Simulating terahertz quantum cascade lasers: Trends from samples from different labs

Abstract

We present a systematic comparison of the results from our non-equilibrium Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum cascade lasers previously published in the literature. Employing identical material and simulation parameters for all samples, we observe that discrepancies between measured and calculated peak currents are similar for samples from a given group. This suggests that the differences between experiment and theory are partly due to a lacking reproducibility for devices fabricated at different laboratories. Varying the interface roughness height for different devices, we find that the peak current under lasing operation hardly changes, so that differences in interface quality appear not to be the sole reason for the lacking reproducibility.

Ignition of quantum cascade lasers in a state of oscillating electric field domains

“…As input we use the nominal sample parameters together with an exponential interface roughness model [36] with 0.2 nm height and 10 nm lateral correlation length. All these model details agree with [23], where results for a large number of devices are shown. Note, that we define the bias U QCL , electric field F and electric current(density) I(J) with an additional minus sign throughout the paper to compensate for the negative electron charge −e.…”

“…This establishes a further degree of freedom compared to the related superlattices with their rich spectra of nonlinear and chaotic behavior [29][30][31]. In this context the time-resolved measurement of the lasing activity with a fast THz detector [32] Our calculations are based on the NEGF model [22,23] providing us with the current density J(F dc , F ac , ω 0 ) and gain G(F dc , F ac , ω 0 ) which are nonlinear functions of the electric field F (t) = F dc + F ac cos(ω 0 t). Here F dc is the field due to an applied bias and F ac is the electrical component of the lasing field in the waveguide with frequency ω 0 .…”

Control of Electron-State Coupling in Asymmetric Ge/Si−Ge Quantum Wells

“…The growth has to be optimized in order to achieve (i) active epitaxial layers being strain-compensated, relatively thick, and of high crystalline quality; (ii) a high degree of reproducibility in the composition profile of the ultrathin barriers, as required to control the electronic spectrum, the tunneling rates, and the electron subband lifetime; (iii) sharp heterointerfaces to minimize the detrimental impact of interface roughness (IFR) scattering on the QCL gain [22]. Moreover, the development of reliable theoretical tools to model the ISB electronic dynamics in electrically biased structures beyond the naive rate equation approach is another mandatory requirement for the effective design and optimization of the active region stack [23]. This calls for detailed knowledge of the actual material parameters for the Si-Ge system, which can only be obtained through well-designed experiments, supported by an appropriate framework for the theoretical description in a multivalley nonpolar material system of the coupling between electronic states in adjacent quantum structures, tunneling rates, and state lifetimes.…”

Two-well quantum cascade laser optimization by non-equilibrium Green's function modelling

“…In contrast, in this work we will employ an advanced model directly at the optimization stage. Specifically, we shall use a NEGF model 27 , capable of accurately simulating experimental devices 13,26,28 and including the most general treatment of scattering, from all relevant processes.…”