Optical Spectroscopy on Ensembles of QDs

The PL has a rather broad distribution (200 meV) which is typical of self-assembled QD systems, reflecting the distribution in QD size. The first striking feature is that the luminescence is above the GaN band gap [19, 43], which is the normal situation for a confined system. This is different from polar GaN/AlN QDs of similar sizes for which the luminescence is below the 

GaN band gap owing to the quantum confined Stark effect [4]. This is a first indication that the effects of the internal electric field are reduced compared to polar QDs [19, 43]. The PL spectrum does not evolve as a function of excitation power density over 6 orders of magnitude, which shows that no screening or state filling effects are observed in our CW experiments. 

A more sensitive way of probing the effect of the internal electric field is to study the luminescence decay time. Indeed for polar QDs, the reduction in spatial overlap of the electron and hole wavefunctions induced by the internal electric field has a dramatic effect on the radiative recombination rate [4, 44]. For three-dimensionally confined wavefunctions with negligible excitonic effect, the radiative decay rate is proportional to the electron-hole wavefunction overlap. This leads to very long decay times ranging from 5 ns to 

The first striking feature is that the luminescence decay is rather fast, the intensity reaching fo/e after 150 ps and being less than io/100 after 1.5 ns. 

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