Biomedical Engineering Reference
In-Depth Information
Fig. 1.13
AFM topography images of oxide structures fabricated on GaAs (0 0 1) by AFM-LAO
for two different oxidation voltages and the corresponding nanoholes obtained after HF selective
etching: “simple conical structure” for
V
ox
14 V
(
d
-
f
). The profiles drawn on one of the oxides/nanoholes are shown at the
right
for both kinds of
structures
=
−
8V(
a
-
c
); “double structure” for
V
ox
=
−
1.3.2
Epitaxial Growth on Patterned GaAs (0 0 1) Substrates
The MBE Stranski-Krastanov growth of coherent self-assembled InAs QDs ensem-
bles on un-patterned GaAs (0 0 1) substrates leads to randomly located QDs with
a relatively broad size distribution. As shown in the previous section, the droplet
epitaxy etching technique allows for a certain control of the QDs size and density
into a nanohole. However, due to the random nature of Ga droplet formation on the
surface, the QDs site formation cannot be controlled as desired. Many efforts have
been dedicated to control the InAs/GaAs (0 0 1) QDs sites nucleation and study
their formation process [
18
,
21
,
22
,
62
-
65
], whereas less work has been realized
for a simultaneous control of both spatial localization and QDs number in a lateral
molecular-like arrangement by ex situ lithographic techniques [
19
,
22
]. As shown
in this chapter, the aim of such ex situ approaches is to define artificially preferential
nucleation sites (nanoholes) to induce the InAs QDs self-assembling into them. The
main constraints imposed by the use of patterned substrates are basically due to
possible evolutions of the nanoholes size (a few nanometers) through: (1) Ga atom
surface migration on GaAs (0 0 1) surfaces for substrate temperatures close to 500
◦
C
[
66
]; (2) growth of a relatively thick GaAs buffer layer, necessary to avoid proximity