  We report on experimental investigations of an electrically driven WSe2 based
light-emitting van der Waals heterostructure. We observe a threshold voltage
for electroluminescence significantly lower than the corresponding single
particle band gap of monolayer WSe2. This observation can be interpreted by
considering the Coulomb interaction and a tunneling process involving excitons,
well beyond the picture of independent charge carriers. An applied magnetic
field reveals pronounced magneto-oscillations in the electroluminescence of the
free exciton emission intensity with a 1/B-periodicity. This effect is ascribed
to a modulation of the tunneling probability resulting from the Landau
quantization in the graphene electrodes. A sharp feature in the differential
conductance indicates that the Fermi level is pinned and allows for an
estimation of the acceptor binding energy.
