Monolayer MoS2 Bandgap Modulation by Dielectric Environments and Tunable Bandgap Transistors

Abstract

Semiconductors with a moderate bandgap have enabled modern electronic device technology, and the current scaling trends down to nanometer scale have introduced two-dimensional (2D) semiconductors. The bandgap of a semiconductor has been an intrinsic property independent of the environments and determined fundamental semiconductor device characteristics. In contrast to bulk semiconductors, we demonstrate that an atomically thin two-dimensional semiconductor has a bandgap with strong dependence on dielectric environments. Specifically, monolayer MoS2 bandgap is shown to change from 2.8 eV to 1.9 eV by dielectric environment. Utilizing the bandgap modulation property, a tunable bandgap transistor, which can be in general made of a two-dimensional semiconductor, is proposed.

Figure 1. Atomic structures and electronic band structures.

(a–f) Atomic structures of the freestanding ML MoS₂ (a), ML MoS₂ on HfO₂ substrate (b), ML MoS₂ sandwiched by HfO₂ (c), ML MoS₂ on Au metallic substrate (d), ML MoS₂ sandwiched by Au (e), and ML MoS₂ sandwiched by Ag (f). The Mo, S, Hf, O, Au, and Ag atoms are indicated by the purple, yellow, gold, red, yellow, and blue color balls, respectively. (g–l) Calculated LDA band structures of the freestanding ML MoS₂ (g), ML MoS₂ on HfO₂ (h), ML MoS₂ sandwiched by HfO₂ (i), ML MoS₂ on Au (j), ML MoS₂ sandwiched by Au (k), and ML MoS₂ sandwiched by Ag (l). The blue filled dots (g–l) indicate the projected states to the ML MoS₂. (m–r) Calculated GW band structures of the freestanding ML MoS₂ (m), ML MoS₂ on HfO₂ (n), ML MoS₂ sandwiched by HfO₂ (o), ML MoS₂ on Au (p), ML MoS₂ sandwiched by Au (q), and ML MoS₂ sandwiched by Ag (r) are shown. The red filled dots (m–r) indicate the projected states to the ML MoS₂. The arrows (g–r) indicate the direct bandgap at the K valley of the ML MoS₂. The Fermi levels of the Au and Ag containing systems (j–l,p–r) are indicated by the green dashed lines.

Figure 2. Bandgap and absolute band edge levels.

(a) Calculated GW bandgaps at the K valley of the ML MoS₂ on a substrate (red) and in a sandwich structure (black), as a function of the effective dielectric constant κ_E of the environment. (b) Calculated absolute GW band edge levels of the ML MoS₂ sandwiched by the effective dielectric media having κ_E. They are compared to the absolute band edge levels in LDA. The work function levels of Au (at −5.1 eV) and the strained (used in our calculations) Au (at −5.4 eV) are indicated by the dashed and dotted line, respectively.

Figure 3. Schematic figures of screening and band diagrams.

(a) Strong Coulomb interaction (weak screening) between electrons (red lines) in the freestanding ML MoS₂ and the wide bandgap. (b) Moderate Coulomb interaction (moderate screening) between electrons (green lines) in ML MoS₂ with both-side (HfO₂) dielectric environments and the reduced bandgap. (c) Weak Coulomb interaction (strong screening) between electrons (blue lines) in ML MoS₂ with both-side metallic (Au) environments, in conjunction with the ML MoS₂ with both-side (HfO₂) dielectric environments. The band offsets are indicated in the band diagram below, which act as transport barriers to electrons (ΔE_C) and holes (ΔE_V). (d) A hypothetical device structure composed of a ML MoS₂ sandwiched by Au metallic contacts at both the ends and a dielectric material in the central region, of which the dielectric constant [κ_E(V)] is variable with the electric field applied by the gate voltage (V). The band diagram shows the tunable transport barriers to electrons [ΔE_C(V)] and holes [ΔE_V(V)].