RRAM-Based Neuromorphic Computing Systems
393
Different deposition techniques resulted in significantly different initial resistance
values. When both films are deposited via e-beam evaporation (AlO x 3 nm and TaO x
5 nm), the initial resistance of the structure was found to be around 200 , which is
extremely low. This was due to the loss of oxygen content during the deposition. On
the other hand, the films deposited via post-rapid thermal annealing in O 2 ambient
and ALD consistently started from highly insulating state. This agrees with the two
techniques tend to result in stoichiometric films. Other than RESET process required
to initiate the switching operation in the non-stoichiometric structures, the switching
polarity and mechanisms involved during the operation remained the same. From
current-voltage characteristics, area-dependent LRS, and elemental analysis, the
change in conductance during operation was attributed to the tunneling barrier modulation induced by oxygen ions migration across the whole area of TaO x /electrode
interface. With the tunable gradual SET/RESET feature of the device, LTP/LTD,
PPF, and STDP were demonstrated. LTP and LTD were characterized under identical pulse scheme (50 pulses) with different pulse amplitude (4.5–5.5 V) and duration
(1–100 μs). Estimated energy of 50 pJ per spike of programming pulse was recorded.
Increase in linearity of conductance change was observed with the decrease in both
pulse amplitude and duration in the expense of the weight dynamic ratio. Improvement in linearity and dynamic ratio could be achieved under non-identical training
pulse scheme with increasing pulse amplitude (2–6 V of 100 μs pulse). Under this
scheme, rough and fine tuning to achieve certain weight value from any randomly
chosen weight with excellent <1% variation was also shown.
Tungsten Oxide (WO x )-based Devices
Another extensively studied oxide structure with underlying mechanism of homogenous anions migration across device active area is WO x . The migration of the oxygen
ions enables the system to tune the interchanging role of Schottky barrier emission and tunneling as predominant conduction mechanism during the operation. In
Pt/WO x /Ti structure [44], the Schottky barrier formed at the interface of Pt/WO x
due to the higher work function of Pt compared to Ti. During the SET process in
which the Pt electrode was positively biased, the oxygen ions migrated towards the
Pt electrode and got accumulated at the Pt/WO x interface. This reduced the Fermi
level near the WO x surface and at the same time decreased the Schottky barrier
height between Pt and WO x , resulting in the increase of device conductance. This
specific structure was demonstrated on a flexible substrate. The synaptic properties of the device, i.e., excitatory postsynaptic current (EPSC), PPF, STP/LTP, and
STDP were characterized and no performance degradation occurred under large angle
bending or 100 times bending tests. EPSC property of the device was experimentally
obtained through device dynamic response upon receiving 2 V, 50 ms programming
pulse. Immediately after the removal of the electric field, the conductance of the
device started to drop and eventually relaxed back to the initial conductance value
after ~400 ms. PPF was determined through the ratio of EPSC peaks obtained by
sending two identical pulses (2 V, 50 ms). The correlation of the PPF and the interval
between the subsequent pulses was recorded up to 1 s. It was well fitted with double
exponential function containing the initial facilitation magnitudes and characteristic
393
Different deposition techniques resulted in significantly different initial resistance
values. When both films are deposited via e-beam evaporation (AlO x 3 nm and TaO x
5 nm), the initial resistance of the structure was found to be around 200 , which is
extremely low. This was due to the loss of oxygen content during the deposition. On
the other hand, the films deposited via post-rapid thermal annealing in O 2 ambient
and ALD consistently started from highly insulating state. This agrees with the two
techniques tend to result in stoichiometric films. Other than RESET process required
to initiate the switching operation in the non-stoichiometric structures, the switching
polarity and mechanisms involved during the operation remained the same. From
current-voltage characteristics, area-dependent LRS, and elemental analysis, the
change in conductance during operation was attributed to the tunneling barrier modulation induced by oxygen ions migration across the whole area of TaO x /electrode
interface. With the tunable gradual SET/RESET feature of the device, LTP/LTD,
PPF, and STDP were demonstrated. LTP and LTD were characterized under identical pulse scheme (50 pulses) with different pulse amplitude (4.5–5.5 V) and duration
(1–100 μs). Estimated energy of 50 pJ per spike of programming pulse was recorded.
Increase in linearity of conductance change was observed with the decrease in both
pulse amplitude and duration in the expense of the weight dynamic ratio. Improvement in linearity and dynamic ratio could be achieved under non-identical training
pulse scheme with increasing pulse amplitude (2–6 V of 100 μs pulse). Under this
scheme, rough and fine tuning to achieve certain weight value from any randomly
chosen weight with excellent <1% variation was also shown.
Tungsten Oxide (WO x )-based Devices
Another extensively studied oxide structure with underlying mechanism of homogenous anions migration across device active area is WO x . The migration of the oxygen
ions enables the system to tune the interchanging role of Schottky barrier emission and tunneling as predominant conduction mechanism during the operation. In
Pt/WO x /Ti structure [44], the Schottky barrier formed at the interface of Pt/WO x
due to the higher work function of Pt compared to Ti. During the SET process in
which the Pt electrode was positively biased, the oxygen ions migrated towards the
Pt electrode and got accumulated at the Pt/WO x interface. This reduced the Fermi
level near the WO x surface and at the same time decreased the Schottky barrier
height between Pt and WO x , resulting in the increase of device conductance. This
specific structure was demonstrated on a flexible substrate. The synaptic properties of the device, i.e., excitatory postsynaptic current (EPSC), PPF, STP/LTP, and
STDP were characterized and no performance degradation occurred under large angle
bending or 100 times bending tests. EPSC property of the device was experimentally
obtained through device dynamic response upon receiving 2 V, 50 ms programming
pulse. Immediately after the removal of the electric field, the conductance of the
device started to drop and eventually relaxed back to the initial conductance value
after ~400 ms. PPF was determined through the ratio of EPSC peaks obtained by
sending two identical pulses (2 V, 50 ms). The correlation of the PPF and the interval
between the subsequent pulses was recorded up to 1 s. It was well fitted with double
exponential function containing the initial facilitation magnitudes and characteristic