Electrothermal Local Annealing via Graphite Joule Heating on Two-Dimensional Layered Transistors

A simple but powerful device platform for electrothermal local annealing (ELA) via graphite Joule heating on the surface of transition-metal dichalcogenide, is suggested here to sustainably restore intrinsic electrical properties of atomically thin layered materials. Such two-dimensional materials are easily deteriorated by undesirable surface/interface adsorbates and are screened by a high metal-to-semiconductor contact resistance. The proposed ELA allows one to expect a better electrical performance such as an excess electron doping, an enhanced carrier mobility, and a reduced surface traps in a monolayer molybdenum disulfide (MoS₂)/graphite heterostructure. The thermal distribution of local heating measured by an infrared thermal microscope and estimated by a finite element calculation shows that the annealing temperature reaches up to >400 K at ambient condition and the high efficiency of site-specific annealing is demonstrated unlike the case of conventional global thermal annealing. This ELA platform can be further promoted as a practical gas sensor application. From an O₂ cycling test and a low-frequency noise spectroscopy, the graphite on top of the MoS₂ continuously recovers its initial condition from surface adsorbates. This ELA technique significantly improves the stability and reliability of its gas sensing capability, which can be expanded in various nanoscale device applications

Photochemical Reaction in Monolayer MoS₂ <i>via</i> Correlated Photoluminescence, Raman Spectroscopy, and Atomic Force Microscopy

Photoluminescence (PL) from monolayer MoS₂ has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damage and associated oxygen adsorption on the sample in ambient conditions. Three distinct behaviors were observed according to the laser irradiation time: (i) slow photo-oxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity; (ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly; and (iii) photoquenching, with complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage; however, the structural disorder is invoked during the fast photo-oxidation stage, and severe structural degradation is generated during the photoquenching stage. The effect of oxidation is further verified by repeating experiments in vacuum, where the PL intensity is simply degraded with laser irradiation in a vacuum due to a simple structural degradation without involving oxygen functional groups. The charge scattering by oxidation is further explained by the emergence/disappearance of neutral excitons and multiexcitons during each stage

Electrical Transport Properties of Polymorphic MoS₂

The engineering of polymorphs in two-dimensional layered materials has recently attracted significant interest. Although the semiconducting (2H) and metallic (1T) phases are known to be stable in thin-film MoTe₂, semiconducting 2H-MoS₂ is locally converted into metallic 1T-MoS₂ through chemical lithiation. In this paper, we describe the observation of the 2H, 1T, and 1T′ phases coexisting in Li-treated MoS₂, which result in unusual transport phenomena. Although multiphase MoS₂ shows no transistor-gating response, the channel resistance decreases in proportion to the temperature, similar to the behavior of a typical semiconductor. Transmission electron microscopy images clearly show that the 1T and 1T′ phases are randomly distributed and intervened with 2H-MoS₂, which is referred to as the 1T and 1T′ puddling phenomenon. The resistance curve fits well with 2D-variable range-hopping transport behavior, where electrons hop over 1T domains that are bounded by semiconducting 2H phases. However, near 30 K, electrons hop over charge puddles. The large temperature coefficient of resistance (TCR) of multiphase MoS₂, −2.0 × 10^–2 K^–1 at 300 K, allows for efficient IR detection at room temperature by means of the photothermal effect

Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS₂ Field-Effect Transistors

Although monolayer transition metal dichalcogenides (TMDs) exhibit superior optical and electrical characteristics, their use in digital switching devices is limited by incomplete understanding of the metal contact. Comparative studies of Au top and edge contacts with monolayer MoS₂ reveal a temperature-dependent ideality factor and Schottky barrier height (SBH). The latter originates from inhomogeneities in MoS₂ caused by defects, charge puddles, and grain boundaries, which cause local variation in the work function at Au–MoS₂ junctions and thus different activation temperatures for thermionic emission. However, the effect of inhomogeneities due to impurities on the SBH varies with the junction structure. The weak Au–MoS₂ interaction in the top contact, which yields a higher SBH and ideality factor, is more affected by inhomogeneities than the strong interaction in the edge contact. Observed differences in the SBH and ideality factor in different junction structures clarify how the SBH and inhomogeneities can be controlled in devices containing TMD materials