ALD-Based Synthesis of Wafer-Scale Transition Metal Dichalcogenides Towards Nanoelectronics and Optoelectronics Applications

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 have been widely studied due to their unique physical and electronic properties even at one atomic layer thickness. Since the first report on the MoS2 field-effect transistor (FET) device [1], great efforts have been made in the past decade to extend the intriguing features of TMDs to practical applications. However, such integration has been severely bottlenecked by the lack of effective approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.Till now, it is still prevalent to fabricate TMD-based electronic devices using mechanical exfoliation methods. Although single-crystal flakes with least defects can be obtained, the size, thickness and location of the exfoliated TMD flakes are largely uncontrollable, making it unsuitable for large-scale device integration and massive production. So far, various synthetic approaches have been proposed to grow large-area TMD thin films. Chemical vapor deposition (CVD)-based method is one of the major routines to synthesize high-quality monolayer and few-layer TMD flakes [2-4]. For example, in typical CVD processes to grow MoS2, the films can be achieved either by the reduction reaction between the S and MoO3 source powders or by sulfurizing the Mo (or MoOx) films pre-deposited by using physical vapor deposition. However, the nucleation sites in the CVD process are uncontrollable which may lead to the vertical stacking of the MoS2 flakes in triangle shape deteriorating the crystalline properties of the film. On the other hand, the pre-deposited Mo or MoOx film by using e-beam evaporation or sputtering usually suffer from rough surface morphology and unsatisfactory film uniformity which further limit the homogeneity of the large-scale film and device integration.Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a possible route towards the synthesis of TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism [5,6]. As compared to the conventional CVD method, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. In this work, we employed the ALD-based synthesis method to grow wafer-scale MoS2 and WS2 thin films. Non-toxic MoCl5, WCl5 and hexamethyldisilathiane (HMDST) precursors have been used which are different and superior to those used in previous work. Film characterizations have confirmed the wafer-level uniformity, crystallinity and stoichiometry of the synthesized films. Homogeneous electrical behaviors have also been obtained from the fabricated FET device arrays. In addition, optoelectronic devices such as photodetectors and simple logic gates such as n-type inverter arrays have been demonstrated using the wafer-scale MoS2 FET arrays. Furthermore, we have used the substitutional doping in the ALD process to achieve controllable p-type doping of MoS2 films, which can provide solid basis for the complimentary metal-oxide-semiconductor (CMOS) integrated circuit applications. This work shows that the ALD-based synthesis of the 2D TMD thin films is an attractive approach and has provided a promising platform to further push forward the circuit and system level applications of the TMD materials.