Physical and Electrical Properties of Ag Contacts on MoS2
Authors: Yuan Hui , Pan Minghu , Cheng Guangjun , Richter Curt A. , Li Qiliang 
Published: February 2020

Recently, MoS2 has attracted a lot of attention for electronic and optoelectronic applications. Layered MoS2 is a 2-dimensional (2D) semiconductor which has a bandgap ranging from 1.2 eV to 1.8 eV depending on its thickness. In particular, monolayer MoS2 with a direct bandgap of 1.8 eV shows promising applications in optoelectronics. 2D MoS2 is also attractive for flexible electronic applications because of its intrinsic ultrathin body, and robust lattice structure.
 Various work has been done to understand the transport mechanism in MoS2 transistors and to improve their performance for future applications. The Source/Drain (S/D) contacts are very important factors for device performance. A variety of contacts have been used and studied to achieve a good Ohmic contact to MoS2. However, a lot of complications and conflicts in these publications show that metal work function is not a good indicator for forming an Ohmic contact with MoS2. And the mechanism of metal contacts on MoS2 is not well understood.

Although it is well believed that metal wettability plays an important role in metal-MoS2 contacts, to our best knowledge, there is no work showing the direct evidence on how the metal contact morphology affects the metal-MoS2 contacts. In this work, for the first time, we have shown how the metal-MoS2 interface influences the MoS2 FET performance. We compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher on-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5nm Au / 5nm Ag (contact layer) or 5nm Au / 5nm Ti film was deposited onto MoS2 mono- and few-layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.

Then, in order to obtain more details in the mechanism of Ag contacts on MoS2, gate assisted contact-end Kelvin test structures and gate assisted 4-probe structures have been designed and fabricated to measure the field effects of the transfer length and contact resistivity in source/drain contacts of top-gate MoS2 field effect transistors. The transistors exhibited n-type transistor characteristics. The source/drain contact resistance was characterized by using both gate-assisted Kelvin and gate-assisted 4-probe structures. The values of contact resistance measured by these two test structures are significantly different. The contact-front contact resistance obtained from the 4-probe structure is significantly influenced by field effects on current crowding, while the contact-end resistance obtained from the Kelvin test structure is not. The metal-MoS2 contact current transfer length, LT
, can be determined from the comparison between these two measurements: LT
was observed to increase linearly with increasing gate voltage. The contact resistivity was also extracted under different gate bias. It shows strong gate effects where the contact resistivity decreases while gate voltage increases. It indicates that the contact performance also contributes to the switching performance of the MoS2 transistors.

To conclude, this work shows that metal-MoS2 interface morphology is an importance consideration to form a good contact to MoS2. The performance of metal contacts of MoS2 transistors is affected by the gate bias, and will also contribute to the switching performance of MoS2 transistors.