Physical processes and lasing properties of GaAs/(AI,Ga)As quantum-cascade structures and lasers

The physical processes of quantum-cascade structures (QCSs) and the lasing properties of quantum-cascade lasers (QCLs) have been investigated current-voltage characteristics, interband photoluminescence (PL) spectroscopy, and intraband infrared spectroscopy. Undoped QCSs with 20 periods as well as complete QCLs with 30 periods based on GaAs/Al0.33Ga0.67As have been fabricated by molecular-beam epitaxy. The population of both the lower and upper laser level can be directly observed by interband PL above a critical field strength in undoped QCSs, which are photo-excited only in the GaAs contact layers. This occupation of the laser levels is correlated with a negative differential conductance in the dark I-V characteristic at this critical field strength. The PL line of the upper laser level is split into two lines, originating from the resonant coupling of the upper laser level with the injector level. The lasing properties of a set of complete QCLs have been investigated as a function of the injector doping density between 3.5 and 10 x 10(11) cm(-2). The intermediately doped QCLs with a doping density of about 6 x 10(11) cm(-2) exhibit a maximum in the lasing energy, maximum operating temperature. and characteristic temperature parameter, while the threshold current density becomes minimal. For all other QGLs, the threshold current density increases, which is correlated with a decrease in the lasing energy. The frequency dependence of the absorption of free carriers mainly in the waveguides essentially determines the increase of the threshold current density with decreasing lasing energy.