Monte Carlo simulation of terahertz step well quantum cascade laser structures

In this paper, electron transport properties of terahertz (THz) step well quantum cascade laser structures are analyzed. These types of structures can allow for the radiative and LO-phonon transitions to be placed within the same well. Under such an arrangement there are three main energy levels, where the transition from the upper state to the middle state is at the THz radiative spacing and the transition from the middle state to the lower state is at or near the LOphonon energy (~ 36 meV in GaAs). The middle state (upper phonon or lower lasing state) is a single energy state, contrasting to previous LO-phonon based quantum cascade laser (QCL) designs that have doublet states. By having vertical radiative and LO-phonon transitions within the same well, it is possible for these types of structures to yield high oscillator strengths, which can lead to increased gain in the active region provided the upper state lifetime and injection efficiency are maintained. The step in the well allows for high injection efficiency due to the spatial separation of the wavefunctions. Monte Carlo simulations are used to analyze the structure in order to investigate these properties. Subband populations, electron temperatures, gain, and current density are extracted from the simulations. Comparisons are made to other existing conventional square well LO-phonon based QCLs. Our analysis indicates that these types of structures should be comparable to other design approaches and that step well injectors can be used to increase the injection efficiency for THz QCLs.