392
P. A. Dananjaya et al.
pulse with optimized amplitudes and durations. Together with the non-overlapping
conductance states achieved, this showed a promising feature towards an ideal analog
synaptic device characteristic.
The electrode engineering in TiO x system was associated with the symmetry characteristics of the system. TiN/TiO x /Mo system was found to improve the symmetry
of the system as compared to TiN/TiO x /Pt [38]. It was due to the work function
difference between the corresponding two electrodes. 64 conductance levels with
excellent distribution was achieved. The device potentiation exhibited more linear
conductance change as compared to its depression. In order to improve the linearity
of the device depression and thus achieving more symmetrical weight update, current
pulse scheme was adopted. The hybrid scheme of voltage (potentiation) and current
(depression) pulse was able to improve the pattern recognition accuracy by around
10%.
Tantalum Oxide (TaO x )-based Devices
Ta/TaO x /TiO 2 /Ti structure was initially proposed as 3D-integrated storage class
memory [39, 40]. It has high endurance of 10
12 , forming free, self-compliant, and selfrectifying characteristics that significantly simplify the peripheral circuit required
during the operation. It works based on homogenous Schottky barrier modulation due
to oxygen vacancy defects migration at Ta/TaO x interface under external electric field.
In this structure, TiO 2 layer provided diode-like effect that resulted in self-rectifying
characteristics in the structure with rectification ratio of ~10
5 . The switching mechanism was confirmed by simulation to accurately reproduce experimentally obtained
DC and AC characteristics of the device [41]. Moreover, its synaptic characteristics,
i.e., long term potentiation (LTP), long term depression (LTD), STDP and pairedpulse facilitation (PPF), have also been experimentally investigated [41, 42]. The
structure exhibited non-linear gradual potentiation and depression with dynamic
ratio of >2 under identical pulse scheme (LTP: +3 V/5 ms, LTD: −3 V/5 ms, and
read: −1.5 V/1 ms). The training pulse duration was found to linearly scale with pulse
amplitude required to maintain similar synaptic plasticity [42]. Extremely low <10 fJ
per synaptic event was experimentally recorded [43]. The nonlinearity of the weight
update could be improved under two different pulse schemes, i.e., state-independent
unipolar pulse scheme (UPS) and bipolar pulse scheme (BPS) [42]. UPS used single
pulse (positive or negative) to move the weight value up or down, while BPS utilized
a pair of pulses of different polarities (positive-high, negative-low or negative-high,
positive low) to run one cycle of weight update. The linearity of the weight update
was improved from 0.6–0.81 (UPS) to 0.42–0.54 (BPS) with ~50% trade-off in the
dynamic ratio of the weights. This device characteristics were implemented in the
simulation of the training evolution of 8 × 8 binary pattern. BPS achieved ~90%
accuracy, which was significantly higher than ~75% accuracy attained under UPS.
This showed the importance of weight update linearity in the long run to provide
more immunity to input noise.
Other than insertion of TiO 2 layer, non-filamentary TaO x devices have also been
paired with Al 2 O 3 barrier layer. Different deposition techniques were used. i.e.,
electron beam evaporation, post-rapid thermal annealing in O 2 ambient, and ALD.
P. A. Dananjaya et al.
pulse with optimized amplitudes and durations. Together with the non-overlapping
conductance states achieved, this showed a promising feature towards an ideal analog
synaptic device characteristic.
The electrode engineering in TiO x system was associated with the symmetry characteristics of the system. TiN/TiO x /Mo system was found to improve the symmetry
of the system as compared to TiN/TiO x /Pt [38]. It was due to the work function
difference between the corresponding two electrodes. 64 conductance levels with
excellent distribution was achieved. The device potentiation exhibited more linear
conductance change as compared to its depression. In order to improve the linearity
of the device depression and thus achieving more symmetrical weight update, current
pulse scheme was adopted. The hybrid scheme of voltage (potentiation) and current
(depression) pulse was able to improve the pattern recognition accuracy by around
10%.
Tantalum Oxide (TaO x )-based Devices
Ta/TaO x /TiO 2 /Ti structure was initially proposed as 3D-integrated storage class
memory [39, 40]. It has high endurance of 10
12 , forming free, self-compliant, and selfrectifying characteristics that significantly simplify the peripheral circuit required
during the operation. It works based on homogenous Schottky barrier modulation due
to oxygen vacancy defects migration at Ta/TaO x interface under external electric field.
In this structure, TiO 2 layer provided diode-like effect that resulted in self-rectifying
characteristics in the structure with rectification ratio of ~10
5 . The switching mechanism was confirmed by simulation to accurately reproduce experimentally obtained
DC and AC characteristics of the device [41]. Moreover, its synaptic characteristics,
i.e., long term potentiation (LTP), long term depression (LTD), STDP and pairedpulse facilitation (PPF), have also been experimentally investigated [41, 42]. The
structure exhibited non-linear gradual potentiation and depression with dynamic
ratio of >2 under identical pulse scheme (LTP: +3 V/5 ms, LTD: −3 V/5 ms, and
read: −1.5 V/1 ms). The training pulse duration was found to linearly scale with pulse
amplitude required to maintain similar synaptic plasticity [42]. Extremely low <10 fJ
per synaptic event was experimentally recorded [43]. The nonlinearity of the weight
update could be improved under two different pulse schemes, i.e., state-independent
unipolar pulse scheme (UPS) and bipolar pulse scheme (BPS) [42]. UPS used single
pulse (positive or negative) to move the weight value up or down, while BPS utilized
a pair of pulses of different polarities (positive-high, negative-low or negative-high,
positive low) to run one cycle of weight update. The linearity of the weight update
was improved from 0.6–0.81 (UPS) to 0.42–0.54 (BPS) with ~50% trade-off in the
dynamic ratio of the weights. This device characteristics were implemented in the
simulation of the training evolution of 8 × 8 binary pattern. BPS achieved ~90%
accuracy, which was significantly higher than ~75% accuracy attained under UPS.
This showed the importance of weight update linearity in the long run to provide
more immunity to input noise.
Other than insertion of TiO 2 layer, non-filamentary TaO x devices have also been
paired with Al 2 O 3 barrier layer. Different deposition techniques were used. i.e.,
electron beam evaporation, post-rapid thermal annealing in O 2 ambient, and ALD.