Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices

Type-II superlattices (T2SLs) are being investigated as an alternative to traditional bulk materials in infrared photodetectors due to predicted fundamental advantages. Subject to significant quantum effects, these materials require the use of quantum transport methodologies, such as the nonequilibrium Green’s function (NEGF) formalism to fully capture the relevant physics without uncontrolled approximations. Carrier mobility is a useful parameter that affects carrier collection in photodetectors. This work investigates the application of mobility extraction methodologies from quantum transport simulations in the case of T2SLs exemplified using an InAs/GaSb midwave structure. In a resistive region, the average velocity can be used to calculate an apparent mobility that incorporates both diffusive and ballistic effects. However, the validity of this mobility for predicting device properties is limited to cases of diffusive limited transport or when the entire device can be included in the simulation domain. Two methods that have been proposed to extract diffusive limited mobility, one based on approximating the ballistic component of transport and the other which considers the scaling of resistance with simulation size, were also studied. In particular, the resistance scaling approach is demonstrated to be the method most physically relevant to predicting macroscopic transport. We present a method for calculating the mobility from resistance scaling considerations that accounts for carrier density variation between calculations, which is particularly relevant in the case of electrons. Finally, we comment on the implications of applying the different mobility extraction methodologies to device property predictions. The conclusions of this study are not limited to T2SLs, and may be generally relevant to quantum transport mobility studies

Nonequilibrium Green’s Function Modeling of type-II Superlattice Detectors and its Connection to Semiclassical Approaches

Theoretical investigations of carrier transport in type-II superlattice detectors have been mostly limited to simplified semiclassical treatments, due to the computational challenges posed by quantum kinetic approaches. For example, interband tunneling in broken-gap configurations calls for a multiband description of the electronic structure, and spatially indirect optical transitions in superlattice absorbers require fully nonlocal carrier-photon self-energies. Moreover, a large number of iterations is needed to achieve self-consistency between Green’s functions and self-energies in the presence of strongly localized states not directly accessible from the contacts. We demonstrate an accurate, yet computationally feasible nonequilibrium Green’s function model of superlattice detectors by formulating the kinetic equations in terms of problem-matched maximally localized basis functions, numerically generated from few modes representing the main conductive channels of the nanostructure. The contribution of all the remaining modes is folded in an additional self-energy to ensure current conservation. Inspection of spatially and energetically resolved single particle properties offers insight into the complex nature of carrier transport in type-II superlattice detectors, and the connection to semiclassical approaches enables the interpretation of mobility experiments