Modulation characteristics of quantum-dot lasers: the influence of p-type doping and the electronic density of states on obtaining high speed

The influence of p-type modulation doping on self-organized quantum-dot lasers is studied using a quasiequilibrium model that includes the multi-discrete energy levels and the energy levels of the wetting layer. Calculations are presented showing that laser performance can be greatly enhanced through p-doping and that, in contrast to planar quantum-well lasers, the p-doping requirements are moderate. Optimized cavity lengths are found, and the threshold temperature and modulation characteristics are determined for these cavity lengths. The model shows that close energy spacing of the discrete hole levels can severely limit the modulation response, as suggested previously, and that this effect is countered through creation of an excess hole concentration using p-type doping. Good agreement is obtained with the modulation response reported in recent experiments for undoped quantum-dot active regions. The calculations suggest that bandwidths greater than 30 GHz can be obtained with sufficient p-doping. A reduction in the inhomogeneous broadening might increase the laser speed to over 60 GHz.