2025 Theses Doctoral

Study of electrical contacts to encapsulated ultra-clean semiconductor monolayers

Wang, Zhiying

Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are of great interest for electronic and optoelectronic applications. However, a major challenge for both fundamental studies and practical applications of TMD-based devices lies in the difficulty of establishing low-resistance, reliable electrical contacts. Conventional direct metal evaporation methods often bcauses physical damage at the metal-TMD interface due to the high kinetic energy of the metalatoms. This results in the formation of chemical defects at the contact interface, leading to Fermi level pinning and high contact resistance. While strategies like using low-melting-point metals or transferred metal contacts have shown promise in forming van der Waals contacts and reducing contact resistance, most prior research has focused on TMDs grown by chemical vapor deposition (CVD) method and devices built on silicon dioxide/silicon (SiO₂/Si) substrates. In these cases, atomic defects in the TMDs and surface charges from the substrate can significantly degrade device performance.

Our lab has addressed these limitations by growing ultra-high-quality TMD crystals using a two-step flux growth method and minimizing extrinsic disorder in the TMD devices through hexagonal boron nitride (hBN) encapsulation. This thesis aims to study the high-performance electrical contacts to ultra-high-purity monolayer TMDs encapsulated within hBN.

Chapter 1 introduces the fundamentals of 2D TMDs and the challenges associated with achieving low-resistance contacts. Chapter 2 describes high-quality TMD crystals and hBN encapsulation technique developed to minimize disorder in 2D TMD devices. Chapter 3 examines low-melting-point bismuth (Bi) semimetal contacts to ultra-clean MoSe₂ monolayers, demonstrating that Bi forms damage- and strain-free van der Waals contacts to high-purity MoSe₂. To characterize these devices, contact-front and contact-end measurements were combined to unambiguously extract key metal-semiconductor junction parameters that could not be accurately determined using the standard transfer length method (TLM). Additionally, pre-etched hBN was employed as a self-aligned mask, which mechanically stabilizes the weakly coupled van der Waals contacts. This technique enables the scaling of contact lengths from the micron scale down to tens of nanometers. In these deeply scaled contacts, both two-probe resistance and end resistance increased with the characteristic length 𝐿_𝑡 , confirming the calculated transfer length.

Chapter 4 explores a novel in situ via contact technique for air-sensitive monolayer TMDs. This method uses plasma etching and metal deposition to create ’vias’ in the hBN with graphene forming an atomically thin etch-stop. The resulting partially fluorinated graphene (PFG) protects the underlying device layer from air-induced degradation and damage during metal deposition. Using the in situ via technique, an ambipolar contact to air-sensitive monolayer 2H-molybdenum ditelluride (MoTe₂) was achieved with more than one order of magnitude improvement in on-current density compared to previous literature. The complete encapsulation provides high reproducibility and long-term stability. This contact technique was also extended to other air-sensitive materials as well as air-stable materials, offering highly competitive device performance.

In summary, these studies lay a solid foundation for future research on high-quality devices based on TMD monolayers, which are critical for both practical applications and the exploration of their intrinsic properties.