2012-06-17 AachenNature Poster MEM22 Clima
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Transcript of 2012-06-17 AachenNature Poster MEM22 Clima
Sergiu Clima1, Kiroubanand Sankaran1,4, Maarten Mees1,3, Yang Yin Chen1,3,Ludovic Goux1, Bogdan
Govoreanu1, Dirk J.Wouters1,3, Jorge Kittl1, Malgorzata Jurczak1, Geoffrey Pourtois1,2
1imec, B-3001 Leuven, Belgium; 2PLASMANT,University of Antwerp, B-2610 Antwerpen, Belgium;
3Katholieke Universiteit Leuven, B-3001 Leuven, Belgium;
4ETSF and IMCN, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium
Ea - hafnium oxides have the lowest activation energies and slightly increasing with the O content/density in the oxide (0.57-0.66 eV), a larger barrier is observed in Al2O3 (1.22 eV) and on the electrodes sides much higher barriers were computed: 2.50 eV in TiN and 4.13 eV in Hf. These activation energies match the experimental window, as measured for HfOx and Al2O3.
5,6
The movement of the O atoms is a local rearrangement of O around Hf atoms, upon which the electrically active defects (VO) show jumps larger than the displacement of any O atom.
O diffusion is facilitated by the free volume that is increasingly more available in the sub-stoichiometric Hf oxides but very difficult in the metallic Hf electrode.
Somewhat higher diffusion coefficients are computed in TiN and Al2O3 vs Hf, but they are still blocking layers for O, if compared to HfOx.
Transition metal oxide valence change Resistor
Random Access Memory (RRAM) operation
principles are based on oxygen-related defect
migration. The switching mechanism is believed to
be driven by the Joule heating enhanced drift of O2-
ions under the applied electric field through the
oxide from/ towards the metal electrode.1,2 The
kinetics of the oxygen diffusion is, therefore, a key
factor for oxide stoichiometry change, which in turn
is responsible for the resistivity of the RRAM cell.
1) Liu, L. F. et al. Engineering oxide resistive switching materials for memristive device application.
Applied Physics a-Materials Science & Processing 102, 991-996, doi:10.1007/s00339-011-6331-2 (2011).
2) Waser, R., Dittmann, R., Staikov, G. & Szot, K. Redox-Based Resistive Switching Memories – Nanoionic
Mechanisms, Prospects, and Challenges. Advanced Materials 21, 2632-2663, doi:10.1002/adma.200900375
(2009).
3) Miron, R. A. & Fichthorn, K. A. Accelerated molecular dynamics with the bond-boost method. Journal of
Chemical Physics 119, 6210-6216, doi:10.1063/1.1603722 (2003).
4) Govoreanu , B. et al. Investigation of forming and its controllability in novel HfO2-based 1T1R 40nm-
crossbar RRAM cells. Ext. Abstr. SSDM Conf.,Nagoya, Japan, pp.1005-1006 (2011).
5) Zafar, S., Jagannathan, H., Edge, L. F. & Gupta, D. Measurement of oxygen diffusion in nanometer scale
HfO2 gate dielectric films. Applied Physics Letters 98, 152903, doi:10.1063/1.3579256 (2011).
6) Nabatame, T. et al. Comparative studies on oxygen diffusion coefficients for amorphous and gamma-
Al2O3 films using O-18 isotope. Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes
& Review Papers 42, 7205-7208, doi:10.1143/jjap.42.7205 (2003).
Motivation Modeling approach
Accelerated Ab Initio Molecular Dynamics
(AIMD) technique combining the bond-boosted
technique3 is used to compute the diffusion
kinetics of O in a series of materials of RRAM
interest. In a TiN/Hf/HfO2/TiN stack, it is
expected that the sputtering of the Hf capping
layer leads to the generation of interfacial
amorphous sub-oxides.4 Beside the electronic
barrier function (a feature needed for self-
rectifying function in a RRAM cell) Al2O3 could
work as O barrier, therefore pinning the
switching layer at a specific location. The
knowledge of diffusion data in amorphous phase
HfOx (x=2,1,1/2), Al2O3 and crystalline TiN and
Hf metallic electrodes helps us understand
where the O comes from and goes into during
switching, as the short time/length simulations
can model the fast and local atomic
rearrangements that must be responsible for the
sub-ns switching events.
a(HfO2) b(HfO1) c(HfO0.5)
e(Al2O3) f(TiN) g(Hf)
Investigated materials Diffusion
f,g
f
c
b
a
RRAM cell
f,g
f
c
b
a
e
SRC RRAM
t
rtrN
D
N
ii
t6
)0()(1
lim 1
2
Model Ea (eV) D0 (m2/s)
HfO0.5 0.57 5.07E-8 HfO1.0 0.60 5.11E-8 HfO1.97 0.66 3.88E-8 Zafar 0.46-0.6 10-8-10-12
Arrhenius plots
a b c g
Ab Initio Molecular Dynamics
DFT/LDA (DZP) forces at each time
step of Newtonian motion of atoms
with a “time machine”
DV
Artificial
potential
b
V
b ett DDD
Frontiers in Electronic Materials Nature Conference
Aachen, 2012
MEM 22