Design strategies for GaAs-based unipolar lasers: Optimum injector-active region coupling via resonant tunneling

Electron injection into the upper state of the laser transition in quantum cascade lasers is studied by investigating the electrical and optical chara cteristics of a set of electroluminescent devices. These devices exploit th e active region of an (Al)GaAs laser based on a diagonal-anticrossed transi tion scheme with emission wavelength at 9.5 mum, and are identical except f or the injection barrier thickness which varies from 3.9 up to 8.0 nm. We f ind that for thin barriers electron wave functions in the injector are dire ctly coupled with those of the continuum. This leads to a parallel current path which strongly reduces the injection efficiency of electrons into the active region. The current leak is suppressed at low temperatures for sampl es with the thickest barriers, but it is still observable at high temperatu res when electrons are thermally activated from the injector miniband into the continuum. (C) 2001 American Institute of Physics.