Percolative switching in transition metal dichalcogenide field-effect transistors at room temperature

Abstract

We have addressed the microscopic transport mechanism at the switching or "on-off" transition in transition metal dichalcogenide (TMDC) field-effect transistors (FET), which has been a controversial topic in TMDC electronics, especially at room temperature. With simultaneous measurement of channel conductivity and its slow time-dependent fluctuation (or noise) in ultra-thin WSe2 and MoS2 FETs on insulating SiO2 substrates, where noise arises from McWhorter-type carrier number fluctuations, we establish that the switching in conventional backgated TMDC FETs is a classical percolation transition in a medium of inhomogeneous carrier density distribution. From the experimentally observed exponents in the scaling of noise magnitude with conductivity, we observe unambiguous signatures of percolation in random resistor network, particularly in WSe2 FETs close to switching, which crosses over to continuum percolation at a higher doping level. We demonstrate a powerful experimental probe to the microscopic nature of near-threshold electrical transport in TMDC FETs, irrespective of the material detail, device geometry or carrier mobility, which can be extended to other classes of 2D material-based devices as well.The expanding family of atomically thin layers of semiconducting TMDC materials for electronic [1][2][3][4][5][6][7], optoelectronic [8][9][10][11][12][13][14], valleytronic [15][16][17][18][19] and even piezoelectronic [20] applications, has defied a generic framework of electron transport because of diverse material quality, channel thickness dependent band structure, dielectric environment, and nature of substrates. A wide variety of physical phenomena ranging from variable range hopping [5], metal-insulator transition [7] to classical percolative charge flow through inhomogeneous medium [21,22] were reported for MoS 2 , the implications of which often lead to a conflicting microscopic scenario. At low carrier density, for example, hopping via single particle states trapped at short-range background potential fluctuations (∼few lattice constants [23]) is incompatible to the observation of classical percolative conduction that requires long-range inhomogeneity in charge distribution, created when linear screening of the underlying charge disorder breaks down [24][25][26][27]. Observations of metal-insulator transition [7,22] [21,24,25,[40][41][42][43][44], but the experimental difficulty lies in accurately determining the fraction p of the conducting region, or "puddles", embedded inside the insulating matrix. Hence despite compelling evidence of long range inhomogeneity in the charge distribution in MoS 2 FETs [21,22], its manifestation in transport remains indirect and confined only to low temperatures. A way to circumvent this difficulty involves measuring the lowfrequency noise, or 1 f -noise, in the channel conductivity σ, which also scales with p with independent characteristic scaling exponents [40,42], and diverges at the percolation threshold (p c ) [41]. A direct relation between the normalized noise...

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