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

Abstract

trap charges. [7,10] These electrically active interface traps can adversely affect device performance by reducing charge carrier mobility, increasing subthreshold swing, and enhancing off-state current (reducing on/off ratio). [7] In addition, these charge impurities near the surface of 2D materials and different kinds of disorders can strongly suppress electronic interactions and control the electrical characteristics of the system. [11][12][13] Since the pioneering observation of metal-insulator transition (MIT) phenomena in 2D systems, carrier-carrier interaction has been widely considered as the driving force of MIT from the viewpoint of quantum phase transition. [14][15][16] However, the previous theories suggested that the relative strength of D t and carrier-carrier interaction in the system determine the physical origin of MIT. [17][18][19][20] In 2D layer materials, the strength of carrier-carrier interaction and disorders induced by interface traps, and defects can be modulated by controlling the thickness of the 2D channel, substrate engineering, and surface passivation. [2,21] In general, carrier-carrier interactions decrease with reducing thickness while localized D t increases significantly. Therefore, different kinds of transition mechanisms in 2D layered materials are expected depending on the thickness of the flake and the corresponding interface quality. Previously, it has been reported that the D t drastically increases when employing thin flakes since thinner 2D flakes are more susceptible to interface disorders than their thicker counterparts. [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. There is a need to systematically analyze the underlying mechanism of MIT with different thicknesses of 2D channels. Therefore, the objective of this study was to systematically investigate the MIT behavior observed in 2D WSe 2 devices by varying the thickness Localized trap density (D t ) at the 2D channel-gate dielectric interface and its relative strength to carrier-carrier interactions depending on the thickness of the 2D channel can determine the nature of a metal-insulator transition (MIT) in 2D materials. Here, the MIT occurring in WSe 2 devices is systematically analyzed by varying the WSe 2 thickness from ≈20 nm to monolayer to explore the effects of D t on MIT. The corresponding critical carrier density increases from ≈8.30 × 10 11 to 9.45 × 10 12 cm -2 and D t from ≈6.0...

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

“…The n C values of different hBN encapsulated BP devices are almost similar, indicating that our results are consistent and reproducible. It should be noted that the n C in our study is lower than the previously reported n C of BP with ionic liquid gating 33 and even lower than those of MoS 2 , 17,20,22 WSe 2 , 16,24 and ReS 2 . 21 In general, n C depends on the order of disorder in the 2D system.…”

“…This has allowed us to investigate the microscopic origin of 2D MIT by excluding the role of R C on the BP channel. On the other hand, the 2D MIT has been observed in various 2D materials. − However, the microscopic origin of MIT is still not clearly understood. For example, the 2D classical percolation-based transition has been used to demonstrate MIT in monolayer MoS 2 , and multilayer CuIn 7 Se 11 due to density inhomogeneities of electronic states.…”

“…Di Sante et al theoretically demonstrated disorder driven MIT in a deformable lattice where disorder enhanced interaction effects dominated . Moreover, disorder and screening lead to strong charge density inhomogeneities around the transition point, thus resulting in percolation-based 2D MIT. − …”

“…As mentioned previously, the carrier–carrier interactions and disorder are the main candidates for observing the MIT in 2D systems. Recently, transition-metal dichalcogenides (TMDs) have emerged as ideal platforms for realizing MIT in 2D systems because TMDs have strong carrier–carrier interactions and disorder depending on the thickness of the flake. − ,− However, the efforts to study 2D MIT in TMDs have to this point been limited, particularly at low temperatures, because of the high contact resistance ( R C ) and non-Ohmic contacts . Specifically, at low temperatures, the suppression of thermally assisted tunneling and thermal emission current over the Schottky barrier leads to high R C and non-Ohmic contacts behavior, which limits the ability to study 2D MIT in TMDs.…”

Pronounced Optoelectronic Effect in n–n ReS2 Homostructure

“…The carrier densities according to each thickness were calculated using eq where q is the elementary charge. When V g = 0 and V T = −4.94, −5.45, −7.32, and −10.15 V, carrier densities were calculated as n = 3.72 × 10 11 , 4.10 × 10 11 , 5.52 × 10 11 , and 7.69 × 10 11 cm –2 , respectively.…”