Electric Field Domains in Quantum Cascade Lasers

Abstract: Quantum cascade lasers (QCLs) are periodic semiconductor heterostructures, capable of producing laser gain. They have become an important source of coherent radiation in the mid infrared, and THz frequency regions. QCLs are non-linear electric devices, and often have bias regions of negative differential resistivity (NDR). These regions are inherently unstable, and electric field domains (EFDs) form in the structure. They are spatial regions with distinct different electric field strengths. These EFDs have a large impact on the behaviour of the device within the regions of NDR. A theoretical model for describing electron transport in superlattices is discussed and extended upon. This model is also adapted for QCLs, and implemented in a simulation program in order to investigate the dynamics of the EFDs. The program considers few hundred periods, where the behaviour of a single period is obtained beforehand via a program using a non-equilibrium Green's function (NEGF) model. Simulations have been performed using the theoretical model, and compared to experimental results. The simulated results include the electric field strength and electron densities, which are resolved in both space and time, for various externally applied biases. Also the time resolved current and bias across the QCL are calculated. Results have been obtained in good agreement with experiments at higher temperatures, while bias oscillations observed at low temperatures in experiments could not be reproduced in simulations with identical conditions. Simulations with an increased capacitance show a significant impact on pulsed mode operation. Employing an external capacitance in experiments could improve the ability to resolve a plateau with stable EFDs. With a reduced current, bias oscillations have been observed in simulations and analysed. Characteristic shapes of current and bias oscillations have been related to the dynamics of EFDs. Oscillations with good qualitative agreement with experiments have been observed, while the time scale of the oscillations deviated with less than an order of magnitude.