Film thickness-induced optical and electrical modifications in large-area few-layer 2H-MoSe₂ grown by MBE

Here, we report on a detailed study of film thickness-induced effects on optical and electrical characteristics of ultra-thin MoSe₂ films grown using molecular beam epitaxy (MBE) on a c-plane sapphire substrate. The layer-dependent optical and electrical responses are investigated for MoSe₂ films with different thicknesses (1, 2, 4 and 7 layers). Spectroscopic ellipsometry (SE) reveals significant variation in optical constants with film thickness in the spectral range of 5.04 eV to 0.73 eV. As the thickness increases from 1 layer to 7 layers, the band gap of the materials also changes from 1.62 eV to 1.19 eV. The layer-dependent band diagram analysis shows that the conduction band to Fermi level energy gap changes from 0.50 eV to 0.40 eV as the film thickness changes from 1 layer to 7 layers, making thicker films more n-type than thinner ones. I-V measurement shows an increase in current from the order of 10^-9 to 10^-5 ampere at a voltage of 3 V as the film thickness increases from 1 layer to 7 layers, which is explained by the corresponding change in the band diagram and supported by a temperature-dependent I-V study. The findings of the study offer a pathway to tune the optical and electrical characteristics of MoSe₂ by controlling the layer number which can be valuable for its electronic and optoelectronic device applications.