Multiphonon-assisted tunneling through deep levels: A rapid energy-relaxation mechanism in nonideal quantum-dot heterostructures.

Abstract

The presence of a deep-level trap coupled to a quantum-dot heterostructure is shown to provide a rapid energy-relaxation pathway through which electrons may thermalize. A capture process is considered whereby a free conduction-band electron is captured into the ground conduction-band state of a quantum dot by multiphonon-assisted tunneling through the trap. As an example calculation, transition rates for a 5 nm radius ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dot coupled to the defect M1 are calculated as a function of separation between the quantum dot and the deep level. For separations less than \ensuremath{\approxeq}10 nm these rates are found to be in excess of ${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 4.2 K. The result suggests that the presence of point defects may serve to enhance the luminescence efficiency of quantum-dot material. The physical situation described in this paper could only arise if the spatial distribution of defects were strongly correlated with that of the quantum-dot structures, e.g., through formation of interface states or point defects as a consequence of the growth process. With this caveat, the proposed mechanism may possibly explain the failure to observe a significant phonon bottleneck effect in recent work on ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-dot structures [e.g., Appl. Phys. Lett. 64, 2815 (1994)].