DC and low-frequency noise behavior of the conductive filament in bipolar HfO 2 -based resistive random access memory V. Maccaronio a , F. Crupi a,⇑ , L.M. Procel c , L. Goux b , E. Simoen b , L. Trojman c , E. Miranda d a Dipartimento di Elettronica, Informatica e Sistemistica, Università della Calabria, Via P. Bucci, 41C, I-87036 Arcavacata di Rende (CS), Italy b Imec, Kapeldreef 75, B-3001 Leuven, Belgium c Colegio de Ciencias e Ingeniería, Universidad San Francisco de Quito, Diego de Robles s/n, Quito, Ecuador d Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain article info Article history: Received 1 October 2012 Received in revised form 25 December 2012 Accepted 14 February 2013 Available online 5 March 2013 Keywords: Resistive RAM Non-volatile memory Low-frequency noise Quantum point contact Hafnium oxide abstract This paper addresses the low frequency noise (LFN) properties of bipolar HfO 2 -based resistive random access memory cells. It is shown that the devices exhibit a current on–off window up to 70 which is almost independent of the temperature in the range 30–180 °C. The experimental current–voltage curves in both resistance states are well reproduced by the quantum point contact model. LFN spectrum is typically characterized by 1/f noise in the low resistance state (LRS), while individual lorentzian compo- nents are often observed superimposed to the background 1/f noise in the high resistance state (HRS). Both LFN types are ascribed to defects fluctuating between a neutral and a charged state. The LFN level normalized to the square of the DC current in HRS is about two orders of magnitude higher than the cor- responding value for LRS. The higher normalized LFN observed in HRS is ascribed to the smaller cross-sec- tion area of the conductive filament and to the stronger effect of the potential barrier modulation induced by a trapped electron. The normalized LFN is independent of the temperature for both resistance states as well as of the bias voltage in LRS, while it decreases with the bias in HRS, which is well correlated to the corresponding resistance decrease. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Flash memories have experienced an impressive scaling trend in the last years, due to the market request of non-volatile memo- ries with high access speed and high capacity, especially for usage in mobile devices. However this memory type is expected to face multiple difficulties in scaling below 20 nm, mainly related to physical and technological issues. Among the different solutions proposed to overcome these obstacles, memories based on a resis- tive switching mechanism, often referred to as resistive random access memory (ReRAM), are the most promising candidates, con- sidering their high scalability, integration density, switching speed, simple design and compatibility with current CMOS fabrication processes [1–5]. A ReRAM cell has a simple capacitor-like structure, with a dielectric layer in between two metal electrodes. In some of the newest structures an additional metallic layer is inserted in contact with one of the electrodes in order to improve the device perfor- mance [6]. The memory effect in these devices is based on the possibility to electrically switch the cell between two well-defined resistance levels. Depending on the I–V characteristics, the switching behavior can be unipolar or bipolar. In unipolar resistive switching, the switching direction depends on the magnitude of the applied voltage but not on its polarity. On the contrary, in bipolar resistive switching, the change of the state can be just ob- tained by applying voltages of the specific polarity. Understanding the nature of the conducting path is a central issue in the study of these types of structures. In this regard, a number of models based on different physical mechanisms have been proposed in the literature to explain the observed switching characteristics [7–12]. Even though models relying on a single or multiple conductive filaments (CF) are generally considered the most likely in transition metal oxides (TMO), no general consensus has been reached yet about the nature of the electron transport in such filamentary structures. Recent papers [13–16] suggest the quantum point contact (QPC) model as an explanation for the con- duction mechanism. In this paper we investigate the DC and the low frequency noise (LFN) properties of the CF in the low (LRS) and high (HRS) resistance states of ReRAM cells with the aim of gaining further insight into the physics of filamentary conduction in HfO 2 . 2. Instrumentation and sample description The sample structure investigated in this work is depicted in Fig. 1 and consists in crossbar-patterned TiNnHfO 2 (5 nm)nHf 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.076 ⇑ Corresponding author. Tel.: +39 0984 494766. E-mail address: crupi@unical.it (F. Crupi). Microelectronic Engineering 107 (2013) 1–5 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee