Diagnosing of the interface quality and the interactions between insulators and semiconductors is significant to achieve the high performance of nanodevices. Herein, low-frequency noise (LFN) in mechanically exfoliated multilayer molybdenum disulfide (MoS2) (~11.3 nm-thick) field-effect transistors with back-gate control was characterized with and without an Al2O3 high-k passivation layer. The carrier number fluctuation (CNF) model associated with trapping/detrapping the charge carriers at the interface nicely described the noise behavior in the strong accumulation regime both with and without the Al2O3 passivation layer. The interface trap density at the MoS2-SiO2 interface was extracted from the LFN analysis, and estimated to be Nit ~ 10(10) eV(-1) cm(-2) without and with the passivation layer. This suggested that the accumulation channel induced by the back-gate was not significantly influenced by the passivation layer. The Hooge mobility fluctuation (HMF) model implying the bulk conduction was found to describe the drain current fluctuations in the subthreshold regime, which is rarely observed in other nanodevices, attributed to those extremely thin channel sizes. In the case of the thick-MoS2 (~40 nm-thick) without the passivation, the HMF model was clearly observed all over the operation regime, ensuring the existence of the bulk conduction in multilayer MoS2. With the Al2O3 passivation layer, the change in the noise behavior was explained from the point of formation of the additional top channel in the MoS2 because of the fixed charges in the Al2O3. The interface trap density from the additional CNF model was Nit = 1.8 × 10(12) eV(-1) cm(-2) at the MoS2-Al2O3 interface.

WSe 2 , etc.) and other mono-elemental materials (e.g., black phosphorus, tellurene, etc.) have generated significant interest in numerous research fields. [2][3][4][5][6] In particular, atomically thin monolayers have proven to be an ideal platform for the exploration of the unique features associating with 2D electronic systems such as ultrahigh mobility, [7] quantum phase transition, [8] indirect exciton condensation, [9] quantum oscillation, [10] and the quantum Hall effect. [11] Despite the unique merits of monolayers, multilayers, which consist of multiple conducting monolayers, may be a better candidate for most 2D layered materials because of their higher carrier mobility and larger current density compared to monolayers, with respect to applications in electronic devices. [3,6,12,13] Previous theoretical and experimental investigations have clearly demonstrated that the effective mass increases with bandgap energy as thickness reduced. [14] In addition, the adjacent surface phonon and numerous Coulomb scatterers that surround atomically thin channel materials further degrade the intrinsic carrier mobility, [15,16] implying the disadvantage of monolayer platform as a transistor.Among the various families of 2D layered materials, a multilayer rhenium disulfide (ReS 2 ) has recently garnered notable attention because of their three unique properties compared to other TMD materials; i) anisotropic in-plane transport, ii) direct bandgap, and iii) layer-independent electronic band structure. [17] The distorted octahedral (1T′) structure of ReS 2 is responsible for the anisotropic in-plane transport property. [18][19][20] The layer-independent electronic band structure with direct bandgap is linked to a decoupled vdW interaction between adjacent layers, [21] leading to a much higher interlayer resistivity of bulk ReS 2 [22] compared to that of the other TMDs [23] including MoS 2 . [24] This fact implies that few-layered ReS 2 is the potentially a superior platform for studying intrinsic carrier transport features of multilayer systems. This is because the interlayer resistivity governs the charge distribution along the channel thickness [25] and the channel access resistance, in addition to the Thomas-Fermi charge screening length.Herein, we report on the distinctive electron conduction mechanism of few-layered 1T′ ReS 2 by presenting; i) the anisotropic carrier transport, ii) modification of charge distribution Charge carrier transport in multilayer van der Waals (vdW) materials, which comprise multiple conducting layers, is well described using Thomas-Fermi charge screening (λ TF ) and interlayer resistance (R int ). When both effects occur in carrier transport, a channel centroid migrates along the c-axis according to a vertical electrostatic force, causing redistribution of the conduction centroid in a multilayer system, unlike a conventional bulk material. Thus far, numerous unique properties of vdW materials are discovered, but direct evidence for distinctive charge transport behavior in 2D layered materi...

We extracted the interlayer resistance between two layers in multilayer molybdenum disulfide (MoS2) field-effect transistors by confirming that contact resistances (Rcontact) measured using the four-probe measurements were similar, within ∼30%, to source/drain series resistances (Rsd) measured using the two-probe measurements. Rcontact values obtained from gated four-probe measurements exhibited gate voltage dependency. In the two-probe measurements, the Y-function method was applied to obtain the Rsd values. By comparing those two Rcontact (∼9.5 kΩ) and Rsd (∼12.3 kΩ) values in strong accumulation regime, we found the rationality that those two values had nearly the same properties, i.e., the Schottky barrier resistances and interlayer resistances. The Rsd values of devices with two-probe source/drain electrodes exhibited thickness dependency due to interlayer resistance changes. The interlayer resistance between two layers was also obtained as ∼2.0 Ω mm.

The semimetallic, two-dimensional layered transition metal dichalcogenide WTe has raised considerable interest due to its huge, non-saturating magnetoresistance. While for the origin of this effect, a close-to-ideal balance of electrons and holes has been put forward, the carrier concentration dependence of the magnetoresistance remains to be clarified. Here, we present a detailed study of the magnetotransport behaviour of ultrathin, mechanically exfoliated WTe sheets as a function of electrostatic back gating. The carrier concentration and mobility, determined using the two band model and analysis of the Shubnikov-de Haas oscillations, indicate enhanced surface scattering for the thinnest sheets. By the back gate action, the magnetoresistance could be tuned by up to ∼100% for a ∼13 nm-thick WTe sheet.

Tunneling field-effect transistors (TFETs) are of considerable interest owing to their capability of low-power operation. Here, we demonstrate a novel type of TFET which is composed of a thin black phosphorus–tin diselenide (BP–SnSe2) heterostructure. This combination of 2D semiconductor thin sheets enables device operation either as an Esaki diode featuring negative differential resistance (NDR) in the negative gate voltage regime or as a backward diode in the positive gate bias regime. Such tuning possibility is imparted by the fact that only the carrier concentration in the BP component can be effectively modulated by electrostatic gating, while the relatively high carrier concentration in the SnSe2 sheet renders it insensitive against gating. Scanning photocurrent microscopy maps indicate the presence of a staggered (type II) band alignment at the heterojunction. The temperature-dependent NDR behavior of the devices is explainable by an additional series resistance contribution from the individual BP and SnSe2 sheets connected in series. Moreover, the backward rectification behavior can be consistently described by the thermionic emission theory, pointing toward the gating-induced formation of a potential barrier at the heterojunction. It furthermore turned out that for effective Esaki diode operation, care has to be taken to avoid the formation of positive charges trapped in the alumina passivation layer.

Ultrathin sheets of two-dimensional (2D) materials like transition metal dichalcogenides have attracted strong attention as components of high-performance light-harvesting devices. Here, we report the implementation of Schottky junction-based photovoltaic devices through site-selective surface doping of few-layer WSe in lateral contact configuration. Specifically, whereas the drain region is covered by a strong molecular p-type dopant (NDP-9) to achieve an Ohmic contact, the source region is coated with an AlO layer, which causes local n-type doping and correspondingly an increase of the Schottky barrier at the contact. By scanning photocurrent microscopy using green laser light, it could be confirmed that photocurent generation is restricted to the region around the source contact. The local photoinduced charge separation is associated with a photoresponsivity of up to 20 mA W and an external quantum efficiency of up to 1.3%. The demonstrated device concept should be easily transferrable to other van der Waals 2D materials.

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