Interface Studies of Metal/2D Material Contacts

The imminent end of Moore’s Law motivated the exploration of alternative materials for electronic device applications. One category of materials that has attracted interest for nanoelectronics is two dimensional (2D) materials, which are characterized by vertically stacked layers of covalently bonded material held together by weak van der Waals forces in the out-of-plane direction. The isolation of single layer graphene by exfoliation and the subsequent rise of graphene in the 2000s led to renewed interest in other types of layered materials such as transition metal dichalcogenides (TMDs). The unique electronic, optical, and thermal properties of 2D materials make them promising for a variety of applications including low power electronics and photodetectors, as well as solar and thermoelectric energy conversion.
A fundamental component of any type of device is the contact that connects the device to external circuitry and controls the flow of current and heat into and out of the device. Numerous experimental and theoretical studies have concluded that the contact interface is the dominant performance limiting factor for 2D transistors particularly at short channel lengths. Due to processing challenges that are unique to 2D materials, the chemical composition and transport properties of the contact interface become more difficult to control. Furthermore, as electrical energy is converted to heat in resistive electrical contacts, thermal contact resistance becomes extremely important since excessive heat generation can severely compromise device operation, reliability, and lifetime. Like electrical resistance, the thermal resistance in the cross-plane direction of metal/2D systems is limited by the interface and has been found to be highly dependent on the nature of chemical interaction between the metal and the 2D material.
There currently exists a gap in understanding the relationship between contact processing conditions, interface chemistry, and electrical and thermal transport properties. Central to the work presented here is the study of chemical reactions at the metal/2D interface using X-ray photoelectron spectroscopy. Interface studies in this work utilize chemical vapor deposition (CVD) grown graphene, geological and CVD MoS2, as well as molecular beam epitaxy-grown WSe2. In combination with photoelectron spectroscopy and metal depositions in high and ultra-high vacuum, ex-situ electrical and thermal interface characterization methods are employed. Different aspects of contact processing are addressed, including contact deposition conditions, post-deposition heat treatments, and polymer-aided processes. The results obtained have profound implications for both fundamental materials science and 2D device design and fabrication processes.