Random population of InAs-GaAs quantum dots

Abstract

The processes which control the occupation of quantum dot (QD) states have a major influence on the temperature dependence of threshold current and the modulation speed of QD dot lasers. Using variable stripe length measurements we have investigated the occupation of dot states of different energy as a function of temperature using structures which have a bimodal size distribution which allows us to distinguish emission from two groups of dots (large and small) with different energy states located in different dots, and emission from ground and excited states in the same size group being of different energy but located in the same dot. Between 200 K and 80 K spontaneous emission from higher states on the small dots increases relative to the emission at lower energy from larger dots which indicates a transition to nonthermal population. At 80 K and 20 K, we observe a linear relation between the emission of the ground states of the small and large dots as a function of current, even though they are many (kT) apart and the slope indicates that states of different energy are populated with the same probability. From measured gain spectra we find a local minimum at 180 K in the radiative threshold current density, which is due to an increase in recombination from higher lying states of the small dots as the temperature is reduced. The computed threshold current for thermal occupation at 300 K and random population at 20 K is in excellent agreement with these results.