Biomedical Engineering Reference
In-Depth Information
Tabl e 1. 1
PL peak emission
energies ascribed to QD
1
and
QD
2
nanostructures
Peak energy (eV)
InAs (ML) QD
1
InAs (ML) QD
2
QD
1
QD
2
1.2
0.9
1.286
|
1.272
1.4
0.9
1.243
|
1.272
1.5
1.26
A clear dependency of the peak emission energy of QD
1
and the
amount of InAs material deposited is demonstrated
0.9
1.230
Note that this fabrication procedure produces areal densities of nanostructures as
low as 2
10
8
cm
−
2
, which would permit an optimal integration of these QDMs as
active optical emitters in advanced photonic devices, where spectroscopic studies at
the single nanostructure level are required.
×
1.2.2
Lateral QDMs
As commented in the introduction of this chapter, the fabrication of QDMs in a
lateral geometry presents several technological advantages as compared to a vertical
arrangement. In this subsection, we will describe a novel in situ fabrication process
that, based on a preferential nucleation procedure of InAs material into GaAs
nanoholes etched by the droplet “nanodrilling” technique, allows for the formation
of lateral In(Ga)As QDMs with high optical quality [
20
]. Particularly, we will show
that depending on the As
2
pressure at which InAs is deposited, the formation of
single QDs or QD pairs into each of the nanoholes can be obtained.
Similarly to the previously described fabrication process of vertical QDMs, the
formation of lateral QDMs starts with an initial flattening of the GaAs substrate
by growing a 0.5
m-thick buffer layer at a substrate temperature of 580
◦
Cby
MBE. Right afterwards, the droplet etching “nanodrilling” process is performed in
order to obtain a template of nanoholes similar to the one shown in Fig.
1.2
.On
this patterned surface, 1.5 ML of InAs are deposited at a substrate temperature of
510
◦
C and following a growth sequence consisting in the deposition of 0.1 ML
at a growth rate of 0.05 ML s
−
1
followed by a pause of 2 s under an As
2
flux.
Keeping constant the rest of growth parameters, the As
2
pressure during this InAs
deposition and annealing process was varied in two different samples taking values
of 3.5
10
−
7
10
−
6
4) surface reconstruction
was always observed at the used As
2
pressures. In both samples, for optical and
topographic investigation purposes, capped with a 100 nm-thick GaAs layer and
uncapped nanostructures were fabricated.
Figure
1.6
shows the AFM topography corresponding to the deposition of InAs
on the nanoholes template for As
2
pressures of 3.5
×
and 1
×
Torr, respectively. A (2
×
10
−
7
×
Torr (Fig.
1.6
a) and
10
−
7
Torr (Fig.
1.6
a)
a single QD is obtained inside the nanoholes with occupancy of 95%. On the other
10
−
6
Torr (Fig.
1.6
b). When using an As
2
pressure of 5
1
×
×
