Recent progress in two-dimensional transition metal dichalcogenides: Material synthesis for microelectronics

Abstract

The size of modern electronic and optoelectronic devices has been shrunk down to nanoscale and this effort still continues. A lot of concerns focus on whether current silicon-based complementary metal oxide semiconductor (CMOS) technologies can overcome the physical limit at nanoscale. A promising alternative choice of semiconductor material, two-dimensional layered materials (2DLMs) have emerged recently, including graphene, 2D transition-metal dichalcogenides (2D TMDs), black phosphors (BPs), etc. Due to their unique atomic thickness nature, 2DLMs are likely to have the greatest potential to overcome the dilemma of geometric scaling. 2DLMs also exhibit a wide range of unique properties owing to their unusual atomically thin defined structures. Most 2D TMDs have a MX2 structure, where M represents transition metal (Mo, W, Ti, Pt, Hf, etc.) and X is the chalcogen (S, Se, or Te). Contrary to graphene which displays many superior properties but lacks a bandgap, 2D TMDs exhibit a wide range of electronic properties, from semiconducting to metallic or even superconducting, depending on the combination of M and X. For most Mo and W based 2D TMDs, the intrinsic band gap is greater than 1 eV so that the high on/off-current ratio is more easily achieved at room temperature. Moreover, 2D TMDs possess band structures that are known to be highly sensitive to various effects that arise from surfaces and interfaces, thus providing versatile knobs for tuning 2D TMD devices. Various modification methods have been applied to further physically or chemically tune 2DLMs based on their unique intrinsic properties. To date these include dimensional sizing, ion-intercalation, application of an external field, tuning the stacking order, and strain engineering, etc. These fundamental modification methods provide useful engineering tools to improve TMD electronic devices. So far, most research results of 2D TMDs have been based on mechanically exfoliated sheets which are single crystalline with few defects. But the exfoliation method has poor reproducibility, relatively low yield, and gives rise to a lateral size no more than hundreds of microns. Thus, the main bottleneck for practical application of 2D TMDs in electronic devices still remains at the stage of wafer scale uniform growth. Compared to chemical vapor deposition (CVD) synthesized graphene film with high carrier mobility and large grain size, the existing synthetic methods of 2D TMDs can only produce films with limited spatial uniformity and fair electrical performance. Thus, there is still significant room for improving current CVD methods, or exploring new synthetic methods. In this review, we summarize and compare various preparation methods of 2D TMD materials. All these methods can be actually classified by two strategies: ″Top-down″ methods, where the bulk forms are exfoliated into few-layer or monolayer sheets; and the ″bottom-up″ methods, that chemically synthesize 2D TMDs using CVD, atomic layer deposition (ALD), molecular beam epitaxy (MBE), etc. We first introduce several important ″Top-down″ methods including mechanical exfoliation, liquid phase exfoliation, ion intercalation and laser thinning. We then focus on the CVD approaches that are suitable for wafer-scale synthesis of high-quality 2D TMD films. We also introduce the latest development of 2D TMDs application in electronic devices, and provide a look at the future of this field.