Synthesis of Few-Layered Transition-Metal Dichalcogenides by Ion Implantation of Chalcogen and Metal Species into Sapphire

The growth of transition-metal dichalcogenides (TMDCs)has beenperformed so far using most established thin-film growth techniques(e.g., vapor phase transport, chemical vapor deposition, molecularbeam epitaxy, etc.). However, because there exists no self-limitingmechanism for the growth of TMDCs, none of these techniques allowsprecise control of the number of TMDC layers over large substrateareas. Here, we explore the ion implantation of the parent TMDC atomsinto a chemically neutral substrate for the synthesis of TMDC films.The idea is that once all of the ion-implanted species have reactedtogether, the synthesis reaction stops, thereby effectively stoppinggrowth. In other words, even if there is no self-limiting mechanism,growth stops when the nutrients are exhausted. We have co-implantedMo and S ions into c-oriented sapphire substrates using various dosescorresponding to 1- to 5-layer atom counts. We find that the subsurfaceregion of the sapphire substrates is amorphized by the ion implantationprocess, at least for implanted doses of 2-layer atom counts and over.For all doses, we have observed the formation of MoS2 materialinside the sapphire after postimplantation annealing between 800 and850 & DEG;C. We report that the order of implantation (i.e., whetherS or Mo is implanted first) is an important parameter. More precisely,samples for which S is implanted first tend to yield thin crystalswith a large lateral extension (more than 200 nm for 5-layer doses)and mainly located at the interface between the amorphized and crystallinesapphire. When Mo is first implanted, the MoS2 crystalsstill predominantly appear at the amorphous-crystalline interface(which is much rougher), but they are much thicker, suggesting a differentnucleation mechanism.

Automated summary

This study explores the synthesis of TMDC films by ion implantation of parent TMDC atoms into a chemically neutral substrate. The results show that the implantation dose and order of implantation are important parameters for the formation of MoS2 crystals with different thicknesses and locations.