  Metal-graphene-metal photodetectors utilize photocurrent generated near the
graphene/metal junctions and have many advantages including high speed and
broad-band operation. Here, we report on photocurrent generation at ABA/ABC
stacking domain junctions in tri-layer graphene with a responsivity of 0.18
A/W. Unlike usual metal-graphene-metal devices, the photocurrent is generated
in the middle of the graphene channel, not confined to the vicinity of the
metal electrodes. The magnitude and the direction of the photocurrent depend on
the back-gate bias. Theoretical calculations show that there is a built-in band
offset between the two stacking domains, and the dominant mechanism of the
photocurrent is the photo-thermoelectric effect due to the Seebeck coefficient
difference.
