Anomalous kinetic roughening in growth of MoS2 films under pulsed laser deposition

Abstract

Growth dynamics of thin films expressed by scaling theory is a useful tool to quantify the statistical properties of the surface morphology of the thin films. To date, the growth mechanism for 2D van der Waals materials has been rarely investigated. In this work, an experimental investigation was carried out to identify the scaling behavior as well as the growth mechanism of 2D MoS₂ thin films, grown on glass substrates by pulsed laser deposition for different deposition time durations, using atomic force microscopy images. The growth of MoS₂ films evolved from layer-by-layer to layer plus island with the increase in deposition time from 20 s to 15 min. The film surface exhibited anisotropic growth dynamics in the vertical and lateral directions where RMS roughness varied with deposition time as w ∼ t^β with the growth exponent β = 0.85 ± 0.11, while the lateral correlation length ξ was ξ = t^1/z with 1/z = 0.49 ± 0.09. The films showed a local roughness exponent α_loc = 0.89 ± 0.01, global roughness exponent α = 1.72 ± 0.14 and spectral roughness exponent α_s = 0.85 ± 0.03, suggesting that the growth of MoS₂ thin films followed intrinsic anomalous scaling behavior (α_s < 1, α_loc = α_s ≠ α). Shadowing owing to conical incoming particle flux distribution towards the substrate during deposition has been attributed to the anomalous growth mode. The optical properties of the films, extracted from ellipsometric analysis, were also correlated with RMS roughness and cluster size variation which unveiled the important role played by surface roughness and film density.