Origin of Temperature-Dependent Threshold Current in p-Doped and Undoped In(Ga)As Quantum Dot Lasers

We examine the temperature dependence of threshold current in self-assembled quantum dot lasers focusing on the factors that produce an increase of threshold current at high temperatures. Nonradiative recombination makes up the majority of threshold current and the temperature dependence of threshold current at higher temperatures. We measure radiative efficiencies at low current density for an undoped structure of 18% and 9% at 300 and 360 K, respectively. These values decrease at higher currents and are even lower for p-doped structures. In undoped structures, the incomplete population of the ground state due to the thermal distribution of holes limits the gain, and this is exacerbated at higher temperatures. In p-doped structures, the gain is increased, but the degree of improvement over an undoped sample is reduced at elevated temperatures. The increasing nonradiative current density at high temperatures is a result of both driving the device harder to maintain the threshold gain requirement (~50% of the increase for an uncoated, 2-mm long laser) and due to an increase in nonradiative recombination at fixed injection. The latter is similar in both p-doped and undoped structures with the most likely origin being the population of higher lying dot and wetting layer states.