Impact of Injector Doping on Threshold Current of Mid-Infrared Quantum Cascade Laser–Non-Equilibrium Green’s Function Analysis

Nonequilibrium Green´s function modeling is used to study the mechanism through which doping of the core region influences threshold current of quantum cascade laser. For devices emitting in mid-infrared utilizing two-phonon resonance depopulation scheme thermal backfilling of lower laser state is identified as the main interaction channel. Empirical-simulation based-relation between lower thermal population, doping density, bias, and temperature is found. This relation allows to propose new scaling rule that couples threshold bias with temperature and doping density. Scaling rule was tested against experimental threshold-current-voltage-temperature data collected for InGaAs/AlInAs quantum cascade lasers MBE grown on InP. Devices used in this experiment utilize two phonon resonance scheme, emit at either 4.7 or 9.3 μm, and achieve high hot-operating-temperature performance. Test of scaling was passed by the device which has threshold currents limited by thermal backfilling of lower state. The other device contains spurious state in the injector minigap and so its threshold currents are limited by the intersubband absorption. Data collected for this device do not obey the scaling rule.