Vertical transport in InAs/GaSb superlattices: model results and relation to in-plane transport

Operation of InAs/GaSb superlattice-based devices requires efficient transport of carriers perpendicular to superlattice layers by drift and/or diffusion. While transverse mobility measurements are performed routinely, vertical transport measurements are difficult and nonstandard, so that very little is known about their value and dependence on material quality, which is important in device modeling. In such a situation, model calculations can help fill the void. In this work, both the horizontal and vertical electron transport in InAs/GaSb superlattices qua superlattices, not quantum wells, as in Gold's model or its extensions, are modeled. The respective Boltzmann equations in the relaxation time approximation are solved, using the interface roughness scattering as the dominant mobility-limiting mechanism. In absence of screening, a universal relation that the vertical relaxation rates are always smaller than horizontal relaxation rates is derived; hence vertical mobilities are generally smaller than horizontal mobilities. We calculate vertical and horizontal mobilities as a function of such superlattice parameters as layer widths and the correlation length of interface roughness. The calculated ratios of the vertical to horizontal mobilities can be used to estimate vertical mobilities from measurements of horizontal mobilities.