We have made quantum wells laser diodes by Molecular Beam Epitaxy with emission wavelengths from 2.3 &mgr;m to 3.1
&mgr;m. With growing wavelength, threshold current densities increase almost exponentially. We obtained threshold values
as low as 65 A/cm2 at 2.3 &mgr;m and 156 A/cm2 at 2.62 &mgr;m. At the same time, the valence-band offset decrease from 132
meV (at 2.3 &mgr;m) to 78 meV (at 2.6 &mgr;m). A threshold current density study shows that Auger effect is not the only
responsible for the augmentation of Jth. The reduction of internal efficiency ηi has a greater impact on the increase of Jth. The diminution of the holes confinement is incriminated for the degradation of ηi with growing wavelength. Therefore, to improve Jth at higher wavelengths another kind of barrier has to be utilized (for example, thanks to the use of the quinary material AlGaInAsSb).
We review here our results concerning laser diodes emitting at 2.38 µm and 2.60 μm. We present an original method allowing to determine the monomolecular, radiative and Auger recombination coefficients A, B and C, as well as the transparency carrier density Ntr, the internal loss αi and the gain coefficient go from the differential efficiency and the threshold current density obtained with different laser diodes. We show how these parameters can be used to optimize the number of quantum wells and explain the differences existing between laser diodes emitting at 2.38 and 2.60 μm. At 2.38 μm, we obtained a threshold current density of 76 A/cm2 with a single quantum well laser diode and at 2.60 μm, a Jth of 152 A/cm2 with a double quantum well laser diode. These threshold current densities can be compared favorably to the best reported values in the 0.85-3.0 mu;m range.
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