RRAM-Based Neuromorphic Computing Systems
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in device conductance, i.e., interfacial type switching. During the SET or RESET
operation, it also alters the thickness of tunneling gap in the system, allowing higher
or lower number of electrons flowing through the device. These enable gradual
conductance change during the switching operation, mitigating the issue encountered
in most of the filamentary synaptic devices. Non-filamentary switching has been
widely reported in many oxide structures, e.g., TiO x , TaO x , WO x , and PCMO.
Titanium Oxide (TiO x )-based Devices
Analog characteristic in TiO x systems have been demonstrated by the tuning the
oxide layer stoichiometry and the use of oxygen gathering electrode. One of the
early structures investigated for synaptic device was TiO x /TiO y bilayer oxides system
[32]. It was composed of ~50 nm sol-gel TiO x layer grown on top of 6 nm TiO y
layer with defect ratio of ~0.23 and ~0.17, respectively. This created active interface
between the two oxide layers in which the exchange of oxygen content occurred
under external electric field. Gradual potentiation (4 MV cm
−1 , 10 ms) and depression
(−2 MVcm
−1 ,10 ms) were obtained with dynamic ratio of ~10. The excellent device
characteristics enabled its implementation on weight change, STDP, and STDP triple
model.
Engineering at the interface of the between the electrode and the active TiO x
switching layer have been reported to successfully improve the dynamic ratio as
well as reduce the switching current of the TiO x -based devices. Insertion of thin
Al 2 O 3 layer (~2 nm) at the interface between TiO 2 and TiN electrode could achieve
memory window of >100 with <10 μA switching current [33]. Further improvement
of switching current, i.e., down to ~1 μA was demonstrated by replacing Al 2 O 3 with
a-Si layer [34, 35]. a-Si played role as an oxygen gathering layer facilitating the
movement of oxygen ions at the interface. The semi-insulating property of a-Si layer
enabled nonlinear IV cell characteristics, which caused amplification of the energy
barrier modulation leading to the large dynamic ratio of the device. The synaptic
characteristics of a-Si/TiO 2 -based devices were input into a simulated 3-layer ANN
and the pattern recognition accuracy of the NN was tested using MNIST database.
The focus on the demonstration was on investigating the effect of read noise, i.e.,
random telegraph noise (RTN), on the pattern recognition accuracy. a-Si/TiO 2 -based
device achieved much better accuracy compared to filamentary TaO x -based devices
due to lower RTN amplitude value and distribution with much less noise occurrence
rate [36].
TiO x has also been implemented on various bilayer oxide systems [37], i.e., AlO x ,
TaO x , WO x , HfO x , ZnO x , and SiO x . Different dynamic ratio and multibit capability performances were achieved with different pairing oxide with TiO 2 . The most
promising multilevel states property was found in AlO y /TiO x bilayer oxide structure, which was able to achieve non-overlapping conductance states of 6.5 bit per cell
despite of less than 10 dynamic ratios. This was attributed to AlO x property being
the oxide with lowest oxygen ions mobility among the pairing oxide layers tested.
Although no synaptic characteristics were especially discussed, well control conductance update can be achieved with different pulse schemes. Specific conductance level
can be achieved from the same starting value using train of identical pulses or single
391
in device conductance, i.e., interfacial type switching. During the SET or RESET
operation, it also alters the thickness of tunneling gap in the system, allowing higher
or lower number of electrons flowing through the device. These enable gradual
conductance change during the switching operation, mitigating the issue encountered
in most of the filamentary synaptic devices. Non-filamentary switching has been
widely reported in many oxide structures, e.g., TiO x , TaO x , WO x , and PCMO.
Titanium Oxide (TiO x )-based Devices
Analog characteristic in TiO x systems have been demonstrated by the tuning the
oxide layer stoichiometry and the use of oxygen gathering electrode. One of the
early structures investigated for synaptic device was TiO x /TiO y bilayer oxides system
[32]. It was composed of ~50 nm sol-gel TiO x layer grown on top of 6 nm TiO y
layer with defect ratio of ~0.23 and ~0.17, respectively. This created active interface
between the two oxide layers in which the exchange of oxygen content occurred
under external electric field. Gradual potentiation (4 MV cm
−1 , 10 ms) and depression
(−2 MVcm
−1 ,10 ms) were obtained with dynamic ratio of ~10. The excellent device
characteristics enabled its implementation on weight change, STDP, and STDP triple
model.
Engineering at the interface of the between the electrode and the active TiO x
switching layer have been reported to successfully improve the dynamic ratio as
well as reduce the switching current of the TiO x -based devices. Insertion of thin
Al 2 O 3 layer (~2 nm) at the interface between TiO 2 and TiN electrode could achieve
memory window of >100 with <10 μA switching current [33]. Further improvement
of switching current, i.e., down to ~1 μA was demonstrated by replacing Al 2 O 3 with
a-Si layer [34, 35]. a-Si played role as an oxygen gathering layer facilitating the
movement of oxygen ions at the interface. The semi-insulating property of a-Si layer
enabled nonlinear IV cell characteristics, which caused amplification of the energy
barrier modulation leading to the large dynamic ratio of the device. The synaptic
characteristics of a-Si/TiO 2 -based devices were input into a simulated 3-layer ANN
and the pattern recognition accuracy of the NN was tested using MNIST database.
The focus on the demonstration was on investigating the effect of read noise, i.e.,
random telegraph noise (RTN), on the pattern recognition accuracy. a-Si/TiO 2 -based
device achieved much better accuracy compared to filamentary TaO x -based devices
due to lower RTN amplitude value and distribution with much less noise occurrence
rate [36].
TiO x has also been implemented on various bilayer oxide systems [37], i.e., AlO x ,
TaO x , WO x , HfO x , ZnO x , and SiO x . Different dynamic ratio and multibit capability performances were achieved with different pairing oxide with TiO 2 . The most
promising multilevel states property was found in AlO y /TiO x bilayer oxide structure, which was able to achieve non-overlapping conductance states of 6.5 bit per cell
despite of less than 10 dynamic ratios. This was attributed to AlO x property being
the oxide with lowest oxygen ions mobility among the pairing oxide layers tested.
Although no synaptic characteristics were especially discussed, well control conductance update can be achieved with different pulse schemes. Specific conductance level
can be achieved from the same starting value using train of identical pulses or single