Analysis of high-temperature continuous wave operation of type-II interband cascade lasers

In mid-infixed quantum well lasers the optical gain gop,(n,7) is a strong fkction of both the charge carrier concentration and the temperature. At threshold, the condition Tgopr = a,,,, implies a certain relationship between the threshold concentration nth and temperature &. On the other hand, the carrier concentration in active quantum wells is determined by the injection current, which in turn strongly affects the electron temperature via both lattice and electron heating. In equilibrium, this implies another relationship Te(ne), which represents all possible steady states of the system on the n,-T, plane. Comparison of the two dependencies yields a simple and illustrative method for analyzing the temperature performance of semiconductor lasers in the continuous-wave (CW) operation regime [l]. The purpose of this work is to highlight the physical effects most critical for the high-temperature operation of novel type-I1 interband cascade lasers (ICL). The model includes the material gain, optical mode confinement, carrier and lattice overheating, as well as the electrical bias conditions. Auger recombination is essentially responsible for threshold current in type-I1 mid-IR lasers and, is therefore, the basic reason for limiting CW operation of these devices to low temperatures. We show, however, that high value of the specific thermal resistance, which is typical for antimonide based ICL [2], primarily determines the ultimate cause of the device failure. Figure 1 shows 3D surface of the modal gain g, = rgopf. Straight lines on the coordinate plane n,-T, below are isogain curves. We assume that optical loss is not a strong fkction of ne and T, in type-II interband lasers [3], so that the threshold “isogain” line g,= aopf would represent the dependence Tfh(nrh); see also bold straight line in Figure 2. Auger recombination process dominates the current at the threshold, however, the overall device overheating is mostly due to the rise of the lattice temperature; see dashed curves in Figure 2. The intersection of the electron temperature curve with the threshold isogain curve in Figure 2 defines the laser threshold operation point. It is readily seen that for low values of the Auger recombination rate there is a robust lasing generation (lower pair of bold curves), while a 4-fold increase of the Auger coefficient already brings the device out of operation (upper pair of bold curves). It is evident that the electron heating plays a secondary role in the overall device overheating, while the lattice temperature rise dominates the process. Remarkably, just a 3-fold decrease of the thermal resistance brings the laser back into operation range, cf. the thin curves in Fig. 2. -