Thickness Scaling Effect on Interfacial Barrier and Electrical Contact to Two-Dimensional MoS₂ Layers

Abstract

This manuscript has been published as ACS Nano, 8 (2014) ABSTRACTUnderstanding the interfacial electrical properties between metallic electrodes and low dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS 2 interfaces. For MoS 2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS 2 thickness. By contrast, the contact resistivity sharply increases with reducing MoS 2 thickness below 5 layers, mainly governed by the quantum confinement effect. It is found that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS 2 thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities. KEYWORDS: two-dimensional material, chalcogenide, field-effect transistor, electrical contact, Schottky barrier, quantum confinementThis manuscript has been published as ACS Nano, 8 (2014) 12836-12842 3 Modern microelectronics roots in a fine control with gate bias on the height of potential barriers and the flow of charges at the interfaces between metallic contacts and active semiconductor channels, 1 which led to a great success of the semiconductor industry and revolutionized our life. The formation of ohmic contacts and high-efficient carrier transfer is the first step to construct high-performance devices. 2 Recently, layered transition-metal dichalcogenides (TMDs) 3-5 have attracted great interest not only for post-silicon electronics, 6-8 but also for optoelectronic 9-14 and photovoltaic 15,16 applications. The concurrence of atomic thickness and sizable bandgap promises them next-generation transistor channels after silicon. In addition, the exotic symmetry breaking in band structure, high optical absorption and mechanical flexibility can be exploited for valleytronic and photovoltaic devices. Undoubtedly, all the electrical systems begin with carrier transfer from electrodes to semiconductor channels; a profound understanding on the interfacial behavior between them is truly essential. 2In conventional bulk materials, the interfacial properties are basically independent on their dimensions. However, the physical scenario totally changes in the low dimensional systems. Generally speaking, the reduced material dimension increases bandgap (E g ) due to quantum confinement, a ubiquitous phenomenon in low-dimensional systems, such as quantum dots 17 and carbon nanotubes (E g ∝1/diameter). 18 The abnormal E g variation was extensively investigated in optical studies 19 but seldom studied in electrical experiments, although it is very important for contact design because an expanded E g may increase interfacial barrier height and suppress charge trans...

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