Low-Temperature 2D/2D Ohmic Contacts in WSe₂ Field-Effect Transistors as a Platform for the 2D Metal–Insulator Transition

Abstract

We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multiterminal WSe 2 Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe 2 devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBNencapsulated WSe 2 devices in ideal Hall-bar geometry enable us to accurately determine arXiv:2012.02873v1 [cond-mat.mes-hall] 4 Dec 2020 the carrier density. Measurements of the temperature (T ) and density (n s ) dependence of the conductivity σ(T, n s ) demonstrate scaling behavior consistent with a metalinsulator quantum phase transition driven by electron-electron interactions, but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering.

Metal–Insulator Transition Driven by Traps in 2D WSe2 Field‐Effect Transistor

“…[8,9] More specifically, carrier transport within a channel is significantly influenced by the localized state commonly known as the trap states at the 2D channelgate dielectric interface by inducing transport through interface of the WSe 2 flake from ≈20 to 0.73 nm (monolayer), different from previous studies. [23,27,29] We carried out multifrequency capacitance-voltage (MFC-V) and transport measurements with temperatures ranging from 300 to 75 K to determine the effect of interface quality and correspondingly localized D t on MIT. Since MFC-V measurements carried out by applying different excitation frequencies (1 kHz to 1 MHz in this study) provide direct insight about localized D t , they are considered the most appropriate and powerful technique to analyze the channel-gate dielectric interface and its impact on device characteristics.…”

“…[6,7,22] Thus, it is difficult to correctly identify the origin of MIT in a 2D material system when both carrier-carrier interactions and a large amount of localized trap states (≈10 13 cm -2 eV -1 ) induced from the channel and channel-gate dielectric interface effects come into play.In the past, some efforts have been made to explain the MIT in 2D MoS 2 , [23][24][25][26] ReS 2 , [27] CuInSe, [28] and WSe 2 . [29] However, most of these studies were limited to a certain channel thickness. Although a few studies have reported MIT in 2D materials, [23,[26][27][28][29][30] how localized D t interplays near a transition point with varying thickness of 2D flakes remains unclear.…”

“…[29] However, most of these studies were limited to a certain channel thickness. Although a few studies have reported MIT in 2D materials, [23,[26][27][28][29][30] how localized D t interplays near a transition point with varying thickness of 2D flakes remains unclear. There is a need to systematically analyze the underlying mechanism of MIT with different thicknesses of 2D channels.…”

“…In the past, some efforts have been made to explain the MIT in 2D MoS 2 , [23][24][25][26] ReS 2 , [27] CuInSe, [28] and WSe 2 . [29] However, most of these studies were limited to a certain channel thickness. Although a few studies have reported MIT in 2D materials, [23,[26][27][28][29][30] how localized D t interplays near a transition point with varying thickness of 2D flakes remains unclear.…”

Gate-Controlled Metal to Insulator Transition in Black Phosphorus Nanosheet-Based Field Effect Transistors

“…The inset in Figure 2a shows that activation energy vanishes at V G = −21 V, further confirming the MIT point. 24 Below the MIT point, the conductivity increases with decreasing temperature, thus indicating metallic behavior (Δσ/ΔT < 0), as shown in Figure 2b. However, above the MIT point, the conductivity decreases with decreasing temperature, thereby showing the insulating phase (Δσ/ΔT > 0) as can be seen in Figure 2b.…”