verticaltransport

Vertical transport is the movement of charge carriers, energy, or signals in the direction perpendicular to the plane of a layered material or device. This contrasts with in-plane (lateral) transport, which occurs along the surface of a layer. Vertical transport is central to devices where current must cross interfaces between stacked or layered structures, such as quantum wells, heterojunctions, or van der Waals stacks.

The mechanisms governing vertical transport include drift and diffusion across interfaces, quantum tunneling through thin barriers, thermionic emission over barriers, and, in some cases, ballistic or hopping transport between adjacent layers. The dominant mechanism depends on layer thickness, barrier height, material quality, and temperature. In ultrathin structures, tunneling and resonant tunneling can strongly influence current–voltage behavior, while thicker barriers favor thermionic or hopping processes.

Common device contexts include resonant tunneling diodes, which rely on vertical transport through quantum wells; vertical-cavity surface-emitting lasers, where carriers are injected perpendicularly into active regions; and vertical transistors, such as vertically oriented field-effect or tunneling transistors, which capitalize on conduction across stacked layers. In nanostructured memories and 3D integrated circuits, vertical transport enables access to multiple storage or logic layers within compact footprints. Two-dimensional material heterostructures (for example, graphene combined with transition metal dichalcogenides) use vertical transport across van der Waals gaps, where barrier engineering and interlayer alignment are critical.

Characterization typically involves current–voltage measurements across interfaces, temperature dependence studies, and spectroscopic techniques. Modeling employs quantum transport methods such as non-equilibrium Green’s functions and transfer-matrix approaches to capture tunneling and interlayer interactions.

Materials considerations include interface quality, barrier materials and heights, band alignment, and contact resistances, all of which strongly affect the efficiency and variability of vertical transport in devices.