Biomedical Engineering Reference
In-Depth Information
Fig. 2.2
Formation mechanism of self-assembled QRs by droplet epitaxy
process using both droplet epitaxy and strain relaxation would give more complex
nanostructures of QDMs which are composed of both QD and QR at the same site.
In this article, we focus on the formation mechanism of InP ring-shaped QDMs
which is quite different from the above-mentioned explanations in terms of both
the material system and the outcome of combined QR/QD. In order to create InP
quantum nanostructures, InGaP with In:Ga content of 50:50 layer is firstly prepared
on GaAs substrate by conventional MBE growth method. This InGaP buffer layer
is lattice matched to GaAs substrate; therefore, no strain exists in this bi-layer
structure. Droplet epitaxy is applied for the whole process in creating InP ring-
shaped QDMs. The very first step starts from In droplet deposition. Then, In droplets
are exposed to P pressure at optimal crystallization temperature to form InP QRs. It
is found that at small thickness of In droplets, only InP QRs are formed. These
InP QRs have symmetrical circular shape due to isotropic crystallization in all
crystallographic directions in the plane of growth. The ring robe of QR is uniform
in terms of both width and height along the whole circumference of QR. When
the thickness of In droplets becomes bigger, the InP QRs are transformed to InP
ring-shaped QDMs during the crystallization process of those thick In droplets. The
formation mechanism of InP QRs is similar to the explanation of InGaAs QRs.
The only difference is their symmetrical circular shape of InP QRs compared to
nonsymmetrical elliptical shape of InGaAs QRs. However, at thick In droplets, InP
QRs are initially formed. The lattice mismatch between InP and InGaP is about
3.8%. This accumulates a strain in the InP QRs. When thickness of ring robe reaches
a critical value, the same strain relaxation works and creates InP QD chain on the
ring robe. The number of QDs on the ring robe depends on the droplet size which
determines the dimension of ring diameter of InP ring-shaped QDMs. Smaller ring
diameter has less number of QDs on the ring robe. The bigger ring diameter gives
more QDs on the ring robe.
With single droplet epitaxy growth process, InP ring-shaped QDMs are created
by the formation of QR as commonly being obtained in most of droplet epitaxy, and
at the same time QDs are created on the ring robe by strain relaxation mechanism
due to lattice mismatch between InP and InGaP epitaxial layers (Fig.
2.3
).
