Resistive switching phenomena in thin films: Materials, devices, and applications
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Introduction Resistive switching (“memristive”) in thin-film devices* under electrical stress (i.e., voltage or current bias) has been observed in a variety of material systems, and different physical mechanisms have been identified that contribute to this effect. In this article, we review the prospective applications of the resistive switching phenomenon. Recent reviews of this topic can be also found.1–5 Note that material systems and physical mechanisms are not discussed in this article but will be reviewed in the articles in this issue. In its simplest form, a memristive device consists of three layers: top and bottom (metallic) electrodes and a thin film (Figure 1). By applying a voltage bias across the electrodes of such a device, the electrical conductivity of the thin film can be changed reversibly and retained for a sufficiently long time
* The most common names used in the context of devices exploiting hysteretic resistive switching effect and that are used interchangeably in the article are resistive random access memory (RRAM),7 memistor,11,12 and memristor or memristive device.14 Note that the latter term has been originally introduced in a circuit theory and, in general, describes a device with pinched hysteresis I–V response.22 Often, a more explicit acronym is used in which the word resistive in RRAM is replaced with a specific material system or mechanism (e.g., conductive bridge RAM [CBRAM], ferroelectric tunnel junction RAM [FTJ-RAM], and phase change RAM [PCRAM]) (see the articles by Lu et al., Tsymbal et al., and Raoux et al. in this issue).
between a highly conductive (ON) state and a highly resistive (OFF) state. Very often, the switching could be continuous between these two extreme states so that intermediate states are also possible. Figure 1b schematically shows the I–V curves for a bipolar switching device. For a detailed classification of the observed switching behaviors, see Figure 1 of the Yang et al. article in this issue. In bipolar devices, an electrical stress of opposite polarity is required to switch the device between ON and OFF states. Note that the particular shape of the I–V curve in the ON/OFF states might vary and will depend on the additional layers integrated in the device stack. In general, nonlinearity of the ON state is required for most of the applications (e.g., to suppress leakage current in crossbar memories) and is achieved by either intrinsic properties of the device (e.g., internal tunnel barrier gap or Schottky barrier) or explicit integration in the device stack of a tunnel barrier (diode, or specific structures such as an ovonics threshold switch) (see the article by Raoux et al. in this issue). Also, Figure 1 does not show the so-called “forming step,” which might be required for metaloxide, solid-state electrolyte, and organic devices before the devices can be switched reversibly (see the articles by Yang et al., Lu et al., and Lee et al. in this issue). Such a forming step is essentially a one-time application of relatively large voltage bias and can be elimina
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