Thickness-Dependent Nonlinear Electrical Conductivity of Few-Layer Muscovite Mica

We report on the direct current (dc) current-voltage (I-V) characteristics of few-layer muscovite mica (MuM) flakes exfoliated and transferred onto SiO2/Si substrate, under different substrate dc bias voltages. Contrary to usual observations in conventional two-dimensional systems, we observe an increase in the in-plane electrical conductivity with a reducing thickness of MuM flakes. At a given voltage, the electrical conductivity of approximately five-layered MuM flake (T3) is 3 orders of magnitude larger than that in approximately ten-layered MuM flake (T2). The I-V characteristics are used to analyze the mechanism of conduction. The model-based analysis reveals the hopping-conduction mechanism to be dominant as compared to the Poole-Frenkel effect. The thickness-dependent work function is measured using Kelvin probe force microscopy for a MuM flake on Si substrate. Assuming that the measured work function is correlated with the Fermi level, we report an upward movement of the Fermi level, toward the conduction band with the reducing thickness of MuM flakes, indicating an increase in the conduction-band carrier density. The observed increase in conductivity in T3 when compared to T2 may be attributed to surface doping due to the increased contribution from K+ ions and lattice relaxation. Our results show that there is a possibility of using few-layer mica as a wide-band-gap semiconductor and that it can open up different avenues for two-dimensional electronic devices.