Electronic energy levels and energy relaxation mechanisms in self-organized InAs/GaAs quantum dots

We report a spectroscopic investigation of the electronic energy levels and carrier-relaxation mechanisms in self-organized InAs/GaAs quantum dots. Power-dependent photoluminescence (PL) and photoluminescence excitation (PLE) are used to study the energy-level structure. Two excited states, 74 and 120 meV above the luminescent ground state, are identified. As expected for a zero-dimensional system, it is not possible to observe PL from the ground state of the dots when exciting between the energies of the ground and first excited state due to the discrete, atomiclike nature of the electronic states. Selectively excited PL and PLE reveal two mechanisms for the relaxation of carriers from the excited states to the ground state: a nonresonant mechanism dominant in the upper state, and a resonant mechanism, involving the emission of one or more LO phonons of well-defined energy, which is dominant in the lower excited state. The resonant mechanism is shown to be a consequence of the distribution of energy-level spacings in the inhomogeneous ensemble of dots; preferentially selecting dots with an energy-level spacing close to an integer multiple of the LO phonon energy.