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Article
Synergistic Resistive switching Mechanism of Oxygen Vacancies and Metal Interstitials in TaO 2
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Linggang Zhu, Jian Zhou, Zhonglu Guo, and Zhimei Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11080 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 16, 2016
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The Journal of Physical Chemistry
Synergistic Resistive Switching Mechanism of Oxygen Vacancies and Metal Interstitials in Ta2O5 Linggang Zhu1, 2, Jian Zhou1, Zhonglu Guo1, 2 and Zhimei Sun1, 2,* 1
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, China *corresponding author:
[email protected] 2
Abstract Ta2O5 is extensively studied as a data storage material for resistance random access memory (RRAM). The resistive switching (RS) in Ta2O5 based RRAM is generally believed to be due to the diffusion of oxygen vacancy (Vo) inside the oxide, while the role of metal interstitials is paid less attention. Here on the basis of first-principles calculations, we show that the role of interstitial Ta (Tai) is competitive under the oxygen poor condition and should also contributes to RS in Ta2O5. This is obvious by our calculated comparable energy barriers for the diffusion of Vo and Tai, which are 3.5 eV and 3.7 eV, respectively. Furthermore, the presence of electric field in working devices will enhance the migration of Tai due to its higher charge states compared to Vo. Meanwhile, Tai will introduce more defect states closer to the conduction bands, and thus is more effective on tuning the electronic structure of Ta2O5. The present work unravels the contribution of Ta cations in RS of tantalum oxides based RRAM, presenting a synergistic RS mechanism of oxygen vacancies and metal interstitials, which should be helpful for optimizing and designing of novel RRAM devices.
1. Introduction Resistance random access memory (RRAM) is one of the most promising candidates for next generation non-volatile devices for data storage, logic and neuromorphic application.1-3 The appealing advantages of RRAM include the simple structure, good scalability, fast switching speed and good compatibility with the popular CMOS (complementary metal-oxide-semiconductor) technology. The working basis of RRAM is quite straightforward: the high and low resistance state of the recording medium corresponds to logic 0 and 1, respectively. Various mechanisms account for the resistive switching (RS) phenomena, such as the carrier trapping,
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migration and clustering of point defect, metal-insulator-transition, etc. A large variety of materials have been found applicable as the recording mediums in RRAM, which can be grouped into binary/ternary oxides, chalcogenides, amorphous-silicon/carbon and so on. Among these materials, transition metal binary oxides have been extensively studied, due to their easy fabrication process and good performance. More details about the emerging RRAM can be found in recent review articles.4-7 As for the binary oxides used in RRAM, tantalum oxides (TaOx) show great promise in recent studies.8-17 Two types of tantalum oxides are normally involved, i.e., dioxide and pentoxide, which can be denoted as TaO2-x and Ta2O5-x, taking into account of the oxygen deficiency and off-stoichiometry. Electrode engineering11 and introducing the intermediate layer into the metal/insulator/metal sandwich structures13 have been proved to be the effective strategies to improve the performance of the TaOx based RRAM. For Ta2O5 and other TaOx based RRAM, the experimental researches so far mainly focus on the amorphous structure, and the generally accepted microscopic mechanism for resistive switching is the formation of conductive path due to the migration of anion, i.e., oxygen vacancy/ions.12, 14-15, 18-19 Based on these conclusions from the experiments, theoretical calculations employing density functional theory have been widely used to study the formation and diffusion of oxygen vacancy in the polymorphs of Ta2O5,20-26 namely β-Ta2O5 and δ-Ta2O5. Although experimentalists used amorphous structure, it is worth noting that the theoretical studies on the properties of crystalline Ta2O5 are essential since it has been demonstrated that the local structure of amorphous Ta2O5 film is consistent with that of crystalline δ-Ta2O5.24 More recently, Lübben et al13 reported their latest experimental research on the switching mode of Ta/TaOx based RRAM. Minimizing the effects of oxygen vacancy by inserting a graphene layer between the oxide and the electrode, the electrochemical metallic memories like switching can still be observed, and this phenomenon is interpreted by considering the migration of Ta-cation (assumed to be interstitial Ta) in Ta2O5. Moreover, they further demonstrated the contribution of mobility of host metal cations to resistive switching in HfOx and TiOx, the other two typical VCMs (valence change memories) materials where migration of oxygen vacancy was believed to take the charge.10 Given the extensive discussion of oxygen vacancy in tantalum oxides in the literature, the proposal of the significant effects of Ta-cation on the resistive switching is quite surprising. As a consequence of these latest discoveries, a detailed 2
