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HL: Halbleiterphysik

HL 57: Poster IIa

HL 57.21: Poster

Dienstag, 8. März 2005, 16:30–19:00, Poster TU E

Population inversion in GaAs/Al_0.33Ga_0.67As and GaAs/ Al_0.45Ga_0.55As quantum-cascade structures — •L. Schrottke, S. L. Lu, R. Hey, M. Giehler, H. Kostial, and H. T. Grahn — Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5–7, 10117 Berlin, Germany

The laser level population in undoped GaAs/(Al,Ga)As quantum-cascade structures (QCS) is investigated by interband photoluminescence spectroscopy. We compare QCSs with different Al content in the barriers (x = 0.33 and 0.45). The larger conduction band offset for the GaAs/Al_0.45Ga_0.55As structure improves the electron confinement for the upper laser level and significantly decreases the leakage current into the quasi-continuum, which results in a reduction of the threshold current density. From the calculated lifetimes of the laser levels, the population ratio for the two structures is estimated to be similar. However, the experimental results show that the population ratio for x = 0.45 appears to be larger than for x = 0.33. At the same time, the line width of the upper laser level is narrower than the one for x = 0.33, which agrees with the smaller variation of the lasing energies of the corresponding QC lasers. We discuss possible mechanisms for this broadening and the apparent reduction of the population ratio in structures with x = 0.33 as well as the consequences for the population inversion with respect to the threshold current densities.