**1. Introduction**

Although still dominated by silicon technology, information storage devices—and more generally speaking nanoelectronic devices—are now facing the challenge of finding new materials and paradigms, in order to further improve features such as computation and write speed, data density, operation voltages, and fabrication costs. A variety of alternatives to traditional information processing devices have been proposed, boosting new scientific research in semiconductor principles and technologies [1]. In this frame, memristors—or resistive switching

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

materials, as the two terms identify the same switching behavior [2, 3]—were identified as valuable candidates for alternative nanoelectronic devices [4–7], with particular reference to nonvolatile memories and neuromorphic applications.

namely, compact thin films and nanotubular films; the switching behavior of anodic oxides

Ion migration that allows oxide growth during anodizing takes place in a solid film tens, or hundreds, of nanometers thick; therefore, it is associated with very high electric fields, in the order of 107 V/cm. To achieve such conditions, current densities of some tens or

and 100 V [12, 16]. A very large number of electrolytes can be employed, from diluted acids to neutral salts, to alkaline solutions [17–19]. Such oxides generally show an amorphous, or predominantly amorphous, structure, especially at low voltages, where only some non-

Oxide thickness increases linearly with applied cell voltage: anodizing ratios are in the range of 2 ± 0.5 nm/V depending on metal composition, electrolyte and growth mode—either galvanostatic or with potential ramp [12, 17–19]. The thicker the oxide already formed, the more onerous its further thickening: indeed, at growing voltages—and hence oxide thicknesses other parasitic processes may kick in, consuming part of the current supplied to the electrode. As a consequence, if the amount of charge employed in the process is used to estimate oxide thickness by coulometry [20, 21], the so-calculated thickness is affected by parasitic reactions, since a portion of current is dissipated, mostly in oxygen evolution, to an increasing extent with increasing cell voltage (**Figure 1**) [19, 20]. Most of research studies on the growth of thin films by anodic oxidation refer to titanium and its alloys and to aluminum [12–14, 22–25], given their relevance in already mature industrial applications. Some works are also proposed on other metals, such as zirconium, niobium, hafnium; yet, they generally focus on the obtaining of high specific surface area morphologies, such as nanotubes [26], which are

**Figure 1.** Thickness versus voltage curve of a typical galvanostatic anodic oxidation process performed in acid electrolyte: Measured oxide thickness grows linearly with voltage, while coulometry exponentially overestimates thickness due to

stoichiometric crystal phases like Magnéli phases may appear.

are used, and cell voltages to produce thin compact films are between 10

Memristive Anodic Oxides: Production, Properties and Applications in Neuromorphic Computing

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47

will be addressed in Section 3.3.

**2.1. Thin compact films**

hundreds of A/m<sup>2</sup>

described in next paragraph.

parasitic reactions.

Indeed, several oxides are capable of resistive switching, that is, their resistance can be switched through a suitable voltage pulse between at least two different values—a high resistance state, HRS, also addressed as OFF state, and a low one, LRS, identified as ON state, by operations of "set" (OFF-ON) and "reset" (ON-OFF). Among them, SiO<sup>2</sup> [8, 9], TiO<sup>2</sup> [3], HfO<sup>2</sup> [10] and Ta2 O5 [11] are the most studied.

Oxide properties—thickness, composition, stoichiometry and defectiveness—are crucial to determine whether it shows memristive properties, and the values of main switching parameters. Hence, the production technique plays a major role, as in turn it determines oxide characteristics; yet, the most commonly employed oxide synthesis/deposition techniques imply high investment costs and rather long deposition times to achieve satisfactory results.

We here present and summarize current knowledge on the growth of oxides with resistive switching capability by anodic oxidation, a low-cost electrochemical technique that may find a new niche of application in the production of memristive metal oxides. In Paragraph 2, the principles of anodic oxidation are described to highlight the typical oxide characteristics that can be achieved. The discussion will be limited to thin oxide layers, and no reference will be made to thicker ceramic oxides produced in sparking regime, as they are not pertinent to the present application [12]. Paragraph 3 provides a comparison of the characteristics of different metal oxides that show memristive properties, focusing on those that can be obtained by anodic oxidation, and then specifically focuses on anodic oxides. Finally, in Paragraph 4, the potential application of these materials in neuromorphic computing is discussed.
