Nanoscale inhomogeneity of the Schottky barrier and resistivity in ${\mathrm{MoS}}_2$ multilayers

Conductive atomic force microscopy (CAFM) is employed to investigate the current injection from a nanometric contact (a Pt coated tip) to the surface of ${\text{MoS}}_2$ thin films. The analysis of local current-voltage characteristics on a large array of tip positions provides high spatial resolution information on the lateral homogeneity of the $\mathrm{tip}/{\text{MoS}}_2$ Schottky barrier ${\mathrm{\Phi }}_B$ and ideality factor $n$, and on the local resistivity ${\rho }_{\text{loc}}$ of the ${\text{MoS}}_2$ region under the tip. Here, ${\mathrm{\Phi }}_B=300\pm 24\text{meV}, n=1.60\pm 0.23$, and ${\rho }_{\text{loc}}=2.99\pm 0.68\mathrm{\Omega }\text{cm}$ are calculated from the distributions of locally measured values. A linear correlation is found between the ${\rho }_{\text{loc}}$ and ${\mathrm{\Phi }}_B$ values at each tip position, indicating a similar origin of the ${\rho }_{\text{loc}}$ and ${\mathrm{\Phi }}_B$ inhomogeneities. These findings are compared with recent literature results on the role of sulfur vacancy clusters on the ${\text{MoS}}_2$ surface as preferential paths for current injection from metal contacts. Furthermore, their implications on the behavior of ${\text{MoS}}_2$ based transistors are discussed.

Figure 1

(a) AFM morphology of the edge of a multilayer ${\text{MoS}}_2$ flake and (b) corresponding height line profile, showing the thickness with respect to the ${\text{SiO}}_2$ substrate. (c) AFM image in the flake central area where CAFM measurements were performed. (d) Scheme of the experimental setup for CAFM measurements. (e) OM image of the fabricated test structure. (f) Circuital schematic representation of the measured system.

Figure 2

Set of 25 $I-V_{\text{tip}}$ characteristics measured on a $500\text{nm}\times 500\text{nm}$ array of tip positions with $\sim 100$ nm spacing on ${\text{MoS}}_2$. (b) Representative forward bias $I-V_{\text{tip}}$ characteristic from this set of measurements and fit with the thermionic emission law to extract the SBH and ideality factor. Inset: Scheme of the metal/${\text{MoS}}_2$ band structure. (c) $H$ function plot for the determination of the series resistance $R$.

Figure 3

Histograms of (a) the local SBH ${\mathrm{\Phi }}_B$, (b) ideality factors $n$, and (c) resistivities ${\rho }_{\text{loc}}$ extracted from the full set of $I-V_{\text{tip}}$ characteristics of Fig. 2. (d) Plot of ${\rho }_{\text{loc}}$ vs ${\mathrm{\Phi }}_B$.