Indirect Band Gap Emission by Hot Electron Injection
in Metal/MoS<sub>2</sub> and Metal/WSe<sub>2</sub> Heterojunctions
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Abstract
Transition metal dichalcogenides
(TMDCs), such as MoS<sub>2</sub> and WSe<sub>2</sub>, are free of
dangling bonds and therefore make more “ideal” Schottky
junctions than bulk semiconductors, which produce Fermi energy pinning
and recombination centers at the interface with bulk metals, inhibiting
charge transfer. Here, we observe a more than 10× enhancement
in the indirect band gap photoluminescence of transition metal dichalcogenides
(TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the
direct band gap emission remains unchanged. We believe the main mechanism
of light emission arises from photoexcited hot electrons in the metal
that are injected into the conduction band of MoS<sub>2</sub> and
WSe<sub>2</sub> and subsequently recombine radiatively with minority
holes in the TMDC. Since the conduction band at the K-point is 0.5
eV higher than at the Σ-point, a lower Schottky barrier exists
for the Σ-point band, making electron injection more favorable.
Also, the Σ band consists of the sulfur <i>p</i><sub><i>z</i></sub> orbital, which overlaps more significantly
with the electron wave functions in the metal. This enhancement in
the indirect emission only occurs for thick flakes of MoS<sub>2</sub> and WSe<sub>2</sub> (≥100 nm) and is completely absent in
monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness
must exceed the depletion width of the Schottky junction, in order
for efficient radiative recombination to occur in the TMDC. The intensity
of this indirect peak decreases at low temperatures, which is consistent
with the hot electron injection model
