Biomedical Engineering Reference
In-Depth Information
Fig. 1.2
AFM topography image of the nanoholes template formed by the droplet epitaxy
“nanodrilling” technique. The
inset
shows a 3D rendered image of a single nanohole. Adapted
from [
42
]. Copyright 2008 American Institute of Physics
of the higher Ga atoms diffusion in this direction. The mean nanoholes depth is
4.4
±
±
±
0.8 nm with mean diameter values of 95
5 nm and 43
3 nm along the
−
GaAs [1 1 0] and [1
1 0] directions, respectively.
On this template of nanoholes, three samples are fabricated depending on
the amount of InAs material deposited to form QD
1
. Particularly, 1.2, 1.4, and
1.5 ML of InAs were deposited at a substrate temperature of 500
◦
C, a growth
rate of 0.01 ML s
−
1
,andBEPAs
4
10
−
7
Torr. As an intermediate tunneling
barrier for carriers, a 4-nm-thick GaAs epitaxial layer is grown by ALMBE
[
41
] at a low substrate temperature of 450
◦
C, growth rate of 0.5 ML s
−
1
,and
BEPAs
4
=
×
5
10
−
6
Torr. This growth mode allows for growing atomically flat
GaAs epitaxial layers at low substrate temperatures, thus reducing possible material
inter-diffusion processes (i.e., abrupt interfaces can be achieved). Finally, for
completing the molecule structure, a second layer of QDs (QD
2
)isgrownonthe
surface by depositing 0.9 ML of InAs under the same growth conditions than QD
1
.
Note that due to the large strain field built-in on the surface by QD
1
, the amount
of InAs material needed for the formation of 3D nanostructures is much lower than
the critical thickness for QDs formation in a self-assembled Stranski-Krastanov
process. In order to study their optical emission, these molecules were finally capped
with a 155 nm-thick GaAs layer.
The structure of the resulting nanostructures is shown in the dark field (cross-
sections) transmission electron microscopy (TEM) images of Fig.
1.3
[
43
]. The
contrast in the images is given by changes in material composition, being the
bright areas InAs-rich zones. The formation of a double structure consisting of
two InAs nuclei separated by a thin and darker GaAs intermediate layer is clearly
observed. The corresponding InAs wetting layers (WLs) are also observed in the
image. Interestingly, the nanostructure QD
1
forms at a lower level than its respective
WL, contrarily to what is observed for the nanostructure QD
2
formed on top,
=
×
2
