Biomedical Engineering Reference
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Fig. 2.6 AFM image of InP ring-shaped QDMs grown from In thickness 3.2 ML (crystallization
temperature 200 C)
indium volume to each droplet spreading onto InGaP buffer layer. In this series
of droplet epitaxy, the lowest indium thickness of 1.6 ML can provide only InP
QR shape due to the limit volume of group III element. The ring robe is thin
only around 1 nm, which does not give enough strain at InP/InGaP interface for
QD formation. When thicker indium is deposited, QR becomes bigger in diameter.
Strain is exhibited on the ring robe and then is relaxed by forming circular QD chain
on the ring. QD uniformity is optimized at appropriate indium thickness. Further
increase of indium thickness, QD size becomes larger and nonuniform.
Evolution of In droplets to InP ring-shaped QDMs by droplet epitaxy is shown
step-wise in Fig. 2.8 . The InP ring formation is originated from the crystallization at
the outer peripheral of In droplets under P pressure together with the out diffusion
of In atoms from the center of the droplets. At the same time, InP/InGaP lattice
mismatch becomes critical when In thickness reaches a certain value. The strain
relaxation happens at the ring robe and gives birth to circular QD chain formation
on the ring.
Deposition temperature is a key parameter to control the ring diameter, dot size,
and number of QDs on the ring robe. Deposition temperature is varied from 120 to
290 C for In deposition rate of 0.8 ML/s and In thickness of 3.6 ML. At a higher
deposition temperature, a longer ring diameter, larger QD size, and larger number
of QDs on the ring are obtained due to larger droplets being created by merging of
small indium droplets. The density of InP ring-shaped QDMs is therefore decreased.
The experimental results in this series of droplet epitaxy are displayed in Fig. 2.9 .
The uniformity of QD on the ring is optimized at a deposition temperature of 250 C.
This value of deposition temperature is therefore widely used in other series of
droplet epitaxy.
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