InP Ring-Shaped Quantum Dot Molecules by Droplet Epitaxy - Quantum Dot Molecules - page 31

ring-shaped QDMs can provide also narrow spectrum of red emission at 0.7 m.

Photoluminescence experiment of InP ring-shaped QDMs is also conducted and
reported in this article.

2.2 The Formation Mechanism of InP Ring-Shaped QDMs

Semiconductor QDs are normally created by strain relaxation at the interface
between lattice-mismatched semiconductor epitaxial layers such as InAs/GaAs.
This growth technique is called Stranski-Krastanov (S-K) growth mode and is
widely used in solid source molecular beam epitaxy (MBE) to prepare InAs
QD nanostructure having good crystal quality for various nanoelectronic and
nanophotonic applications.
This growth mechanism is self-assembly providing random individual InAs QDs
spreading on the plane of epitaxial layer after the critical thickness of InAs wetting
layer is reached (Fig. 2.1 ). InAs QDMs can be self-assembled by modification
of growth process such as thin capping of initial InAs QDs leading to particular
templates for InAs QDMs formation [ 26 ]. Partially capped InAs QDs can also
provide QR templates for InAs bi-QDMs using gas source MBE [ 27 ]. The origin
of InAs QDMs in these cases is based on elastic strain relaxation.
Droplet epitaxy is another alternative technique to prepare quantum nanostruc-
tures from both lattice-matched system such as GaAs on GaAs and small-lattice
mismatched system such as InGaAs on GaAs. The growth process consists of two
steps, i.e., droplet deposition and crystallization. The crystallization of the droplets
begins at the outer edge of the droplets leading to the formation of QRs such as
GaAs QRs and InGaAs QRs on GaAs substrates [ 28 , 29 ]. Out diffusion of Ga and
InGa atoms in the droplets under As pressure is the main growth mechanism of
respective QRs (Fig. 2.2 ).
It is also found that there is some etching effect in the middle part of the Ga
and InGa droplets in contact with the surface of GaAs buffer layer, which leads to
deeper nanoholes inside the QRs. There will remain some residual strain field inside
those deep nanoholes which originates InAs QD formation around and inside QRs
[ 30 , 31 ]. Anisotropy also occurs during InGaAs QR formation and gives birth to
square-shaped nanoholes which also become templates for InAs quadra QDMs [ 7 ].
Combination of QR and QD formations in the same consecutive MBE growth

Fig. 2.1 Formation mechanism of self-assembled QDs by S-K growth mode