By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament (CF) features controlling TiN/HfO2/TiN resistive memory (RRAM) operations. The leakage current through the dielectric is found to be supported by the oxygen vacancies, which tend to segregate at hafnia grain boundaries. We simulate the evolution of a current path during the forming operation employing the multiphonon trap-assisted tunneling (TAT) electron transport model. The forming process is analyzed within the concept of dielectric breakdown, which exhibits much shorter characteristic times than the electroforming process conventionally employed to describe the formation of the conductive filament. The resulting conductive filament is calculated to produce a non-uniform temperature profile along its length during the reset operation, promoting preferential oxidation of the filament tip. A thin dielectric barrier resulting from the CF tip oxidation is found to control filament resistance in the high resistive state. Field-driven dielectric breakdown of this barrier during the set operation restores the filament to its initial low resistive state. These findings point to the critical importance of controlling the filament cross section during forming to achieve low power RRAM cell switching.

Thermal decomposition behavior of the HfO 2 / SiO 2 / Si system J. Appl. Phys. 94, 928 (2003); 10.1063/1.1578525 Interfacial properties of ZrO 2 on silicon J. Appl. Phys. 93, 5945 (2003); 10.1063/1.1563844Yttrium silicate formation on silicon: Effect of silicon preoxidation and nitridation on interface reaction kineticsThe interfacial chemistry of the high-k dielectric HfO 2 has been investigated on nitrided and un-nitrided Si͑100͒ using x-ray photoelectron spectroscopy ͑XPS͒ and secondary ion mass spectroscopy ͑SIMS͒. The samples are prepared by sputter depositing Hf metal and subsequently oxidizing it. A 600°C densification anneal is critical to completing Hf oxidation. These spectroscopic data complement electrical testing of metal oxide semiconductor capacitors fabricated with ϳ50 Å HfO 2 on nitrided and un-nitrided Si͑100͒. Capacitors with interfacial nitride show reduced leakage current by a factor of 100 at a Ϫ1 V bias. Concurrently, interfacial nitride increased capacitance 12% at saturation. XPS shows that an interfacial layer composed of nonstoichiometric hafnium silicate (HfSi x O y ), forms at both the HfO 2 /Si and HfO 2 /SiN x interfaces. Differences in the Si 2p and O 1s XP spectra suggest more silicate forms at the un-nitrided interface. HfO 2 films on un-nitrided Si show more O 1s and Si 2p photoemission intensity characteristic of HfSi x O y . SIMS depth profiles through the buried interface are consistent with interfacial silicate formation, as shown by a HfSiO ϩ ion signal, that is sandwiched between HfO 2 and SiN x . SiN x is suggested to minimize interfacial HfSi x O y formation by limiting the amount of Si available to interact with the HfO 2 layer.

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The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance.

An interface dipole model explaining threshold voltage (Vt) tuning in HfSiON gated n-channel field effect transistors (nFETs) is proposed. Vt tuning depends on rare earth (RE) type and diffusion in Si∕SiOx∕HfSiON∕REOx/metal gated nFETs as follows: Sr<Er<Sc+Er<La<Sc<none. This Vt ordering is very similar to the trends in dopant electronegativity (EN) (dipole charge transfer) and ionic radius (r) (dipole separation) expected for a interfacial dipole mechanism. The resulting Vt dependence on RE dopant allows distinction between a dipole model (dependent on EN and r) and an oxygen vacancy model (dependent on valence).