Determination of interlayer electron mobility in multilayer MoS₂ flake using impedance spectroscopy

Abstract

Layered molybdenum disulfide (MoS2) is emerging as a promising candidate for novel electronic applications, though the focus has largely been on few-layered thin films and in-layer electrical properties. MoS2 shows various nontrivial departures in its electrical properties in comparison to isotropic crystalline materials, however the understanding of anisotropic properties, particularly of charge carrier mobility in MoS2 flakes, is meagre. We experimentally study inter-layer mobility in mechanically exfoliated MoS2 flakes. The choice of a suitable sandwich device structure enables us to study the inter-layer transport using temperature dependent current–voltage (J–V) and impedance spectroscopy. The separation of peaks in the imaginary part of impedance i.e. Im (Z) spectra due to defects and transport in the space charge limited regime allows measurement of mobility un-encumbered by other effects. We study flakes obtained from both naturally occurring and synthetic crystals. The calculated inter-layer mobility ( μ ⊥ ) exhibits μ ⊥ ∼ T δ temperature dependence with δ = 1.6 and thereby revealing that impurity scattering is the dominant mechanism. A further confirmation that the mobility is limited by charged impurity scattering comes from the observation that it improves for temperatures lower than a characteristic temperature, which marks deionization of donor impurities. Our results provide a direct estimation of the inter-layer mobility in exfoliated MoS2 flakes and would lead to carrier transport engineering and design strategies for applications based on MoS2 flakes.

Defect density of states in natural and synthetic MoS2 multilayer flakes

“…It is known that S-vacancies the common cause of the n-type doping in MoS 2 samples. The carrier concentrations from our earlier studies have been determined to be approximately 10 15 cm −3 and 10 14 cm −3 for natural and synthetic samples, respectively, [20,21].…”

“…The contact sizes for the top and bottom electrodes in the Au|m-MoS 2 |Au device were identical to the size of the contacts in the Au|m-MoS 2 |ZnO device. The Au|m-MoS 2 |Au device depicts ohmic behavior indicating that, in both the Au|m-MoS 2 |ZnO devices, the asymmetric behavior in the J-V characteristics arises due to the conduction properties of the interface between MoS 2 |ZnO [20,21]. In the natural MoS 2 flake device, the current shows power-law dependence on the applied voltage (J ∼ V m ).…”

“…Both electrodes were deposited at room temperature, where the ZnO electrode was deposited using a pulsed DC magnetron sputtering technique and the Au contact was deposited through a thermal evaporation method. Finally, the MoS 2 flakes with ZnO and Au contacts were mounted on a transistor outline (TO) header for the temperature dependent currentvoltage and admittance measurements [20,21]. A schematic of the fabricated device is shown in figure 1(a).…”