Identifying and Engineering the Electronic Properties of the Resistive Switching Interface

Abstract

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].

GGA-1/2 self-energy correction for accurate band structure calculations: the case of resistive switching oxides

“…On the other hand, the CF/HfO 2 Schottky barriers are sufficiently large, no matter whether pure Hf or conductive sub-oxides were used as the CF model. GGA-1/2 is very useful for calculating the electronic structures of these models since previous calculations either suffered from band gap inaccuracy [78], or were forced to adopt short models for the dielectric to enable heavy hybrid functional calculations [31].…”

“…Therefore, realistic ab initio RRAM device calculations require both a high computational speed and relatively accurate band gaps. Traditionally, one resolves the band gap problem in DFT-LDA/DFT-GGA by employing LDA+U and GGA+U [27][28][29][30], or resorts to hybrid functionals [31]. While the adoption of hybrid functional increases the computational load considerably, LDA+U is almost as fast as conventional LDA.…”

Structure and properties of a model conductive filament/host oxide interface in HfO2 -based ReRAM

“…This suggests that for this kind of device, significant positive charging and band bending should occur in the CF near the CF-HfO 2 interface. Such band alignment is not extensively debated in the literature [16][17][18] even though it is key to understanding properties such as charge transport [19]. We also show that the energy cost to transfer oxygen from HfO 2 lattice sites into interstitial sites in HfO increases almost linearly with the concentration of oxygen atoms (1.2 eV per transferred oxygen).…”