Nanotechnol Rev 2016; 5(3): 311–329 Review Baker Mohammad*, Maguy Abi Jaoude, Vikas Kumar, Dirar Mohammad Al Homouz, Heba Abu Nahla, Mahmoud Al-Qutayri and Nicolas Christoforou State of the art of metal oxide memristor devices DOI 10.1515/ntrev-2015-0029 create a ubiquitously connected world and the realization Received May 3, 2015; accepted September 30, 2015; previously of the internet of things. Since the establishment of Moore’s published online January 21, 2016 law in the 1960s, device evolution can be mainly traced by Abstract: Memristors are one of the emerging technolo- incessant micro-sizing to reach higher processing speed gies that can potentially replace state-of-the-art inte- and to facilitate the production of sizeable packing den- grated electronic devices for advanced computing and sities at lower fabrication costs. Particularly, “memory” digital and analog circuit applications including neuro- constitutes nowadays more than 60% of the modern pro- morphic networks. Over the past few years, research and cessor area, which makes it a major target for device min- development mostly focused on revolutionizing the metal iaturization. Microprocessors today provide unmatched oxide materials, which are used as core components of the computing power owing to state-of-the-art complementary popular metal-insulator-metal memristors owing to their metal-oxide-semiconductor (CMOS) technology, which highly recognized resistive switching behavior. This paper ultimately enables the so-called “non-volatile memory”. outlines the recent advancements and characteristics of Nonetheless, further projection of the current CMOS such memristive devices, with a special focus on (i) their design to satisfy the growing needs for higher processing established resistive switching mechanisms and (ii) the capacity and larger data size while miniaturizing device key challenges associated with their fabrication processes scale has now reached its practical limits. Key challenges including the impeding criteria of material adaptation for arise from extending the performance capstone of actual the electrode, capping, and insulator component layers. devices in terms of leakage current, power consumption, Potential applications and an outlook into future develop- and switching speed, when the main constraint is often ment of metal oxide memristive devices are also outlined. attributed to structure, outside the peculiar capabilities of the CMOS material itself [1]. Hence, technological break- Keywords: memory technology; memristor; RRAM; thin throughs pushing forward novel device configurations film. are highly desirable to achieve new scalable platforms that would outperform the classic CMOS design. Nowa- days, researchers in the field of micro or nano electronics 1 Introduction are focusing their attention on “memristor” technology as a viable alternative to early CMOS-based approach for Progress in the field of semiconductor electronics con- device miniaturization. The concept of a “memristor” (or tinues to have profound influence on human society. In memory-resistor) device was initially interpreted by Leon particular, this has led to an unprecedented growth in the Chua in 1971, as the fourth fundamental circuit element information communication technology field, as well as based on symmetry arguments [2–4]. The device was pro- virtually every other field of engineering and technology. posed as a missing passive element that could link the Hence, the demand for faster and more efficient informa- magnetic flux to the electric charge, a property that cannot tion processing systems continues to increase at a high be obtained by any combination of the other three funda- rate. Typical trending is primarily driven by the quest to mental elements, namely, the resistor, the capacitor, and the inductor. An elementary memristor can be perceived *Corresponding author: Baker Mohammad, Khalifa University of as a two-terminal device with a sandwiched metal/insula- Science Technology and Research, PO Box 127788, tor/metal (MIM) structure, which is generally integrated in Abu Dhabi, UAE, e-mail: [email protected] an elementary crossbar circuit, as illustrated in Figure 1. Maguy Abi Jaoude, Vikas Kumar, Dirar Mohammad Al Homouz, Heba Abu Nahla, Mahmoud Al-Qutayri and Nicolas Christoforou: Typical configuration allows for smaller interconnec- Khalifa University of Science Technology and Research, tion and higher composite density than the one achieved PO Box 127788, Abu Dhabi, UAE using conventional three-terminal transistors [5]. Another 312 B. Mohammad et al.: Metal oxide memristor devices robust and predictive understanding of its fundamental mechanisms [7]. Impeding difficulties on correlating basic mathematical models with performance data