  Two-dimensional flat-band systems have recently attracted considerable
interest due to the rich physics unveiled by emergent phenomena and correlated
electronic states at van Hove singularities. However, the difficulties in
electrically detecting the flat band position in field-effect structures are
slowing down the investigation of their properties. In this work, we employ
Indium Selenide (InSe) as a flat-band system due to a van Hove singularity at
the valence band edge in a few-layer form of the material without the
requirement of a twist angle. We investigate tunneling photocurrents in gated
few-layer InSe structures and relate them to ambipolar transport and
photoluminescence measurements. We observe an appearance of a sharp change in
tunneling mechanisms due to the presence of the van Hove singularity at the
flat band. We further corroborate our findings by studying tunneling currents
as a reliable probe for the flat-band position up to room temperature. Our
results create an alternative approach to studying flat-band systems in
heterostructures of 2D materials.
