We analyze theoretically several crucial performance aspects of terahertz quantum cascade lasers, such as the impact of doping on the threshold current, the relative importance of the various scattering mechanisms, and the balance of coherent transport and realistic energy dissipation. We have developed a fully self-consistent model for stationary charge transport based on nonequilibrium Green's function theory that takes into account incoherent scattering with phonons, impurities, and rough interfaces as well as electron-electron scattering in the Hartree approximation, but does not a priori assume the electron distributions to follow the periodicity of the quantum cascade laser ͑QCL͒ structure. The theoretical results show excellent quantitative agreement with experimental data. We find scattering at rough interfaces to strongly affect electronic transport and efficiently limit the optical gain. Our results also indicate that a large portion of the current is maintained by coherent multibarrier tunneling. We show that this dominant coherent transport may lead to electron distributions that do not follow the periodicity of the QCL.

We have developed a self‐consistent non‐equilibrium Green's function theory for charge transport and optical gain in quantum cascade lasers (QCL). Acoustic, polar‐optical phonon scattering, impurity, Hartree electron‐electron and interface roughness scattering within the self‐consistent Born approximation are taken into account. The incorporation of the full momentum and energy dependence of the scattering mechanisms turns out to be vital for the QCL device characteristics. The optical gain is calculated in linear optical response, taking into account the non‐equilibrium state occupations. For low‐doping THz GaAs/AlGaAs QCLs, we find the carrier transport at low bias to be dominated by multi‐barrier tunneling rather than by sequential tunneling. Importantly, interface roughness is shown to increase the current density drastically and up to a factor of 1.5 but suppress the gain by a factor of 10 for a realistic interface roughness. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We have developed a self-consistent nonequilibrium Green's function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al .15 Ga .85 As QCL structure. Phonon scattering, impurity, Hartree electronelectron and interface roughness scattering within the selfconsistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.

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