Next, we calculated the current-voltage (I-V) curves of these three
heterostructures, which are shown in Fig. 18c. The results further confirm the
above features of conduction seen in the transmission spectra. For example, at the
bias voltage of 1.0 V, a much larger current (~57 μA) flows in the heterostructure
with a continued CF, as compared to the case with a discontinued CF (6 μA). The
current value (~57 μA) is comparable to that in the real device [4], though this
agreement may be accidental.
In summary of this subsection, the structures and electronic properties of a-TaO x
(x ¼ 2.85, 2.67, 2.50, 2.25, 2, 1.5, 1 and 0.75) were examined via first-principles
calculations. Our results reveal that there is a strong correlation between the Ta
(O) coordination number and the O-Ta (Ta-Ta) bond length. More importantly, we
suggested that the formation of Ta-Ta bonding structures, but not V O , is mainly
responsible for the LRS in the Pt/a-TaO x /Pt resistive switches.
4.2 Diffusion of Metal and Oxygen Ions in a-TaO x Based
Resistive Switch
Almost all the models proposed so far for the switching mechanism of a-TaO x -based
atomic switches are based on the diffusion of O ions or vacancies in a-TaO x
[45]. Very recently, it was reported that the diffusion of Ta ions could also play an
important role in the switching process of a-TaO x -based resistive switches [46]. It
should be noted that the diffusion of both Ta and O ions have already been observed
during the growth of a-TaO x (x < 2.5) film [47, 48]. However, details of the
behaviors of Ta and O ion diffusion in a-TaO x during the switching process are
still unclear. Thus, to deepen the understanding on the switching mechanism of
a-TaO x -based resistive switches, we performed atomistic simulations to clarify the
diffusion behaviors of both Ta and O ions during the switching processes [10].
4.2.1 Diffusion Coefficients and Barriers of Ta and O Ions in a-TaO x
To examine the diffusion behaviors of Ta and O ions in a-TaO x (x ¼ 2, 1.5 and 1),
we calculated the time-average mean square displacement (MSD) at the temperatures of 873 K, 1073 K, 1273 K, 1473 K and 1673 K. From the time evolution of
MSDs of Ta and O ions, we extracted the corresponding diffusion coefficients (D).
Then from the temperature dependence of D shown in Fig. 19, we have evaluated the
activation energies (E a ) according to Arrhenius equation. Our results revealed that D
and E a of Ta and O ions have strong correlation with the O concentration in a-TaO x :
D (E a ) values of Ta and O ions increase (decrease) with the decrease of O content in
a-TaO x .
Our calculated values of E a for O ions in a-TaO x (x ¼ 2, 1.5 and 1) are 0.59, 0.41
and 0.31 eV, respectively, which agrees with the experimental results (from 0.29 to
Atomistic Simulations for Understanding Microscopic Mechanism of. . .
119
heterostructures, which are shown in Fig. 18c. The results further confirm the
above features of conduction seen in the transmission spectra. For example, at the
bias voltage of 1.0 V, a much larger current (~57 μA) flows in the heterostructure
with a continued CF, as compared to the case with a discontinued CF (6 μA). The
current value (~57 μA) is comparable to that in the real device [4], though this
agreement may be accidental.
In summary of this subsection, the structures and electronic properties of a-TaO x
(x ¼ 2.85, 2.67, 2.50, 2.25, 2, 1.5, 1 and 0.75) were examined via first-principles
calculations. Our results reveal that there is a strong correlation between the Ta
(O) coordination number and the O-Ta (Ta-Ta) bond length. More importantly, we
suggested that the formation of Ta-Ta bonding structures, but not V O , is mainly
responsible for the LRS in the Pt/a-TaO x /Pt resistive switches.
4.2 Diffusion of Metal and Oxygen Ions in a-TaO x Based
Resistive Switch
Almost all the models proposed so far for the switching mechanism of a-TaO x -based
atomic switches are based on the diffusion of O ions or vacancies in a-TaO x
[45]. Very recently, it was reported that the diffusion of Ta ions could also play an
important role in the switching process of a-TaO x -based resistive switches [46]. It
should be noted that the diffusion of both Ta and O ions have already been observed
during the growth of a-TaO x (x < 2.5) film [47, 48]. However, details of the
behaviors of Ta and O ion diffusion in a-TaO x during the switching process are
still unclear. Thus, to deepen the understanding on the switching mechanism of
a-TaO x -based resistive switches, we performed atomistic simulations to clarify the
diffusion behaviors of both Ta and O ions during the switching processes [10].
4.2.1 Diffusion Coefficients and Barriers of Ta and O Ions in a-TaO x
To examine the diffusion behaviors of Ta and O ions in a-TaO x (x ¼ 2, 1.5 and 1),
we calculated the time-average mean square displacement (MSD) at the temperatures of 873 K, 1073 K, 1273 K, 1473 K and 1673 K. From the time evolution of
MSDs of Ta and O ions, we extracted the corresponding diffusion coefficients (D).
Then from the temperature dependence of D shown in Fig. 19, we have evaluated the
activation energies (E a ) according to Arrhenius equation. Our results revealed that D
and E a of Ta and O ions have strong correlation with the O concentration in a-TaO x :
D (E a ) values of Ta and O ions increase (decrease) with the decrease of O content in
a-TaO x .
Our calculated values of E a for O ions in a-TaO x (x ¼ 2, 1.5 and 1) are 0.59, 0.41
and 0.31 eV, respectively, which agrees with the experimental results (from 0.29 to
Atomistic Simulations for Understanding Microscopic Mechanism of. . .
119