Biomedical Engineering Reference
In-Depth Information
3
0.6
2.5
0.4
2
0.2
1.5
0
1
−0.2
0.5
−0.4
0
−0.6
−0.5
−1
−0.8
−10
−5
0
5
10
x [nm]
Fig. 6.1
tot
for a
c
-plane
lens-shaped InN/GaN QD. The potential is shown for a slice through the QD center. The
z
-axis is
parallel to the
Contour plot of the total (spontaneous + piezoelectric) built-in potential
φ
[
0001
]
-direction. [From [
67
]]
discuss the overall variation of the built-in potential and how it affects the electronic
structure and consequently the optical properties. In a second step we then analyze
how the aspect ratio of a
c
-plane nitride-based heterostructure affects the built-in
potential drop across such a system. This analysis reveals that the built-in potential
across a
c
-plane nitride-based QD is significantly reduced compared to that across
a QW of the same height and with the same indium composition. This opens the
possibility to use QDs with higher indium composition to achieve more efficient
radiative carrier recombination in optoelectronic devices operating in the green and
yellow spectral region.
6.3.1
Built-In Potential in an Isolated QD
Figure
6.1
shows a contour plot of the total built-in potential
φ
tot
calculated for a
model
c
-plane InN/GaN QD with a diameter of
d
2nm,
chosen to illustrate the typical properties of the polarization potential in nitride-
based QDs. First of all, one observes a large potential difference,
=
18 nm and a height of
h
=
5V,
between the top and the bottom of the dot. This potential difference causes a
spatial separation of electron and hole wave functions, with the negatively charged
electrons moving towards the top of the dot while the positively charged holes move
towards the bottom of the QD. This behavior has two main consequences. Firstly,
Δφ
≈
1
.
tot
