Charge transfers and charged defects in WSe₂/graphene-SiC interfaces

Abstract

We report on Kelvin Probe Force Microscopy (KPFM) and Density Functional Theory (DFT) investigations of charge transfers in vertical heterojunctions between tungsten diselenide (WSe2) layers and graphene on silicon carbide substrates. The experimental data reveal the existence of an interface dipole, which is shown by DFT to originate from the neutralization of the graphene n-doping by an electron transfer towards the transition metal dichalcogenide (TMD) layer. The relative vacuum level shift probed by KPFM between the TMD and the substrate stays constant when passing from monolayer to bilayer graphene, which confirms that the Schottky-Mott model can be rigorously applied to these interfaces by taking into account the charge transfer from the substrate to the TMD. DFT calculations show that the first TMD layer absorbs almost all the excess charges contained in the graphene, and that the second TMD layer shall not play a significant role in the electrostatics of the system. Negatively charged defect at the TMD edges contribute however to the electrostatic landscape probed by KPFM on both TMD layers.

Evidence for highly p-type doping and type II band alignment in large scale monolayer WSe2/Se-terminated GaAs heterojunction grown by molecular beam epitaxy

“…Considering a quasi-particle bandgap of 2.00 eV in monolayer WSe 2 , the VBM position indicates an electron transfer from the graphene to the WSe 2 and a n-type character for majority carriers in the WSe 2 monolayer. 56 Conversely, the case of WSe 2 /Se-terminated GaAs shows the opposite behavior with a VBM at the K point at energy E − E F = −0.57 ± 0.01 eV, indicating an electron transfer from the WSe 2 to the Se-terminated GaAs and thus a p-type doping of the WSe 2 . When such charge transfer operates at the interface, its creates a permanent interface dipole, which is characterized by a potential step Δ V .…”

Probing Nanoscale Schottky Barrier Characteristics at WSe2/Graphene Heterostructures via Electrostatic Doping

“…When graphene and WSe 2 are contacted, electron transfer from graphene to WSe 2 occurs, as identified by our experimental results and predicted by DFT simulations in literature. [ 32 ] The charge transfer aligns the Fermi level equilibrium and concomitantly leads to a rearrangement of the vacuum levels. The rearrangement is achieved via a vertical interface dipole forming across 2D layers, as opposed to band bending in bulk junctions.…”

“…This observation is consistent with recent studies investigating the local interface properties of heterostructures between WSe 2 and epitaxial graphene containing regions of monolayer and bilayer graphene. [31,32] There, a Fermi level pinning free interface was largely inferred from the Fermi level shift of the respective heterostructures displaying the same energy shift as the underlying monolayer and bilayer graphene. In the present work, Fermi level pinning is locally studied on the same structure via independently modulating the graphene work function at multiple doping levels.…”

“…[ 31 ] By employing Kelvin force probe microscopy (KPFM) measurements on similar heterostructures, the band offsets could be directly linked to a fixed work function shift. [ 32 ] The work function offsets of TMDs on monolayer and bilayer graphene and the resulting differences in Schottky barrier height were further associated with local variations in photoluminescence spectra, namely local populations of exciton and trion emission. [ 33 ] Band offsets reported by the aforementioned techniques appear to directly relate to the local charge transfer characteristics, however an understanding of the recorded band offsets in the context of the Schottky–Mott model is still lacking.…”

Unexpected Electron Transport Suppression in a Heterostructured Graphene–MoS2Multiple Field-Effect Transistor Architecture

“…In particular, devices typically include only two contacts, which makes the effects on the transport characteristics due to the interface not easy to distinguish from those due to the resistivity of the 2D materials, since typically only the global conductance of the device can be measured. Charge transfer phenomena, strain, and charge trapping in defects − might also play an important role. Furthermore, in the case of field-effect devices, the low density of states in the vicinity of the Dirac point leads to weak screening properties despite the metallic nature of graphene .…”