This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON ELECTRON DEVICES 1 Resistive Switching with Self-Rectifying Tunability and Inﬂuence of the Oxide Layer Thickness in Ni/HfO 2 /n + -Si RRAM Devices Alberto Rodriguez-Fernandez, Samuel Aldana, Francesca Campabadal, Jordi Suñé, Fellow, IEEE, Enrique Miranda, Senior Member, IEEE , Francisco Jiménez-Molinos, Juan Bautista Roldán, and Mireia Bargallo Gonzalez Abstract — The impact of the dielectric thickness, form- ing polarity, and current compliance on the self-rectifying current–voltage (I –V ) characteristics of Ni/HfO 2 /n + -Si resis- tive random access memory (RRAM) devices was inves- tigated. The obtained results indicate that these three aspects not only play a role in the postforming currents but also affect the switching properties of the devices. In the case of 5-nm-thick oxide devices, a self-rectifying ratio of about three orders or magnitude is observed after substrate injection forming (SIF) with current compliance below 500 μA. However, similar devices subjected to gate injection forming (GIF) do not exhibit such rectifying fea- ture. This distinctive behavior for SIF is ascribed to the formation of a Schottky-like contact in between the Ni-based conducting ﬁlament and the semiconductor electrode. For 20-nm-thick oxide devices, the forming voltage under GIF and the subsequent dielectric degradation are higher than for thinner oxide layers, resulting in a less resistive state, and a negligible role of the referred Schottky barrier. The effect of the temperature on the diffusion of the Ni ions that form the conducting path is also discussed. Index Terms— HfO 2 , memristor, resistive switching (RS), resistive random access memory (RRAM). Manuscript received March 13, 2017; revised May 18, 2017; accepted June 15, 2017. This work was supported in part by the Spanish Ministry of Economy and Competitiveness through FEDER under Project PCIN2013-076, Project TEC2014-52152-C3-1-R, Project TEC2014-52152-C3-2-R, and Project TEC2014-54906-JIN, in part by the ENIAC Joint Undertaking under Project PANACHE, in part by the DURSI of the Generalitat de Catalunya under Grant 2014SGR384, and in part by the Spanish ICTS Network MICRONANOFABS. The review of this paper was arranged by Editor Y.-H. Shih (Corresponding author: Alberto Rodriguez-Fernandez.) A. Rodriguez-Fernandez, J. Suñe, and E. Miranda are with the Departamento Enginyeria Electrònica Ediﬁci, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain (e-mail: alberto.rodriguez@uab.es; jordi.sune@uab.es; Enrique.miranda@uab.es). S. Aldana, F. Jiménez-Molinos, and J. B. Roldán are with the Depar- tamento de Electrónica y Tecnología de Computadores, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain (e-mail: samaldana@ugr.es; jmolinos@ugr.es; jroldan@ugr.es). F. Campabadal and M. B. Gonzalez are with the Institut de Microelectrònica de Barcelona, 08193 Bellaterra, Spain (e-mail: mireia.bargallo.gonzalez@csic.es; francesca.campabadal@csic.es). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TED.2017.2717497 I. I NTRODUCTION R ESISTIVE random access memory (RRAM) is one of the most promising technologies for nonvolatile mem- ory devices due to its signiﬁcant advantages concerning low power consumption, fast switching speed, high scalability, and 3-D integration feasibility over the widely used silicon- based ﬂash memories [1]–[6]. However, one major issue that requires serious attention is the occurrence of multiple sneak paths between RRAM cells when placed in cross- bar array architectures. This can lead to severe readout errors [4], [7], [8]. To overcome this problem, an additional selector device, such as a rectifying diode or pass transistor needs to be integrated into each memory cell to suppress the crosstalk effect [8], [9]. This selector, however, signiﬁcantly increases the unitary cell size and enhances the fabrication process complexity [10], [11]. In last years, an alterna- tive solution to the use of a selector device, consisting in a RRAM cell with intrinsic self-rectifying properties, has emerged [10]–[16]. In this regard, several works have reported resistive switching (RS) behavior in CMOS compatible Ni-based HfO 2 /n + -Si [10]–[13] or HfO 2 /n + Ge [14] structures presenting Schottky-diode behavior, i.e., asymmetric electron transport. The origin of the self-rectifying conduction characteristics in RRAM devices is often associated with the formation of nanocontacts between the conducting ﬁlaments (CF) and the electrodes. It is widely accepted that the unipolar RS behavior of Ni/HfO 2 /n + -Si devices relies on the formation and rupture of these oxygen vacancy atomic bridges and the subsequent Ni diffusion/migration from the metal electrode towards the bottom electrode [13], [17], [18]. The CF dissolution is often ascribed to thermally enhanced diffusion induced by Joule heating effects [19]–[22]. In addition, the forming polarity [23] and current compliance [17], [18] are known to signiﬁcantly affect the size and composition of the CFs, which in turn determines the self-rectifying behavior. In this paper, an in-depth investigation of the low-resistive state (LRS) currents and switching properties of Ni/HfO 2 /n + - Si devices with HfO 2 thickness ranging from 5 to 20 nm 0018-9383 © 2017 IEEE. 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