Study of electron transport through graphene homojunctions and optoelectronic characterization of 2D perovskites

Abstract

Over the last half century microelectronic devices mainly based on semiconducting materials have transformed our lives. The downsizing of primary components such as transistors provide us with fast, reliable, hand-full and low cost electronic devices. Moore’s law describes the scaling process of the electronic devices over the last decades showing that the size of their primary electronic components is minimized by half every 2 years. In the last decades microelectronics has reduced in size down to nano-meter scale. At this size the working principles of the devices are no longer solely governed by classical physical principles due to the emergence of prominent quantum mechanical properties. This is underpinning the development of conceptually novel devices based on three-dimensional topological insulators-heterojunctions, one- (nanowires) and zero-dimensions (quantum dots). To this end, the discovery of graphene and more generally layered materials has marked another significant stepforward in science and technologies. Graphene is just an atomic thick layer of carbon atoms and it presents a set of extraordinary properties. It has high mechanical robustness and it is a good thermal and electrical conductor presenting high carriers mobility even at room temperature (RT). Thus, it has been used for ultra-fast electronics, however, the lack of band gap limits its application into conventional electronics. On the other hand, its exceptional electronic properties allow the fabrication of new electronic devices the function of which is based on completely new principles. To this end, graphene electron optical devices that allow the control of charge flow in a similar way as the optical apparatus do with optical rays have been suggested. The initial aim of this thesis was to take advantage of the unique charge carrier properties found in graphene to develop quantum electrical analogues of optical elements and circuits. Realistic electron optic devices require ballistic transport in which charge carriers propagate in straight trajectories, similarly to straight line propagation of light rays. Thus, Chapter 4 is focused on the fabrication and the study of ballistic metal-induced lateral graphene p-n junction (GPNJ). Therefore, a comprehensive Raman characterization of the metal-graphene interface is presented while electrical measurements in combination with theoretical models are also used to characterize the electron transport throughout the metal-induced GPNJ. After the successful completion of the first part of the thesis, COVID 2019 pandemic completely stopped our lab work for an extended period of time (close to 1 year) and imposed limited access to our institute’s clean room facilities. Thus, it was almost impossible to achieve the initial goal of this thesis to be achieved and a graphene-based electron optical device to be demonstrated. Therefore, the aim of this thesis was reconsidered and the new research topic was to study the electrical and opto-electronic properties of 2D perovskites which had in the meanwhile attracted exponentially growing interest by a wide community. Consequently, the second large part of this thesis is devoted to the optoelectronic characterization of 2D perovskites. Chapter 5 presents the temperature dependence of the deep trapping effect into these layered structures. In Chapter 6 a temperature dependent comprehensive study of the dominant photoconduction mechanism into the 2D perovskites takes place, and, in the last part of chapter, 2D peroskites are also characterized as active materials in photodetectors.