Role of the wetting layer in the carrier relaxation in quantum dots

Abstract

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures-grown by modified droplet epitaxy-where no wetting layer is connecting the dots. We find a fast carrier relaxation time ͑30 ps͒ to the dot ground state, which becomes even faster for increasing the photogenerated carrier injection. This shows that the two-dimensional character of the wetting layer is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in literature. We discuss the role of the barrier states as well as the possibility of Auger processes involving the zero-dimensional levels of the quantum dots.

Role of the continuum background for carrier relaxation in InAs quantum dots

“…The rise time values decrease with a 1 / P dependence, with P the excitation density, which is common for a two-particle scattering process. 10,32 From these measurements it is clear, that the carrier capture process is dominated by different mechanisms for the two density regimes. The results as depicted in Fig.…”

“…1 showing the single LOphonon line at the onset of the continuum background, in accordance with observed PLE spectra. 1,6,7 Despite intensive investigations on the carrier capture and relaxation in QDs, the various explanations 1,10,11,[14][15][16][17][18] for the relaxation channel which have been proposed do not provide a single coherent picture. In particular, a clear consensus for the detailed carrier relaxation mechanism is still lacking.…”

“…Toda et al 1 initially associated the continuum to the tail of wetting layer ͑WL͒ defect states, to which the carriers can relax their excess energy. However, Sanguinetti et al 10 showed that even without the presence of the WL, fast carrier relaxation into the QD energy ground state ͑GS͒ is possible with similar relaxation times as observed in Stranski-Krastanow-͑SK-͒grown QDs. More recently, Vasanelli et al 11 explained the continuum background in the PLE spectrum by the existence of crossed transitions between the bound QD states and delocalized states associated with the WL or barrier layer, hence, ascribing the continuum background as due to an intrinsic property of QDs.…”

Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory

“…We do not expect the Al content of the barrier to influence the obtained GaAs nanostructure morphology as the AlGaAs barrier is buried under a 1.75 ML GaAs top layer which forms on the surface during the following Ga deposition step 6,32 . Prior to the deposition of Ga, the As cell is closed and the background pressure reduced below 10 −9 Torr.…”

Optical transitions in quantum ring complexes

“…The captured carriers then relax into the lower lying quantum ring levels where they radiatively recombine. Our previous study showed the recombination lifetime in GaAs/(Al,Ga)As QDs as ∼ 400 ps, whereas the characteristic time of intra-dot relaxation was much shorter -less than 30 ps -depending on excitation density 26,27 . Because of the rapid intra-dot relaxation, we can expect that an electron and a hole recombine after they are in quasi-equilibrium.…”