Graphene Heterostructures with Wide Bandgap Semiconductors

Abstract

Graphene (Gr) is a two dimensional material constituted by an atomically thin carbon membrane, characterized by a unique combination of excellent electrical, optical, thermal and mechanical properties. Its main limitation for microelectronic applications is related to the lack of a bandgap, leading to a poor Ion/Ioff ratio when it is considered as channel material for MOSFET devices. Novel device concepts based on Gr heterostructures with semiconductors are currently under consideration, in order to overcome these limitations. These devices are based on the vertical current transport through Gr interfaces with semiconductors or thin insulators. Their working principle exploits some unique properties of Gr, such as the atomic thickness and the field effect modulation of its workfunction. Some demonstrations of these devices, recently reported in the literature, have been based on the integration of Gr with Si technology. This thesis work has been focused on the fabrication and the electrical characterization of high quality heterostructures of Gr with wide bandgap semiconductors (WBS), such as Silicon Carbide (4H-SiC), Gallium Nitride (GaN) and related alloys (AlxGa1-xN), which present superior properties for high power and high frequency electronic applications. At first, the fabrication methods have been discussed, i.e. (i) the controlled graphitization of the surface by high temperature thermal annealing, for 4H SiC; (ii) a highly reproducible transfer method to move Gr, grown by CVD on copper foils, to the surface of AlGaN/GaN heterostructures. A detailed structural, morphological and spectroscopic characterization of these Gr/WBS heterostructures has been carried out by the joint application of several analytical techniques, such as AFM; TEM, micro-Raman spectroscopy. Secondly, the current transport mechanisms through these heterostructures have been investigated in details by properly fabricated test devices and by nanoscale resolution electrical characterization techniques (CAFM, SCM). A correlation between the nanoscale structural, morphological and electrical properties of the interfaces with the devices average electrical behavior has been achieved. Basing on these results, some potential devices applications (such as the Gr/SiC Schottky diode with a gate modulated barrier and the hot electron transistor constituted by a Gr/AlGaN/GaN heterostructures with a Gr base) have been discussed and their advantages with respect to the Si counterparts have been estimated.