How the dynamic of photo-induced gate screening complicates the investigation of valley physics in 2D materials

TL;DR

This paper reveals that photo-induced gate screening effects and dielectric leakage currents complicate the interpretation of gate-dependent measurements in 2D materials, impacting the study of valley physics.

Contribution

It highlights the influence of trap state charging dynamics and leakage currents on gate-dependent optical measurements in 2D materials, which was previously underappreciated.

Findings

Gate sweep direction and rate affect charge carrier density.

Leakage currents reduce gate tunability.

Photo-excitation alters trap state charging behavior.

Abstract

An in-depth analysis of valley physics in 2D materials like transition metal dichalcogenides requires the measurement of many material properties as a function of Fermi level position within the electronic band structure. This is normally done by changing the charge carrier density of the 2D material via the gate electric field effect. Here, we show that a comparison of gate-dependent measurements, which were acquired under different measurement conditions can encounter significant problems due to the temporal evolution of the charging of trap states inside the dielectric layer or at its interfaces. The impact of, e.g., the gate sweep direction and the sweep rate on the overall gate dependence gets especially prominent in optical measurements due to photo-excitation of donor and acceptor states. Under such conditions the same nominal gate-voltage may lead to different gate-induced charge carrier densities and, hence, Fermi level positions. We demonstrate that a current flow from or even through the dielectric layer via leakage currents can significantly diminish the gate tunability in optical measurements of 2D materials.