Acoustic phonon assisted free-carrier optical absorption in an n-type monolayer MoS2 and other transition-metal dichalcogenides

The theory of free-carrier absorption (FCA) is given for monolayers of transition-metal dichalcogenides, particularly for molybdenum disulphide (MoS2), when carriers are scattered by phonons. Explicit expressions for the absorption coefficient alpha are obtained and discussed for acoustic phonon scattering via screened deformation potential and piezoelectric coupling taking polarization of the radiation in the plane of the layer. It is found that alpha monotonously decreases with the increasing photon frequency Omega, increases with the increasing temperature T, and linearly depends on two-dimensional electron concentration n(s). Effect of screening, which is ignored in all the earlier FCA studies, is found to reduce alpha significantly, attributing to the larger effective mass of the electrons. Results are also obtained in the classical and quantum limit giving the power laws alpha similar to Omega(-2) and T. Comparison of the results is made with those in bulk semiconductors and semiconductor quantum wells. (C) 2015 AIP Publishing LLC.