How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials

Abstract

One important goal in the field of 2D materials is the investigation of valley physics in semiconducting transition metal dichalcogenides (TMDs). [1,2] As valley dynamics are governed by a delicate interplay of different electron-electron, electronphonon, and many-body interactions, an overall understanding of valley physics is only possible, and physical models can only be tested when different device properties such as valley lifetimes, exciton lifetimes, spin and momentum scattering times, or phonon and electron dispersion relations are analyzed as a function of Fermi level position-ideally for the same device. To accomplish this, a variety of different measurements are required, most importantly, the combination of both optical and electrical techniques, e.g., only the combination of gate-dependent electrical transport measurements, photoluminescence (PL) spectroscopy, and time-resolved Kerr rotation (TRKR) measurements recently enabled us to identify the dynamics which are responsible for the transfer of a polarization from optically excited bright trions to a valley polarization of free charge carriers in monolayer WSe 2. [3] However, this necessary prerequisite for the investigation of valley physics encounters practical obstacles, as not every measurement system can simultaneously conduct optical and electrical measurements. Even worse is the fact that some measurement techniques are mutually exclusive, e.g., in valley-sensitive TRKR measurements, the pump and probe energies normally have to be set to neutral or charged exciton energies (i.e., energies below the band gap), [3-5] but for this, first PL measurements with above-bandgap excitation are necessary to determine the energetic position of these exciton features. [6,7] Therefore, it is important to compare gate-dependent measurements recorded under different conditions.

Valley lifetimes of conduction band electrons in monolayer WSe2

“…The hysteresis can result from a screening effect of the gate electric field by photo-excited charged defects in the dielectric layer. [11][12][13][14] We demon- strate that disregarding the adaptation of the measurement scheme can lead to erroneous conclusions drawn from gate-dependent measurements and can completely mask the true gate-dependent valley dynamics. First, we discuss the "conventional" measurement technique in which the gate-voltage is set to a fixed value and then the full TRKR curve is measured by varying the time delay ∆t between pump and probe pulses over laboratory time.…”

“…With this measurement technique, once the gatevoltage is set to a certain value, the photo-induced screening of the gate-electric field will start to a change the gate-induced charge carrier densities over time. 14 Although all data points within a Kerr rotation curve are therefore measured at a nominal identical gate voltage, each single point is effectively measured at a different Fermi level position.…”

“…To achieve the maximum comparability between different measurements, the same gate sweep direction and velocity should be used for all TRKR, PL, and transport measurements. 14 This ensures that the photodoping effect impacts all measurements in the same way.…”

Two-Dimensional Dielectrics for Future Electronics: Hexagonal Boron Nitride, Oxyhalides, Transition-Metal Nitride Halides, and Beyond

“…(iv) Small physical thickness: to maximize capacitive coupling, a small physical thickness is required in addition to a high dielectric constant. (v) Defect-free: defects introduce new electronic states in the bandgap, causing charge trapped in defects to change the gate threshold voltage . Defects can also increase leakage current , and play a pivotal role in mobility engineering .…”

“…(v) Defect-free: defects introduce new electronic states in the bandgap, causing charge trapped in defects to change the gate threshold voltage . Defects can also increase leakage current , and play a pivotal role in mobility engineering . (vi) Adoption in a manufacturing process .…”