InP Ring-Shaped Quantum Dot Molecules by Droplet Epitaxy - Quantum Dot Molecules - page 39

Fig. 2.8 Schematic displays of each processing steps of InP ring-shaped QDMs on InGaP/GaAs
by droplet epitaxy technique
is observed, compared to the effect of primary step in the deposition process.
Uniform QD is optimized at a crystallization temperature of 200 ◦ C.
In several series of experiments on droplet epitaxy, we can optimize the growth
conditions to give fine InP ring-shaped QDMs by setting the deposition temperature
and crystallization temperature at 250 and 200 ◦ C, respectively. The ring shape
and density as well as the dot size are mainly defined by indium thickness during
deposition step.
The number of QDs per QDM is another feature that is needed to be controlled
for QD device applications. QDMs with eight QDs in ring shape are one of our
aims in fabricating InP ring-shaped QDMs. This octa-quantum dot molecule will be
a quantum dot set applicable for extended quantum dot cellular automata (EQCA).
The concept of QCA will be discussed in detail in the following section.
When InP ring-shaped QDMs are capped by 100 nm thick InGaP for analytical
photoluminescence measurement and for multi-stacked nanostructure, ring-shaped
QDMs should be little affected by InP/InGaP matrix by using first InGaP capping
layer grown by migration enhanced epitaxy technique at a low temperature of
300 ◦ C. To confirm the existence of ring-shaped QDMs, the sample is then prepared
for TEM observation of their cross-sectional profiles. TEM image of InP ring-
shaped QDMs grown at a deposition temperature of 250 ◦ C and a crystallization
temperature of 200 ◦ C with indium deposition rate of 1.6 ML/s and indium thickness
of 3.2 ML is shown in Fig. 2.11 . TEM image can give actual dimensions of the
nanostructure as follows. The outer and inner ring diameters are 140 and 80 nm,
respectively. The basement of QD is approximately 40 nm with the dot height of
few nm. These data are collected from the cross-sectional profile of the topmost
nanostructure which has no thermal annealing effect by the capping process.