps with a range of detrapping energies are involved. A model is proposed which accurately predicts the detrapping rate for an arbitrary temperature-time profile. Its parameters are the initial distribution of ions among the trapping states, n0(E), and a quantity, β, characterizing the attempt-to-escape frequencies for the traps. n0(E) can be derived from analysis of a TSIC curve, given a value for β which can itself be determined by performing the detrapping in two stages. Escape frequencies are typically of the order 4×1011 s−1, in line with theoretical predictions. In the (111) samples studied, n0(E) at a detrapping field of 106 V cm−1 for Na+ ions at the Si-SiO2 interface is found to have a sharp maximum at about 0.75 eV, with a tail extending upwards in energy to at least 1.5 eV. The same distribution is found irrespective of the conditions under which the ions were trapped. At the Al-SiO2 interface, n0(E) is found to depend strongly on the maximum temperature at which the ions were trapped, higher temperatures causing the maximum in the distribution to be shifted to higher energies. This is explained in terms of thermal detrapping of ions initially captured in shallow traps followed by retrapping at the same interface in deeper traps. The total number of available trapping sites at the Si-SiO2 interface is greater than 5×1012 cm−2. It is inferred that the total density of traps at the Al-SiO2 interface is much greater still.