4.2.2 Diffusion Mechanism of Ta and O Ions in a-TaO x
To deepen our understanding on the diffusion behaviors of Ta and O ions in a-TaO x ,
here we discuss them in the light of the dependence of the structural parameters (such
as CN and bond length) on x value of a-TaO x (x ¼ 2.85, 2.67, 2.50, 2.25, 2, 1.5,
1 and 0.75) [10]. The x dependence of the Ta-O bond lengths and CNs, both Ta
(O) and O(Ta), are plotted in Fig. 14. The results show that the Ta-O bond length
increases with the decrease of x, which expands the free atomic volume around both
metal (Ta) and O atoms, and consequently enhances their mobility. In particular, the
CN of O(Ta) increases by ~1 as x decreases from 2.85 to 0.75 as seen in Fig. 14. This
corresponds to the change of the component unit from OTa 2 to OTa 3 , and as a result
suppresses the mobility of O ions in some degree due to the increased steric
hindrance around themselves. In contrast, the CN of Ta(O) shows obvious decrease
(by ~3) with the decrease of x as seen in Fig. 14. This corresponds to the change of
the component unit from octahedral TaO 6 to TaO 3 , and will dramatically enhance
the mobility of Ta ions due to the decreased steric hindrance around themselves.
Thus, the enhancement of Ta ion mobility in a-TaO x is more obvious than that of O
ions as the decrease of O concentration, and consequently the mobility of Ta and O
ions is close to each other under low O concentration.
It is noted that our theoretical results agree with the previous experiments on the
growth of TaO x (x < 2.5), which shows that the diffusion of Ta ions in a-TaO x layer
is non-negligible [47]. Accordingly, from the diffusion behaviors of metal ions
during the growth of metal oxide films, we may be able to obtain useful information
to understand the switching mechanism of metal oxide based resistive switches. In
experiments, the metal ion diffusion has already been observed in metal sub-oxide
layer during oxide film growth for Ti, Al, Nb and W [47]. On the basis of these
results, we can speculate that the metal ion diffusion in TaO x , TiO x , AlO x , NbO x and
WO x could occur during the switching of resistive switches based on these materials.
In summary of this subsection, we investigated the diffusion behavior of Ta and O
ions in a-TaO x with various O concentrations. Our results reveal that the diffusion of
Ta ions is enhanced with the decrease of O concentration, and approaches to that of
O ions at the extremely low O concentration cases. Such phenomenon can be
attributed to the combined effect of the changes in the Ta/O coordination numbers
and the Ta-O/Ta-Ta bond strengths with the charge of O concentration in a-TaO x .
5 Concluding Remarks
In this chapter, we have discussed several issues related to the switching mechanism
of amorphous TaO x (a-TaO x ) based resistive switches on the basis of our simulations
within the density functional theory. Our results reveal that (1) Cu nanowires with a
diameter of three atoms or larger can work as conduction filaments (CFs) in the Cu/aTa 2 O 5 /Pt atomic switch; (2) Cu atoms tend to be ionized at the Cu/a-Ta 2 O 5 interface,
Atomistic Simulations for Understanding Microscopic Mechanism of. . .
121
To deepen our understanding on the diffusion behaviors of Ta and O ions in a-TaO x ,
here we discuss them in the light of the dependence of the structural parameters (such
as CN and bond length) on x value of a-TaO x (x ¼ 2.85, 2.67, 2.50, 2.25, 2, 1.5,
1 and 0.75) [10]. The x dependence of the Ta-O bond lengths and CNs, both Ta
(O) and O(Ta), are plotted in Fig. 14. The results show that the Ta-O bond length
increases with the decrease of x, which expands the free atomic volume around both
metal (Ta) and O atoms, and consequently enhances their mobility. In particular, the
CN of O(Ta) increases by ~1 as x decreases from 2.85 to 0.75 as seen in Fig. 14. This
corresponds to the change of the component unit from OTa 2 to OTa 3 , and as a result
suppresses the mobility of O ions in some degree due to the increased steric
hindrance around themselves. In contrast, the CN of Ta(O) shows obvious decrease
(by ~3) with the decrease of x as seen in Fig. 14. This corresponds to the change of
the component unit from octahedral TaO 6 to TaO 3 , and will dramatically enhance
the mobility of Ta ions due to the decreased steric hindrance around themselves.
Thus, the enhancement of Ta ion mobility in a-TaO x is more obvious than that of O
ions as the decrease of O concentration, and consequently the mobility of Ta and O
ions is close to each other under low O concentration.
It is noted that our theoretical results agree with the previous experiments on the
growth of TaO x (x < 2.5), which shows that the diffusion of Ta ions in a-TaO x layer
is non-negligible [47]. Accordingly, from the diffusion behaviors of metal ions
during the growth of metal oxide films, we may be able to obtain useful information
to understand the switching mechanism of metal oxide based resistive switches. In
experiments, the metal ion diffusion has already been observed in metal sub-oxide
layer during oxide film growth for Ti, Al, Nb and W [47]. On the basis of these
results, we can speculate that the metal ion diffusion in TaO x , TiO x , AlO x , NbO x and
WO x could occur during the switching of resistive switches based on these materials.
In summary of this subsection, we investigated the diffusion behavior of Ta and O
ions in a-TaO x with various O concentrations. Our results reveal that the diffusion of
Ta ions is enhanced with the decrease of O concentration, and approaches to that of
O ions at the extremely low O concentration cases. Such phenomenon can be
attributed to the combined effect of the changes in the Ta/O coordination numbers
and the Ta-O/Ta-Ta bond strengths with the charge of O concentration in a-TaO x .
5 Concluding Remarks
In this chapter, we have discussed several issues related to the switching mechanism
of amorphous TaO x (a-TaO x ) based resistive switches on the basis of our simulations
within the density functional theory. Our results reveal that (1) Cu nanowires with a
diameter of three atoms or larger can work as conduction filaments (CFs) in the Cu/aTa 2 O 5 /Pt atomic switch; (2) Cu atoms tend to be ionized at the Cu/a-Ta 2 O 5 interface,
Atomistic Simulations for Understanding Microscopic Mechanism of. . .
121