Biomedical Engineering Reference
In-Depth Information
of InAs QDs to non-radiative recombination centers present at the interface with the
substrate that can degrade the QDs optical quality [
67
]. This means that it would be
desirable to maintain the maximum temperature of the whole fabrication process as
close as possible to 500
◦
C and the thickness of the buffer layer to a few nanometers
thick.
Accordingly, to proceed with the preferential epitaxial growth of QDs into the
fabricated nanoholes (Fig.
1.13
b, e), the GaAs native oxide resulting after exposure
to HF was removed in the MBE chamber at a low substrate temperature of 450
◦
C.
This process was carried out by exposing the surface to an atomic hydrogen (H)
flux using a Ta H
2
thermal cracker with a H
2
base pressure of 10
−
5
Torr together
10
−
7
Torr) [
68
,
69
]. This is a low temperature process that
avoids using the conventional oxide thermal desorption process at 600
◦
C. On the
other hand, it is known that performing a long H
with As
4
(BEPAs
4
=
5
×
As
4
treatment followed by the
growth of a very thin GaAs buffer layer is an efficient strategy to obtain high optical
emission efficiencies in QDs located in close proximity to the interface with the
substrate [
67
]. Hence, in this work a H
+
As
4
treatment during 30 min preceded
the growth of a 7-nm-thick GaAs buffer layer by ALMBE [
41
] at a substrate
temperature of 450
◦
C, BEPAs
4
=
+
10
−
6
Torr and growth rate of 0.5 ML s
−
1
.The
effect of growing this GaAs buffer layer on the selectivity of the patterned nanoholes
for InAs growth was investigated by depositing 1.5 ML of InAs at 0.01 ML s
−
1
,
substrate temperature of 500
◦
CandBEPAs
4
=
2
×
10
−
7
4) surface
reconstruction was observed] on patterned samples with and without the growth
of a GaAs buffer layer [
22
].
For an initial oxide with a “simple conical structure,” Fig.
1.14
shows the AFM
topography of the resulting nanoholes after HF wet chemical etching (Fig.
1.14
a),
after HF wet chemical etching and H
5
×
Torr [a (2
×
+
As
4
exposure (Fig.
1.14
b), and after HF
wet chemical etching, H
As
4
exposure, and the growth of a 7-nm-thick GaAs
buffer layer (Fig.
1.14
c). Profiles along the [1 1 0] and [1
+
1 0] directions of the
nanoholes are also shown in Fig.
1.14
d. The nanohole evolution due to the previ-
ously described processes and before InAs deposition is summarized in Table
1.2
.
Particularly, the average relative differences in depth and width of the obtained
nanohole along the [1 1 0] and [1
−
1 0] directions with respect to the initial nanohole
(after HF wet chemical etching) are shown. A positive (negative) value means an
increase (decrease) of the specified nanohole dimension. The observed nanoholes
depth increases after the treatment with atomic H is a well-known and reported
effect [
62
].
After HF wet chemical etching (Fig.
1.14
a), the nanohole shows a clear round
shape with a diameter of 160 nm and a depth of 10 nm. This round shape evolves
to a hexagonal one (Fig.
1.14
b) after performing the atomic H
−
As
4
treatment,
showing two sides parallel to the [1 1 0] direction that correspond to the intersection
of the B-type facets (As-terminated) with the (0 0 1) surface plane [
70
]. When a
7-nm-thick GaAs buffer layer is grown, a clear nanohole enlargement along the
[1 1 0] direction occurs (Fig.
1.14
c). The observed decrease of the nanohole width
along the [1
+
−
1 0] direction is attributed to the presence of well-defined B-type
