switching materials [28–30]. According to previous reports, the widely accepted
switching mechanism of O-deficient-material (such as TaO x and TiO x ) based resistive switches is based on the drift/diffusion of O vacancies (V O s) driven by applied
electric fields [31, 32]. In the case of TiO x -based resistive switch, previous experimental and theoretical studies revealed that the conduction filament (CF) is composed of the V O s and the conduction is based on the electron hopping in CF, which
causes the negative temperature coefficient of resistance (TCR) in both high resistance state (HRS) and low resistance state (LRS) [32, 33]. Keeping this in mind,
several studies explained the switching in the a-TaO x based resistive switches from
the electron hopping conduction and/or the V O -based CF [34, 35]. Recently, Lee
et al. [5] and Choi et al. [36] in detail investigated the component of CF in a-TaO x
based resistive switches, and their results show that a considerable amount of
continued Ta-rich region (TaO 1-x ) exist in the switching-on state, accompanied by
a positive TCR [30, 37]. This implies the strong qualitative difference in the
switching behavior between the TiO x and a-TaO x systems.
Thus, we examined the origin of switching mechanism in the a-TaO x -based
resistive switches via first-principles simulations [7]. Since the switching processes
of a-TaO x based devices strongly correlate with the change of O concentration [5],
we examined a-TaO x structures under various O concentrations to understand the
switching mechanism.
4.1.1 Structures and Electronic Properties of Single O Vacancies
in a-Ta 2 O 5
Firstly, a stoichiometric a-TaO 2.5 structure was generated via the melt quenching
method. As is well known, the structural and electronic properties of defects are
sensitive to their local environments, while the local environments vary site by site in
amorphous structures. Therefore, to understand the properties of single V O in
a-TaO 2.5 , all the possible vacancy sites were considered.
As can be seen from Fig. 12a, the stability of vacancy correlates with the Ta-Ta
bond length, that is, a structure with a shorter bond length has a lower energy
[7]. The length of the Ta-Ta dimer bond formed at the vacancy site is shorter than
that in a-TaO 2.5 (3.20 Å). In particular, the bond length of Ta-Ta dimer (2.84 Å) in
the most stable V O structure is comparable with the one (2.86 Å) in Ta metal. The
short bond length suggests good stability of the Ta dimer at V O . To further confirm
this, MD simulations were performed at different temperature as shown in
Fig. 12b, c. The results reveal that the Ta dimer is very stable at room temperature,
while the position of such Ta dimer structure migrates into its adjacent sites at high
temperature (i.e., 873 K). Thus, the motion of such a single Ta dimer structure is
likely to occur under external electric fields in real devices.
To understand the electronic properties of the V O in a-TaO 2.5 , we have calculated
the DOS for the energetically most stable V O structure with HSE06 hybrid functional
for the exchange-correlation term [38]. The results show that the defect state (mainly
coming from V O ) appears near the Fermi level, and its location (0.8 eV and 2.5 eV
110
S. Watanabe and B. Xiao
switching mechanism of O-deficient-material (such as TaO x and TiO x ) based resistive switches is based on the drift/diffusion of O vacancies (V O s) driven by applied
electric fields [31, 32]. In the case of TiO x -based resistive switch, previous experimental and theoretical studies revealed that the conduction filament (CF) is composed of the V O s and the conduction is based on the electron hopping in CF, which
causes the negative temperature coefficient of resistance (TCR) in both high resistance state (HRS) and low resistance state (LRS) [32, 33]. Keeping this in mind,
several studies explained the switching in the a-TaO x based resistive switches from
the electron hopping conduction and/or the V O -based CF [34, 35]. Recently, Lee
et al. [5] and Choi et al. [36] in detail investigated the component of CF in a-TaO x
based resistive switches, and their results show that a considerable amount of
continued Ta-rich region (TaO 1-x ) exist in the switching-on state, accompanied by
a positive TCR [30, 37]. This implies the strong qualitative difference in the
switching behavior between the TiO x and a-TaO x systems.
Thus, we examined the origin of switching mechanism in the a-TaO x -based
resistive switches via first-principles simulations [7]. Since the switching processes
of a-TaO x based devices strongly correlate with the change of O concentration [5],
we examined a-TaO x structures under various O concentrations to understand the
switching mechanism.
4.1.1 Structures and Electronic Properties of Single O Vacancies
in a-Ta 2 O 5
Firstly, a stoichiometric a-TaO 2.5 structure was generated via the melt quenching
method. As is well known, the structural and electronic properties of defects are
sensitive to their local environments, while the local environments vary site by site in
amorphous structures. Therefore, to understand the properties of single V O in
a-TaO 2.5 , all the possible vacancy sites were considered.
As can be seen from Fig. 12a, the stability of vacancy correlates with the Ta-Ta
bond length, that is, a structure with a shorter bond length has a lower energy
[7]. The length of the Ta-Ta dimer bond formed at the vacancy site is shorter than
that in a-TaO 2.5 (3.20 Å). In particular, the bond length of Ta-Ta dimer (2.84 Å) in
the most stable V O structure is comparable with the one (2.86 Å) in Ta metal. The
short bond length suggests good stability of the Ta dimer at V O . To further confirm
this, MD simulations were performed at different temperature as shown in
Fig. 12b, c. The results reveal that the Ta dimer is very stable at room temperature,
while the position of such Ta dimer structure migrates into its adjacent sites at high
temperature (i.e., 873 K). Thus, the motion of such a single Ta dimer structure is
likely to occur under external electric fields in real devices.
To understand the electronic properties of the V O in a-TaO 2.5 , we have calculated
the DOS for the energetically most stable V O structure with HSE06 hybrid functional
for the exchange-correlation term [38]. The results show that the defect state (mainly
coming from V O ) appears near the Fermi level, and its location (0.8 eV and 2.5 eV
110
S. Watanabe and B. Xiao