[show abstract][hide abstract] ABSTRACT: 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.
Journal of Applied Physics 12/2012; 110:124518. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The evolution over time of the leakage current in HfO2-based MIM capacitors under continuous or periodic constant voltage stress (CVS) was studied for a range of stress voltages and temperatures. The data were analyzed based on the results of conductive atomic force microscopy (AFM) measurements demonstrating preferential current flow along grain boundaries (GBs) in the HfO2 dielectric and ab initio calculations, which show the formation of a conductive sub-band due to the precipitation of oxygen vacancies at the GBs. The simulations using the statistical multi-phonon trap-assisted tunneling (TAT) current description successfully reproduced the experimental leakage current stress time dependency by using the calculated energy characteristics of the O-vacancies. The proposed model suggests that the observed reversible increase in the stress current is caused by segregation of the oxygen vacancies at the GBs and their conversion to the TAT-active charge state caused by reversible electron trapping during CVS.
[show abstract][hide abstract] ABSTRACT: The time evolution of the leakage current in HfO<sub>2</sub> based MIM capacitors under constant voltage stress applied either continuously or with periodic interruptions was studied for a range of stress voltages and temperatures. The data analysis was performed based on the results of the conductive AFM measurements demonstrating preferential current flow along grain boundaries (GB) in the HfO<sub>2</sub> dielectric and ab initio calculations, which show the formation of a conductive sub-band due to oxygen vacancy precipitation at GBs. The proposed model suggests that the observed reversible increase in leakage current is caused by the defects segregation at GBs and electron trapping/detrapping at these defects. The energy characteristics of the electrically active defects extracted from the electrical measurements match well with those calculated for neutral oxygen vacancies in hafnia.
Solid-State Device Research Conference (ESSDERC), 2010 Proceedings of the European; 10/2010
[show abstract][hide abstract] ABSTRACT: By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament features controlling TiN/HfO2/TiN resistive memory operations. The forming process is found to define the filament geometry, which in turn determines the temperature profile and, consequently, the switching characteristics. The findings point to the critical importance of controlling filament dimensions during the forming process (polarity, max current/voltage, etc.).
IEEE International Electron Devices Meeting; 01/2010
[show abstract][hide abstract] ABSTRACT: The results in this paper suggest that migration of the electrons, captured during the fast charging process, to other available traps represents the major process responsible for the intrinsic V<sub>t</sub> instability in the high-k NMOS transistors. The extracted trap characteristics are consistent with those of the oxygen vacancies in the monoclinic hafnia
VLSI Technology, Systems, and Applications, 2006 International Symposium on; 05/2006
[show abstract][hide abstract] ABSTRACT: Properties of the defects responsible for fast transient electron trapping phenomenon in high-k gate dielectrics were investigated.
It was shown that electrons might be reversibly trapped at shallow, delocalized gap states. Ab initio calculations have indicated
that Zr substitutional impurities and fourcoordinated negatively charged O vacancies can fit the electrical profile of the
electron traps. The relationship between trap characteristics and reduction of trapping observed in ultra-thin high-k gate
stacks are discussed.
[show abstract][hide abstract] ABSTRACT: Using ab initio density functional calculations we study implications of film growth kinetics on the structure and defects at the HfO2/Si(001) interface region. We introduce a simplified model for the atomic layer deposition with HfCl4 as a metal precursor and H2O as the oxidant. The model suggests that the initial interface growth results in a typically less than 1 monolayer of Hf in the first metallic layer. As a consequence, the oxygen coordination at the interface is either two (one Si and one Hf atom) or three (two Hf and one Si atoms), which partially supports the kinetically motivated interface models but is in contrast with the epitaxially motivated models suggested previously. Further calculations show the oxygen vacancy formation energy at the Si-O-Hf sites is ∼1 eV lower than that in the bulk HfO2. This makes the Si-Hf bond a likely interface defect, which is predicted to be a shallow hole trap. Calculations also predict interstitial oxygen to migrate from HfO2 into Si substrate resulting in the formation of the SiOx interface layer.