Biomedical Engineering Reference
In-Depth Information
lateral formation of QDMs gives more variety of geometrical configurations of
QDMs depending on their number of QDs per QDM like bi-QDMs [
6
], quadra-
QDMs [
7
], QDM chains [
8
] which have potential applications for single electron
transistors, spintronics, quantum computation based on quantum cellular automata
concept [
912
].
Beside the 0D quantum dot nanostructure, quantum ring (QR) is another feature
which exhibits uniqueness due to 1D circular electron transport in ring-shaped
nanostructure for magnetic sensitive behavior [
13
]. Most of the quantum rings are
grown by droplet epitaxy magnetic sensitive behavior technique where group III
elements like Ga, In in droplets are deposited and are followed by crystallization
under pressure of group V elements like As, P [
14
-
16
]. During the crystallization
process, the droplets will be transformed to ring shape due to out diffusion of group
III elements from the center of the droplets because the crystallization is preferably
started from the outer rim of the droplets. This technique has a growth merit that it
can be conducted for either lattice-matched or lattice-mismatched system. GaAs
single quantum rings and GaAs double quantum rings are examples of lattice-
matched nanostructures grown by droplet epitaxy [
17
,
18
]. InGaAs quantum rings
grown on GaAs are realized from lattice-mismatched system [
19
,
20
].
Combination of QDs and QRs in the same nanostructure is quite interesting due
to dual characteristics of both 0D and 1D electron behaviors. Therefore, ring-shaped
QDMs become our research target in this article. In order to do so, isotropic ring
structure is required to create good ring-shaped QDMs. Isotropic out diffusion of
group III elements during crystallization under the pressure of group V elements is
a key selection of this ring-shaped QDMs where circular and uniform ring robe can
be obtained. It is found that GaAs quantum rings and InGaAs quantum rings exhibit
anisotropic property and show noncircular ring shape having difference ring robes
along [110] and [1-10] crystallographic directions [
21
,
22
].
InGaP having 0.5 In and 0.5 Ga compositions is lattice matched to GaAs [
23
].
Conventional growth technique of Stranski-Kastanov (S-K) is used to prepare
InGaP buffer layer on GaAs substrate. InP quantum rings are then created on
InGaP buffer layer by droplet epitaxy. Isotropic and circular quantum rings are
obtained using InP/InGaP material system. Symmetrical and uniform ring robe of
quantum rings is very important condition to further growth of ring-shaped QDMs.
InP and InGaP have 3.8% lattice mismatching [
24
], and this condition can lead
to the formation of QDs on the ring robe of QRs. Ring-shaped QDMs are then
possible when the growth process continues. Control of some growth parameters
such as droplet deposition temperature, crystallization temperature, and growth rate
can define the number of QDs on the QRs. The dimension as well as the density
of ring-shaped QDMs is also affected from those growth parameters. Experimental
results will be discussed in the following section of the article.
InP is a direct band-gap semiconductor having good thermal, optical, and
electrical properties for high performances of high power photonic and electronic
devices [
25
]. InP nanostructure would further improve the device performances by
the quantum effects like carrier and optical confinements. InP QDs as well as InP
