Low-Temperature Nonthermal Population of InAs–GaAs Quantum Dots

Measurements of the unamplified spontaneous emission spectra from 80 K to 350 K of a dot ensemble show clear evidence for increased population of higher lying states in the inhomogeneous distribution as the temperature is reduced from 200 K to 80 K, indicating a nonthermal population at low temperature and confirming that the recombination processes are localized in individual dots. These conclusions are supported by modeling an inhomogeneous ensemble of 2 times 10⁶ dots. From simultaneous measurements of optical gain, our data show that the increase in threshold current density with decreasing temperature below about 200 K (which is commonly observed) is due to increased population of higher lying states associated with the transition to nonthermal behavior.