Quantum dots are often referred to as artificial atoms. Nanoscale materials such as semiconductor quantum dots (SQDs) have electronic and optical properties between those of much larger macro and micro scale bulk semiconductor crystals, and much smaller atoms and molecules. Some of the fundamental atom-like properties of optically active quantum dots, such as photon ant bunching and presence of absorption/emission lines predominantly broadened by radiative recombination, have already been confirmed experimentally. In contrast to atoms and electrically-defined quantum dots, optically active quantum dots suffer from spatial and spectral in homogeneity.
There is considerable advancement in quantum dot forming technology but still there seems to be some problems of application of a quantum dot structure to semiconductor devices. One of them is a low efficiency of injecting carriers in quantum dots and the other is phonon bottleneck phenomenon which lowers the efficiency of injecting carriers to the ground level.
Phonon bottleneck
In a quantum dot, the electron inter-sub-level separation may not match the phonon spectrum. This could reduce or even suppress the electron relaxation via the emission of phonon(s) severely This is the "phonon bottleneck" effect. It could keep the electron gasses in the conduction and intermediate bands well separated.
The phonon bottleneck effect reduces the electron relaxation and keeps the electron gasses in the conduction and intermediate bands well separated. Phonon bottleneck effect makes also the radiative recombination between the conduction and the intermediate bands potentially dominant. The recombination between electrons and holes confined within the dots is predominantly radiative because of their high wave function overlap integrals. In the same way, photon absorption is also strong because of the great value of the overlap integrals involved.
The phonon bottleneck effect, or slowed relaxation of electrons through phonon emission in quantum dots is detrimental to their application as optical devices because it degrades the threshold current and the external quantum efficiency.