3 Switching Mechanism of Cu/a-Ta 2 O 5 /Pt Atomic Switch
3.1 Conduction Path in Cu/a-Ta 2 O 5 /Pt Atomic Switch
The widely accepted switching mechanism of the atomic switch is the formation/
rupture of a metal atomic bridge in the insulator between the two electrodes, which
has been identified in experiments [17–19]. In the case of the a-Ta 2 O 5 -based atomic
switch, composition analyses of a-Ta 2 O 5 film before and after applying a voltage
show that Cu ions migrate from the Cu electrode into the a-Ta 2 O 5 film, which agrees
well with the speculation of the formation of a conducting Cu filament in a-Ta 2 O 5 .
On the other hand, previous studies in our group have shown that a conduction
channel is formed in the crystal Ta 2 O 5 film via interstitial Cu atoms, but not via
oxygen vacancies [20, 21]. However, it is desirable to examine the case of a-Ta 2 O 5
since, as mentioned before, the amorphous phase is usually adopted in experiments
and prototype devices [12, 13].
3.1.1 Single Cu Atomic Chains in a-Ta 2 O 5
In our study to explore the possible conductive paths in the a-Ta 2 O 5 structure [6],
both atomic positions and lattice constants of Cu doped a-Ta 2 O 5 structures were
fully relaxed. Keeping the previous studies using the Cu-doped crystalline Ta 2 O 5
[20, 21] in mind, we examined both alternate Ta-Cu atomic chain and continued Cu
atomic chain as the candidates of conductive path in a-Ta 2 O 5 via interstitial Cu
atoms or substituting O by Cu atoms. After structural optimization, both the alternate
Ta-Cu and continued Cu atomic chains were found in a-Ta 2 O 5 via substituting O
with Cu atoms as shown in Fig. 2a,b. The analysis of the density of states (DOS)
indicates the formation of conductive, delocalized defect states near the Fermi level,
which can serve as conductive paths. However, both structures were easily destroyed
after MD simulations at 500 K. Cu atoms prefer bonding together to single atomic
chain arrangements, and tend to form cluster or nanowire structures. This gives us a
hint for the formation of Cu filaments in a-Ta 2 O 5 .
3.1.2 Cu Nanowires in a-Ta 2 O 5
Cu structures with bigger diameters such as Cu nanowires have been extensively
studied both theoretically and experimentally in environments other than the
a-Ta 2 O 5 matrix [22, 23]. In the present studies [6], two relatively thin Cu nanowires
with the interlaced trigonal and tetragonal packing were chosen to be inserted into
the a-Ta 2 O 5 along c-axis (perpendicular to the a-b plane in the unit cell). Although
the structures of these two Cu nanowires changed in some degree after structural
relaxation, the continued Cu-Cu bonding structures were still observed, and the
subsequent MD simulations further confirm their stability. However, we should note
98
S. Watanabe and B. Xiao
3.1 Conduction Path in Cu/a-Ta 2 O 5 /Pt Atomic Switch
The widely accepted switching mechanism of the atomic switch is the formation/
rupture of a metal atomic bridge in the insulator between the two electrodes, which
has been identified in experiments [17–19]. In the case of the a-Ta 2 O 5 -based atomic
switch, composition analyses of a-Ta 2 O 5 film before and after applying a voltage
show that Cu ions migrate from the Cu electrode into the a-Ta 2 O 5 film, which agrees
well with the speculation of the formation of a conducting Cu filament in a-Ta 2 O 5 .
On the other hand, previous studies in our group have shown that a conduction
channel is formed in the crystal Ta 2 O 5 film via interstitial Cu atoms, but not via
oxygen vacancies [20, 21]. However, it is desirable to examine the case of a-Ta 2 O 5
since, as mentioned before, the amorphous phase is usually adopted in experiments
and prototype devices [12, 13].
3.1.1 Single Cu Atomic Chains in a-Ta 2 O 5
In our study to explore the possible conductive paths in the a-Ta 2 O 5 structure [6],
both atomic positions and lattice constants of Cu doped a-Ta 2 O 5 structures were
fully relaxed. Keeping the previous studies using the Cu-doped crystalline Ta 2 O 5
[20, 21] in mind, we examined both alternate Ta-Cu atomic chain and continued Cu
atomic chain as the candidates of conductive path in a-Ta 2 O 5 via interstitial Cu
atoms or substituting O by Cu atoms. After structural optimization, both the alternate
Ta-Cu and continued Cu atomic chains were found in a-Ta 2 O 5 via substituting O
with Cu atoms as shown in Fig. 2a,b. The analysis of the density of states (DOS)
indicates the formation of conductive, delocalized defect states near the Fermi level,
which can serve as conductive paths. However, both structures were easily destroyed
after MD simulations at 500 K. Cu atoms prefer bonding together to single atomic
chain arrangements, and tend to form cluster or nanowire structures. This gives us a
hint for the formation of Cu filaments in a-Ta 2 O 5 .
3.1.2 Cu Nanowires in a-Ta 2 O 5
Cu structures with bigger diameters such as Cu nanowires have been extensively
studied both theoretically and experimentally in environments other than the
a-Ta 2 O 5 matrix [22, 23]. In the present studies [6], two relatively thin Cu nanowires
with the interlaced trigonal and tetragonal packing were chosen to be inserted into
the a-Ta 2 O 5 along c-axis (perpendicular to the a-b plane in the unit cell). Although
the structures of these two Cu nanowires changed in some degree after structural
relaxation, the continued Cu-Cu bonding structures were still observed, and the
subsequent MD simulations further confirm their stability. However, we should note
98
S. Watanabe and B. Xiao