Thermally stimulated ionic current (TSIC) measurements have been used to study the kinetic behavior of mobile ions in AI-Si0 2 -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 temperaturetime profile. Its parameters are the initial distribution of ions among the trapping states, nll(E), and a quantity, /3, characterizing the attempt-to-escape frequencies for the traps. n(l(E) can be derived from analysis of a TSIC curve, given a value for /3 which can itself be determined by performing the detrapping in two stages. Escape frequencies are typically of the order 4 X 1011 S 1, in line with theoretical predictions. In the (111) samples studied, nllE) at a detrapping field of 10" V cm 1 for Na' ions at the Si-SiO, interface is found to have a sharp maximum at about 0.75 eV, with a tail extending upwards in energy to at least l.5 eV. The same distribution is found irrespective of the conditions under which the ions were trapped. At the AI-Si0 2 interface, nfl(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-SiO, interface is greater than 5 X 10 12 cm 2. It IS inferred that the total density of traps at the AI-Si0 2 interface is much greater still. P ACS numbers: 73.40.Qv, 66.30.Jt, 82.20.Pm
Experiments have been done to measure the trapping and neutralization kinetics of sodium ions in Al-SiO2-Si structures using oxides grown with HCl-Ar/O mixtures. The trapping of sodium at the Si-SiO2 interface was quantified by thermally stimulated ionic current (TSIC) measurements made at negative applied electric fields following positive bias temperature stress. There were three distinct types of behavior depending on the chlorine content of the oxide. For chlorine contents ≳3.0×1015 atoms cm−2 the TSIC results showed two sodium peaks, with maxima at about 0.8 and 1.5 eV, corresponding to states at the Si-SiO2 interface in which the ions were on the whole charged and neutral, respectively. Complete neutralization of the sodium, and good stability with respect to detrapping from the Si-SiO2 interface under typical device operating conditions, was possible only in samples containing ≳3.0×1015 Cl atoms cm−2. A Schottky-type variation of the neutral peak trap energy with applied field was measured, with coefficient (19±1)×10−5 eV V−1/2 cm1/2. For chlorine contents in the range (0.9–2.8)×1015 cm−2 the TSIC spectra showed two overlapping peaks, with maxima at 0.8 and about 1.25 eV. In this case only part of the second peak at energies above 1.25 eV corresponded to ions which were completely neturalized. For chlorine contents <0.8×1015 cm−2, the TSIC spectra showed only one sodium peak, at about 0.8 eV, similar to that observed in dry-grown oxides which do not contain chlorine. It was found that a fraction of the ions, about 20%, were neutralized even in dry-grown oxides. The amount of sodium neutralization was measured as a function of time and temperature of positive-bias stress, magnitude of applied electric field, and oxide chlorine content. A method was used which separated the rate of neutralization at the Si-SiO2 interface from the overall process. For samples with chlorine content ≳3.0×1015 cm−2, the time dependence of the interfacial neutralization process was consistent with a model in which lateral diffusion of sodium, to chlorine-containing ’’islands’’ at the Si-SiO2 interface, is the rate-limiting process. The activation energy of the interfacial neutralization process, (0.87±0.15)eV, is in agreement with that of lateral diffusion, (0.8±0.2)eV.
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