Photoreflectance study of the near-band-edge transitions of chemical vapor deposition-grown mono- and few-layer MoS₂ films

Room-temperature photoreflectance (PR) and reflectance (R) spectroscopy are utilized to investigate the near-band-edge transitions of molybdenum disulfide (MoS₂) thin films grown on sapphire substrates by a hot-wall chemical vapor deposition system. The layer thickness and optical properties of the MoS₂ thin films are confirmed by Raman spectroscopy, atomic force microscope, and photoluminescence (PL) analysis. The B exciton shows relatively weak PL intensity in comparing with the A exciton even for monolayer MoS₂ films. In the R spectrum of few-layer MoS₂, it is not possible to clearly observe exciton related features. The PR spectra have two sharp, derivative-like features on a featureless background. Throughout the PR lineshape fitting, the transition energies are designated as the A and B excitons at the K-point of the Brillouin zone, but at room temperature there seems to be no distinguishable feature corresponding to an H-point transition for the mono- and few-layer MoS₂ films unlike in bulk. These transition energies are slightly larger than those obtained by PL, which is attributed to the Stokes shifts related to doping level. The obtained values of valence-band spin-orbit splitting are in good agreement with those from other experimental methods. By comparing the PR lineshapes, the dominant modulation mechanism is attributed to variations of the exciton transition energies due to change in the built-in electric field. On the strength of this study, PR spectroscopy is demonstrated as a powerful technique for characterizing the near-band-edge transitions of MoS₂ from monolayer to bulk.