Improved Contacts to MoS₂ Transistors by Ultra-High Vacuum Metal Deposition

The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (<i>R</i>_C). Here we present a systematic study of scaling MoS₂ devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10¹² to 10¹³ cm^–2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10^–9 Torr) yields three times lower <i>R</i>_C than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10^–7 Ω·cm², stable for over four months. Modeling reveals separate <i>R</i>_C contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS₂ contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the “14 nm” technology node

Role of Remote Interfacial Phonon (RIP) Scattering in Heat Transport Across Graphene/SiO₂ Interfaces

Heat transfer across interfaces of graphene and polar dielectrics (e.g., SiO₂) could be mediated by direct phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand the relative contribution of each component, we develop a new pump–probe technique called voltage-modulated thermoreflectance (VMTR) to accurately measure the change of interfacial thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-effect transistors and find that the thermal conductance of SiO₂/graphene/SiO₂ interfaces increases by up to Δ<i>G</i> ≈ 0.8 MW m^–2 K^–1 under electrostatic fields of <0.2 V nm^–1. We propose two possible explanations for the small observed Δ<i>G</i>. First, because the applied electrostatic field induces charge carriers in graphene, our VMTR measurements could originate from heat transfer between the charge carriers in graphene and RIPs in SiO₂. Second, the increase in heat conduction could be caused by better conformity of graphene interfaces under electrostatic pressure exerted by the induced charge carriers. Regardless of the origins of the observed Δ<i>G</i>, our VMTR measurements establish an upper limit for heat transfer from unbiased graphene to SiO₂ substrates via RIP scattering; for example, only <2% of the interfacial heat transport is facilitated by RIP scattering even at a carrier concentration of ∼4 × 10¹² cm^–2

Stacked Graphene-Al₂O₃ Nanopore Sensors for Sensitive Detection of DNA and DNA–Protein Complexes

We report the development of a multilayered graphene-Al₂O₃ nanopore platform for the sensitive detection of DNA and DNA–protein complexes. Graphene-Al₂O₃ nanolaminate membranes are formed by sequentially depositing layers of graphene and Al₂O₃, with nanopores being formed in these membranes using an electron-beam sculpting process. The resulting nanopores are highly robust, exhibit low electrical noise (significantly lower than nanopores in pure graphene), are highly sensitive to electrolyte pH at low KCl concentrations (attributed to the high buffer capacity of Al₂O₃), and permit the electrical biasing of the embedded graphene electrode, thereby allowing for three terminal nanopore measurements. In proof-of-principle biomolecule sensing experiments, the folded and unfolded transport of single DNA molecules and RecA-coated DNA complexes could be discerned with high temporal resolution. The process described here also enables nanopore integration with new graphene-based structures, including nanoribbons and nanogaps, for single-molecule DNA sequencing and medical diagnostic applications