  Auger recombination in semiconductors is a many-body phenomenon in which
recombination of electrons and holes is accompanied by excitation of other
charge carriers. Being nonradiative, it is detrimental to light emission. The
excess energy of the excited carriers is normally rapidly converted to heat,
making Auger processes difficult to probe directly. Here, we employ a technique
in which the Auger-excited carriers are detected by their ability to tunnel out
of the semiconductor through a thin barrier, generating a current. We employ
vertical van der Waals (vdW) heterostructures with monolayer WSe2 as the
semiconductor and the wide band gap hexagonal boron nitride (hBN) as the tunnel
barrier to preferentially transmit high-energy Auger-excited carriers to a
graphite electrode. The unambiguous signatures of Auger processes are a rise in
the photocurrent when excitons are created by resonant excitation, and negative
differential photoconductance resulting from the shifts of the exciton
resonances with voltage. We detect holes Auger-excited by both neutral and
charged excitons, and find that the Auger scattering is surprisingly strong
under weak excitation. The selective extraction of Auger carriers at low,
controlled carrier densities that is enabled by vdW heterostructures
illustrates an important addition to the techniques available for probing
relaxation processes in 2D materials.
