(Invited) Internal Photoemission of Electrons from 2-Dimensional Semiconductors
Authors: Afanas'ev Valery V. , Chiappe Daniele , Leonhardt Alessandra , Houssa Michel , Huyghebaert Cedric , Radu Iuliana , Stesmans Andre 
Published: February 2020

Few monolayer (ML) thin transition metal dichalcogenide (TMD) semiconductors attract considerable interest because of their tunable properties relevant to a broad range of device applications. In particular, few-layer MoS2 has already been shown to deliver significant advantages in terms of transistor channel downscaling. The performance of the MoS2-based devices critically depends on the band alignment with other materials since the alignment determines the electrostatics of the stack, height of the tunneling barriers, contact resistance, etc. However, barrier characterization at interfaces of few-layer two-dimensional (2D) materials represents an experimental challenge: The electronic transport across these interfaces is strongly affected by hydrocarbon residues of the flake transfer process, non-homogeneities (e.g., domain boundaries), and defects in the material itself. Therefore, it is important to find a technique allowing one to reliably determine the 2D-semiconductor bandgap edge energies with respect to the bands of other materials. In the present work we will show that this goal can be achieved using the spectroscopy of Internal PhotoEmission (IPE) of electrons from MoS2 and WS2 into an insulating layer. By using 2- and 4-ML thick films of these TMDs grown on top of a SiO2 layer as the prototype interfaces, we observe electron IPE from the 2D photo-emitters and determine the corresponding energy barriers.
The studied samples were prepared by thermal evaporation of a thin Mo or W film on SiO2/Si or HfO2(2 nm)/SiO2/Si substrates. Next, the samples were transferred to a furnace and sulfurized in pure H2S(10 or 100 mbar) at 800 °C resulting in the formation of a large-area (in the range of cm2) poly-crystalline MoS2 or WS2 films with crystallites of 50 nm size [1]. Cross-sectional TEM images reveal the characteristic layered structure. The Raman spectrum exhibits two distinct peaks corresponding to the in-plane (E2g) and the out-of-plane (A1g) vibrations in TMD layer, respectively. This observation reveals conversion of the metal film into 2H-MoS2 or 2H-WS2 hexagonal polytypes. By varying the thickness of the initially deposited metal, it appears possible to produce samples with 4 or 2 ML thick MoS2 and WS2 films without contamination of the sulfide/oxide interfaces. Electrical contacts were fabricated by evaporation of 100-nm thick Au or Al pads on top of the TMD films [2].
Fabrication of large area laterally uniform MoS2 and WS2 few-ML thin films enabled us to perform IPE experiments which address one of the fundamental issues concerning energy band alignment between different TMD materials. By observing electron photoemission from the valence band (VB) of MoS2 or WS2 into the conduction band (CB) of SiO2 underlayer, one can find the energy barriers. These correspond to the VB top energies of the two photo-emitting TMD materials referenced to the same energy level of the oxide CB bottom edge [3]. The IPE quantum yield spectra reveal the same field-dependent energy onset of the IPE current in the energy range 3.5-4.0 eV for both MoS2 and WS2 as well as in their stacked heterojenctions. Since the same photocurrent spectra were also obtained when using Al as the contact pad material instead of Au, the TMD layer can be identified as the sole source of photoelectrons [2]. This conclusion is independently affirmed by the observed significant decrease of the quantum yield if reducing the thickness of MoS2 electrode from 4 ML to 2 ML. The observed field dependence of the IPE spectra agrees well with the image-force barrier lowering at the TMD/SiO2 interface. The energy barrier of Fe =4.2±0.1 eV between the top of the MoS2 VB and the bottom of the SiO2 CB can be found by extrapolating this dependence to zero field. No significant difference between spectral thresholds from MoS2 and WS2 is found suggesting that the VBs of of these TMDs are nearly aligned. The same conclusion can be reached if analyzing the IPE yield spectra from MoS2/WS2 and WS2/MoS2 heterostructures. This result would suggest the validity of the “common anion rule” for sulfides of metals similarly to the earlier studied case of insulating metal oxides [3]. Recent IPE experiments addressing the effects of TMD layer transfer on top of insulating substrate will also be discussed.

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