Resistive Switching in Nano-Optoelectronic Devices: Towards an Optical Memristor

Abstract

[eng] Resistive switching devices have been a topic of great interest in the last two decades, as they could lead the next generation of memories and processors. These devices present a behavior that allows them to modify their electrical resistance between two or more states and retain them without the need for external energy. The possibility of having two resistance states (high resistance interpreted as 0 and low resistance as 1) already serves as a digital memory, with the advantage of faster switching speeds, lower dimensions for single devices and lower power consumption, when compared to current memories. Since their first physical realization in 2008, great advances have been made in terms of materials employed, device structure, modelling and integration and scaling into arrays and chips. In addition, the properties of resistive switching devices have opened the door for other applications beyond pure memory and the conventional von Neumann architecture. Within the context of resistive switching research, this Doctoral Thesis proposes one new field that can be benefited in the future by the inclusion of such devices: Optoelectronics. The main objective of this Doctoral Thesis is the development of a new concept of devices, which we have called optical memristors. Two types of devices have been attempted and realized: memristors with light emission or absorption. Notwithstanding, both had a particular requirement: transparent materials where necessary for light to be transmitted not only through the electrodes but also through the active layers of the devices. The first approach to light emitting memristors presented explores the possibilities of light emitting devices based on rare earth ions. These elements are commonly employed in displays for the fabrication of phosphorus layers that are excited by a blue emitting device. When properly used as dopants, these elements are optically active and can be electrically excited within a matrix of an oxide material. Thus, the emission of such devices based on Al/Tb/Al/SiO2 layers is studied. A reduction of emission efficiency is also identified with resistive switching capabilities of these devices, though a low number of cycles is possible. A second approach starts from an already transparent conductive oxide (TCO) that has shown resistive switching properties in the literature: ZnO. This material presents advantages when compared to the most employed TCO, ITO, in the form of a non-toxic and abundant compound. In addition, it can be doped with rare earth ions that are optically active. In the same way as the previous approach, resistive switching of these devices is possible, but the inclusion of rare earth ions highly diminishes their endurance. Finally, a different strategy allows for the objective results to be achieved. Silicon oxide is employed as an already reported material with resistive switching properties, where Si nanocrystals (NCs) are embedded as luminescent centers. Their combination results optimal for the target application, yielding durable devices with differentiated emissions dependent on the resistance state and that avoid its overwriting when read. Furthermore, the range of optical properties that become available to these devices through the presence of Si NCs is extended to that of light absorption. The devices become optically-readable taking profit of the photovoltaic effect of their tandem solar cell structure, distinguishing high and low current extractions dependent on the resistance state. Last, the effect of resistive switching and the presence of conductive filaments in these solar cells is explored, achieving increased efficiencies when compared to pristine devices.

[spa] Los dispositivos conmutadores de resistencia han sido un tema de gran interés en las últimas dos décadas, postulándose como la próxima generación de memorias y procesadores. Estos dispositivos presentan un comportamiento que les permite modificar su resistencia eléctrica entre dos o más estados y retenerlos sin necesidad de aportar energía externa. Desde su primera realización física en 2008, se han logrado muchos avances en términos de materiales, estructuras de dispositivos, modelado e integración en chips. En el contexto de esta Tesis Doctoral se propone un nuevo campo que se puede beneficiar introduciendo estos dispositivos: la Optoelectrónica. El principal objetivo de esta Tesis Doctoral es el desarrollo de un nuevo concepto de dispositivos llamado memristores ópticos, que combinen la conmutación de resistencia con propiedades ópticas. Para ello, dos tipos de dispositivos se han investigado y llevado a cabo: memristores con emisión o absorción de luz. Ambos tienen como requisito la necesidad de capas transparentes a la luz para que pueda ser transmitida a través de los electrodos y las capas activas. Un primer intento comienza con dispositivos emisores basados en iones de tierras raras, donde se identifica una reducción de la intensidad de emisión con su conmutación de resistencia, aunque finalmente solo unos pocos ciclos son posibles. De manera similar se parte de ZnO, del que ya es conocida su capacidad de conmutación, se dopa con las tierras raras. Sin embargo, el número de ciclos posibles también disminuye al incluir los iones emisores. Finalmente se opta por cambiar a óxido de silicio con nanocristales de silicio embebidos. Esta combinación resulta óptima, lográndose una gran durabilidad y emisiones que permiten diferenciar el estado de resistencia sin peligro de sobrescribirlo. El rango de propiedades ópticas que permiten los nanocristales de silicio se extiende a la absorción de luz, consiguiendo una lectura del estado a través del efecto fotovoltaico y una extracción de portadores diferenciada. Por último, se estudia el efecto de los filamentos conductores presentes en el dispositivo sobre la aplicación de estos como células solares, consiguiendo altos incrementos de eficiencia que podrían ser aplicados a otros dispositivos similares.