Effects of insertion of an h-AlN monolayer spacer in Pt-WSe2-Pt field-effect transistors

Abstract

The growth of two-dimensional hexagonal aluminum nitride (h-AlN) on transition metal dichalcogenide (TMD) monolayers exhibits superior uniformity and smoothness compared to HfO $_2^{}$ on silicon substrate. This makes an h-AlN monolayer an ideal spacer between the gate oxide material and the WSe₂ monolayer in a two-dimensional field effect transistor (FET). From first principles approaches, we calculate and compare the transmission functions and current densities of Pt-WSe₂-Pt nanojunctions without and with the insertion of an h-AlN monolayer as a spacer in the gate architecture. The inclusion of h-AlN can alter the characteristics of the Pt-WSe₂-Pt FET in response to the gate voltage ( $V_{\text{g}}$ ). The FET without (or with) h-AlN exhibits the characteristics of a P-type (or bipolar) transistor: an on/off ratio of around $2.5\times {10}^6$ (or $1.7\times {10}^6$ ); and an average subthreshold swing (S.S.) of approximately 109 mV/ $\text{dec.}$ (or 112 mV/ $\text{dec.}$ ), respectively. We observe that $V_{\text{g}}$ shifts the profile of the transmission function by an energy of $\alpha \left(e{V}_{\text{g}}\right)$ , where $\alpha$ represents the gate-controlling efficiency. We observed that ${\alpha }_{\text{in}}=83\%$ and ${\alpha }_{\text{out}}=33\%$ , corresponding to whether the Fermi energy is located inside or outside the band gap. Therefore, we construct an effective gate model based on the Landauer formula, with the transmission function at $V_{\text{g}}=0$ as the baseline. Our model generates results that are consistent with those obtained through first principles calculations. The relative error in current densities between model and first-principles calculations is within $\left[\frac{ln\left(10\right)}{S.S.}\right]\left|\Delta V_{\text{G}}^{\text{eff}}\right|$ . The 2D atomistic FETs show excellent device specifications and the ability to compete with existing transistors based on traditional silicon technology. Our findings could help advance the design of TMD-based FETs.

Keywords: 2D field effect transistor; Density functional theory; Non-equilibrium Green’s function; Subthreshold swing.