Among the oxide-based resistive switching devices, those including amorphousTaO x (a-TaO x ) were widely studied, which exhibit excellent performance [3–5]. In
general, the switching of these devices could be attributed to the forming/rupture of
conduction filaments (CFs). However, the atomistic details of switching mechanisms
of a-TaO x -based resistive switches has been unclear until recently. More specifically,
(1) in a-TaO x -based bipolar switches such as Cu/a-Ta 2 O 5 /Pt, the component of CF
had been experimentally identified to be Cu [5], while the atomic structure of CF was
unclear: the ionization of interface Cu had been found to be the rate-limiting step
during the switching process of the device, while the detail atomic structure near the
interface region was still unknown, too [5]. (2) In the Pt/a-TaO x /Pt resistive switch,
the atomic component of CF, that is, whether it consists of O vacancies or Ta
clusters, was still unknown [4]; on the other hand, how the active ions (such as O
and Ta) diffuse in a-TaO x was also unclear.
We tackled with the above issues via first-principles simulations based on the
density functional theory (DFT) [6–11]. In this chapter, we discuss the atomic
structures of the CFs, the interface structures, and the diffusion behaviors of active
ions in a-TaO x -based resistive switching devices on the basis of our simulation
results.
2 Computation Methods and Models
2.1 Methods
The calculation of structure relaxation, electronic properties and molecular dynamics
(MD) simulations were carried out using the Vienna ab initio simulation package
(VASP) [12, 13]. The projector augmented-wave (PAW) [14] method and the
generalized gradient approximation (PW91) [15] were adopted to describe the
atomic core electrons and electron-electron interactions, respectively.
The calculations of electronic transport properties were carried out via the
Atomistix Tool-kit (ATK) program [16]. A numerical atomic basis set, a single-ζ
basis with polarization, was used to solve the Kohn–Sham equations. The PerdewBurke-Ernzerhof (PBE) functional form within the generalized gradient approximation (GGA) was adopted for electron-electron interaction [11].
2.2 Structure of Amorphous Ta 2 O 5
In real tantalum-oxide-based resistive switching devices, TaO x is always in the
amorphous phase. Therefore, we performed most of our simulations assuming the
amorphous TaO x (a-TaO x ) phase. We first built an a-Ta 2 O 5 (i.e. a-TaO 2.5 ) structure
by melt-quenching method using VASP. More specifically, the initial crystalline
δ-Ta 2 O 5 structure was melted by MD simulation at 6000 K for 9 ps, and
96
S. Watanabe and B. Xiao
general, the switching of these devices could be attributed to the forming/rupture of
conduction filaments (CFs). However, the atomistic details of switching mechanisms
of a-TaO x -based resistive switches has been unclear until recently. More specifically,
(1) in a-TaO x -based bipolar switches such as Cu/a-Ta 2 O 5 /Pt, the component of CF
had been experimentally identified to be Cu [5], while the atomic structure of CF was
unclear: the ionization of interface Cu had been found to be the rate-limiting step
during the switching process of the device, while the detail atomic structure near the
interface region was still unknown, too [5]. (2) In the Pt/a-TaO x /Pt resistive switch,
the atomic component of CF, that is, whether it consists of O vacancies or Ta
clusters, was still unknown [4]; on the other hand, how the active ions (such as O
and Ta) diffuse in a-TaO x was also unclear.
We tackled with the above issues via first-principles simulations based on the
density functional theory (DFT) [6–11]. In this chapter, we discuss the atomic
structures of the CFs, the interface structures, and the diffusion behaviors of active
ions in a-TaO x -based resistive switching devices on the basis of our simulation
results.
2 Computation Methods and Models
2.1 Methods
The calculation of structure relaxation, electronic properties and molecular dynamics
(MD) simulations were carried out using the Vienna ab initio simulation package
(VASP) [12, 13]. The projector augmented-wave (PAW) [14] method and the
generalized gradient approximation (PW91) [15] were adopted to describe the
atomic core electrons and electron-electron interactions, respectively.
The calculations of electronic transport properties were carried out via the
Atomistix Tool-kit (ATK) program [16]. A numerical atomic basis set, a single-ζ
basis with polarization, was used to solve the Kohn–Sham equations. The PerdewBurke-Ernzerhof (PBE) functional form within the generalized gradient approximation (GGA) was adopted for electron-electron interaction [11].
2.2 Structure of Amorphous Ta 2 O 5
In real tantalum-oxide-based resistive switching devices, TaO x is always in the
amorphous phase. Therefore, we performed most of our simulations assuming the
amorphous TaO x (a-TaO x ) phase. We first built an a-Ta 2 O 5 (i.e. a-TaO 2.5 ) structure
by melt-quenching method using VASP. More specifically, the initial crystalline
δ-Ta 2 O 5 structure was melted by MD simulation at 6000 K for 9 ps, and
96
S. Watanabe and B. Xiao