Current Spreading Efficiency and Fermi Level Pinning in GaInNAs–GaAs Quantum-Well Laser Diodes

The role of the current spreading efficiency, $\eta_s$, on the degradation of the figures of merit of stripe geometry GaInNAs–GaAs quantum well (QW) laser diodes (LDs) is studied as a function of the N content in the range from 0 to 3.3%. It is found that, in N-containing devices, $J_{\rm th}$ is strongly dependent on the current injection location along the stripe. This is attributed to a poor spreading of carriers along the length of the laser stripe when N is present in the diodes. If the current is injected using two parallel probes, the threshold current turns out to be nearly independent on the position of the current injection sites and carrier distribution along the laser stripe is similar in N-free and N-containing devices. A model is proposed to explain this phenomenon in which the low-populated portions of the QW are pumped optically by reabsorption of the photons emitted by the high-populated portions of the QW. Local heating in N-containing devices would cause a temperature gradient along the stripe that hinders this optical pumping of the lowly-injected portion of the cavity. The value of the lateral component of $\eta_s,$ $\eta_{s}^{\rm lat}$, is evaluated by measuring the degree of above-threshold Fermi level pinning in the QW using two probes so any variation in $\eta_s$ will arise from variations in $\eta_{s}^{\rm lat}$ only. To do so, the partially amplified spontaneous emission from the laser diodes is measured above and below threshold, and the result is used to calculate $\eta_s$ . It is found that $\eta_s$ decreases by ${\sim} \,$18% upon addition of N. This reduction can account for half of the observed reduction in the internal quantum efficiency, $\eta_i$, in N-containing LDs with respect to N-free devices. The rest of the degradation of $\eta_i$ could be accounted for by another recombination mechanism such as non-radiative recombination at defects in the barriers. The physical mechanisms responsible for the degradation of $\eta_s$ are discussed and various alternative models are proposed.