Universal understanding of direct current transport properties of ReRAM based on a parallel resistance model
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We propose a parallel resistance model (PRM) in which total resistance (Rtotal) is given by the parallel connection of resistance of a filament (Rfila) and that of a film excluding the filament (Rexcl)—that is, 1/Rtotal ⳱ 1/Rfila + 1/Rexcl—to understand direct current (dc) electric properties of resistive random-access memory (ReRAM). To prove the validity of this model, the dependence of the resistance on temperature, R(T), and the relative standard deviation (RSD) of RHRS of Pt/NiO/Pt on the area of a top electrode, S, are investigated. It is clarified that both the R(T) and RSD depended on S, and all such dependencies can be explained by the PRM. The fact that Rtotal is decided by the magnitude relation between Rfila and Rexcl makes transport properties S-dependent and hinders the correct understanding of ReRAM. Smaller S is essential to observe the intrinsic transport properties of ReRAM filaments.
I. INTRODUCTION
Resistive RAM (ReRAM), which consists of binary transition metal oxides (TMOs) such as NiO and TiO2, is a promising candidate to replace flash memory as consequences of its simple cell structure, compatibility with the complementary metal oxide semiconductor (CMOS) process, and multilevel applications.1 However, although extensive research on binary metal oxides has been conducted for more than 40 years,2,3 the mechanism of resistance switching and memory (RSM) effect has not yet been clarified. Here, the RSM effect is the effect of switching between bistable resistance states, i.e., the high-resistance state (HRS) and the low-resistance state (LRS), by applying appropriate bias voltages.1 Because the RSM effect occurs after the forming process, which is similar to a dielectric breakdown,2,3 defining the formation process is a key problem in distinguishing each model to explain its mechanism. Possible mechanisms have been suggested by (i) Hickmott et al.,4 (ii) Simmons et al.,5 and (iii) Dearnaley et al.6: (i) Hickmott et al. assumed there were immobile neutral impurities in the oxide. In the forming process, the impurities are ionized by the Poole–Frenkel process, giving an impurity band roughly in the middle of the band gap. The electric current flows by the space-charge limited conduction in the impurity band. The conductivity is reduced by the neutralization of impurity centers by electrons tunneling from a lower-lying hole level in the band gap. The memory effects can be explained by redistributions of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0093 812 J. Mater. Res., Vol. 23, No. 3, Mar 2008 http://journals.cambridge.org Downloaded: 18 Mar 2015
electrons over the impurity and hole levels.2 (ii) Simmons et al. suggested that the forming process is caused by the homogeneous injection of the upper electrode ions into the oxide, thus forming an impurity band. Because the impurity density is high, the electric current flows through the oxide by the tunneling from one impurity center to another on levels at the same energy. The condu
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