Caracterización eléctrica de dispositivos de conmutación resistiva para su aplicación en el ámbito de memorias no volátiles y de circuitos neuromórficos

Laburpena

In this thesis it is investigated the behavior of Resistive Switching devices based on MIM (Metal-Insulator-Metal) structures, which configuration therefore consists in two metallic electrodes separated by an insulating material or dielectric. Indeed, the dielectric of each one of these structures is the main element in our study with the aim of grouping and comparing them. The operation of a resistive switching memory (RRAM) is based on the property to modulate the electric resistance of the insulating material which is part of it, and consequently on the ability to regulate the current which flows between both electrodes. The film of dielectric material, given its scarce thickness (usually between 3 and 100 nm), can experiment a dielectric breakdown when it is under an electric stress. This fact generates current paths between the electrodes which make the dielectric to behave as a memory thanks to a partial reversibility of the conductance levels in its inner part. As a result, through the formation and rupture of one or more conductive filaments in the dielectric, the state of the device can be controlled in such a way that it switches between two resistance levels (low and high, respectively), which fundaments its application in the scope of non-volatile memories. Moreover, as the value of the effective electric resistance of the structure is related to the existence of conductive filaments in the core of the dielectric, it is also possible to get an analogical functioning by controlling the number and thickness of the filaments. This generates the potential presence of multiple intermediate states among low and high resistance ones. Thus, this property avails to emulate the behavior of the neurons synaptic connections and opens a window of opportunity to the applications in the neuromorphic circuits area, where these devices act as electronic synapses. As a consequence, our study consists in the electric characterization of the resistive switching devices behavior from two perspectives: digital and analogical. The digital one is based on controlling two well-differentiated states, while the analogical is more complex: the repetitivity of an intermediate states quasicontinuum and the existence of effective procedures to go across those states, conform a large-scale scientific and technological challenge. As it will be described along these pages, the universe of applications which appears behind these devices comprises a wide range of possibilities. This locates them in a focus of interest which covers a multidisciplinary field from materials science to bio-inspired circuits, passing through the criptography (where they can fit into, because of the probabilistic switching which permits to perform unclonable functions) and the development of neural networks and deep-learning-based applications. The main contribution of this work has been the accomplishment of a systematic study of a group of resistive switching devices based on a wide gamut of dielectric materials, by using electric characterization techniques (where some of them are genuine from our research group). The analysis of both DC and small-signal parameters, the setting of variables like frequency and temperature, the usage of current and charge injection techniques, and the development of methods to precisely control the itinerary through the intermediate states, all constitute the biggest original scientific value of this thesis. As it will be shown, the collaboration with research groups of renowned trajectory has been decisive to undertake this ambitious task. In this dissertation, the main content has been split in three distinct parts: MIM structures based on hafnium oxide, MIM structures fabricated with diverse functional oxides, and more advanced structures with 1-transistor-1-resistor configuration (1T1R) with dielectric of hafnium oxide and aluminium-doped hafnium oxide. The hafnium oxide devices, which exhibit a great repetitivity, have been fabricated in the Microelectronics Institute of Barcelona. The functional oxides section presents some less-conventional oxides combinations as dielectric material. The laboratory samples characterized in that section come from the University of Tartu (Estonia) and from the University of Helsinki (Finland). On the other hand, the 1T1R structures part emerged from my two pre-doctoral stays which I spent in IHP research centre located in Frankfurt-Oder (Germany). Chapters 4, 5 and 6 have the longest extension and represent the true core of this work, because they agglutinate the full compendium of results which belong to the three parts previously mentioned. The precedent chapters apport a scientific context by exposing the state-of-the-art: chapters 1 and 2 address the basic fundaments of the high-k dielectrics and the resistive switching memories, and Chapter 3 mentions the electric characterization techniques of memristors, putting the focus on those which we systematically apply in our research group. Finally, in Chapter 7 the principal contributions are explained along with a general valuing of the work I have carried out during my pre-doctoral journey