4.1 I–V Characteristics in the Atomic Contact Regime
I–V measurements were performed for a Ag/PEO/Pt device with a 40 nm-thick PEO
film, at room temperature in air [25, 26]. Quantized conductance can be observed by
carefully tuning the stop voltage (V S ) and voltage sweep rate in positive bias.
Figure 6a and b depict the typical I–V and corresponding conductance curves at
different V S with a voltage sweep rate of 3.5 mV s
À1 . For these measurements, a
10 kΩ resistor was inserted in series with the device to regulate the on state current,
as illustrated in the inset of Fig. 7a. With V S ¼ 0.27 V, the device conductance
jumped from zero to 1G 0 (Fig. 6a). This conductance jump to 1G 0 represents the
formation of a single atom point contact in the narrowest region of the metal filament
formed. As the bias voltage was subsequently swept back, the conductance state was
initially retained at the ~1G 0 level, but dropped to zero before the bias voltage
reached 0 V, indicating volatile switching behavior. On the other hand, with
V S ¼ 0.95 V, the device conductance increased in a stepwise fashion and finally
reached 8G 0 (Fig. 6b). The conductance state changed not only to integer multiples
but also to half-integer multiples. In addition, the conductance state held its level
even if the bias voltage was swept back to 0 V and dropped to zero in negative bias,
indicating nonvolatile behavior.
The retention characteristics of different conductance values were also investigated. Figure 6c plots I–V curves measured with a constant sweep rate (3.5 mV s
À1 )
and various V S (0.18–1.1 V) to realize quantized conductance states with a specific
value. Then, the spontaneous conductance decay (retention) was measured as a
function of time for each quantized conductance state (Fig. 6d). Conductance states
of 1G 0 dropped to zero before starting the retention measurement and thus did not
show any retention characteristics. In contrast, conductance states of !2G 0 exhibited
different retention time depending on the conductance value. The retention time
became longer and increased exponentially for higher conductance values.
The conductance state distribution was evaluated from the I–V data with different
V S . Figure 7 depicts conductance-state histograms by counting the number of states
for different V S . The Ag/PEO/Pt device exhibited large peaks at integer multiples of
G 0 , clearly demonstrating quantized conductance behavior. The peaks of the state
distributions shift to the higher conductance side with increasing V S . In addition,
distinct peaks appear at half-integer multiples of G 0 , and fractional conductance
variations are also observed between peaks.
4.2 Transport Simulations for Atomic Point Contacts
Detailed atomistic simulations were performed to investigate the correlation between
the observed quantized conductance and the atomic point contact structure
[25, 26]. The quantized conductance originates from the narrowest region of a
metal filament between electrodes, which consists of the number of atoms
150
T. Tsuruoka et al.
I–V measurements were performed for a Ag/PEO/Pt device with a 40 nm-thick PEO
film, at room temperature in air [25, 26]. Quantized conductance can be observed by
carefully tuning the stop voltage (V S ) and voltage sweep rate in positive bias.
Figure 6a and b depict the typical I–V and corresponding conductance curves at
different V S with a voltage sweep rate of 3.5 mV s
À1 . For these measurements, a
10 kΩ resistor was inserted in series with the device to regulate the on state current,
as illustrated in the inset of Fig. 7a. With V S ¼ 0.27 V, the device conductance
jumped from zero to 1G 0 (Fig. 6a). This conductance jump to 1G 0 represents the
formation of a single atom point contact in the narrowest region of the metal filament
formed. As the bias voltage was subsequently swept back, the conductance state was
initially retained at the ~1G 0 level, but dropped to zero before the bias voltage
reached 0 V, indicating volatile switching behavior. On the other hand, with
V S ¼ 0.95 V, the device conductance increased in a stepwise fashion and finally
reached 8G 0 (Fig. 6b). The conductance state changed not only to integer multiples
but also to half-integer multiples. In addition, the conductance state held its level
even if the bias voltage was swept back to 0 V and dropped to zero in negative bias,
indicating nonvolatile behavior.
The retention characteristics of different conductance values were also investigated. Figure 6c plots I–V curves measured with a constant sweep rate (3.5 mV s
À1 )
and various V S (0.18–1.1 V) to realize quantized conductance states with a specific
value. Then, the spontaneous conductance decay (retention) was measured as a
function of time for each quantized conductance state (Fig. 6d). Conductance states
of 1G 0 dropped to zero before starting the retention measurement and thus did not
show any retention characteristics. In contrast, conductance states of !2G 0 exhibited
different retention time depending on the conductance value. The retention time
became longer and increased exponentially for higher conductance values.
The conductance state distribution was evaluated from the I–V data with different
V S . Figure 7 depicts conductance-state histograms by counting the number of states
for different V S . The Ag/PEO/Pt device exhibited large peaks at integer multiples of
G 0 , clearly demonstrating quantized conductance behavior. The peaks of the state
distributions shift to the higher conductance side with increasing V S . In addition,
distinct peaks appear at half-integer multiples of G 0 , and fractional conductance
variations are also observed between peaks.
4.2 Transport Simulations for Atomic Point Contacts
Detailed atomistic simulations were performed to investigate the correlation between
the observed quantized conductance and the atomic point contact structure
[25, 26]. The quantized conductance originates from the narrowest region of a
metal filament between electrodes, which consists of the number of atoms
150
T. Tsuruoka et al.