Kinetics of charge trapping in dielectrics

Abstract

Transient electronic conduction in thermally grown SiO2 has been shown to be limited by space-charge evolution. The space charge originates from trapping of the injected species. It induces a field which affects the emission of charges at the injecting electrode. The trapping of charge has been analyzed on the basis of three, essentially different, mechanisms: (1) first order trapping, (2) first order trapping which takes into account that trapped charges repel injected charges, and (3) trapping which increases during injection due to the generation of states. It is shown that implementation of the three trapping mechanisms yields a current versus injected charge plot which is given in the asymptotes by simple logarithmic functions. Intersection of the asymptotes directly yields the value of the capture cross section. From the slopes of the asymptotes the surface density(ies) of the trap(s) can be calculated. The method can be used without, a priori, assuming either the injection mechanism or the trapping mechanism. From the relative position of the intersection points the applicable mechanism can be derived, however. For thermal SiO2 on Si we determined, by this method, values of trap densities and capture cross sections which are typical for water-related traps. Furthermore, we derived that trapping mechanism (2) fits our data best.

Analysis of breakdown in ferromagnetic tunnel junctions

“…We have no indication that the failure of the constant-Q bd model for our junction breakdown could be related to the occurrence of a current density exceeding a certain critical value, J cr , above which the density of occupied trap states is very high. In such a case the conductivity would be observed to change an order of magnitude with time in a constant applied voltage experiment, 18 a phenomenon that we did not observe.…”

“…For the case of tunneling across SiO 2 strong experimental support in favor of the wearing mechanism has been obtained by Wolters and van der Schoot, 18 who observed that for 8-40 nm SiO 2 -based capacitors under certain conditions the breakdown probability density depends on the total amount of tunneled electrons, the ͑electron͒ charge to breakdown, Q bd , which is the electron current density integrated until breakdown. The quantity Q bd ͑averaged over a large ensemble͒ was found to be independent of the type of experiment employed ͑e.g., constant voltage or current, or ramped voltage or current͒.…”

“…The quantity Q bd ͑averaged over a large ensemble͒ was found to be independent of the type of experiment employed ͑e.g., constant voltage or current, or ramped voltage or current͒. The authors 18 presented evidence that damage to the SiO 2 lattice due to energy dissipation of the tunneled electrons starts at the positive electrode, and gradually forms a low resistance path through the oxide, finally leading to breakdown. The energy available for creating damage to the oxide is argued to be independent of the applied voltage or current density because the tunneled electrons always lose most of their energy (Ͼ3 eV͒ when they enter the anode.…”

“…It has been found for SiO 2 -based systems that the constant Q bd model is valid within a good approximation for low current densities J at the moment of breakdown, but that above a critical current density, J cr , Q bd decreases rapidly with increasing J. 18 This was explained from the ͑independent͒ observation that for JϾJ cr the oxide contains a significant fraction of occupied traps ͑relatively localized electron states which at Vϭ0 are empty because their energy is higher than E F , but whose occupation increases with increasing J). The trapped electronic charge modifies the originally relatively flat equipotential planes, leading to confinement of the injected electron current to regions with a low space charge density, thereby enhancing the local current density and decreasing Q bd .…”

Charge trapping and detrapping in polymeric materials

“…It has been suggested that some defects can trap both electron and hole although the trap may have different energy levels for each type of charge carrier. 8 There have been a series of attempts to understand trapping behavior over the years as the formation of space charge in solid dielectrics has been considered as a key component to understand ageing phenomenon in the insulating materials.…”