  Electron transport in graphene is along the sheet but junction devices are
often made by stacking different sheets together in a "side-contact" geometry
which causes the current to flow perpendicular to the sheets within the device.
Such geometry presents a challenge to first-principles transport methods. We
solve this problem by implementing a plane-wave based multiple scattering
theory for electron transport. This implementation improves the computational
efficiency over the existing plane-wave transport code, scales better for
parallelization over large number of nodes, and does not require the current
direction to be along a lattice axis. As a first application, we calculate the
tunneling current through a side-contact graphene junction formed by two
separate graphene sheets with the edges overlapping each other. We find that
transport properties of this junction depend strongly on the AA or AB stacking
within the overlapping region as well as the vacuum gap between two graphene
sheets. Such transport behaviors are explained in terms of carbon orbital
orientation, hybridization, and delocalization as the geometry is varied.
