Materials selection for oxide-based resistive random access memories

Guo, Yuzheng; Robertson, John

doi:10.1063/1.4903470

Cited by 108 publications

(114 citation statements)

References 44 publications

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“…4. For Ta2O5, Ov at the two-fold coordinated intralayer oxygen site has the lowest formation energies for all charge states, in agreement with the result of Guo et al [56]. The charge transition level (+2/0) and (0/-2) lies 0.2 eV below and 0.2 eV above the conduction band (CB) edge, respectively.…”

Section: Ov Formation Energies In Ta2o5 and Tio2supporting

confidence: 78%

“…In the last part, we have seen that the system Fermi energies for both Ta2O5/TaO2 and TiO2/Ti4O7 interfaces lie above the CB edge (or just slightly below according to [52]) of respective stoichiometric oxides, which suggests that negatively charged (or neutral according to [52]) Ov are stable. Guo et al [56] has studied the effect of scavenging metal on tuning the Fermi level of oxide/parent metal electrode interface. The idea was that scavenging metal with higher work function sets the effective system Fermi level to near the mid-gap where +2 charged Ov is stabilized.…”

Section: Ov Formation Energies In Ta2o5 and Tio2mentioning

confidence: 99%

“…Experimentally, scavenger metal layer has been used to optimize the performance of RS memory cells [59,60]. Guo et al [56] rationalized that scavenger metal of larger work function than the parent metal electrode lowers the Fermi energy to ensure Ov in its positively charged state. In the following part, we propose the use of dopants as another route for this purpose.…”

Section: Ov Formation Energies In Ta2o5 and Tio2mentioning

confidence: 99%

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Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

Hong

Zhang

Shi

2015

Journal of Elec Materi

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We study the electronic structures of TiO2/Ti4O7 and Ta2O5/TaO2 interfaces using the screened exchange (sX-LDA) functional. Both of these bilayer structures are useful infrastructures for high performance RRAMs. We find that the system Fermi energies of both interfaces are just above the conduction band edge of the corresponding stoichiometric oxides. According to the charge transition levels of the oxygen vacancies, the oxygen vacancies are stabilized at the -2 charged state in both Ta2O5 and TiO2. We propose to introduce interfacial dopants to shift the system Fermi energies downward so that the +2 charged oxygen vacancy can be stable, which is important for the controlled resistive switching under the electrical field. Several dipole models are presented to account for the ability of Fermi level shift due to the interfacial dopants. IntroductionMetal oxides have been widely used in the emerging nonvolatile memory technologies like the resistive random access memory (RRAM) [1,2] and the memristor [3][4][5]. Both of these devices are characterized by the reversible resistance switching (RS) which is commonly due to the growth and rupture of the conducting filaments (CFs) consisting of charged oxygen vacancies (Ov) [6][7][8]. Numerous reports have been published on metal oxides operating through this RS mechanism like TiO2 [9,10], Ta2O5 [11], SrTiO3 [12], NiO [13,14], and so on. Among various oxide candidates, TiO2 demonstrates both unipolar [9,10] and bipolar [15,16] RS. Transmission electron microscopy of the unipolar RS TiO2 has suggested that the CFs in unipolar switching are composed of Magneli phase Ti4O7 [10]. Recently, real time identification of the evolution of Ti4O7 CFs in TiO2 has been reported [17]. However, the problem of TiO2 is the co-existence of more than one stable substoichiometric Magneli phases which is unfavorable for long device endurance [18]. On the other hand, Ta2O5 keeps the longest endurance record among these oxides [11] and it operates in a bipolar manner. The long endurance of Ta2O5 based devices is partly attributed to only one stable substoichiometric phase, Ta, under 1000°C [18] despite of large amount of metastable substoichiometric phases such as TaO2. The in situ observation of CFs in Ta2O5 has recently been reported [19].One of the biggest challenges of these oxide based devices is the requirement for the electroforming process [20,21]. This could induce physical damages to the device due to oxygen gas release [22]. Besides, wide variance of device properties depends on the details of the electroforming which are yet to be understood. Therefore, it is essential to achieve electroforming free RS so as to eliminate the device variance for applicable computer circuits [20,21].

