Electron-phonon interaction in the dynamics of trap filling in quantum dots

Abstract

We analyze theoretically the effects of electron-phonon interaction in the dynamics of an electron that can be trapped to a localized state and detrapped to an extended band state of a small quantum dot (QD) using a simple model system. In spite of its simplicity the time dependent model has no analytical solution but a numerically exact one can be found producing a rich dynamics. The electronic motion is quasi-periodic in time, with oscillations around a mean value that are basic characteristics of the weak and strong coupling regimes of electron-phonon interaction and set the time scales of the system. Using values of the parameters appropriate for defects in semiconductor QDs, we find these time scales to range typically from tenths of picoseconds to a few picoseconds. The values of the time averaged trap occupancy strongly depend on the the strength of the electron-phonon interaction and can be as large as 40$\%$ when the coupling is most efficient, independently of other parameters. An interesting result of the present work is the formation of resonances at specific values of the electron-phonon coupling parameter that only exist when several levels are allowed to coherently cooperate in the filling of the trap. They are characterized by a trap occupancy that is a periodic function of time with large amplitude and period picturing an electron that is periodically trapped and detrapped. We conclude that the formation of these resonances is a robust consequence of electron-phonon interaction in small systems. Electron-phonon interaction is an efficient mechanism that can provide ca. 50$\%$ filling of a deep trap state on a subpicoseconds to picoseconds time scale, much faster than radiative decay occurring in time scales of tens of picoseconds to nanoseconds, while the occupancy of this state will be smaller than ca. 1$\%$ in the absence of electron-phonon coupling.