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comparison between oxygen anion and Ta cation, in terms of their effects on the electronic structure and diffusion behavior in Ta2O5 in a microscopic level becomes necessary and urgent, in order to establish a complete mechanism for TaOx-RRAM. Moreover, using Ta as the probe to study the diffusion channels of species via the interstitial mechanism in Ta2O5 is also meaningful for other so-called electrochemical metallization memories (ECMs), such as Cu/Ta2O5/Pt cells where mobile Cu ions in Ta2O5 dominate the formation of conductive path.11, 27 It is worth pointing out that in the complete resistive switching process, the compositions/structures in the very local area of the host are evolving, such as the formation/rupture of the metallic filaments inside the sample. And this evolution should finish within a few nanoseconds for a fast writing/erasing speed of RRAM. All these characteristics make the capturing of all the details using one single research technique unlikely. However, we believe that the diffusion of the point defects in the matrix should be the most basic thereby very vital issue to be investigated, in order to understand the resistive switching process of RRAM. It has been demonstrated that the structure of Ta2O5 films, correlated with its density, has apparent effects on the resistive switching behavior.11 As a matter of fact, the single-crystalline memory materials28-30 and/or intermediate crystalline structures formed in the amorphous matrix during the resistive switching process10 are quite common in RRAM devices. In the present study, we focus on the crystalline Ta2O5, the
thermodynamically
most
stable
compound
in
the
established
Ta-O
phase-diagram.31 Recently, a new structure of tantalum pentoxide, named as λ-Ta2O5, was found,32 which is more stable than the other polymorphs (β-Ta2O5 and δ-Ta2O5). In fact these three polymorphs have very similar structure: consisting of two-dimensional (2D) Ta2O3 layer and On layer (pure oxygen layer), arranging along the z direction. Lee et al32 found that in the thermodynamically ground state of Ta2O5 the local structure of globally disordered O sublattice is similar to the λ model. Moreover, λ-Ta2O5 is the only phase among the polymorphs that has identical bandgap as the amorphous structure32 which exist in actual devices so far. Therefore, here the λ model is used to investigate the point defect behavior in Ta2O5 by using firs-principles calculations. In addition to the oxygen vacancy that normally exists in the oxides, the formation and diffusion of interstitial Ta, which might also take place in oxygen deficient Ta2O5 (Ta2O5-x) and contribute to the resistive switching behavior, are also studied. Considering the special structure of λ-Ta2O5 (arrangement of 2D Ta2O3 layer 3
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and On layer), the quantification of the diffusion networks in this system should be very interesting and useful.
2. Calculation details All the structures are optimized using Vienna ab initio simulation package (VASP).33-34 Exchange-correlation potential is described by generalized gradient approximation (GGA-PBE).35 Pseudopotentials with configurations of 5p65d36s2 and 2s22p4 are used for Ta and O, respectively. The plane-wave cutoff energy is set as 450 eV. A 2×2×3 supercell (containing 168 atoms in perfect Ta2O5) are constructed to model the structures with vacancy and/or interstitial atom. Here the neutral defects are considered, which is quite reasonable in the presence of external electric field that can compensate the extra charge of the system, as in the case of RRAM. Although the whole supercell is kept electroneutral, all the atoms inside the ionic environment including the introduced Tai are actually charged. A k-point mesh of 2×2×2 is used, including four irreducible k-points when no symmetry is imposed during the calculation. The minimum energy path (MEP) for the diffusion is optimized using climbing image nudged elastic band (CI-NEB) method,36 and five intermediate states are sampled along each diffusion path. In the CI-NEB calculations, the shape of the supercell is fixed while all the atoms inside are free to move, and it has been shown that for the large supercell as used here, the relaxation of the supercell shape has very small effects on the energy barrier.37 For the rank of the transition state located by CI-NEB method, early vibration frequency calculations has shown that it is the first-order.38 In the present study, test calculations of the frequency are run in the case of oxygen-vacancy diffusion. Finite difference method is used to construct the Hessian matrix. Given the large supercell used here and the fact that the diffusive specie is mainly oxygen atom, only the displacement of the oxygen atom is considered. And we found that only one imaginary frequency exists in the transition state, i.e., the structure found is first-order saddle point. Traditional DFT often fails to accurately describe the electronic structure of transition metal oxides due to the localization of the d electron, like the underestimation of the band gap. Thus in the present study, hybrid functional HSE0639 is used to calculate the electronic structure. Due to huge computational resources consumption of HSE06 calculation, we used a 1×1×2 supercell to study the electronic structure of the defect-containing Ta2O5. Firstly, the supercell containing 4
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Vo/Tai is relaxed at PBE level, then self-consistent static calculations is performed using HSE06, after which the density of states and partial charge density are obtained. It is worth noting that for the energetic data in the paper, such as the migration energy, only DFT-PBE calculations are used, since it has been shown that the correction on the electron localization has quite small influence on these quantities.37
3. Results and Discussion 3.1. Formation energy of Vo and Tai As mentioned above, two types of defects are considered here, i.e., Vo and Tai. Their formation energy can be calculated using the following equations:
E Vo = E (Ta 2 x O 5 x −1 ) − E (Ta 2 x O 5 x ) + µO f
(1)
i E Ta = E (Ta 2 x +1O5 x ) − E (Ta 2 x O5 x ) − µTa f
(2)
1 2
µTa = [ E (Ta 2 O5 ) − 5µO ]
In the above equations, E represents the energy of specific systems with the component of the system listed in the bracket; µO and µTa are the chemical potential of the oxygen and tantalum atom/ion, respectively. The formation energy of these two types of defects can be expressed as a function of µO , while the range of µO is restricted in the Ta:Ta2O5 equilibrium system:
1 1 1 EO2 + E Ta2O5 ≤ µO ≤ EO2 f 2 5 2 5 Ta E Ta2O5 = E (Ta 2 O5 ) − 2 Ebulk − EO2 f 2
(3)
Where EO2 is the energy of oxygen molecule, E Ta2O5 is the formation energy of Ta2O5, f Ta is energy of each Ta atom in the bulk material. According to equation (3) and our Ebulk
calculations results, the range of µ O is -8.91 eV
