Characterization of Traps in the Transition Region at the HfO[sub 2]∕SiO[sub x] Interface by Thermally Stimulated Currents

Abstract

Thermally stimulated currents ͑TSCs͒ have been measured to investigate electron traps in HfO 2 prepared by reactive sputtering on silicon. Broken planes of the silicon crystal, which may contribute to the occurrence of interface states, were identified between the silicon and SiO x interlayer by transmission electron microscopy ͑TEM͒. A second domain was found between SiO x and HfO 2 constituting a gradual transition region between the two oxides. This interface region was found to be a source of unstable charge traps where captured electrons interact with the silicon energy states through a combined tunneling and thermal process.

Influence of the SiO2 interlayer thickness on the density and polarity of charges in Si/SiO2/Al2O3 stacks as studied by optical second-harmonic generation

“…52 The tunneling process thus likely involves some kind of trap-assisted tunneling (TAT) mechanism, which, given the fact that a post-deposition anneal is required to activate a large negative charge density, might be thermally induced. 53,54 Nevertheless, assuming that the tunneling process can be described by a direct-tunneling-like mechanism, the charge density at the SiO 2 /Al 2 O 3 interface can be written as…”

Electron States in MOS Systems

“…5 (b) from consecutive measurements on the same sample. Repeated measurements gave rise to peak positions at various temperatures, suggesting structural instabilities of these traps thus changing their energy positions and emission properties [31]. The solid TSC curves in this diagram are theoretical while the measured data coincide only up to a certain temperature, where the signal collapses.…”

“…In this transition region, a high concentration of traps is expected. For the HfO 2 /SiO x stack, it was recently demonstrated that these traps are structurally unstable and communicate with the free carriers in the silicon crystal by a combined thermal-tunneling process [30,31].…”

“…We observe that Fowler-Nordheim tunneling into the stoichiometric HfO 2 range is possible only for voltages at about 1.6 V. Above this voltage the tunneling process takes place inside the transition region. Here, a large concentration of acceptor traps has been found to exist [31], which gives the opportunity for increased tunneling processes of trap assisted type. Due to the influence of the lower k-values in the transition region, for the present stack the effective kvalue is 11.9, corresponding to EOT = 1.3 nm.…”

“…From thermally stimulated current (TSC) measurements on metal/HfO 2 /SiO x /Si structures it has been found that traps in the transition region discussed above, communicate with states in the silicon conduction band by a combined thermal-tunneling process [31].…”