A kinetic monte carlo study for mono- and bi-layer growth of mos2 during chemical vapor deposition

Abstract

Controllable synthesis of MoS2 with desired number of layersvia chemical vapor deposition (CVD) remains challenging. Hence, it ishighly desirable to develop a theoretical model that can be used to predict the single- and multilayer growth of MoS2 quantitatively, and provide guidelines for experimental fabrication. Herein we have established a kinetic Monte Carlo (kMC) model to predict the CVD growth ofmono- and bilayer MoS2. First, we proposed that the growth rates of layer 1 and layer 2 were governed by the distribution of the adatom concentration, and the growth kinetics of compact triangular MoS2 followed the kink nucleation-propagation mechanism. The adatom concentration was formulated in terms of adatom flux, effective lifetime of adatoms, growth temperature, binding energies, edge energies, and nucleation criterion. The kink nucleation and propagation were determined byenergy barriers of the adatom attachments to the zigzag and armchair edges. We then employed an analytic thermodynamic criterion to extract these parameters. Using the calibrated model, we found that the growth rate of layer 2 strongly depended on the size of layer 1 and decreased monotonically with increasing size of layer 1, and might even become prohibited at the maximum size of layer 1. Furthermore, we analyzed the size and morphology evolutions of bilayer MoS2 at different growth temperatures and adatom fluxes. Throughout the growth processes of bilayer MoS2, the morphologies of layers 1 and 2 maintained triangular shapes with compact edges, consistent with the kink nucleation-propagation growth mechanism. Our simulations revealed that the growth of bilayer MoS2 was promoted by increasing the growth temperature or decreasing the adatom flux, which corroborated the experimental observations. The increase in growth temperature led to reduced adatom concentration at the edge of layer 2 in accordance with the adatom concentration far from the edge of layer 2, resulting in a consistent difference in the adatom concentration to romote the growth of bilayer MoS2. Similarly, the decrease in adatom flux lowered the differencebetween the adatom concentrations far from the edge and at the edge of layer 1, decelerating the growth of layer 1. The decelerated growthof layer 1 reduced the difference between the adatom concentrations far from the edge and at the edge of layer 2 to zero, permitting the growth of bilayer MoS2. To guide the experimental synthesis, we constructed a phase diagram to delineate the permitted or prohibited growthof bilayer MoS2 at different growth temperatures and adatom fluxes. Hence, this work not only unveils the conditions for the growth of mono- and bi-layer MoS2, but also provides guidelines for controllable synthesis of MoS2 with the desired number of layers.