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Section: Ov Formation Energies In Ta2o5 and Tio2supporting

confidence: 78%

Section: Ov Formation Energies In Ta2o5 and Tio2mentioning

confidence: 99%

Section: Ov Formation Energies In Ta2o5 and Tio2mentioning

confidence: 99%

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Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

Hong

Zhang

Shi

2015

Journal of Elec Materi

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“…An oxygen vacancy at the intralayer 2-fold site has the lowest formation energy over all charge states. We can see from the diagram that a transition from +2 charged state to neutral state at a Fermi level just below the conduction band minimum (CBM), in agreement with Guo et al [35]. Ta2O5/TaO2 bilayer structure is useful for controllable programming current and device stability that TaO2 serves as metallic source of oxygen ions for the switching in the insulating Ta2O5 [36,37].…”

Section: Single Defectsupporting

confidence: 65%

Correlation and ordering of defects in the formation of conducting nanofilaments

Zhang¹,

Li²,

Shi³

2016

J. Phys. D: Appl. Phys.

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We study the dependence of the formation energies of oxygen and metal induced defects in Ta2O5, TaO2, TaO, TiO2 and Ti4O7 on the chemical potential of electron and atomic constitutes. In the study of single defect, metal induced defects are found to be preferable to oxygen induced defects. This is against the experimental fact of the dominant role of oxygen induced defects in the RS process. A simple multiple defects picture without correlated atomic rearrangement does not cure this problem. The problem is resolved under the correlated multiple defect picture where the multiple defects result in correlated atomic rearrangement and the final products show certain atomic ordering. IntroductionResistive switching (RS) in transition metal oxides (TMOs) has attracted great interests for a possible application in nonvolatile memory devices [1,2] and analog memristors [3][4][5]. Defect redistribution under the switching field is generally considered to play prominent role in the reversible RS [6][7][8]. In simple binary TMOs such as Ta2O5 and TiO2, the agglomeration (segregation) of oxygen vacancies and the subsequent growth (rupture) of the conducting filament is one of the most cited RS mechanisms [6][7][8]. These simple binary TMOs have the advantage of easy fabrication. However, the role of oxygen vacancies in the formation of conducting filaments at an atomic level is less well known. Besides the locally formed conducting path, the resistance can also be switched back and forth by the drift of the charged oxygen vacancies towards or away from one of the two electrodes which modifies the width of the interfacial depletion region and consequently the Schottky contact resistance [9]. Oxygen vacancy induced charge trap can also assist bulk like RS through continuous filling or emptying the traps, whose extent of charge occupation varies the resistance [10]. More complicated correlated TMOs like Pr0.7Ca0.3MnO3 [11], SmNiO3 [12] show metal-insulator transition which accounts for the RS. In these correlated TMOs, the oxygen vacancy coordination environment, which is subject to the driving field either electrically or thermally, modulates the local valence state of transition metal by rearranging the hybridization of transition metal d and oxygen p orbitals. Above mentioned TMOs show RS through effective change of the local oxidation state. Therefore, they are categorized as Redox (Reduction-oxidation) RS TMOs [5]. In this work, we focus on the filament type RS. High performance Ta2O5 based nonvolatile memory device [13] and TiO2 memristor based neuromorphic network [14] have recently been demonstrated. In situ characterization revealed that the conducting filaments in TiO2 and Ta2O5 are composed of Magneli phase Ti4O7 [15] and TaO1-x

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“…The selection of appropriate switching layer material is central to the RRAM operation 7 and tantalum oxide is of particular interest because of high endurance of switching cycles 8 . The purpose of N doping is to reduce electronic defect states associated with the Ovac 9 and enabling CFs to be formed with more stable and discrete resistances suitable for MLC.…”

mentioning

confidence: 99%

The role of nitrogen doping in ALD Ta2O5 and its influence on multilevel cell switching in RRAM

Sedghi

Brunell

et al. 2017

Applied Physics Letters

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The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermediate states, improving the stability of states, and providing a gradual reset for multi-level cell switching during reset. The N-doped Ta2O5 devices have relatively small set and reset voltages (< 1 V) with reduced variability due to doping.

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Materials selection for oxide-based resistive random access memories

Cited by 108 publications

References 44 publications

Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

Correlation and ordering of defects in the formation of conducting nanofilaments

The role of nitrogen doping in ALD Ta2O5 and its influence on multilevel cell switching in RRAM

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