Quantum Efficiency of Quantum Dot Lasers

The quantum efficiency relates the calculated to the measured external threshold current of a laser. This quantity is often estimated from the length dependence of the external differential efficiency above threshold, assuming the carrier density is pinned. Often it is also assumed that the internal current varies linearly with the external current; it is shown here this is not the case due to the effect of stimulated emission on the current-voltage relation of the active region. Furthermore, it has been observed that spontaneous emission from inhomogeneous quantum dots does not pin above threshold which questions the determination of their optical loss. This nonpinning is reproduced by a model in which the laser photon rate equation is coupled to rate equations for the occupation of dot states mediated by a thermal phonon distribution. The threshold current from this model agrees with a Fermi-Dirac calculation but the external efficiency above threshold is lower and its length dependence gives a smaller value of mode loss than the input value. The reasons for this behavior are analyzed and it is concluded that a Fermi-Dirac calculation does not represent the light-current characteristics in quantum dot lasers at room temperature.