collected out of physical devices are viewed as the main barrier for practical implementation of memristors in a wide variety of applications. One of the most complicated processes to understand and control at the molecular view is the electrical switching mechanism as a function of physical core parameters including (i) the chemistry of materials Figure 1: Schematic of memristor device structure with metal/ and (ii) the commonly neglected stochastic and interfacial insulator/metal (MIM) configuration. phenomena arising between the sandwiched layers of the device upon physical contact or during electrical operation peculiar feature of a memristor is its memory function, [1]. Access to such contained information to provide better which originates from a resistance state that the device description of the mechanistic operation of physical mem- remembers after being subjected to an electric poten- ristor devices would hence require thorough investigations tial difference over a certain time. Although the theory of of the physico-chemical properties of the materials config- memristive switching was introduced 40 years ago [3, 4], ured down to nanoscale levels. interpretation of the driving mechanism only appeared Hence, this review article aims to present an overview two decades later and remains obscure to date [6]. The first of the recognized ion-transport resistive switching mecha- clear connection between Chua’s theory and the practi- nism in metal oxide memristors. Focus is placed on dis- cal demonstration of a memristor device was achieved by tinguishing between the physical and chemical processes Hewlett-Packard Labs in 2008, when scientists observed a that underlie the functionality of anionic and cationic memristive behavior at the nanoscale level using thin-film devices, to explain the bipolar and unipolar switching titanium dioxide as insulator layer [5]. With a simple math- behavior observed in each case. The review also aims to ematical model, researchers at HP Labs were further able provide a description of existing device configurations to to demonstrate that the memristance phenomenon arises highlight the implication of bulk and interfacial proper- naturally in nanoscale systems. HP prototype memristors ties of the elementary device materials onto the overall have been shown to store data, process logic at nanoscale memristive performance characteristics. The mapping footprint, exhibit long retention time, and offer fast, non- covers the physical and chemical angle to provide guid- volatile, and low-power electrical switching [1, 5]. Memris- ance on the selection criteria of key components involving tors continue to stir up a continuous worldwide research electrodes, capping, and switching metal oxide materials, market growth as promising alternatives to classic CMOS including fabrication processes for the purpose of device- devices, owing to their potential scalability and low power design optimization. The review concludes with various consumption for memory applications. While interest potential applications anticipated for metal oxide memris- in memristive devices is steeply increasing (Figure 2), tors, with an outlook on the pending challenges observed successful commercialization of this technology requires on their integration into the semiconductor market. 2 Switching mechanism The first switching mechanisms were elucidated in the late 1990s with a wide variety of oxide systems [7–10]. Nowadays, common studies depict the memristive switching behavior based on a popular thin-film MIM configuration, where the insulator layer is composed of one or more metal oxides with semiconducting properties [7]. To act as memristor, a Cumulative publication per year from 2008 to 2014. Figure 2: physical MIM device must exhibit a range of internal resis- The number of publications is obtained by searching the follow- ing keywords: memristor, RRAM, and resistive switching from the tive states, which are tunable in a quasi-stable manner. Dif- header “Topic” of the Web of Science electronic site https://webof- ferent factors play a key role on defining the instantaneous knowledge.com/. resistive state of the device, of which the applied electric B. Mohammad et al.: Metal oxide memristor devices 313 field and the compliance current can be externally manipu- [14, 15]. Considerable effort has been made since to study lated during device characterization. Other restricted syn- the switching mechanism in a variety of oxide systems, ergistic determinants, including (i) electron mobility, (ii) ranging from simple binary transition metal oxides (e.g. gradient of species concentrations, and (iii) gradient of HfO2, TiO2, ZnO, Nb2O5, Ta2O5, MoO, WO, MnO, NiO, and temperature within the insulator region, closely depend