12.2 Quantum Dot LED
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concern. This section will focus on semiconductor quantum dot LEDs to introduce
their fabrication methods, optical properties, advantages of quantum dot LEDs over
traditional quantum well LEDs, and the current state of research on quantum dot
LEDs.
12.2.1 Preparation Method of Quantum Dots
There are many methods for preparing quantum dots, among which lithography
and self-organization methods are common. There are also some special preparation methods. The quantum dots can be prepared by photolithography to obtain a
uniform array. As quantum dots generally require small dimensions (on the order of
nanometers), they are typically prepared using fine exposure techniques such as electron beam exposure and focused ion beam techniques. However, the size of quantum
dots obtained by photolithography is still too large due to the resolution of current
photolithography processes. Furthermore, subsequent etching and other processes
can also introduce some contaminations and damages.
In order to avoid the above problems of lithography method, some methods
for forming quantum dots without photolithography and etching steps have been
developed. In other words, quantum dots can be prepared by epitaxy on a patterned
substrate. For example, SiN x square openings are formed on a GaAs substrate by
lithography and wet etching. Then, AlGaAs is epitaxially grown by MOVPE, leading
to a spontaneously formed AlGaAs pyramid in the SiN x opening. Subsequently,
monolayer GaAs is epitaxially grown. GaAs quantum dots can be formed on top of
the AlGaAs pyramid, while GaAs quantum wells are formed on the four side walls of
the AlGaAs pyramid [15]. By adopting this method, a uniformly arranged quantum
dot array with uniform size can be obtained. At the same time, contamination and
etching damage in the process of preparing quantum dots by the photolithography
method are avoided. However, the as-grown quantum dots still have some problems.
First, the quantum dot pitch is defined by lithography, which is generally large. Thus,
it is difficult to obtain dense quantum dot alignment, which is disadvantageous for
high-power LED or LD devices. Second, there are also losses of illumination at side
wall.
Self-organized growth of defect-free high density quantum dot can be achieved
by Stranski-Krastanow (SK), which leads to a great concern. The SK growth mode
refers to a film growth mode where a two-dimensional continuous film (so-called
“wetting layer”) is formed in the epitaxial growth process of the material, followed
by formation of a three-dimensional island (i.e., quantum dots). The transition from
2D growth to 3D growth stems from the lowest energy principle. Because the strain
energy and surface energy should be considered during the growth process, the strain
energy will increase to a certain extent when the thickness of 2D layer is above a
certain value. Despite the increased surface energy when changed into 3D growth, the
strain energy can be reduced to maintain the lowest energy of the system. However,
265
concern. This section will focus on semiconductor quantum dot LEDs to introduce
their fabrication methods, optical properties, advantages of quantum dot LEDs over
traditional quantum well LEDs, and the current state of research on quantum dot
LEDs.
12.2.1 Preparation Method of Quantum Dots
There are many methods for preparing quantum dots, among which lithography
and self-organization methods are common. There are also some special preparation methods. The quantum dots can be prepared by photolithography to obtain a
uniform array. As quantum dots generally require small dimensions (on the order of
nanometers), they are typically prepared using fine exposure techniques such as electron beam exposure and focused ion beam techniques. However, the size of quantum
dots obtained by photolithography is still too large due to the resolution of current
photolithography processes. Furthermore, subsequent etching and other processes
can also introduce some contaminations and damages.
In order to avoid the above problems of lithography method, some methods
for forming quantum dots without photolithography and etching steps have been
developed. In other words, quantum dots can be prepared by epitaxy on a patterned
substrate. For example, SiN x square openings are formed on a GaAs substrate by
lithography and wet etching. Then, AlGaAs is epitaxially grown by MOVPE, leading
to a spontaneously formed AlGaAs pyramid in the SiN x opening. Subsequently,
monolayer GaAs is epitaxially grown. GaAs quantum dots can be formed on top of
the AlGaAs pyramid, while GaAs quantum wells are formed on the four side walls of
the AlGaAs pyramid [15]. By adopting this method, a uniformly arranged quantum
dot array with uniform size can be obtained. At the same time, contamination and
etching damage in the process of preparing quantum dots by the photolithography
method are avoided. However, the as-grown quantum dots still have some problems.
First, the quantum dot pitch is defined by lithography, which is generally large. Thus,
it is difficult to obtain dense quantum dot alignment, which is disadvantageous for
high-power LED or LD devices. Second, there are also losses of illumination at side
wall.
Self-organized growth of defect-free high density quantum dot can be achieved
by Stranski-Krastanow (SK), which leads to a great concern. The SK growth mode
refers to a film growth mode where a two-dimensional continuous film (so-called
“wetting layer”) is formed in the epitaxial growth process of the material, followed
by formation of a three-dimensional island (i.e., quantum dots). The transition from
2D growth to 3D growth stems from the lowest energy principle. Because the strain
energy and surface energy should be considered during the growth process, the strain
energy will increase to a certain extent when the thickness of 2D layer is above a
certain value. Despite the increased surface energy when changed into 3D growth, the
strain energy can be reduced to maintain the lowest energy of the system. However,