Biomedical Engineering Reference
In-Depth Information
6.1
Introduction
The semiconductor materials InN, GaN, and AlN and their alloys have attracted con-
siderable attention due to their promising applications both in quantum information
processing applications, where nitride-based quantum dots (QDs) open new spectral
regions for single-photon sources [
1
], and also for more conventional optoelectronic
devices such as lasers and light-emitting diodes (LEDs) [
2
]. These systems are in
principle able to cover a wide wavelength range from ultra-violet to infrared [
2
].
For energy-efficient solid state lighting, which combines output from blue, green,
and red LEDs, InGaN alloys are promising candidates, since the assistance of
phosphor is theoretically not required for a white light source [
2
]. While nitride-
based heterostructures have already been utilized in blue LEDs [
3
]andlasers[
4
],
the emission efficiency of
c
-plane InGaN/GaN quantum wells (QWs) drops signifi-
cantly when going to longer wavelengths through use of higher indium composition
or thicker QWs [
2
]. This behavior is known in the literature as the “green gap”
problem [
2
,
5
]. One of the main reasons for this reduction in efficiency is the strong
electrostatic built-in field in nitride-based heterostructures grown along the polar
c
-
axis [
6
]. The electrostatic built-in field in wurtzite nitride-based heterostructures is
of the order of MV/cm, which is at least an order of magnitude larger than that in
more conventional zinc blende InGaAs nanostructures [
6
-
8
]. Therefore, these built-
in fields significantly affect the electronic and optical properties of nitride-based
optoelectronic devices.
Several different approaches have been discussed in the literature to reduce or
even eliminate these built-in fields [
9
-
16
]. One of the strategies to reduce the
built-in field in nitride-based optoelectronic devices is to replace QWs by QDs,
since the built-in potential in a QD compared to a QW of the same height and
composition is significantly reduced [
15
,
16
]. The indium composition can therefore
be increased in a QD compared to a QW, enabling efficient recombination to
longer wavelengths. We review this approach in more detail, presenting an analysis
that gives insight into the key factors that determine the magnitude of the built-
in potential in an isolated nitride-based heterostructure. This analysis can then
be used to understand and predict how the built-in potential behaves in a stack
of QDs. This is of particular interest for a number of reasons. Firstly, different
authors [
14
,
17
,
18
] have recently demonstrated that InGaN QD-based LEDs and
lasers, operating in the amber and green spectral region, show superior performance
compared to their QW-based counterparts. In the active region of laser structures,
stacks of InGaN QDs have been used, since the application of vertically stacked
InGaAs QD structures has been shown to be beneficial for laser applications [
19
].
For these applications a small spacing between the QD layers is sometimes used to
achieve an electronic coupling between the individual dots along the column [
20
].
Secondly, experimental data indicate that, compared to a single nitride-based QD,
a vertical stacking of nitride-based dots also leads to enhanced photoluminescence
(PL) efficiency and efficient emission at room temperature [
21
]. Again, small barrier
thicknesses (
D
≈
2 nm) have been chosen to achieve a stronger coupling between
