Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlOx Capping.

Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS2, grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe2 in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe2 grown by CVD and capped by sub-stoichiometric AlOx. The 2L channels exhibit ∼20× lower contact resistance (RC) and ∼30× larger current density compared with 1L channels. RC is further reduced by >5× with AlOx capping, which enables improved transistor current density. Overall, 2L AlOx-capped MoSe2 transistors (with ∼500 nm channel length) achieve improved current density (∼65 µA/µm at VDS = 4 V), a good Ion/Ioff ratio of >106, and an RC of ∼60 kΩ·µm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.

Dry Transfer of van der Waals Crystals to Noble Metal Surfaces To Enable Characterization of Buried Interfaces.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.

Ultrahigh thermal isolation across heterogeneously layered two-dimensional materials.

Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS2, and WSe2 with thermal resistance greater than 100 times thicker SiO2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.