RESONANT-TUNNELING IN QUANTUM CASCADE LASERS

Experimental evidence that in quantum cascade lasers electron injectio n into the active region is controlled by resonant tunneling between t wo-dimensional subbands is discussed. A quantitative analysis is carri ed out using an equation for the current density based on a tight-bind ing approximation, Electron injection into the active region is optimi zed when the current density is limited by the lifetime of the excited state of the laser transition. In this regime, quasi-equilibrium is r eached between the population of the injector ground state and that of the excited state of the laser transition characterized by a common q uasi-Fermi level. The design of the injector depends on the selected l aser active region; in particular, the choice of physical parameters, such as doping concentration and injection barrier thicknesses, is in general different for vertical or diagonal transition lasers, The pape r concludes with an investigation of the transport properties at thres hold and its dependence on stimulated emission; a relationship between the differential resistance above threshold and the value of the slop e efficiency is deduced.