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

Quantum mechanical model of crossing and anti-crossing points in 3D full-band Monte Carlo simulations

This work presents a 3D quantum mechanics based model to address the physics at band structure crossing/anti-crossing points in full band Monte Carlo (FBMC) simulations. The model solves the Krieger and Iafrate (KI) equations in real time using pre-computed coefficients at k-points spatially sampled within the first Brillouin zone. Solving the KI equations in real time makes this model applicable for all electric fields, which enables its use in FBMC device simulations. In this work, a two-level refinement scheme is used to aggressively sample regions in proximity to band crossings for accurate solutions to the KI equations and coarsely sample everywhere else to limit the number of k-points used. The presented sampling method is demonstrated on the band structure of silicon but is effective for the band structure of any semiconductor material. Next, the adaptation of the fully quantum KI model into the semi-classical FBMC method is discussed. Finally, FBMC simulations of hole transport in 4H silicon carbide with and without the KI model are performed. Results along different crystallographic directions for a wide range of electric fields are compared to previously published simulation and experimental values