InP Ring-Shaped Quantum Dot Molecules by Droplet Epitaxy - Quantum Dot Molecules - page 45

Fig. 2.15 PL spectra of InP QRs and InP ring-shaped QDMs with different indium thicknesses
(1.6-6.4 ML) excited by 40 mW Ar + laser at 20 K

It is found from this series of experiment that the higher indium deposition rate
yields the better QD quality, and QD uniformity can be achieved. At 1.6 ML/s
indium deposition rate, PL peak is blue-shifted to 740 nm wavelength with FWHM
of 43 meV. We find also that, at this 1.6 ML/s deposition rate of indium, the
percentage of creating octa-QDMs is as high as 46%. It is unfortunate that higher
deposition rate than 1.6 ML/s could not be conducted due to the limitation of In cell
temperature in our MBE equipment.
Final growth parameter to be optimized is indium thickness which gives more
volume to the formation of InP ring-shaped QDMs. At a low indium thickness of
1.6 ML, only InP QRs are formed. Densities of QDs and of ring-shaped QDMs,
number of QDs per ring-shaped QDMs and dimensions of QDs and of ring-shaped
QDMs are increased with indium thickness greater than 1.6 ML. InP QRs and
InP ring-shaped QDM samples with different indium thicknesses (1.6-6.4 ML) are
evaluated for their crystal quality and homogeneity via PL measurement. At higher
indium thickness than 3.2 ML, QDMs lose their dot uniformity. PL peak begins to
split into two peaks because of the QD inhomogeneity attributed to two variations of
QD height which can be confirmed from their respective AFM images. PL spectra
have broader FWHM as shown in Fig. 2.15 .
From the whole series of PL experiments, we optimize the growth conditions
for InP ring-shaped QDM formation by droplet epitaxy as follows. The deposition