Dielectric and Gate Metal Engineering for Threshold Voltage Modulation in Enhancement Mode Monolayer MoS₂ Field Effect Transistors.

Abstract

Excellent gate electrostatics in field effect transistors (FETs) based on 2D transition metal dichalcogenide (2D TMD) channels can dramatically decrease static power dissipation. Energy-efficient FETs operate in enhancement mode with a small and positive threshold voltage (V_th) for n-type devices. However, most state-of-the-art FETs based on monolayer MoS₂ channel operate in depletion mode with negative V_th due to doping from the underlying dielectric substrate. In this work, we identify key properties of the semiconductor/dielectric interface (MoS₂ on industrially relevant high dielectric constant (k) HfO₂, ZrO₂ and hBN for reference) responsible for realizing enhancement-mode operation of 2D MoS₂ channel FETs. We find that hBN and ZrO₂ dielectric substrates provide low defect interfaces with MoS₂ that enables effective modulation of the V_th using gate metals of different work functions (WFs). We use photoluminescence (PL) and synchrotron X-ray photoelectron spectroscopy (XPS) measurements to investigate doping levels in monolayer MoS₂ on different dielectrics with different WF gate metals. We complement the FET and spectroscopic measurements with capacitance-voltage analysis on dielectrics with varying thicknesses, which confirms that V_th modulation in ZrO₂ devices is correlated with WF of the gate metals - in contrast with HfO₂ devices that exhibit signatures of V_th pinning induced by oxide/interface defect states. Finally, we demonstrate FETs using a 2D MoS₂ channel and a 6 nm of ZrO₂ dielectric, achieving a subthreshold swing of 87 mV dec^-1 and a threshold voltage of 0.1 V. Our results offer insights into the role of dielectric/semiconductor interface in 2D MoS₂ based FETs for realizing enhancement mode FETs and highlight the potential of ZrO₂ as a scalable high-k dielectric.